JP3712897B2 - Image processing apparatus and image forming apparatus having the same - Google Patents

Description

【０１１３】
ｗｉ ＝ ｓｉ＋ｔｉ＋ｕｉ＋ｖｉ (ｉ＝１，２，…，Ｎ） …（１）
また複数の調整量が入力される場合、中間調出力階調処理部２８は、各調整量に基づいてそれぞれ決定される補正量の総和を、第２補正量とすればよい。たとえば表１に示すように３つの調整量が設定される場合、或る指定濃度値に対応する補正量ｗｉは、式１に示すように、該指定濃度値に対応する第１補正量ｓｉと、第１の調整量に基づいて定められる該指定濃度値に対応する補正量ｔｉと、第２の調整量に基づいて定められる該指定濃度値に対応する補正量ｕｉと、第３の調整量に基づいて定められる該指定濃度値に対応する補正量ｖｉとの和になる。これによって、各種の調整量が設定される場合、調整量に基づいた第２補正量が容易に算出される。
【０１１４】
図１１は、画像形成装置１１における原稿複写時の中間調出力階調処理部２８の主制御処理を示すフローチャートである。なお原稿複写の指示に先立ち、基準補正曲線の選択処理と補正量の設定処理とが行われているので、１本の基準補正曲線が既に選択されており、かつ補正量が補正量記憶部３２に記憶されている。
【０１１５】
使用者が操作パネル１６から原稿複写の指示を入力すると、中間調出力階調処理部２８は、ステップＡ２で、曲線記憶部３１内の選択された基準補正曲線と補正量記憶部３２内の１組の補正量とに基づいて補正曲線を作成し、作成された補正曲線に基づいてγ補正テーブルを作成する。γ補正テーブルは、作成された補正曲線に基づいて作られるディザマトリクス内の各画素毎の補正テーブルである。ステップＡ２の詳細な処理は後述する。
【０１１６】
γ補正テーブルが作成された後、中間調出力階調処理部２８は、ステップＡ３で、処理対象の画像のデータを構成する入力濃度値を、空間フィルタ処理部２７から読込む。中間調出力階調処理部２８は、ステップＡ４で、読込まれた入力濃度値に対して、γ補正テーブルを用いたγ補正処理を行う。これによって、読込まれた入力濃度値が、ディザマトリクス内のどの位置の画素かに応じて、その位置の画素の補正テーブルにより補正される。
【０１１７】
中間調出力階調処理部２８は、ステップＡ５で、処理対象の画像データを構成する全入力濃度値を読込んだか否かを判断する。未だ読込まれていない入力濃度値が残っているならば、ステップＡ５からステップＡ３に戻る。画像データの全処理濃度値が読込まれているならば、主制御処理を終了する。
【０１１８】
以上図１１で説明したように、中間調出力階調処理部２８は、階調補正処理に用いるべき補正曲線を、基準補正曲線と１組の補正量とを用いて原稿複写時に作成する。作成される補正曲線の形状は、補正量に応じて変化する。ゆえに画像処理装置１４は、補正量記憶部３２が複数組の補正量を記憶し、中間調出力階調処理部２８が原稿複写時にいずれか１組の補正量を選択して補正曲線の作成に用いる構成になっていることが好ましい。これによって中間調出力階調処理部２８は、複数の目標の特性曲線にそれぞれ応じた階調補正処理を行うことができる。この際、１組の補正量はたとえば１６個の値から構成されるので、１組の補正量のデータ量は１本の補正曲線のデータ量よりも少ない。すなわち補正量記憶部３２は限られた所定量のデータである補正量の組を、処理の過程で蓄えていれば良い。ゆえに基準補正曲線および複数組の補正量を記憶するのに必要な記憶容量は、複数の目標の特性曲線にそれぞれ応じた補正曲線を記憶するのに必要な記憶容量よりも少ない。これによって第１の実施の形態の画像処理装置１４は、階調補正処理のためデータを記憶するために必要な記憶容量を、補正曲線自体を常に記憶する構成である従来技術の画像形成装置よりも、削減することができる。
【０１１９】
指定濃度値に対応する補正量が画像出力装置１５固有の出力特性に基づいた第１補正量である場合、中間調出力階調処理部２８は、実質的には、画像出力装置１５固有の出力特性に基づいた適切なγ補正処理を、階調補正処理として行うことができる。指定濃度値に対応する補正量が、第１補正量と調整量に基づいた第２補正量との和である場合、中間調出力階調処理部２８は、実質的には、調整量に基づいた濃度値の補正処理と画像出力装置１５固有の出力特性に基づいたγ補正処理とを、階調補正処理として行うことができる。また指定濃度値に対応する補正量は、第２補正量だけであっても良い。この場合中間調出力階調処理部２８は、実質的には、基準補正曲線を用いたγ補正処理と調整量に基づく補正値の補正処理とを、階調補正処理として行うことができる。さらにまた指定濃度値に対応する補正量が第２補正量だけである場合、濃度値の補正に関する調整量の代わりに、画像出力装置１５固有の出力特性に応じた調整量が、画像処理装置１４に与えられても良い。この場合、第２補正量が画像出力装置１５固有の出力特性に応じた量になるので、中間調出力階調処理部２８は、画像出力装置１５固有の出力特性に基づいたγ補正処理を、階調補正処理として行うことができる。
【０１２０】
図１１のステップＡ２の処理において、前述したように補正量は指定濃度値に関してだけ求められているので、補正曲線の作成時に、指定濃度値以外の残りの入力濃度値に対する補正値を、指定濃度値に対する補正値と同じ手順で求めることは難しい。このために残りの入力濃度値に対する補正値として、図１２で説明するように、指定濃度値に対する補正値を補間して得られる値が使用される。また指定濃度値に対応する補正量は既に求められているので、図１５で説明するように、指定濃度値に対応する補正量を補間することによって、残りの入力濃度値に対応する補正量を求めてもよい。これによって基準補正曲線上の全補正値に対応する補正量を得ることができるので、補正曲線を作成することができる。
【０１２１】
図１２は、図１１の中間調出力階調処理部２８の主制御処理のステップＡ２の処理の第１の手順を示すフローチャートである。使用者が操作パネル１６から原稿複写の指示を入力した後、ステップＢ２に進む。中間調出力階調処理部２８は、ステップＢ２で、指定濃度値に対する基準補正曲線上の補正値と指定濃度値に対応する補正量との和を求め、求められた和を指定濃度値に対する補正値にする。中間調出力階調処理部２８は、ステップＢ３で、指定濃度値以外の残りの入力濃度値に対する補正値を、指定濃度値に対する補正曲線上の補正値を用いた補間処理によって求める。前記補間処理は、どのような補間法が用いられても良く、たとえば線形補間法またはスプライン補間法が用いられる。このようにステップＢ２およびステップＢ３の処理によって、γ補正処理で実際に用いるべき補正曲線が作成される。中間調出力階調処理部２８は、ステップＢ４で、作成された補正曲線に基づいてγ補正テーブルを作成し、処理を終了する。このように中間調出力階調処理部２８は、図１２で説明した処理によって、補正曲線を作成することができる。
【０１２２】
図１３は、図１２のステップＢ３の補間処理に線形補間法が用いられた場合に作成される補正曲線である。図１４は、図１２のステップＢ３の補間処理にスプライン補間法が用いられた場合に作成される補正曲線である。図１３および図１４において、黒丸は、指定濃度値に対する補正曲線上の補正値を示している。線形補間法が用いられる場合、補間処理を非常に簡便に行うことができる。スプライン補間処理が用いられる場合、非常に滑らかな補間曲線を作成することができるので、より適正なγ補正を行うことができる。ステップＢ３の補間処理に実際に用いられる補間法は、使用者が希望するγ補正の程度に応じて、適宜選択されればよい。また指定濃度値に対応する補正量が第１補正量と第２補正量との和である場合、図１２で説明した第１の手順を用いるならば、階調補正処理のためのデータを記憶するために必要な記憶容量をさらに削減することができる。
【０１２３】
図１５は、図１１の中間調出力階調処理部２８の主制御処理のステップＡ２の処理の第２の手順を示すフローチャートである。使用者が操作パネル１６から原稿複写の指示を入力した後、ステップＣ２に進む。中間調出力階調処理部２８は、ステップＣ２で、指定濃度値以外の残りの入力濃度値に対応する補正量を、指定濃度値に対応する補正量を用いた補間処理によって求める。補正量の補間処理では、線形補間法が用いられることが好ましい。これによって全ての入力濃度値にそれぞれ対応する補正量が得られる。
【０１２４】
中間調出力階調処理部２８は、ステップＣ３で、入力濃度値に対する基準補正曲線上の補正値と該入力濃度値に対応する補正量との和を求め、求められた和を入力濃度値に対する補正曲線上の補正値にする。このようにステップＣ２およびステップＣ３の処理によって、γ補正処理で実際に用いるべき補正曲線が作成される。中間調出力階調処理部２８は、ステップＣ４で、作成された補正曲線に基づいてγ補正テーブルを作成し、処理を終了する。中間調出力階調処理部２８は、図１５で説明した第２の手順を用いかつ線形補間法を用いる場合、図１２で説明した第１の手順を用いかつ線形補間法を用いる場合よりも、滑らかな補正曲線を作成することができる。
【０１２５】
前述したように、上述した階調補正処理は、ディザ処理を兼ねている。階調補正処理がディザ処理を兼ねている場合、基準補正曲線と指定濃度値に対応する補正量とは、ディザマトリクスのサイズ毎に、設定されていることが好ましい。
【０１２６】
【表２】

【０１２７】
図１６は、２×２のディザマトリクスを用いた多値ディザ処理が行われる場合、ディザマトリクス内の４つの画素の階調値の組合わせを示す図である。表２は、２×２のディザマトリクス内の４つの画素にそれぞれ割当てられる基準のテーブルを示す。ディザマトリクス内の第１画素Ｄ１に対応する補正テーブルの作成処理には、第１のテーブルが用いられる。第２画素Ｄ２に対応する補正テーブルの作成処理には、第２のテーブルが用いられる。第３画素Ｄ３に対応する補正テーブルの作成処理には、第３のテーブルが用いられる。第４画素Ｄ４に対応する補正テーブルの作成処理には、第４のテーブルが用いられる。
【０１２８】
図１７は、２×２のディザマトリクスを用いた多値ディザ処理が行われる場合第１〜第４のテーブルが補正曲線上でどのような意味をもつかを示すグラフである。なお図１７において、ディザマトリクスのサイズｍｓは、ディザマトリクスが２×２のものなので４である。補正曲線のグラフの縦軸が４等分され、分割結果である領域Ｒ１，Ｒ２，Ｒ３，Ｒ４が、順に、第１テーブル、第２テーブル、第３テーブル、第４テーブルに割当てられる。各テーブルにおける入力濃度値に対する補正値は、補正曲線の割当てられた領域内にある一部分に応じて、定められる。
【０１２９】
たとえばγ補正処理に用いるべき補正曲線Ｇ３４に対して、各テーブルにおける補正値は、以下のようになる。第１のテーブルにおいて、０以上ａ未満の入力濃度値に対する補正値は、該入力濃度値に対する補正曲線Ｇ３４の第１の部分Ｇ３４（１）上の補正値と等しく、ａ以上２５５以下の入力濃度値に対する補正値は、２５５である。第２のテーブルにおいて、０以上ａ未満の入力濃度値に対する補正値は０であり、ａ以上ｂ未満の入力濃度値に対する補正値は、補正曲線Ｇ３４の第２の部分Ｇ３４（２）上の補正値から２５５を減じた値と等しく、ｂ以上２５５以下の入力濃度値に対する補正値は２５５である。第３のテーブルにおいて、０以上ｂ未満の入力濃度値に対する補正値は０であり、ｂ以上ｃ未満の入力濃度値に対する補正値は、補正曲線Ｇ３４の第３の部分Ｇ３４（３）上の補正値から５１０を減じた値と等しく、ｃ以上２５５以下の入力濃度値に対する補正値は２５５である。第４のテーブルにおいて、０以上ｃ未満の入力濃度値に対する補正値は０であり、ｃ以上２５５以下の入力濃度値に対する補正値は、補正曲線Ｇ３４の第４の部分Ｇ３４（４）上の補正値から７６５を減じた値と等しい。図１７において、「ａ」、「ｂ」、「ｃ」は、補正値「２５５」、「５１０」、「７６５」に対する補正曲線Ｇ３４上の入力濃度値である。なお第２のテーブルにおいて０以上ａ未満の入力濃度値に対する補正値が０になっているのは、第２のテーブルに割当てられた領域Ｒ２内に、補正曲線Ｇ３４内の０以上ａ未満の入力濃度値に対応する部分が存在しないためである。
【０１３０】
補正曲線Ｇ３４に基づいた第１〜第４のテーブルを用いると、たとえば入力濃度値８０に対して、第１画素Ｄ１の補正値は２５５であり、第２画素Ｄ２の補正値は１００であり、第３画素Ｄ３の補正値は０であり、第４画素Ｄ４の補正値は０であることが、分かる。このように第１〜第４のテーブルを用いて、入力濃度値に対応する補正曲線上の補正値を第１〜第４画素に振分けた値が求められる。
【０１３１】
以上の説明では、２×２のディザマトリクスが用いられる場合を例として説明したが、ディザマトリクスはこれに限らず、３×３以上のディザマトリクスが用いられても良い。３×３のディザマトリクスが用いられる場合、各画素毎の補正テーブルを作成するには、基準補正曲線の縦軸を９等分すればよく、４×４のディザマトリクスが用いられる場合、各画素毎のテーブルを作成するには、補正曲線の縦軸を１６等分すればよい。すなわちディザマトリクス内の画素の数と同数の領域が得られるように、補正曲線の縦軸を等分割すればよい。基準補正曲線および補正量がディザマトリクスのサイズ毎に設定されている場合、中間調出力階調処理部２８は、ディザマトリクスのサイズに応じて適切な階調補正を行うことができる。
【０１３２】
図１８は、本発明の第２の実施の形態に係る画像処理装置５１の構成を示すブロック図である。第２の実施の形態の説明において、第１の実施の形態で既に説明している装置、処理部およびパラメータには同じ参照符を付し、詳細説明は省略する。第２の実施の形態の画像処理装置５１は、Ａ／Ｄ変換部２１、シェーディング補正部２２、入力階調補正部２３、色補正部２４、像域分離処理部２５、墨生成下色除去部２６、空間フィルタ処理部２７、中間調出力階調処理部５３、曲線記憶部３１、補正量記憶部３２、および補正係数記憶部（補正係数記憶手段）５４を含む。補正係数記憶部５４は、少なくともデータが読出し可能である不揮発性の記憶部、たとえばＲＯＭで実現される。第２の実施の形態の画像処理装置５１を備えた画像形成装置５５は、第１の実施の形態の画像形成装置１１において、第１の実施の形態の画像処理装置１４が第２の実施の形態の画像処理装置５１に置換えられた構成になっている。
【０１３３】
前述した第１の実施の形態の画像処理装置１４の中間調出力階調処理部２８の処理の説明は、画像入力装置１３の特性のばらつき、および原稿の下地濃度の影響がほとんどない場合の説明である。画像入力装置１３の特性ばらつきの影響および原稿の下地濃度の影響は、無視できない場合がある。たとえば原稿が、有彩色の記録紙に画像が形成されている場合、または白い記録紙であっても下地濃度が異なる記録紙に画像が形成されている場合、画像入力装置１３の特性ばらつきの影響や原稿の下地濃度の影響を無視することは難しい。画像入力装置１３の特性ばらつきの影響および原稿の下地濃度の影響を考慮して階調補正処理を行うために、第２の実施の形態の中間調出力階調処理部５３は、以下に説明する処理を含む階調補正処理を行う必要がある。
【０１３４】
第２の実施の形態の中間調出力階調処理部５３における処理は、第１の実施の形態の中間調出力階調処理部２８における処理と比較して、▲１▼指定濃度値に対応する出力測定値と複数の基準特性曲線とを比較し、上記出力測定値に最も近い基準特性曲線を選択する工程と、▲２▼指定濃度値に対応する補正量である第１補正量を求める際に、入力濃度値に対する出力測定値の特性と所定の目標の特性曲線とを比較する工程とが異なるだけであり、他の工程は同じであるので、上記の異なる▲１▼および▲２▼の工程についてだけ説明する。
【０１３５】
まず、指定濃度値に対応する出力測定値に最も近い基準特性曲線を選択する工程について説明する。中間調出力階調処理部５３は、最初に、予め準備された試験画像に階調補正処理を施さずに、上記試験画像を、画像出力装置１５によって、記録媒体である記録紙に印刷する。次に、印刷された試験画像を、画像入力装置１３に読取らせる。このとき中間調出力階調処理部５３は、試験画像の下地濃度の読取りも同時に行い、このとき読取られた濃度値を入力濃度値が０のパッチの出力測定値とする。試験画像の下地濃度の測定箇所は、記録紙表面の試験画像が形成されていない領域ならば、どの領域でもよい。たとえば図１９に示す構成の試験画像が印刷された記録媒体では、破線で示した領域５６が、下地濃度の測定箇所として用いられる。
【０１３６】
このようにして読取られた試験画像の出力測定値は、たとえば図２０に示す黒丸のようになる。図２０は、試験画像の出力測定値の一例を示しており、出力測定値は、画像入力装置１３の特性ばらつきや下地濃度に応じて異なる。この場合、画像入力装置１３の特性ばらつきや下地濃度の影響に起因して、測定された出力測定値は全体に高く観測されている。このとき、高濃度側の出力測定値を基に、最も近い基準特性曲線を選択することもできるが、その場合、低濃度側の階調特性が劣化してしまう。低濃度側は高濃度側と比べて視覚的に目立ちやすくなるので、低濃度側の階調特性が悪くなることは好ましくない。したがって、基準特性曲線を選択する処理を行う前に、低濃度側に着目し、出力測定値に対して、画像入力装置１３の特性ばらつきや下地濃度の影響を除去する補正処理を施す必要がある。
【０１３７】
出力測定値に対する補正処理は、図２１に示すような補正係数と、試験画像の入力濃度値が０のパッチの出力測定値、すなわち画像入力装置１３によって読取られた記録媒体の濃度値とを用い、演算によって行う。出力測定値に対する補正処理は、具体的には、図２１の黒丸で示す１組の補正係数を用いて、濃度が低いほど強い補正を施すような濃度補正量を求め、濃度補正量を出力測定値から引くことによって行う。本明細書では、記録紙の試験画像のパッチが形成されていない領域の濃度を測定して得られる濃度値を「下地濃度値」と記す。画像入力装置１３の特性ばらつきがある場合、あるいは、下地濃度が異なる原稿を用いる場合、要因は異なるが、どちらの場合も下地濃度値はシフトして観測される。
【０１３８】
補正係数は、各指定濃度値に対応して設定されている。指定濃度値に対応する補正係数は、測定された下地濃度値が、対応する指定濃度値に及ぼす影響を表している。下地濃度値に補正係数を掛けた値が、出力測定値の補正処理に用いる濃度補正量となる。補正係数および濃度補正量は、それぞれ、入力濃度値が０のときに最大、入力濃度値が最大のときに最小となる。補正係数の取得る最大値は１、最小値は０である。上記補正係数は、事前に、種々の画像入力装置１３や種々の原稿を用いた画像読取りの試験結果を基に適切な値が求められており、補正係数記憶部５４に格納されている。
【０１３９】
各指定濃度値に対応する出力測定値および下地濃度値が測定されると、補正係数記憶部５４に格納されている補正係数が読出される。まず、各指定濃度値に対応する補正係数を下地濃度値に掛けることによって、各指定濃度値に対応する濃度補正量が求められる。次に、各指定濃度値に対応する濃度補正量を各指定濃度値に対応する出力測定値から差引くことによって、各指定濃度値に対応する補正出力濃度値が得られる。
【０１４０】
このようにして求められた補正出力濃度値を出力測定値の代わりに用い、図６および図７で説明した手順に従って、補正出力濃度値と複数の基準特性曲線とが比較されて、最適な基準特性曲線が選ばれる。そして、選ばれた基準特性曲線に対応する基準補正曲線が選択される。以上の手順で基準補正曲線が選択されるので、第２の実施の形態の中間調出力階調処理部５３は、画像入力装置１３の特性ばらつきおよび試験画像が印刷された記録媒体の下地濃度の影響を除いて、画像出力装置１５固有の出力特性に対して最も適切な基準補正曲線を選択することができる。
【０１４１】
中間調出力階調処理部５３は、第１補正量の設定のために、上記の選択された基準補正曲線を用いて試験画像に階調補正処理を施し、処理後の試験画像を画像出力装置１５によって記録紙に印刷する。印刷された試験画像は、再度、画像入力装置１３によって読取られる。これによって、図２２の黒丸で示すような出力測定値が得られる。選択された基準補正曲線を用いた階調補正処理を施した試験画像が記録紙に印刷され、この試験画像の濃度が読取られた場合も、出力測定値はシフトして観測される。ゆえに第１補正量の設定時にも、出力測定値は、基準補正曲線の選択時に説明した出力補正値の補正処理と同様の手法を用いて補正される。このとき下地濃度値は、新たに画像入力装置１３によって読取った値を用いてもよい。あるいは、記録紙の種類が等しければ読取られる下地濃度値はほとんど変わらないので、基準特性曲線を選択する際の値を用いても構わない。
【０１４２】
このようにして求められた補正出力濃度値を出力測定値の代わりに用い、図９および図１０で説明した手順に従って、補正出力濃度値と目標の特性曲線とが比較されて、第１の補正量が求められる。算出された第１補正量が、そのまま指定濃度値に対する補正量として用いられても良く、算出された第１補正量と前述した第２補正量との和が、指定濃度値に対する補正量として用いられても良い。以上の手順で補正量が設定されるので、第２の実施の形態の中間調出力階調処理部５３は、画像入力装置１３の特性ばらつきおよび試験画像が印刷された記録媒体の下地濃度の影響を受けずに、最も適切な補正量を算出することができる。
【０１４３】
以上述べたように、画像入力装置１３に特性ばらつきがあったり、原稿の下地濃度が異なる場合でも、各指定濃度値毎に予め定められた補正係数と測定された下地濃度値を用いて出力測定値の補正を行うことによって、中間調出力階調処理部５３は、これらの影響を排除して適切な階調補正処理を行うことが可能となる。なお上記のように説明した各指定濃度値の補正係数は、シアン（Ｃ）・マゼンタ（Ｍ）・イエロー（Ｙ）・黒（Ｋ）の各色成分毎に求めておくことが好ましい。これによって色成分毎に適した補正量を求めることが可能となるので、中間調出力階調処理部５３はより適切な階調補正処理を施すことができる。
【０１４４】
上記の説明では、補正係数が一意的に定められており、画像入力装置１３の特性ばらつきや原稿の下地濃度の差を１組の補正係数を用いて補正する場合について述べた。デジタルカラー複写機等では様々な色の下地濃度の原稿を複写する場合があり得る。
【０１４５】
このような場合、それぞれの下地濃度に応じた第１の補正量を用いれば、さらに適切な階調補正処理が可能となる。この場合、下地濃度の異なる複数の代表的な記録紙に対する補正係数を予め求めて補正係数記憶部５４に格納しておき、各記録紙に対する第１の補正量を求める際に対応する補正係数を補正係数記憶部５４から読出して補正処理を行えばよい。このようにして、様々な色の下地濃度の記録紙に対応して求められた第１の補正量が補正量記憶部３２に格納される。
【０１４６】
上記第１の補正量の選択に用いられる操作パネル５６は、たとえば、図２３に示すように、記録紙の下地のサンプル６１（下地濃度）を表示する表示部６２・記録紙の下地サンプル６１を順次表示させる操作ボタン６３および記録紙の下地を設定する決定ボタン６４を有する構成のものが挙げられる。操作ボタンは、次のサンプルの表示を指示するボタンと、表示されるサンプルを１つ前に戻すことを指示するボタンとから構成される。使用者は原稿の下地とサンプルとを比較しながら、順次サンプルを表示させ、最も近いサンプルを選択すればよい。中間調出力階調処理部５３は、階調補正処理時に、選択されたサンプルに対応する第１補正量を読出して用いる。階調補正処理時に、原稿に適した第１の補正量が補正量記憶部３２から読出されて用いられるので、中間調出力階調処理部５３は適切な階調補正処理を施すことができる。
【０１４７】
図２４は、本発明の第３の実施の形態に係る画像処理装置７１の構成を示すブロック図である。第３の実施の形態の説明において、第１の実施の形態で既に説明している装置、処理部およびパラメータには同じ参照符を付し、詳細説明は省略する。第３の実施の形態の画像処理装置７１は、Ａ／Ｄ変換部２１、シェーディング補正部２２、入力階調補正部２３、色補正部２４、像域分離処理部２５、墨生成下色除去部２６、空間フィルタ処理部２７、中間調出力階調処理部７３、曲線記憶部３１、補正量記憶部３２、および最大補正値記憶部（最大補正値記憶手段）７４を含む。最大補正値記憶部７４は、少なくともデータが読出し可能である不揮発性の記憶部、たとえばＲＯＭで実現される。第３の実施の形態の画像処理装置７１を備えた画像形成装置７５は、第１の実施の形態の画像形成装置１１において、第１の実施の形態の画像処理装置１４が第３の実施の形態の画像処理装置７１に置換えられた構成になっている。
【０１４８】
前述した第１の実施の形態の画像処理装置１４の中間調出力階調処理部２８の処理の説明は、画像出力装置１５の最大出力値が安定して制御されている場合の説明である。第１の実施の形態の画像処理装置１４において、複数の指定濃度値に最大の入力濃度値である２５５が含まれない場合がある。この場合でも、指定濃度値のうちの最も濃度の高い指定濃度値に対応する画像出力装置１５の出力値（以後「第１出力値」と記す）は、所定の値に補正される。複数の指定濃度値に２５５が含まれない場合、最も高い入力濃度値は、最も濃度の高い指定濃度値以上となる。最も高い入力濃度値に対応する画像出力装置１５の出力値（以後「第２出力値」と記す）は、画像出力装置１５の最大出力値に依存して補正される。この結果、画像出力装置１５の状態によっては、第２出力値は一定の値にならない。これによって、高濃度領域における濃度差、すなわち第２出力値と第１出力値との差は、画像出力装置１５の最大出力値が大きくなるほど大きくなり、画像出力装置１５の最大出力値が小さくなるほど小さくなる。
【０１４９】
画像処理装置は、画像出力装置１５の状態、たとえば感光体の表面電位や現像特性の変動に起因して、最大出力値が異なる場合であっても、高濃度領域における濃度差が大きくならないような適切な階調補正処理を行うことが望まれている。画像出力装置１５の変動が大きく画像出力装置１５の最大出力値の変化が無視できない場合には、画像出力装置１５に与える補正曲線上の補正値に制限を設けるなどのさらなる補正処理が必要になる。このために第３の実施の形態の中間調出力階調処理部７３は、（１）画像出力装置に与える補正曲線上の補正値に制限を加える工程と、（２）補正値の制限に用いる閾値である最大補正値を決定する工程とを、さらに行う。第３の実施の形態の中間調出力階調処理部７３における処理は、第１の実施の形態の中間調出力階調処理部２８における処理の全工程と比較して、（１）の補正値の制限工程と（２）の最大補正値の設定工程とが追加されたことが異なるだけであり、他の工程は同じであるので、上記の異なる（１）および（２）の工程についてだけ説明する。
【０１５０】
図２５は、基準補正曲線の選択を行う工程における、試験画像の各パッチの出力測定値と、該試験画像の各パッチを出力する際に用いられる補正値との関係をグラフ化したものである。基準補正曲線の選択工程では、階調補正を施さない状態で試験画像の出力を行うため、パッチの出力に用いられる補正値は、試験画像のパッチ部分の画素の濃度値のｍｓ倍になる。図２５では、画像出力装置１５の最大出力値が通常の場合と最大出力値が極端に高くなった場合との出力測定値の例を示している。画像出力装置１５が印刷した画素の濃度値が画像入力装置１３によって測定されている場合、画像入力装置１３に特性ばらつきがなくかつ下地濃度の影響がなければ、画像出力装置１５により印刷された画素の濃度値は画像入力装置１３の出力値と等価になっている。
【０１５１】
図２５の説明では、通常の場合の画像出力装置１５の最大出力値をＯｕｔ−ｍａｘ１、極端に高くなった場合の画像出力装置１５の最大出力値をＯｕｔ−ｍａｘ２、基準補正曲線の選択工程において最も濃度の高い指定濃度値ｘｈに対する目標特性曲線上の出力値を目標出力値Ｏｕｔ−ｚとする。また、通常の場合における画像出力装置１５の出力特性をＧ４１、最大出力値が極端に高くなった場合における画像出力装置１５の出力特性をＧ４２、最も高い濃度の指定濃度値ｘｈに対する、通常の場合の出力特性Ｇ４１上の補正値をＺ１（ｘｈ）、最も高い濃度の指定濃度値ｘｈに対する、最大出力値が極端に高くなった場合の出力特性Ｇ４２上の補正値をＺ２（ｘｈ）とする。
【０１５２】
画像出力装置１５の最大出力値が通常の場合と極端に高くなった場合とのどちらでも、最も高い濃度の指定濃度値ｘｈでの補正曲線上の補正値に対応する画像出力装置１５の出力値は、おおよそ目標出力値Ｏｕｔ−ｚに制御される。すなわち、画像出力装置１５に与えられる補正値が、最も高い濃度の指定濃度値ｘｈに対する、通常の場合の出力特性Ｇ４１上の補正値Ｚ１（ｘｈ）、あるいは、最も高い濃度の指定濃度値ｘｈに対する、最大出力値が極端に高くなった場合の出力特性Ｇ４２上の補正値Ｚ２（ｘｈ）のどちらであったとしても、画像出力装置１５の出力値がおよそ目標出力値Ｏｕｔ−ｚになるように、補正曲線は構成されている。
【０１５３】
しかし画像出力装置１５の最大出力値は、補正値に関して制限が設けられていない状態では、２５５×ｍｓの補正値が与えられて出力される。このため、通常の場合および最大出力値が極端に高くなった場合にそれぞれ２５５×ｍｓの補正値が与えられたならば、画像出力装置１５は、最大出力値Ｏｕｔ−ｍａｘ１，Ｏｕｔ−ｍａｘ２の濃度の画素をそれぞれ出力することとなる。よって、上記２通りの場合、最も濃度の高い指定濃度値ｘｈに対する補正値に対応する出力値と最大出力値との差はΔＯｕｔ１とΔＯｕｔ２となり、濃度の高い部分の画像出力装置１５からの出力が大きく異なることとなる。最小の入力濃度値から最も高い濃度の指定濃度値ｘｈまでの入力濃度値と出力値との関係は、画像出力装置１５の変動があってもほぼ同様になるように制御されるが、最も高い濃度の指定濃度値ｘｈ以上の入力濃度値に対しては、画像出力装置１５の最大出力値に依存するので、高濃度領域において濃度差が生じることとなる。高濃度領域における濃度差の発生を防止するためには、画像出力装置１５に与える補正値に関して制限を設ける必要がある。補正値に制限を設ける最大補正値Ｚｌｉｍは、基準補正曲線の選択を行う工程において与えられる。
【０１５４】
図２６の曲線Ｇ５２は、画像出力装置１５に与える補正値に制限として最大補正値を設け、補正値に制限を加える場合の補正値と入力濃度値の関係（補正曲線）をグラフ化したものである。最大補正値で制限を加えない場合、最大の入力濃度値２５５に対して２５５×ｍｓの補正値が対応するような補正曲線Ｇ５１に基づいて、画像出力装置１５に補正値が与えられる。基準補正曲線の選択を行う工程で求められた最大補正値Ｚｌｉｍを用いて制限を加える場合、入力濃度値２５５に対して最大補正値Ｚｌｉｍが対応するような補正曲線Ｇ５２に基づいて、画像出力装置１５に補正値が与えられる。
【０１５５】
最大補正値の設定の方法には、以下の方式（１）〜方式（３）がある。方式（１）では、固定された最大補正値が予め設けられる。補正曲線上の補正値は、既定の最大補正値以下に制限される。方式（２）では、最大補正値として、最も濃度の高い指定濃度値に対する補正曲線上の補正値の既定割合の値が設定される。補正曲線上の補正値は、この最大補正値以下に制限される。方式（３）では、既定の最大出力目標値と各出力測定値とに基づいて、最大出力目標値に対応する最大補正値が求められる。補正曲線上の補正値は、この最大補正値以下に制限される。これらの方式において最大補正値の決定を行う手順について説明する。
【０１５６】
方式（１）の場合、画像出力装置１５の最大出力値が変動しても問題が生じない程度の最大補正値を予め求めておき、この値を固定値として使用する。このようにして求められた最大補正値は、図２４に示す最大補正値記憶部（最大補正値記憶手段）７４に格納される。補正曲線が求められ、補正値に制限を設ける必要がある場合、最大補正値記憶部７４から上記最大補正値が読出されて、補正曲線が修正される。
【０１５７】
方式（２）の場合、中間調出力階調処理部７３は、最も濃度の高い指定濃度値ｘｈに対する補正曲線上の補正値Ｚ（ｘｈ）と、最大補正値決定係数Ｋｌｉｍとを用いて、計算式（２）に基づいて最大補正値Ｚｌｉｍを求める。最大補正値決定係数Ｋｌｉｍは、種々の原稿を基に予め求められる最適な値である。方式（２）の場合、式（２）に基づいて求められた最大補正値Ｚｌｉｍが、最大補正値記憶部７４に格納される。
Ｚｌｉｍ ＝ Ｋｌｉｍ × Ｚ（ｘｈ） …（２）
【０１５８】
最大補正値に代わって、最大補正値決定係数が最大補正値記憶部７４に格納されていてもよい。この場合、最大補正値は、画像形成装置７５に備えられる調整量入力手段から入力される調整量に基づいて求められる最も濃度の高い指定濃度値Ｘｈに対する第２の補正量を画像出力装置固有の出力特性に基づいて定められる最も濃度の高い指定濃度値Ｘｈに対する第１の補正量に加算して指定濃度値Ｘｈに対応する補正量を求め、基準補正曲線と該補正量とに基づいて得られる最も濃度の高い指定濃度値Ｘｈに対する補正値に上記最大補正値決定係数を乗算することによって、求められる。最大補正値決定係数が格納されているならば、求められた補正曲線を修正する必要がある場合、最大補正値記憶部７４から最大補正値決定係数が読出され、読出された最大補正値決定係数を用いて上記の手順で最大補正値が求められ、求められた最大補正値を用いて補正値に制限が加えられる。このように調整量が入力される場合、補正曲線上の補正値は第２の補正量に応じて異なるので、最大補正値決定係数を最大補正値記憶部７４に格納し、画像を出力する毎に最大補正値を算出し補正曲線を修正することが好ましい。
【０１５９】
方式（３）の場合、事前に、最適な状態に調整された画像形成装置７５を用いて最大出力値の画像パターンを出力し、出力された画像パターンを画像入力装置１３によって読取り、その測定値である出力測定値の標準的な値を最大出力目標値Ｏｕｔ−ｌｉｍ−ｔａｒｇとして規定する。最大出力値の画像パターンは、たとえば、図４に示した試験画像の同様のものであり、最大出力値に相当するパッチを含む試験画像等で実現される。試験画像の出力および読取りは複数台の画像形成装置７５を用いてそれぞれ複数回行われる。出力測定値の標準的な値としては、平均値または中央値等が用いられる。
【０１６０】
最大出力目標値を基に最大補正値を求める手順を、図２５を用いて説明する。まず、▲１▼基準補正曲線の選択を行う工程で得られる指定濃度値のパッチの出力測定値と、指定濃度値のパッチの画素値のｍｓ倍の値、すなわち指定濃度値のｍｓ倍の値である補正値とを基に、線形補間またはスプライン補間を用いて、指定濃度値のｍｓ倍の補正値と画像出力装置１５の出力値との関係を表すグラフを作成する。今の場合、画像形成装置７５は最適な状態に調整されているので、補正値と出力値との関係は通常の場合の画像出力装置の出力特性Ｇ４１となる。次いで、▲２▼作成されたグラフと最大出力目標値との交点（Ｚｌｉｍ，Ｏｕｔ−ｌｉｍ−ｔａｒｇ）を求める。最後に、▲３▼求められた交点（Ｚｌｉｍ，Ｏｕｔ−ｌｉｍ−ｔａｒｇ）の補正値Ｚｌｉｍを、最大補正値とする。
【０１６１】
方式（３）の最大補正値を求める手順において、出力測定値が安定して得られない場合には、複数の最大補正値が求められる可能性がある。出力測定値が安定して得られない場合としては、たとえばパッチの濃度が単調に減少するような試験画像を出力しているにも関わらず、補正値と出力値との関係を表すグラフに出力測定値が増加するような部分が存在する場合が挙げられる。複数の最大補正値が求められることを防止するための方法としては、複数の最大出力目標値を設ける方法が挙げられる。
【０１６２】
最大出力目標値を２つ設ける場合の最大補正値の設定手順を、図２７を用いて説明する。図２７のグラフＧ４４は、図２５と同じく、指定濃度値のｍｓ倍の値である補正値と基準補正曲線の選択を行う工程で得られる画像出力装置１５の出力測定値との関係をグラフ化したものである。なお、図２７では、出力値と補正値の関係を線形補間した例を示している。
【０１６３】
まず、▲１▼予め定められた第１最大出力目標値Ｏｕｔ−ｌｉｍ−ｔａｒｇ１および第２最大出力目標値Ｏｕｔ−ｌｉｍ−ｔａｒｇ２とグラフＧ４４との交点を求める。次いで、▲２▼求められた第２最大出力目標値Ｏｕｔ−ｌｉｍ−ｔａｒｇ２とグラフＧ４４との交点の中の補正値が最も小さい交点である第１の交点（Ｚｌｉｍ２，Ｏｕｔ−ｌｉｍ−ｔａｒｇ２）と、第１最大出力目標値とグラフＧ４４との交点の中の、補正値が第１の交点の補正値Ｚｌｉｍ２以下である交点のうち、最も補正値が大きい交点である第２の交点（Ｚｌｉｍ１，Ｏｕｔ−ｌｉｍ−ｔａｒｇ１）とを求める。最後に、式（３）に示すように、求められた第１の交点および第２の交点の補正値の平均値を求め、該平均値を最大補正値Ｚｌｉｍとする。
Ｚｌｉｍ ＝ (Ｚｌｉｍ１＋Ｚｌｉｍ２）÷２ …（３）
【０１６４】
最大出力目標値が２つ設けられる場合、最大出力目標値ならびに第１最大出力目標値・第２最大出力目標値が、最大補正値記憶部７４に格納される。最大補正値記憶部７４には、最大出力目標値ならびに第１最大出力目標値および第２最大出力目標値全ての値を格納しておき、これらの値と、画像出力装置１５における出力測定値と補正値との関係を表す近似曲線との交点を求め、その結果に応じてどの最大出力目標値を使用するのかを決めればよい。すなわち、第１最大出力目標値および第２最大出力目標値との交点の数がそれぞれ１つである場合は、近似曲線は単調に増加する特性であると判断されるので、最大出力目標値を用いて最大補正値を求める。第１最大出力目標値および第２最大出力目標値との交点が２点以上ある場合は、近似曲線は単調増加特性を示していないと判断されるので、第１最大出力目標値および第２最大出力目標値を用いて、式（３）に基づいて最大補正値を求めればよい。最大出力目標値が複数設定される場合、最大出力目標値は通常２つあれば充分である。
【０１６５】
最大補正値を求める方法としては、上記の３通りの方法のうちの何れを用いても構わない。方式（３）の場合、最適な状態に調整された画像形成装置を用いて最大補正値の決定を行うので、方式（１）および方式（２）の場合と比較して、より適切な階調補正処置を施すことができるので好ましい。
【０１６６】
最大補正値を用いた補正曲線の制限工程は、中間調出力階調処理部７３に画像が与えられ、補正曲線Ｇ５１が作成された後に、行われる。図２６に示すように制限後の補正曲線Ｇ５２は、最小の入力濃度値０から所定の入力濃度値ｘαまでの第１の範囲内では、制限が加えられない補正曲線Ｇ５１と等しい。所定の入力濃度値ｘαに対する制限前の補正曲線Ｇ５１上の補正値Ｚ（α）は、最大補正値Ｚｌｉｍ未満である。所定の入力濃度値ｘαから最大の入力濃度値２５５までの第２の範囲内において、各入力濃度値に対する制限後の補正曲線Ｇ５２上の補正値は、最大補正値Ｚｌｉｍ以下に保たれている。第２の範囲内の制限前の補正曲線Ｇ５１上の補正値が単調増加している場合、第２の範囲内の制限後の補正曲線Ｇ５２上の補正値も単調増加するように、補正値は制限されている。第２の範囲内の各入力濃度値に対する制限後の補正曲線Ｇ５２上の補正値は、たとえば、該各入力濃度値に対する制限前の補正曲線Ｇ５１上の補正値から、最大出力値と最大補正値との差に各入力濃度値に応じた既定の係数を掛けた積を減算して得られる。
【０１６７】
上記で述べた最大補正値は、各色成分毎に定められるのがよい。方式（２）のように最大補正値決定係数を用いる場合、方式（３）のように最大出力目標値や第１最大出力目標値および第２最大出力目標値等を使用する場合、これらの値がそれぞれ各色成分毎に定められ、各色成分毎に最大補正値が求められる。このように、最大補正値を各色成分毎に定めることによって、各色成分毎に補正曲線を修正することができるので、より適切な階調処理を施すことが可能となる。
【０１６８】
また複写する原稿の種類や使用者の好みによって、上述の手順で設定された最大補正値を用いて補正値に制限を加える方がよい場合とよくない場合とがある。印画紙写真や印刷写真などの原稿の階調性が滑らかな原稿を複写する場合には補正値に制限を加える方がよい場合が多く、文字原稿などに関しては補正値に制限を加えない方がよい場合が多い。このような理由に基づき、複写する原稿の種類に応じて自動的に補正値に制限を加えるか加えないかを切換えるか、あるいは、使用者が手動で切換え可能にすることが好ましい。これによって、より好ましい画像を得ることが可能となる。
【０１６９】
補正値に制限を加えるか否かを自動的に切換える方法としては、画像形成装置７５に備えられる操作パネル１６からの複写モードを基に制御する方法が挙げられる。たとえば、操作パネル１６から写真モードやカラー画像を出力するカラーモードが選択されているときは、補正曲線上の補正値に制限を加える処理を施し、文字モードや白黒画像モードが入力されているときは、補正値に制限を加える処理を施さないようにすればよい。また、使用者が任意に処理を切換えるようにするには、画像形成装置７５に備えられる操作パネル１６を用いて、補正値に制限を加えるか否かの指示を任意に入力できるように構成すればよい。たとえば、操作パネル１６に手動入力モードを設定し、液晶ディスプレイ等より成る表示部に「滑らかな画像にしますか？」等のメッセージを表示し、使用者に入力を促すようにする方法が挙げられる。
【０１７０】
第１〜第３の実施の形態の説明で用いた各種のパラメータの具体値は例示であり、他の値であってもよい。たとえば、基準補正曲線は、５本に限らず、少なくとも１本あればよい。指定濃度値は、１６個に限らず、複数あればよい。入力濃度値の上限値は２５５に限らない。入力濃度値の下限値は０に限らない。階調補正処理がディザ処理の一部分を兼ねている場合、基準補正値の上限値は入力濃度値の上限値のｍｓ倍であればよく、兼ねていない場合、基準補正値の上限値は入力濃度値の上限値と等しければ良い。
【０１７１】
第１〜第３の実施の形態では、画像処理装置１４，５１，７１は、中間調出力階調処理部２８，５３，７３以外の他の処理部を含んでいる。第１の実施の形態の画像処理装置１４は、中間調出力階調処理部２８と曲線記憶部３１と補正値記憶部３２とを少なくとも含んでいれば良く、他の処理部は適宜省略されてもよい。第２の実施の形態の画像処理装置５１は、中間調出力階調処理部５３と曲線記憶部３１と補正値記憶部３２と補正係数記憶部５４とを少なくとも含んでいればよく、他の処理部は適宜省略されてもよい。第３の実施の形態の画像処理装置７１は、中間調出力階調処理部７３と曲線記憶部３１と補正値記憶部３２と最大補正値記憶部７４とを少なくとも含んでいればよく、他の処理部は適宜省略されてもよい。また第１〜第３の実施の形態の階調補正処理は、中間調生成処理を兼ねている。第１〜第３の実施の形態の階調補正処理と中間調生成処理とは、個別に実行されてもよい。また第１〜第３の実施の形態の階調補正処理は、入力階調補正処理を行う際に適用されてもよい。
【０１７２】
第３の実施の形態で説明したような、画像出力装置１５の最大出力値の変動の影響を除去するための補正値の制限工程および最大補正値の設定工程は、第２の実施の形態において説明したような、画像入力装置１３の特性ばらつきや下地濃度の影響を除去するための出力測定値の補正処理を有する階調補正処理に追加してもよい。画像処理装置は、階調補正処理に伴い、出力測定値の補正処理と補正値の制限工程との両方を行うことがさらに好ましい。これによって画像入力装置１３の特性ばらつきや下地濃度の影響と画像出力装置１５の最大出力値の変動の影響との両方を階調補正処理から除くことができるので、より適切な階調補正処理が可能になる。
【０１７３】
さらにまた第１〜第３の実施の形態の中間調出力階調処理部２８，５３，７３が行う処理は、コンピュータが行っても良い。このために、中間調出力階調処理部２８，５３，７３の処理を中央演算処理回路に行わせるための制御プログラムと、曲線記憶部２１および補正量記憶部２２に記憶されるデータとが、コンピュータが読取り可能な記憶媒体に記憶される。前記制御プログラムとデータとは、階調補正処理のための制御ソフトウエアを構成する。記憶媒体内の制御ソフトウエアがコンピュータにインストールされた後、コンピュータの中央演算処理回路が制御プログラムを実行すると、コンピュータは中間調出力階調処理部２８，５３，７３と同じ処理を行うことができる。
【０１７４】
【発明の効果】
以上のように本発明によれば、画像処理装置において、複数の指定濃度値に対応する補正量と基準補正曲線とだけが常に記憶されており、階調補正処理手段は、階調補正処理に用いられる補正曲線を、画像が与えられる毎に作成する。これによって画像処理装置は、階調補正処理に用いられる補正曲線が変更可能である場合、複数組の補正量と基準補正曲線だけを記憶すればよいので、記憶手段の記憶容量を削減することができる。
また本発明によれば、指定濃度値に対応する補正量および基準補正曲線は、ディザマトリクスの大きさ毎にそれぞれ設定されている。これによって階調補正処理手段は、各ディザマトリクスサイズ毎に適切な階調補正処理を行うことができる。
【０１７５】
また本発明によれば、複数の基準補正曲線が記憶される場合、補正曲線の作成に用いられる基準補正曲線は、画像出力手段固有の出力特性に応じて選ばれる。これによって階調補正処理手段は、画像出力手段固有の出力特性により適した補正曲線を作成することができる。さらにまた本発明によれば、基準補正曲線は、指定濃度値の画像パターンをそのまま画像出力装置に出力させた結果に基づいて選ばれる。これによって階調補正処理手段は、画像出力手段固有の出力特性に対して最も適切な基準補正曲線を選択することができる。
【０１７６】
また本発明によれば、指定濃度値に対応する補正量は、画像出力手段固有の出力特性に基づいて定められる。これによって階調補正処理手段は、画像出力手段固有の出力特性に基づいた適切なγ補正処理を行うことができる。さらにまた本発明によれば、指定濃度値に対応する補正量は、調整量に基づいて定められる。これによって階調補正処理手段は、調整量に基づいた濃度値の補正処理をγ補正処理と同時に行うことができる。また本発明によれば、指定濃度値に対応する補正量は、画像出力手段固有の出力特性と調整量とに基づいて定められる。これによって階調補正処理手段は、画像出力手段固有の出力特性に基づいた適切なγ補正処理を行うことができ、かつ調整量に基づいた補正処理を行うことができる。
【０１７７】
さらにまた本発明によれば、画像出力手段固有の出力特性に基づく補正量は、基準補正曲線を用いた階調補正処理が施された指定濃度値の画像パターンの出力結果に応じて設定される。これによって階調補正処理手段は、画像出力手段固有の出力特性に対して最も適切な第１の補正量を設定することができる。また本発明によれば、複数の調整量が入力される場合、調整量に基づいて定められる補正量は、各調整量に基づいた補正量の総和になっている。これによって前記階調補正処理手段は、複数の調整量が入力される場合、複数の調整量に基づいた補正量を容易に算出することができる。
【０１７８】
さらにまた本発明によれば、指定濃度値に対する補正曲線上の補正値は、指定濃度値に対する基準補正曲線上の補正値に指定濃度値に対する補正量を加算して求められ、指定濃度値以外の入力濃度値に対する補正曲線上の補正値は、指定濃度値に対する補正曲線上の補正値を用いた補間処理によって求められる。これによって階調補正処理手段は、１組の補正量と基準補正曲線だけを用いて、補正曲線を作成することができる。また本発明によれば、補正曲線の作成時に用いられる補間処理は、線形補間処理である。これによって前記階調補正処理手段は、補正曲線を容易に作成することができる。さらにまた本発明によれば、補正曲線の作成時に用いられる補間処理は、スプライン補間処理である。これによって階調補正処理手段は、滑らかな補正曲線を作成することができる。
【０１７９】
また本発明によれば、指定濃度値以外の入力濃度値に対応する補正量が、指定濃度値に対応する補正量を用いた線形補間処理によって求められ、指定濃度値および指定濃度値以外の入力濃度値に対する補正曲線上の補正値は、これら濃度値に対する基準補正曲線上の補正値にこれら濃度値の補正量を加算して求められる。これによって階調補正処理手段は、より滑らかな補正曲線を容易に作成することができる。
【０１８０】
さらにまた本発明によれば、指定濃度値に対応する補正量および基準補正曲線は、各色成分毎にそれぞれ設定されている。これによって階調補正処理手段は、各色成分の濃度値毎に適切な階調補正処理を行うことができる。
【０１８１】
また以上のように本発明によれば、画像処理装置には、複数の指定濃度値に対応する補正量と基準補正曲線とに加えて、補正量設定に用いる適切な補正係数が予め求められて記憶されている。階調補正処理手段は、階調補正処理に用いる補正曲線を、画像が与えられるたびに作成する。これによって階調補正処理手段は、画像入力手段に特性ばらつきがある場合や下地濃度の異なる原稿が複写される場合でも、これらの影響を除いて補正曲線を作成することができるので、適切な階調補正処理を行うことができる。
【０１８２】
また本発明によれば、複数の基準補正曲線が記憶されている場合、基準補正曲線は、指定濃度値の画像パターンをそのまま画像出力手段に出力させた結果に対し、画像パターンが形成されていない領域の記録媒体の濃度値と補正係数とに基づいた補正を施した結果に基づいて選ばれる。これによって階調補正処理手段は、画像入力手段の特性ばらつきや原稿の下地濃度の影響を除き、画像出力手段固有の出力特性に対して最も適切な基準補正曲線を選択することができる。さらにまた本発明によれば、画像出力手段固有の出力特性に基づく補正量は、基準補正曲線を用いて階調補正処理が施された指定濃度値の画像パターンの読取られた濃度値に対して、画像パターンが形成されていない領域の記録媒体の濃度値と補正係数とに基づいて補正を施した結果に応じて設定される。これによって階調補正処理手段は、画像入力手段の特性ばらつきや記録媒体の下地濃度の影響を受けずに、最も適切な補正量を算出することができる。
【０１８３】
また本発明によれば、補正係数は、記録媒体の種類に応じてそれぞれ記憶されており、操作パネルの表示を参考にした手動操作によって任意に選択される。これによって階調補正処理手段は、下地濃度の異なる記録媒体に対しても、適切な補正量を用いて適切な階調補正処理を行うことができる。また補正量が複数組備えられている場合、選択された補正係数と対になる補正量が選択されるので、階調補正処理手段は、下地濃度の異なる記録媒体に対しても、適切な補正量を用いて適切な階調補正処理を行うことができる。さらにまた本発明によれば、補正係数は色成分毎にそれぞれ求められているので、色成分毎に補正量を求めることが可能であり、階調補正処理手段はより適切な階調補正処理を行うことができる。
【０１８４】
また以上のように本発明によれば、画像出力手段の最大出力値の変化幅が無視できない程度に変動する場合においても、画像処理装置の階調補正処理手段は、画像出力手段に与える補正値に制限を加えることによって、高濃度領域における濃度差を抑制し適切な階調補正処理を行うことができる。さらにまた本発明によれば、最も濃い指定濃度値に対応する補正曲線上の補正値に最大補正値決定係数を乗じて最大補正値が求められるので、事前に好ましい最大補正値決定係数を定めて、定められた最大補正値決定係数を最大補正値記憶手段に格納しておけばよい。画像が出力されるたびに出力に先立って最大補正値を算出可能なので、階調補正処理手段は適切な階調補正処理を行うことができる。
【０１８５】
また本発明によれば、階調補正処理手段は、試験画像を出力し、該試験画像を画像入力手段によって読取らせて得られる出力測定値に基づいて最大補正値を求める。これによって出力された試験画像を読取るたびに最大補正値が求められるので、階調補正処理手段はより適切な階調補正処理を行うことができる。さらにまた本発明によれば、出力測定値が安定して得られない可能性がある場合にも、最大補正値の算出に用いる最大出力目標値を複数設けることによって、階調補正処理手段は最大補正値を安定して求めることが可能になる。これによって階調補正処理手段は、より適切な階調補正処理を行うことができる。
【０１８６】
また本発明によれば、画像の濃淡やコントラスト等の調整が行われる場合、これらの調整量に応じて最大補正値が求められるので、階調補正処理手段はより適切な階調補正処理を行うことができる。さらにまた本発明によれば、最大補正値は色成分毎にそれぞれ求められているので、階調補正処理手段は色成分毎に適切な階調補正処理を行うことができる。また本発明によれば、最大補正値を使用するか否かが選択可能になっているので、原稿の種類や使用者の好みに応じて、階調補正処理手段はより適切な階調補正処理を行うことができる。
【図面の簡単な説明】
【図１】本発明の実施の一形態に係る画像処理装置が備えられた画像形成装置の構成を示す正面断面図である。
【図２】本発明の第１の実施の形態に係る画像処理装置の構成を示すブロック図である。
【図３】本発明の階調補正処理に使用される基準補正曲線を示すグラフである。
【図４】本発明の階調補正処理に必要な基準補正曲線を選択するために用いられる基準特性曲線を示すグラフである。
【図５】本発明の階調補正処理に用いられる試験画像を示す図である。
【図６】基準特性曲線の選択にあたり、指定濃度値に対応する出力測定値と基準特性曲線との差を求める手法を説明するためのグラフである。
【図７】指定濃度値に対応する出力測定値と基準特性曲線とを比較して最も近い基準特性曲線を選択する手法を説明するためのグラフである。
【図８】基準補正曲線に基づいて指定濃度値のパッチを出力してスキャナで読込んだ際の出力測定値と、目標の特性曲線とを示すグラフである。
【図９】出力測定値に対する目標特性曲線上の入力濃度値を求める手法を説明するためのグラフである。
【図１０】出力測定値に対する目標特性曲線上の入力濃度値に対する補正値を設定する手法を説明するためのグラフである。
【図１１】図２の画像処理装置内の中間調出力階調処理部の主制御処理を示すフローチャートである。
【図１２】本発明の階調補正処理に用いられるγ補正テーブルの第１の作成手法を示すフローチャートである。
【図１３】本発明の階調補正処理において、指定濃度値以外の入力濃度値に対する補正曲線上の補正値が指定濃度値に対する補正曲線上の補正値を線形補間して求められた場合の補正曲線を示すグラフである。
【図１４】本発明の階調補正処理において、指定濃度値以外の入力濃度値に対する補正曲線上の補正値が指定濃度値に対する補正曲線上の補正値をスプライン補間して求められた場合の補正曲線を示すグラフである。
【図１５】 本発明の階調補正処理に用いられるγ補正テーブルの第２の作成手法を示すフローチャートである。
【図１６】本発明の階調補正処理をディザ処理に適用し、基準補正曲線と指定濃度値に対応する補正量とをディザマトリクスのサイズ毎に設定する手法を示すために、２×２のディザマトリクスにおける階調値の組合わせを示す図である。
【図１７】本発明の階調補正処理をディザ処理に適用し、補正曲線からディザマトリクス内の各画素毎の補正テーブルを設定する手法を示すために、２×２のディザマトリクスが用いられる場合の各画素のテーブルと補正曲線との関係を示すグラフである。
【図１８】本発明の第２の実施の形態に係る画像処理装置の構成を示すブロック図である。
【図１９】本発明の第２の実施の形態において、階調補正処理に用いられる試験画像を示す図である。
【図２０】画像入力装置に特性ばらつきがある場合や原稿の下地濃度が異なる場合、測定された指定濃度値に対応する出力測定値と基準特性曲線との関係を表したグラフである。
【図２１】画像入力装置に特性ばらつきがある場合や原稿の下地濃度が異なる場合の影響を補正する際に用いる補正係数のグラフである。
【図２２】画像入力装置に特性ばらつきがある場合や原稿の下地濃度が異なる場合、基準補正曲線に基づいて指定濃度値のパッチを出力して画像入力装置で読込んだ際の出力測定値と目標の特性曲線との関係を示すグラフである。
【図２３】原稿の下地濃度が異なる場合、サンプルを表示して、それに対応する補正量を任意に選択するための操作パネルの構成例を示す図である。
【図２４】本発明の第３の実施の形態に係る画像処理装置の構成を示すブロック図である。
【図２５】画像出力装置に変動がある場合、高濃度領域において濃度差が生じる要因を説明するためのグラフである。
【図２６】補正値に制限を設ける場合の補正曲線と設けない場合の補正曲線とを示したグラフである。
【図２７】出力値が補正値に対して単調増加特性を示さない場合、最大補正値を求める方法を説明するためのグラフである。
【図２８】従来技術の画像作成装置で使用される濃淡階調の各濃度のパッチからなる画像を示す図である。
【図２９】従来技術の画像作成装置におけるγ補正処理の様子を説明するためのグラフである。
【符号の説明】
１１，５５，７５ 画像形成装置
１３ 画像入力装置
１４，５１，７１ 画像処理装置
１５ 画像出力装置
１６，５６ 操作パネル
２８，５３，７３ 中間調出力階調処理部（階調補正処理手段・中間調補正処理手段）
３１ 基準補正曲線記憶部（基準補正曲線記憶手段）
３２ 補正量記憶部（補正量記憶手段）
５４ 補正係数記憶部（補正係数記憶手段）
７４ 最大補正値記憶部（最大補正値記憶手段）[0001]
[Technical field to which the invention belongs]
  The present invention relates to an image processing apparatus used in a color image forming apparatus such as a digital color copying machine.And image forming apparatus including the sameIn particular, the present invention relates to tone correction processing.
[0002]
[Prior art]
A conventional image forming apparatus that performs gamma correction processing is disclosed in Japanese Patent Laid-Open No. 6-205217. The digital color copying machine disclosed in the publication includes a scanner, a print control unit, and an electrophotographic printer. The print control unit includes a memory, and the memory stores reference image data composed of image patterns of density values of light and shade gradations as shown in FIG. 28 and data of target gradation characteristics G1 (FIG. 29). To do. A user or service person of the digital copying machine causes the printing machine to print the reference image and read the printed matter to the scanner prior to copying the document with γ correction. The printer control unit creates γ correction data G3 by calculation using the data of the reference gradation characteristic G2 of the printed matter as the read result and the data of the target gradation characteristic G1 desired by the user, Remember me. When original copying is performed in the copying machine after the γ correction data G3 is stored, the gradation characteristics of the image of the original read by the scanner are corrected using the γ correction data G3.
[0003]
As shown in FIG. 29, a point A0 corresponding to the image input level L1 on the graph of the reference gradation characteristic G2 is a point A1 corresponding to the image input level L1 on the graph of the target gradation characteristic G1 by the γ correction processing. Should be corrected. For such correction, in the γ correction using the γ correction data G3, the laser exposure amount P (used for outputting the point A2 on the graph of the reference gradation characteristic G2 with respect to the output image density equal to the point A1. L1) is employed during image printing. Similarly, in order to correct the point B0 on the graph of the reference gradation characteristic G2 with respect to the image input level L2 to the point B1 on the graph of the target gradation characteristic G1, the point B2 on the graph of the reference gradation characteristic G2 The laser exposure amount P (L2) used for the output is used at the time of image printing. Thus, every time the serviceman or the user newly creates the γ correction data G3, the printer control unit performs image processing corresponding to the aging change of the electrophotographic process conditions or the change of the characteristics of the toner used. .
[0004]
[Problems to be solved by the invention]
In the above-described digital copying machine disclosed in Japanese Patent Laid-Open No. 6-205217, when changing the target gradation characteristics, or when changing various settings such as shading or contrast of the printed image, In addition, γ correction data must be created based on the above-described method. In order to hold the generated γ correction data, the memory needs to have a certain capacity, and there is a limit to the reduction of the memory capacity.
[0005]
An image forming apparatus includes a conventional image input apparatus realized by a scanner, an image processing apparatus including a gradation correction processing unit that performs gradation correction processing including γ correction, and an image output apparatus realized by a printing press. It consists of. In such an image forming apparatus, when there are variations in the characteristics of the image input apparatus, and when an original having a background density different from that of a white recording paper is copied, an image that is originally a low density becomes dark, or On the other hand, it is desirable that an image having a high density is suppressed from being thinned and an image close to a document to be visually copied is formed. Documents with a different background density from white recording paper include originals with images formed on color (chromatic) recording paper, originals with images formed on recording paper with different paper quality even on white recording paper, and newspapers Etc.
[0006]
Further, in the image forming apparatus, when copying a predetermined type of original, it is desired that no significant density difference occurs in the high density region depending on the maximum density that can be output from the image output apparatus. . For example, when copying an original with smooth gradation such as a photograph, there is no sudden change in gradation in the high density area, and there is a difference in density between the image on the original and the output image from the image forming apparatus. It is hoped that this will not occur.
[0007]
  An object of the present invention is to provide an image processing apparatus capable of reducing the storage capacity necessary for storing data for gradation correction processing.And image forming apparatus including the sameIs to provide.
[0008]
  Another object of the present invention is to perform image processing capable of correcting an image input apparatus and performing appropriate gradation correction processing on the image even if there are variations in characteristics unique to the image input apparatus and differences in the background density of the document. apparatusAnd image forming apparatus including the sameIs to provide.
[0009]
  Still another object of the present invention is to perform correction so that the density difference in the high density region does not become large even when the maximum output density of the image output apparatus is different due to the state of the image output apparatus. Image processing apparatus capable of performing tone correction processing on imageAnd image forming apparatus including the sameIs to provide.
[0010]
[Means for Solving the Problems]
  The present invention provides an image input means for inputting an image.Input density values of pixels constituting the image fromGradation correction processing means for performing gradation correction processing using a correction curve on the input image;
  The gradation correction processing means gives a correction value on the correction curve for the input density value,It is a configuration that provides an image output means with an image subjected to gradation correction processingIn the image processing apparatus,
  A reference correction curve storage means for storing a reference correction curve indicating a predetermined change in the correction value with respect to the input density value;
  In correspondence with each of a plurality of specified density values, which are input density values that are specified with an interval between them in the range of the input density value,A difference between a correction value on the reference correction curve for the specified density value and a correction value on the correction curve for the specified density value;The reference correction curveBaseThe preset correction amount is stored inCorrection amount storage means;
  Halftone correction processing means for performing a dither halftone correction process on the image,
  The gradation correction processing means is used for gradation correction processing based on the reference correction curve and the correction amount corresponding to the designated density value every time an image is output.SaidCreate correction curveAnd
  The correction amount corresponding to the reference correction curve and the designated density value is set for each dither matrix size that can be used by the halftone correction processing unit.
[0011]
  According to the present invention, in the image processing apparatus, only the correction amount corresponding to the designated density value and the reference correction curve are always stored, and the correction curve used for the gradation correction processing is configured to output the image from the image output means. Created at the point of output. This eliminates the need for the image processing apparatus to always store all data of the correction curve actually used in the gradation correction processing in the storage means. When the correction curve used for the gradation correction process can be changed, the correction amount storage unit need only store a plurality of sets of correction amounts. Based on these reasons, the image processing apparatus of the present invention has a smaller storage capacity for storing data necessary for gradation correction processing than the conventional image forming apparatus that stores all data of a plurality of correction curves. can do.
  The gradation correction processing means in the image processing apparatus can perform appropriate gradation correction processing for each dither matrix size.
[0012]
In the image processing apparatus of the present invention, when the reference correction curve storage unit stores a plurality of the reference correction curves, the gradation correction processing unit includes a plurality of reference units according to output characteristics unique to the image output unit. One of the correction curves is selected, and the selected reference correction curve is used to create the correction curve.
[0013]
According to the present invention, in the image processing apparatus, the reference correction curve selected according to the output characteristics unique to the image output means is used to create the correction curve. Thereby, the gradation correction processing means can create a correction curve more suitable for the output characteristics unique to the image output means.
[0014]
In the image processing apparatus of the present invention, the gradation correction processing means may select the reference correction curve when
The image pattern of the plurality of designated density values is output by the image output means without performing gradation correction processing,
The density value of the output image pattern is read by the image input means, and the read density value is compared with a predetermined reference characteristic curve associated with each reference correction curve,
A reference correction curve associated with the reference characteristic curve closest to the read density value is selected.
[0015]
According to the present invention, the gradation correction processing means in the image processing apparatus can select the most appropriate reference correction curve for the output characteristics unique to the image output means.
[0016]
The image processing apparatus of the present invention is characterized in that the correction amount corresponding to the specified density value is a first correction amount that is a correction amount based on an inherent output characteristic of the image output means.
[0017]
According to the present invention, the gradation correction processing means in the image processing apparatus can perform appropriate γ correction processing based on output characteristics unique to the image output means.
[0018]
The image processing apparatus of the present invention further includes an adjustment amount input means for inputting an adjustment amount set for the gradation correction processing of the image,
The correction amount corresponding to the specified density value is a second correction amount that is a correction amount based on the input adjustment amount.
[0019]
According to the present invention, in the image processing apparatus, the correction amount corresponding to the designated density value is determined based on the adjustment amount. When the adjustment amount is set according to the output characteristics specific to the image output means, the gradation correction processing means can perform appropriate γ correction processing based on the output characteristics specific to the image output means. When the adjustment amount is an adjustment amount related to the density value correction processing desired by the user of the image forming apparatus, the gradation correction processing unit can perform the density value correction processing based on the adjustment amount simultaneously with the γ correction processing. it can. In the present invention, correction based on the adjustment amount is not included, and correction of output characteristics unique to the image output means is referred to as γ correction processing.
[0020]
The image processing apparatus of the present invention further includes an adjustment amount input means for inputting an adjustment amount set for the gradation correction processing of the image,
The correction amount corresponding to the specified density value is a first correction amount that is a correction amount based on the unique output characteristics of the image output means, and a second correction amount that is a correction amount based on the input adjustment amount. It is the sum of the quantity.
[0021]
According to the present invention, the gradation correction processing means in the image processing apparatus can perform appropriate γ correction processing based on output characteristics unique to the image output means, and correct the density value based on the adjustment amount. Processing can be performed.
[0022]
In the image processing apparatus according to the aspect of the invention, the gradation correction processing unit may determine the first correction amount when determining the first correction amount.
The image pattern of the specified density value is subjected to gradation correction processing using the reference correction curve,
The image pattern subjected to gradation correction processing is output by the image output unit, the density value of the output image pattern is read by the image input unit, and the read density value is compared with a predetermined target output curve. The first correction amount is determined based on the comparison result.
[0023]
According to the present invention, the gradation correction processing means in the image processing apparatus can set the most appropriate first correction amount for the output characteristics unique to the image output means.
[0024]
The image processing apparatus of the present invention is characterized in that, when a plurality of adjustment amounts are input from the adjustment amount input means, the second correction amount is a sum of correction amounts determined for each adjustment amount. To do.
[0025]
According to the present invention, the gradation correction processing means in the image processing apparatus can easily calculate the second correction amount when the correction amount is input in duplicate.
[0026]
In the image processing apparatus of the present invention, the gradation correction processing unit may generate the correction curve when
Obtaining a correction value on the correction curve for the specified density value by adding a correction value on the reference correction curve for the specified density value and a correction amount corresponding to the specified density value;
A correction value on the correction curve for an input density value other than the specified density value is obtained by an interpolation process using a correction value on the correction curve for the specified density value.
[0027]
According to the present invention, the gradation correction processing means of the image processing apparatus can create a correction curve using only the reference correction curve and one set of correction amounts.
[0028]
The image processing apparatus according to the present invention is characterized in that the interpolation processing is linear interpolation processing.
[0029]
According to the present invention, in the image processing apparatus, the interpolation process used when creating the correction curve is a linear interpolation process. As a result, the interpolation calculation becomes simpler than when an interpolation process other than the linear interpolation process is used, so that the gradation correction processing means can easily create a correction curve.
[0030]
The image processing apparatus according to the present invention is characterized in that the interpolation processing is spline interpolation processing.
[0031]
According to the present invention, in the image processing apparatus, the interpolation process used when creating the correction curve is a spline interpolation process. As a result, the gradation correction processing means can create a smooth correction curve as compared with the case where interpolation processing other than the spline interpolation processing is used. When gradation correction processing is performed using a correction curve created using spline interpolation processing, the gradation characteristics of the output after γ correction in the gradation correction processing means become smoother.
[0032]
In the image processing apparatus of the present invention, the gradation correction processing unit may generate the correction curve when
A correction amount corresponding to an input density value other than the specified density value is obtained by linear interpolation using a correction amount corresponding to the specified density value,
Obtaining a correction value on the correction curve for the specified density value by adding a correction value on the reference correction curve for the specified density value and a correction amount corresponding to the specified density value;
A correction value on the correction curve for an input density value other than the specified density value is obtained by adding a correction value on the reference correction curve for the input density value and a correction amount obtained by the linear interpolation process. Features.
[0033]
According to the present invention, the gradation correction processing means in the image processing apparatus can create a smoother correction curve. Therefore, the tone characteristics of the output after γ correction in the tone correction processing means become smoother. In addition, since linear interpolation processing is used for interpolation processing of the correction amount, interpolation calculation is easier than in the case where interpolation processing other than linear interpolation processing is used. Based on these reasons, the gradation correction processing means can easily create a smooth correction curve.
[0034]
The image processing apparatus of the present invention is characterized in that, when the image is a color image, a correction amount corresponding to the reference correction curve and the specified density value is set for each of a plurality of color components.
[0035]
According to the present invention, the gradation correction processing means in the image processing apparatus can perform appropriate gradation correction processing for each density value of each color component.
[0038]
  The present invention includes a gradation correction processing unit that performs gradation correction processing using a correction curve on an image input from the image input unit;
  Indicates a predetermined change in the correction value for the input density value of the pixels making up the image.A plurality of output characteristics specific to the image output means.Reference correction curve storage means for storing a reference correction curve;
  A correction amount set in advance based on the reference correction curve is stored in correspondence with each of a plurality of specified density values which are input density values which are specified with an interval between them in the range of the input density value. Correction amount storage means,
  The gradation correction processing means creates a correction curve used for gradation correction processing based on the reference correction curve and the correction amount corresponding to the designated density value every time an image is output, and the input image In the image processing apparatus having a configuration in which the correction value on the correction curve corresponding to the input density value of the constituent pixels is provided to the image output unit,
  An area in which the image pattern having a plurality of designated density values is output by the image output means on the recording medium without performing gradation correction processing, and the density value of the output image pattern and the image pattern are not formed. The density value of the recording medium is read by the image input means, and is multiplied by the background density value obtained by measuring the density of the area where the image pattern is not formed. A coefficient for obtaining a density correction amount for correcting the output measurement value which is the density value of the image pattern.Memorize correction coefficientYouCorrection coefficient storage meansWhen,
  Halftone correction processing means for performing a dither halftone correction process on the image,
  The gradation correction processing means subtracts each density correction amount corresponding to each designated density value from an output measurement value corresponding to each designated density value, thereby obtaining a corrected output density value corresponding to each designated density value. The corrected output density value is compared with a predetermined reference characteristic curve associated with each of the reference correction curves, and a reference correction curve associated with the reference characteristic curve closest to the density value of the corrected image pattern is obtained. Selected,
  The correction amount corresponding to the reference correction curve and the designated density value is set for each dither matrix size that can be used by the halftone correction processing unit.
[0039]
According to the present invention, the image processing apparatus stores an appropriate correction coefficient obtained in advance in addition to a correction amount and a reference correction curve corresponding to a plurality of designated density values. The correction coefficient is used to correct the influence of variations in image input means and the background density of the document on the density of the image on the document, and is determined based on the input result of a known image. Since such correction coefficients are stored, the gradation correction processing means can set the correction amount based on the reference correction curve and the correction coefficient prior to the gradation correction processing. The gradation correction processing means uses the reference correction curve and the set correction amount, and creates a correction curve used for the gradation correction processing every time an image is given. As a result, when there is a variation in the characteristics of the image input means, the gradation correction processing means can create a correction curve excluding the influence of the characteristic variation, so that appropriate gradation correction processing can be performed. In addition, when an image on the document having a background density different from that of the white background recording paper is input from the image input means, the gradation correction processing means can create a correction curve excluding the influence of the difference in background density. Therefore, appropriate gradation correction processing can be performed.
[0041]
According to the present invention, when a plurality of reference correction curves are stored in the image processing apparatus, the gradation correction processing means selects one reference correction curve, and selects the selected reference correction curve as the correction amount. This is used in the setting process and the correction curve creation process. To this end, the gradation correction processing means reads the image pattern of the designated density value output as it is on the recording medium by the image output means with the image input means, and an image pattern is formed with respect to the read density value. Correction is performed based on the density value and correction coefficient of the recording medium in the non-existing area. Then, among the plurality of reference correction curves, a reference correction curve that is associated with the reference characteristic curve closest to the density value distribution of the corrected image pattern is selected. As a result, the gradation correction processing means can remove the influence of the variation in the characteristics of the image input means or the influence of the background density of the original document from the density value of the read image pattern. The most appropriate reference correction curve can be selected.
[0042]
  The present invention also providesThe correction amount corresponding to the specified density value is a first correction amount that is a correction amount based on an inherent output characteristic of the image output means,
  When the gradation correction processing means determines the first correction amount,
    The image pattern of the specified density value is subjected to gradation correction processing using the reference correction curve,
    The image pattern subjected to the gradation correction process is output on the recording medium by the image output means,
    The image input means reads the density value of the output image pattern and the density value of the recording medium in the area where the image pattern is not formed,
    The density value of the read image pattern is corrected using the density value of the read recording medium and the correction coefficient.To obtain the corrected output density value,
    Correction outputThe density value is compared with a predetermined target characteristic curve, and the first correction amount is determined based on the comparison result.
[0043]
According to the present invention, in the image processing apparatus, the gradation correction processing means performs the gradation correction processing using the reference correction curve and outputs the image pattern of the designated density value output on the recording medium by the image input means. The density value thus read is corrected based on the density value and the correction coefficient of the recording medium in the area where the image pattern is not formed. Then, the first correction amount is set for each specified density value by comparing the density values of the plurality of corrected image patterns with the target characteristic curve. As a result, the gradation correction processing means can calculate the most appropriate first correction amount without being affected by variations in the characteristics of the image input means or the background density of the recording medium on which the image pattern is output. .
[0044]
In the image processing apparatus of the present invention, the correction coefficient is determined for each of a plurality of types of recording media,
The gradation correction processing means is configured to be able to select one of correction coefficients corresponding to a plurality of types of recording media.
[0045]
According to the present invention, the correction coefficient is set for each of a plurality of types of recording media. The plural types of recording media have different base densities, for example. The gradation correction processing means is configured to be able to select one of the correction coefficients corresponding to a plurality of types of recording media used for processing. Therefore, the gradation correction processing means can set the correction amount using the correction coefficient corresponding to the type of the recording medium on which the input image is formed. Thereby, the gradation correction processing means can set a correction curve using an appropriate correction amount according to the type of the recording medium when an image on a recording medium of a different type from the white background recording paper is input. Appropriate gradation correction processing according to the type of recording medium can be performed.
[0046]
When correction coefficients corresponding to a plurality of types of recording media are stored, correction amounts corresponding to the correction coefficients corresponding to the respective recording media can be set in advance and stored in the correction amount storage means. It is. In this case, the gradation correction processing means selects a correction amount that is paired with the selected correction coefficient, and creates a correction curve using the selected correction amount. Thereby, the gradation correction processing means can set a correction curve using an appropriate correction amount according to the type of the recording medium when an image on a recording medium of a different type from the white background recording paper is input. Appropriate gradation correction processing according to the type of recording medium can be performed.
[0047]
The image processing apparatus of the present invention is characterized in that, when the image is a color image, the correction coefficient is determined for each of a plurality of color components.
[0048]
According to the present invention, in the image processing apparatus, the correction coefficient is determined for each of a plurality of color components. The gradation correction processing means can set a correction amount using each correction coefficient for each color component, and can create each correction curve using a correction amount corresponding to each color component. As a result, the tone correction processing means can perform more appropriate tone correction processing for each color component.
[0049]
  The present invention includes a gradation correction processing unit that performs gradation correction processing using a correction curve on an image input from the image input unit;
  A reference correction curve storage means for storing a reference correction curve indicating a predetermined change of a correction value with respect to an input density value of pixels constituting an image;
  A correction amount set in advance based on the reference correction curve is stored in correspondence with each of a plurality of specified density values which are input density values which are specified with an interval between them in the range of the input density value. Correction amount storage means,
  The gradation correction processing means creates the correction curve used for the gradation correction processing based on the reference correction curve and the correction amount corresponding to the designated density value every time an image is output, and the input image In the image processing apparatus configured to provide the image output means with a correction value on the correction curve corresponding to the input density value of the pixels constituting
  Maximum correction value storage means for storing a maximum correction value that is an upper limit value of the correction value on the correction curve set based on the maximum output value of the image output means used when the correction curve is created With
  The gradation correction processing means performs the gradation correction processing using a correction curve that is limited so that a correction value on the correction curve is equal to or less than a maximum correction value.Yes,
  The image further includes halftone correction processing means for performing dither halftone correction processing on the image,
  The correction amount corresponding to the reference correction curve and the specified density value is set for each dither matrix size that can be used by the halftone correction processing means.An image processing apparatus characterized by this.
[0050]
According to the present invention, in the image processing apparatus, the maximum correction value on the correction curve is stored in the maximum correction value storage unit. The maximum correction value is the upper limit value of the correction value on the correction curve, and is set according to the maximum output value corresponding to the maximum density that can be output by the image output means. Since such a maximum correction value is stored, the gradation correction processing means creates a correction curve based on the reference correction curve and the correction amount corresponding to the designated density value each time an image is output, It is possible to limit the correction curve so that the correction value on the created correction curve is equal to or less than the stored maximum correction value, and perform the tone correction process using the limited correction curve. When the maximum output value of the image output unit fluctuates with a change width that cannot be ignored by adding a limitation using the maximum correction value to the correction value given to the image output unit, the image processing apparatus Thus, the tone difference can be suppressed and appropriate gradation correction processing can be performed.
[0051]
  The present invention also providesThe maximum correction value is on the correction curve corresponding to the specified density value of the darkest density.SaidCorrection value,Indicates the default ratio for the correction valueIt is determined based on the maximum correction value determination coefficient.
[0052]
  According to the present invention, in the image processing apparatus, the gradation correction processing means sets the correction value on the correction curve corresponding to the specified density value of the darkest density among all the specified density values.Indicates the default percentage of the correction valueThe maximum correction value can be calculated by multiplying the maximum correction value determination coefficient. In order to calculate the maximum correction value, a preferable maximum correction value determination coefficient may be determined and stored in the maximum correction value storage unit prior to calculation of the correction value on the correction curve. Since the maximum correction value is determined based on the correction value on the correction curve corresponding to the darkest designated density value and the maximum correction value determination coefficient, the maximum correction value can be calculated every time an image is output. As a result, even if the correction curve before the correction value is limited differs every time the image is output, the gradation correction processing means can perform appropriate gradation correction processing.
[0053]
  The present invention also providesThe reference correction curve storage means stores a plurality of the reference correction curves;
  When the gradation correction processing unit selects one of the plurality of reference correction curves to be used for processing according to output characteristics specific to the image output unit,
    The image pattern of the plurality of designated density values is output by the image output means without performing gradation correction processing,
    The density value of the output image pattern is read by the image input means,
    The density value of the read image pattern is compared with a predetermined standard characteristic curve associated with each of the standard correction curves, and the standard correction curve associated with the standard characteristic curve closest to the density value of the read image pattern Select
  It is a standard measurement value of an image pattern that is output from the image output unit adjusted to an optimum state and indicates the maximum output value among the image patterns read by the image input unit.Maximum output target value andSaidThe maximum correction value is obtained based on the density value of the read image pattern.
[0054]
According to the present invention, in the image processing apparatus, the gradation correction processing means outputs an image pattern of each designated density value without performing gradation correction processing when setting the maximum correction value, and each image pattern is output as an image. The maximum correction value is obtained based on the density value of the read image pattern. The gradation correction processing unit creates a curve indicating a change in the density value of the image pattern read by the image input unit with respect to the correction value given to the image output unit, on the curve corresponding to the maximum output target value. The correction value can be set as the maximum correction value. As described above, since the maximum output value is obtained every time the image pattern is output, the gradation correction processing means can perform more appropriate gradation correction processing. When the reference correction curve storage means stores only one reference correction curve, the gradation correction processing means does not select the reference correction curve, but uses the stored reference correction curve to obtain the maximum correction value. Only for the setting, the image pattern of the specified density value is output and read, and the maximum correction value may be set according to the reading result.
[0055]
The image processing apparatus according to the present invention is characterized in that a plurality of the maximum output target values are determined.
[0056]
According to the present invention, in the image processing apparatus, when the maximum correction value is set using the read density value of the image pattern having the specified density value, a plurality of maximum output target values are set. The gradation correction processing means obtains a maximum correction value candidate value based on the read image pattern density value and each maximum output target value, and statistically processes the plurality of candidate values to obtain the maximum correction value. It can be calculated. If there is a plurality of maximum output target values, there is a possibility that the density value of the read image pattern may not be stably obtained (for example, output using an image pattern of a specified density value whose density monotonously decreases) The gradation correction processing means can stably obtain the maximum correction value even when there is a portion where the read density value increases despite being performed. Thereby, the gradation correction processing means can perform more appropriate gradation correction processing.
[0057]
The image processing apparatus of the present invention further includes an adjustment amount input means for inputting an adjustment amount set for the gradation correction processing of the image,
The maximum correction value is obtained according to a second correction amount that is a correction amount based on the input adjustment amount.
[0058]
According to the present invention, when an adjustment other than the γ correction is performed, the gradation correction processing unit obtains the maximum correction value according to the adjustment amount input with respect to the other adjustments, and uses the obtained maximum correction value. Use to limit the correction value on the correction curve. Thereby, the gradation correction processing means can perform more appropriate gradation correction processing.
[0059]
The image processing apparatus of the present invention is characterized in that, when the image is a color image, the maximum correction value is determined for each of a plurality of color components.
[0060]
According to the present invention, in the image processing apparatus, the maximum correction value is determined for each color component. The gradation correction processing means can create a correction curve for each color component and limit the correction value on the correction curve using the maximum correction value corresponding to each color component. As a result, the gradation correction processing means can perform appropriate gradation correction processing for each color component.
[0061]
Further, the image processing apparatus of the present invention is characterized in that the gradation correction processing means can be switched to use or not to use the maximum correction value.
[0062]
  According to the present invention, in the image processing apparatus, the gradation correction processing means is configured to be able to switch whether or not to use the maximum correction value when creating the correction curve. Only when it is determined that the maximum correction value is to be used, the gradation correction processing means uses the correction value on the correction curve created based on the reference correction curve and the correction amount corresponding to the specified density value as the maximum correction value. The limited correction curve is used for gradation correction processing. When it is determined that the maximum correction value is not used, a correction curve created based on the reference correction curve and the correction amount corresponding to the designated density value can be used as it is for the gradation correction processing. Since whether or not to use the maximum correction value can be switched, the gradation correction processing means can perform more appropriate gradation correction processing according to the type of document or user preference. It is.
  The present invention also provides an image forming apparatus including the above-described image processing apparatus.
[0063]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a schematic front sectional view showing a configuration of an image forming apparatus 11 provided with an image processing apparatus of the present invention. The image forming apparatus 11 is realized by a digital color copying machine. Inside the main body of the image forming apparatus 1, an image input device 13 as an image input unit, an image processing device 14, and an image output device 15 as an image output unit are provided. A transparent document table 111 and an operation panel 16 are provided on the upper surface of the main body of the image forming apparatus 11.
[0064]
A double-sided automatic document feeder (RADF) 112 is mounted on the top surface of the document table 111. The double-sided automatic document feeder 112 is supported so as to be openable and closable with respect to the document table 111, and has a predetermined positional relationship with the surface of the document table 111. The double-sided automatic document feeder 112 first transports a document so that one surface of the document faces the image input device 13 at a predetermined position on the document table 111, and after reading the one surface, the document is finished. The document is conveyed toward the document table 111 while the document is turned upside down so that the other side faces the image input device 13 at a predetermined position of the document table 111. The document conveying operation and the front / back reversing operation in the duplex automatic document feeder 112 are controlled in relation to the operation of the entire image forming apparatus 11.
[0065]
The image input device 13 is disposed below the document table 111 in order to read an image of the document conveyed on the document table 111 by the double-sided automatic document feeder 112. The image input device 13 includes a document scanning body that reciprocates in parallel with the lower surface of the document table 111, an optical lens 115, and a CCD (Charge Coupled Device) line sensor 116 that is a photoelectric conversion element.
[0066]
The document scanning body includes a first scanning unit 113 and a second scanning unit 114. The first scanning unit 113 includes an exposure lamp that exposes the surface of the document and a first mirror that deflects a reflected light image from the document in a predetermined first direction. The first scanning unit 113 reciprocates in parallel with the lower surface of the document table 111 while maintaining a predetermined distance with respect to the lower surface of the document table 111. The second scanning unit 114 includes a second mirror and a second mirror for further deflecting the reflected light image from the document deflected by the first mirror of the first scanning unit 113 in a predetermined second direction. Has 3 mirrors. The second scanning unit 114 reciprocates in parallel with the lower surface of the document table 111 while maintaining a predetermined speed relationship with the first scanning unit 113.
[0067]
The optical lens 115 reduces the reflected light image deflected by the third mirror of the second scanning unit, and forms the reduced reflected light image at a predetermined position on the CCD line sensor 116. The CCD line sensor 116 sequentially photoelectrically converts the formed reflected light image and outputs an analog image signal which is an electrical signal. The CCD line sensor 116 provided in the image forming apparatus 11 is specifically a three-line color CCD line sensor. The 3-line color CCD line sensor reads a black and white image or a color image, separates the reflected light image into red (R), green (G), and blue (B) color component images, and outputs these three color components. A reflectance signal composed of a signal corresponding to the image is output. The reflectance signal generated by the CCD line sensor 116 is given to the image processing device 14. In the following description, the three colors of red, green, and blue are collectively referred to as “RGB”.
[0068]
The image processing device 14 performs predetermined processing described later on the given RGB reflectance signal. As a result, RGB reflectance signals are digital image signals (hereinafter referred to as “image data”) composed of signals corresponding to cyan (C), magenta (M), yellow (Y) and black (K) color component images, respectively. Is referred to as). In the following description, the four colors of cyan, magenta, yellow, and black are collectively referred to as “CMYK”.
[0069]
Below the image output device 15, a paper feed mechanism 211 having a paper tray is provided. In the image forming apparatus 11 of the present embodiment, cut sheet-like paper is used as the paper P. In general, the paper feed mechanism 221 separates the paper P in the paper tray one by one and supplies it to the image output device 15. When duplex printing is performed, the paper feed mechanism 211 transports the paper P on which an image is formed on one side so that the paper P is re-supplied to the image output device 15 in accordance with the image formation timing of the image output device 15. . In the paper feed mechanism 221, when the paper P delivered from the paper tray is supplied into the guide of the paper feed transport path in the paper feed mechanism 221, a sensor provided in the paper feed transport path is used to detect the paper P. Detecting the tip part of the sensor and outputting a detection signal.
[0070]
A pair of registration rollers 212 is disposed in front of the image output device 15. The registration roller 212 temporarily stops the transported paper P based on a detection signal from a sensor in the paper feed transport path. The separately supplied paper P is conveyed to the image output device 15 at a supply timing controlled by the registration roller 212.
[0071]
In the image output device 15, the first image forming portion Pa, the second image forming portion Pb, the third image forming portion Pc, the fourth image forming portion Pd, the transfer conveyance belt mechanism 213, and the fixing device 217 are included. And are provided. The transfer / conveying belt mechanism 213 is disposed in a lower portion in the image output device 15. The transfer / conveying belt mechanism 213 includes a driving roller 214, a driven roller 215, and a transfer / conveying belt 216 stretched between these rollers 214 and 215 so as to extend substantially in parallel. A pattern image detection unit 232 is provided in the vicinity of the lower side of the transfer conveyance belt 216.
[0072]
The first image forming unit Pa, the second image forming unit Pb, the third image forming unit Pc, and the fourth image forming unit Pd are located above the transfer conveyance belt 216 in the image output device 15 and close to the transfer conveyance belt 216. Thus, they are arranged in order from the upstream side of the sheet conveyance path. The fixing device 217 is disposed on the downstream side of the transfer conveyance belt mechanism 213 in the sheet conveyance path. A paper suction charger 228 is provided between the first image forming portion Pa and the paper feed mechanism 21. A static eliminator 229 for separating the paper is provided between the fourth image forming unit Pd and the fixing device 217 and at a position almost directly above the driving roller 214.
[0073]
The transfer / conveying belt mechanism 213 is generally configured to convey the paper P supplied by the registration rollers 212 while electrostatically attracting the sheet P to the transfer / conveying belt 216, and specifically operates as follows. To do. The sheet suction charger 228 charges the surface of the transfer conveyance belt 216. The transfer conveyance belt 216 is driven in the direction indicated by the arrow Z in FIG. Therefore, the transfer / conveying belt 216 holds the paper P fed through the paper feeding mechanism 211, and the first image forming unit Pa, the second image forming unit Pb, the third image forming unit Pc, and the fourth image forming unit. The paper P is sequentially conveyed to Pd. Since the surface of the transfer / conveying belt 216 is charged by the sheet adsorbing charger 228, the transfer / conveying belt mechanism 213 securely adsorbs the sheet P supplied from the paper feeding mechanism 211 to the transfer / conveying belt 216. The paper P can be stably conveyed from the first image forming portion Pa to the fourth image forming portion Pd so as not to cause a positional shift. In order to separate the electrostatically attracted paper P from the transfer conveyance belt 216, an AC voltage is applied to the static eliminator 229 for paper peeling.
[0074]
The first image forming unit Pa, the second image forming unit Pb, the third image forming unit Pc, and the fourth image forming unit Pd have substantially the same configuration. Each image forming unit Pa. Pb, Pc, and Pd include photosensitive drums 222a, 222b, 222c, and 222d that are driven to rotate in the direction indicated by arrow F in FIG. Around the photosensitive drums 222a to 222d, chargers 223a, 223b, 223c, 223d, developing devices 224a, 224b, 224c, 224d, transfer members 225a, 225b, 225c, 225d, and cleaning devices 226a, 226b, 226c and 226d are sequentially arranged along the rotation direction F of the photosensitive drums 222a to 222d.
[0075]
Laser beam scanner units 227a, 227b, 227c, and 227d are provided above the photosensitive drums 222a to 222d, respectively. The laser beam scanner units 227a to 227d include semiconductor laser elements, polygon mirrors 240a, 240b, 240c, and 240d that are deflection devices, fθ lenses 241a, 241b, 241c, and 241d, and two mirrors 242a, 242b, 242c, and 242d. ; 243a, 243b, 243c, 243d, respectively. In FIG. 1, the semiconductor laser element is not shown.
[0076]
A signal corresponding to the black color component image of the color document image in the image data from the image processing device 14 is input to the laser beam scanner unit 227a of the first image forming unit Pa. A signal corresponding to the cyan color component image of the color document image in the image data is input to the laser beam scanner unit 227b of the second image forming unit Pb. A signal corresponding to the magenta color component image of the color original image in the image data is input to the laser beam scanner unit 227c of the third image forming unit Pc. A signal corresponding to the yellow color component image of the color original image in the image data is input to the laser beam scanner unit 227d of the fourth image forming unit Pd.
[0077]
The chargers 223a to 223d uniformly charge the photosensitive drums 222a to 222d, respectively. The semiconductor laser elements of the laser beam scanner units 227a to 227d emit laser beams that are modulated in accordance with signals given to the units. Polygon mirrors 240a to 240d deflect laser light from the semiconductor laser element in a predetermined main scanning direction. The fθ lenses 241a to 241d and the mirrors 242a to 242d; 243a to 243d image the laser beams deflected by the polygon mirrors 240a to 240d on the surfaces of the charged photosensitive drums 222a to 222d. As a result, electrostatic latent images corresponding to the four color components of the color original image are formed on the photosensitive drums 222a to 222d.
[0078]
The developing device 224a of the first image forming unit Pa contains black toner. The developing device 224b of the second image forming unit Pb contains cyan toner. The developing device 224c of the third image forming unit Pc contains magenta toner. The developing device 224d of the fourth image forming unit Pd contains yellow toner. The developing devices 224a to 224d develop the electrostatic latent images on the photosensitive drums 222a to 222d with toner stored therein. As a result, the image output device 15 reproduces the original image as black, cyan, magenta, and yellow toner images.
[0079]
The transfer members 225a to 225d transfer the toner images on the photosensitive drums 222a to 222d onto one surface of the paper P conveyed by the transfer conveyance belt 216. The cleaning devices 226a to 226d remove toner remaining on the photosensitive drums 222a to 222d after the transfer, respectively.
[0080]
After the transfer of the toner image is completed in the fourth image forming unit Pd, the sheet P is sequentially peeled from the transfer conveyance belt 219 from the leading end portion by the static eliminator 229 and guided to the fixing device 217. The fixing device 217 fixes the toner image transferred onto the paper P onto the paper P.
[0081]
The sheet P that has passed through the nip between the fixing rollers provided in the fixing device 217 passes through the conveyance direction switching gate 218. The transport direction switching gate 218 passes the transport path of the paper P after the toner image is fixed, the first path for discharging the paper P out of the main body of the image forming apparatus 11, and the paper P toward the image output apparatus 15. Selectively switch between a second path for resupply. When the transport path is switched to the first path by the transport direction switching gate 218, the paper P is discharged onto the paper discharge tray 220 attached to the outer wall of the main body of the image forming apparatus 11 by the discharge roller. When the transport path is switched to the second path by the transport direction switching gate 218, the paper P is transported to the switchback transport path 221, the front and back are reversed by the switchback transport path 221, and then passed through the paper feed mechanism 211. Then, it is supplied again to the image output device 15.
[0082]
In the above description, optical writing is performed on the photoconductors 222a to 222d by scanning and exposing the laser beams by the laser beam scanner units 227a to 227d. Instead of the laser beam scanner units 227a to 227d, an LED (Light Emitting Diode) head which is a writing optical system including a light emitting diode array and an imaging lens array may be used. The LED head is silent compared to the laser beam scanner unit and is silent because there are no moving parts. Therefore, an LED head is preferably used in a tandem digital color image forming apparatus that requires a plurality of optical writing units. In the above description, the image output device 15 is realized by an electrophotographic printer. The image output device 15 is not limited to an electrophotographic printer, and may be realized by another device, for example, an ink jet printer.
[0083]
FIG. 2 is a block diagram showing the configuration of the image processing apparatus 14 according to the first embodiment of the present invention provided in the image forming apparatus 11 of FIG. The image processing apparatus 14 includes an analog / digital (hereinafter abbreviated as “A / D”) conversion unit 21, a shading correction unit 22, an input tone correction unit 23, a color correction unit 24, an image area separation processing unit 25, and black generation. Under color removal unit 26, spatial filter processing unit 27, halftone output tone processing unit 28, reference correction curve storage unit (reference correction curve storage means: hereinafter abbreviated as “curve storage unit”) 31, and correction amount storage unit (Correction amount storage means) 32 is included. The curve storage unit 31 is realized by a nonvolatile storage unit from which at least data can be read, for example, a ROM (Read Only Memory). The correction amount storage unit 32 is realized by a storage unit that can write and read data, for example, a RAM (Random Access Memory), and preferably a storage unit that can write and read data and is nonvolatile. Realized.
[0084]
The A / D converter 21 converts the RGB reflectance signal provided from the image input device 13 into a digital signal. The shading correction unit performs a shading correction process on the A / D converted reflectance signal. The shading correction process is performed to remove various distortions that occur in the image signal due to the configuration of the illumination system, the imaging system, and the imaging system of the image input device 13.
[0085]
The input tone correction processing unit 23 performs input tone processing on the reflectance signal that has been subjected to the shading correction processing. The input tone correction process is a process for converting the reflectance signal into a signal that can be easily handled by the image processing apparatus 14 such as a density signal. The input tone correction unit 23 may further perform color balance processing on the reflectance signal.
[0086]
The color correction unit 24 converts the RGB density signal into a CMY density signal, and performs color correction processing on the CMY density signal in order to realize faithful color reproduction in the image output device 15. Specifically, the color correction processing is processing for removing color turbidity based on the spectral characteristics of CMY toners each including unnecessary absorption components from the CMY density signal.
[0087]
The image area separation processing unit 25 performs area separation processing based on the CMY density signals output from the color correction unit 24. The separation result in the image area separation processing unit 25 is given to the black generation undercolor removal unit 26, the spatial filter processing unit 27, and the halftone output gradation processing unit 28. The separation result in the image area separation processing unit 25 may be given to the image output device 15.
[0088]
The black generation under color removal unit 26 performs black generation processing for generating a black color signal based on the CMY color signals constituting the density signal output from the color correction unit 24. The black generation under color removal unit 26 performs under color removal processing on the CMY color signal. The under color removal process is a process of obtaining a new CMY color signal by subtracting the black color signal generated by the black generation process from the CMY color signal. As a result of these processes, the CMY density signal is converted into image data which is an image signal composed of CMYK color signals.
[0089]
The spatial filter processing unit 27 performs spatial filter processing using a digital filter on the CMYK image data obtained by the black generation and under color removal unit 26. As a result, the spatial frequency characteristic of the image is corrected, so that it is possible to prevent the image output from the image output device 15 from being blurred or deteriorated in graininess.
[0090]
The halftone output gradation processing unit 28 performs gradation correction processing and halftone generation processing on the CMYK image data after the spatial filter processing while referring to the storage contents of the curve storage unit 31 and the correction amount storage unit 32. Apply. The halftone generation process is a process for dividing an image into a plurality of pixels so that the gradation of each pixel can be reproduced. Further, the halftone output gradation processing unit 28 may perform a process of converting the density value of the image data into a halftone dot area ratio that is a characteristic value of the image output device 15. The density signal processed by the halftone output gradation processing unit 28 is supplied to the image output device 15.
[0091]
If characters and photographs are mixed in the original image read by the image input device 13, the image region separation processing unit 25 separates the images into character regions extracted as black characters and photograph regions identified as photographs. Is done. The character area includes color characters in some cases. When an image is separated into a character area and a photographic area, preferably, for the character area, the spatial filter processing unit 27 performs sharpness enhancement processing with a large enhancement amount of a high frequency, and a halftone output gradation processing unit. 28 performs high-resolution dither processing suitable for high-frequency reproduction. This is to improve the reproducibility of black characters or color characters in the image. When the image is separated into a character area and a photographic area, the photographic area is preferably subjected to low-pass filter processing by the spatial filter processing unit 27, and the halftone output gradation processing unit 28 places importance on gradation reproducibility. Dither processing is performed. This is to remove halftone dot components in the photographic area.
[0092]
The image processing apparatus 14 according to the first embodiment is characterized by the gradation correction processing in the halftone output gradation processing unit 28. A rough execution procedure of the gradation correction processing is as follows. For gradation correction processing, a reference correction curve as shown in FIG. 3 is stored in advance in the curve storage unit 31, and a plurality of correction amounts are preset and stored in the correction amount storage unit 32. . The halftone output gradation processing unit 28 creates a correction curve to be actually used by using the reference correction curve and the correction amount at the time when the CMYK image data is given, and uses the created correction curve. Perform gradation correction processing. As described above, in the gradation correction processing, the reference correction curve is not used as it is, but the created correction curve is used because output characteristics unique to the image output device 15 have manufacturing variations. is there.
[0093]
The reference correction curve (hereinafter referred to as “reference correction curve”) is used to correct the value output in accordance with the density value of the input image data in the halftone output tone processing unit 28. This is the standard for determining the above. Both the reference correction curve and the created correction curve show changes in the correction value with respect to the input density value. The input density value is a density value to be processed that is input to the halftone output tone processing unit 28. The correction value is a value output from the halftone output gradation processing unit 28 and corresponds to the pixel density.
[0094]
The correction amount corresponds to the designated density value on a one-to-one basis. The designated density value is an input density value that is designated in advance with an interval between them within the range of the input density value. The correction amount corresponding to a certain specified density value is the difference between the correction value for the specified density value on the correction curve actually used in the gradation correction process and the correction value on the reference correction curve for the specified density value. The correction amount is set based on the reference correction curve corresponding to the specified density value. In order to reduce the amount of data related to the gradation correction processing, the number of designated density values is smaller than the total number of numerical values acquired from the input density values. In the first embodiment, the designated density value is 16.
[0095]
The reference correction curve and the set of correction amounts are preferably set for each color component of CMYK. In this case, the halftone output gradation processing unit 28 is configured to use a reference correction curve and a correction amount set for each color component when performing gradation correction processing on the density value of each color component. . Thus, the halftone output gradation processing unit 28 can perform an appropriate gradation correction process for each color component density value.
[0096]
The gradation correction process according to the first embodiment will be described in detail below. In the following description, the halftone generation process is realized by a dither process, and the gradation correction process also serves as a part of the dither process. Therefore, in the following description, the halftone output gradation processing unit 28 serves as both a gradation correction processing unit that performs gradation correction processing and a halftone correction processing unit that performs halftone generation processing. In the gradation correction process for the density values of CMYK color components, only the specific values of the reference correction curve and the correction amount used are different from each other. Therefore, in the following description, for the density value of any one color component, Only the tone correction processing is described. In this embodiment, the input density value is an integer in the range of 0 to 255, and the correction value is an integer in the range of 0 to 255 times ms. “Ms” is the size of the dither matrix used in the dither processing. The range of the correction value is 0 or more and 255 × ms or less because the gradation correction processing also serves as dither processing.
[0097]
It is preferable that a plurality of reference correction curves for a single color component are set. In this case, the halftone output tone processing unit 28 is configured to select and use one of the optimum reference correction curves from among a plurality of reference correction curves based on the output characteristics unique to the image output device 15. It has become.
[0098]
In order to select a reference correction curve, the halftone output gradation processing unit 28 includes a plurality of reference characteristic curves and test image data in advance.
[0099]
For example, when five reference correction curves G21 to G25 shown in FIG. 3 are stored in the curve storage unit 31, reference characteristic curves G11 to G15 showing five input-output characteristics as shown in FIG. 4 are provided. . The input-output characteristic of the reference characteristic curve is a state in which gradation correction processing of an apparatus (hereinafter referred to as “gradation correction output apparatus”) composed of the halftone output gradation processing unit 28 and the image output apparatus 15 is not performed. This corresponds to a change in the value corresponding to the pixel density of the image printed by the image output device 15 with respect to the input density value. As will be described later, since the pixel density of the printed image is measured using the image input device 13, the output value of the image input device 13 is used as a value corresponding to the pixel density of the printed image.
[0100]
There is a one-to-one correspondence between the reference correction curve and the reference characteristic curve. 3 and 4, the reference characteristic curve of the reference symbol G11 corresponds to the reference correction curve of the reference symbol G21, and similarly, the curves having the same numerical value at the end of the reference symbol correspond to each other. The greater the slope of the reference characteristic curve, the smaller the slope of the corresponding reference correction curve. If the input / output characteristic graph of the gradation correction output device matches a certain reference characteristic curve, the gradation correction output device can be obtained by performing gradation correction processing using the reference correction curve corresponding to the reference characteristic curve. The input / output characteristic graph of the second line coincides with a predetermined target characteristic curve.
[0101]
For example, as shown in FIG. 5, the test image is composed of image patterns having densities corresponding to a plurality of designated density values for each color component of CMYK. Such an image pattern is called a patch.
[0102]
The procedure for selecting the reference correction curve will be described below with reference to FIGS. The halftone output gradation processing unit 28 first performs only the halftone generation process on the test image without performing the gradation correction process, and causes the image output device 15 to output the test image after the halftone generation process. . As a result, a test image that has not been subjected to gradation correction processing is printed on a recording sheet as a recording medium. Next, the halftone output tone processing unit 28 causes the printed test image to be read by the image input device 13 and outputs an output value corresponding to the density of each patch of the printed test image (hereinafter referred to as “output measurement value”). ) The output measurement value of each patch is associated with the designated density value of each patch. As a result, output characteristics specific to the image output apparatus are obtained.
[0103]
Next, the halftone output tone processing unit 28 obtains the difference between the output measurement value and each reference characteristic curve in order to determine which reference characteristic curve is closest to the output measurement value. FIG. 6 shows an example in which the difference between the third reference characteristic curve G13 and arbitrary designated density values s (1) and s (2) is obtained. In the example of FIG. 6, for the specified density value s (1), the output characteristic value OUT-m (s (1)) corresponding to the specified density value s (1) and the reference characteristic curve for the specified density value s (1). Difference ΔO with output value OUT−b (s (1)) on G13_{S (1)}Is required. Similarly, for the designated density value S (2), the output measurement value OUT-m (s (2)) for the designated density value s (2) and the output value on the reference characteristic curve G13 for the designated density value S (2). Difference ΔO from OUT−b (s (2))_{S (2)}Is required.
[0104]
After the difference between each reference characteristic curve and the output measurement value is obtained for all the 16 designated density values, the halftone output tone processing unit 28 uses the obtained difference to obtain the most distribution of the output measurement value. One of the closest reference characteristic curves is discriminated, and a reference correction curve corresponding to the discriminated reference characteristic curve is selected. If the output measurement values corresponding to the 16 designated density values are displayed as indicated by the black circles in FIG. 7, for example, based on the weighted sum of the differences or the variance of the differences, the third reference characteristic curve G13 becomes the output measurement value. Since it can be seen that it is the closest, the third reference correction curve G23 corresponding to the reference characteristic curve G13 is selected. When the reference correction curve selected based on the output characteristic unique to the image output device 15 is used for the creation of the correction curve, the halftone output gradation processing unit 28 is appropriate for the output characteristic unique to the image output device 15. A correction curve can be created.
[0105]
A gradation correction process is performed on the test image using the selected reference correction curve as it is, and the processed test image is printed by the image output device 15 and the printed test image is read, as shown in FIG. Thus, the characteristic of the output measurement value with respect to the input density value may deviate from the predetermined target characteristic curve. In such a case, the correction amount corresponding to the designated density value is determined based on output characteristics unique to the image output device 15. In the following description, the correction amount determined based only on the output characteristics unique to the image output device 15 is referred to as a “first correction amount”. The target characteristic curve is stored in advance in a storage unit provided separately in the curve storage unit 31 or the image processing apparatus 14.
[0106]
The first correction amount setting process based on the output characteristics unique to the image output device 15 will be described below with reference to FIGS. First, tone correction processing and halftone generation processing using the reference correction curve as they are are performed on a test image prepared in advance. Next, these two processed test images are printed on a recording sheet by the image output device 15. Next, the printed test image is read by the image input device 13. The black circles in FIG. 8 indicate the output measurement values of the patches obtained as a result of reading the test image. The output measurement value obtained in this way corresponds to the designated density value on a one-to-one basis. In FIG. 9, a black circle P1 indicates an arbitrary designated density value x._iOutput measurement value y corresponding to_m(X_i) Is displayed.
[0107]
Next, the target characteristic curve G31 and a specified density value x_iOutput measurement value y corresponding to_m(X_i) And the measured output value y as shown in FIG._m(X_i) Input density value x on the target characteristic curve G31_b(Y_m) Is required. In the following description, “input density value on target characteristic curve with respect to output measurement value” is abbreviated as “output target density value”. Next, as shown in FIG. 10, the output target density value x_b(Y_m) As a new correction value for the specified density value x on the reference correction curve G32._iCorrection value Z for_N(X_i) Is set. The new correction value is a correction value on the correction curve to be used by the halftone output gradation processing unit 28 in order to output an image according to the target characteristic curve.
[0108]
The new correction value setting process is executed for each of the plurality of output measurement values shown in FIG. Thus, the measured output value y deviates from the target characteristic curve._m(X_i) Based on the specified density value x_iOutput target density value x that is an input density value other than_b(Y_m) Is calculated. After the correction value for the output target density value is set, a new correction value for the specified density value is obtained by interpolation processing using the set correction value.
[0109]
After the interpolation process is completed, a difference between a new correction value for each designated density value and a reference correction value for each designated density value is obtained. The difference obtained in this way is the designated density value x._iIs used as the first correction amount corresponding to. For example, if the input density value change of a new correction value is indicated by a curve G33, the first correction amount s (x_i) Is the correction value Z on the reference correction curve G32._N(X_i) And the correction value Zk (x_i) And the difference. When the correction amount for creating the correction curve is set based only on the output characteristic unique to the image output device 15, the first correction amount is stored in the correction amount storage unit 32 as it is as the correction amount for the specified density value. . By using the procedure described above, the halftone output gradation processing unit 28 can appropriately set the first correction amount based on the output characteristics unique to the image output device 15.
[0110]
As described above, test image data is prepared in advance for the selection process of the reference correction curve and the setting process of the first correction amount. Instead of providing test image data, a test image printed on recording paper may be provided in advance. In this case, the halftone output gradation processing unit 28 causes the image input device 13 to read the printed test image prior to the execution of the selection process or the setting process, and the image signal generated as a result is used as test image data. Use. Comparing the case where the test image data is provided and the case where the printed test image is provided, the former case is preferable because it is not affected by the reading error of the image input device 13 or the like.
[0111]
In the image forming apparatus 11 including the image processing apparatus 14 according to the present embodiment, a user may desire to add a desired shade or contrast to an image output from the image output apparatus. In this case, the user sets an adjustment amount according to the density or contrast to be added, and inputs the adjustment amount from the operation panel 16 to the halftone output gradation processing unit 28. The operation panel 16 is used as adjustment amount input means. When the adjustment amount is input, the halftone output gradation processing unit 28 further obtains a correction amount (hereinafter referred to as “second correction amount”) determined based on the adjustment amount, and the first correction amount and the second correction amount. The sum of the amount and the amount is adopted as a correction amount corresponding to the specified density value, so that gradation correction processing is performed.
[0112]
[Table 1]

[0113]
wi = si + ti + ui + vi (i = 1, 2,..., N) (1)
When a plurality of adjustment amounts are input, the halftone output gradation processing unit 28 may use the total correction amount determined based on each adjustment amount as the second correction amount. For example, when three adjustment amounts are set as shown in Table 1, the correction amount wi corresponding to a certain specified density value is equal to the first correction amount si corresponding to the specified density value as shown in Equation 1. , A correction amount ti corresponding to the specified density value determined based on the first adjustment amount, a correction amount ui corresponding to the specified density value determined based on the second adjustment amount, and a third adjustment amount And the correction amount vi corresponding to the designated density value determined on the basis of. Thus, when various adjustment amounts are set, the second correction amount based on the adjustment amount is easily calculated.
[0114]
FIG. 11 is a flowchart showing main control processing of the halftone output gradation processing unit 28 at the time of document copying in the image forming apparatus 11. Since the reference correction curve selection process and the correction amount setting process are performed prior to the instruction to copy the document, one reference correction curve has already been selected and the correction amount is the correction amount storage unit 32. Is remembered.
[0115]
When the user inputs a document copy instruction from the operation panel 16, the halftone output gradation processing unit 28 selects the selected reference correction curve in the curve storage unit 31 and 1 in the correction amount storage unit 32 in step A 2. A correction curve is created based on the correction amount of the set, and a γ correction table is created based on the created correction curve. The γ correction table is a correction table for each pixel in the dither matrix created based on the created correction curve. Detailed processing of step A2 will be described later.
[0116]
After the γ correction table is created, the halftone output gradation processing unit 28 reads the input density value constituting the image data to be processed from the spatial filter processing unit 27 in step A3. In step A4, the halftone output gradation processing unit 28 performs γ correction processing using the γ correction table on the read input density value. As a result, the read input density value is corrected by the correction table for the pixel at that position in accordance with the pixel at which position in the dither matrix.
[0117]
In step A5, the halftone output tone processing unit 28 determines whether or not all input density values constituting the image data to be processed have been read. If there is an input density value that has not yet been read, the process returns from step A5 to step A3. If all the processing density values of the image data have been read, the main control process is terminated.
[0118]
As described above with reference to FIG. 11, the halftone output gradation processing unit 28 creates a correction curve to be used for the gradation correction process at the time of document copying using the reference correction curve and a set of correction amounts. The shape of the created correction curve changes according to the correction amount. Therefore, in the image processing apparatus 14, the correction amount storage unit 32 stores a plurality of sets of correction amounts, and the halftone output gradation processing unit 28 selects one of the correction amounts at the time of document copying to create a correction curve. It is preferable that the configuration is used. Thereby, the halftone output gradation processing unit 28 can perform gradation correction processing corresponding to each of a plurality of target characteristic curves. At this time, since one set of correction amounts is composed of, for example, 16 values, the data amount of one set of correction amounts is smaller than the data amount of one correction curve. That is, the correction amount storage unit 32 only needs to store a set of correction amounts, which are limited predetermined amounts of data, in the course of processing. Therefore, the storage capacity required to store the reference correction curve and the plurality of sets of correction amounts is smaller than the storage capacity required to store the correction curves respectively corresponding to the plurality of target characteristic curves. As a result, the image processing apparatus 14 according to the first embodiment has a storage capacity necessary for storing data for gradation correction processing as compared with the conventional image forming apparatus that always stores the correction curve itself. Can also be reduced.
[0119]
When the correction amount corresponding to the specified density value is the first correction amount based on the output characteristics unique to the image output device 15, the halftone output gradation processing unit 28 substantially outputs the output unique to the image output device 15. Appropriate γ correction processing based on the characteristics can be performed as gradation correction processing. When the correction amount corresponding to the specified density value is the sum of the first correction amount and the second correction amount based on the adjustment amount, the halftone output tone processing unit 28 is substantially based on the adjustment amount. The density correction process and the γ correction process based on the output characteristics unique to the image output device 15 can be performed as the gradation correction process. Further, the correction amount corresponding to the designated density value may be only the second correction amount. In this case, the halftone output gradation processing unit 28 can substantially perform γ correction processing using the reference correction curve and correction value correction processing based on the adjustment amount as gradation correction processing. Furthermore, when the correction amount corresponding to the specified density value is only the second correction amount, an adjustment amount corresponding to the output characteristic unique to the image output device 15 is used instead of the adjustment amount related to the correction of the density value. May be given to. In this case, since the second correction amount is an amount corresponding to the output characteristic unique to the image output device 15, the halftone output gradation processing unit 28 performs γ correction processing based on the output characteristic unique to the image output device 15. This can be performed as gradation correction processing.
[0120]
In the process of step A2 in FIG. 11, since the correction amount is obtained only for the designated density value as described above, the correction values for the remaining input density values other than the designated density value are used as the designated density when the correction curve is created. It is difficult to obtain in the same procedure as the correction value for the value. Therefore, as a correction value for the remaining input density value, a value obtained by interpolating the correction value for the specified density value is used as will be described with reference to FIG. Since the correction amount corresponding to the designated density value has already been obtained, the correction amount corresponding to the remaining input density value is obtained by interpolating the correction amount corresponding to the designated density value, as will be described with reference to FIG. You may ask for it. As a result, correction amounts corresponding to all correction values on the reference correction curve can be obtained, so that a correction curve can be created.
[0121]
FIG. 12 is a flowchart showing a first procedure of step A2 of the main control process of the halftone output tone processing unit 28 of FIG. After the user inputs a document copy instruction from the operation panel 16, the process proceeds to step B2. In step B2, the halftone output tone processing unit 28 obtains the sum of the correction value on the reference correction curve for the designated density value and the correction amount corresponding to the designated density value, and corrects the obtained sum for the designated density value. Value. In step B3, the halftone output tone processing unit 28 obtains correction values for the remaining input density values other than the specified density value by interpolation processing using correction values on the correction curve for the specified density value. Any interpolation method may be used for the interpolation processing, and for example, a linear interpolation method or a spline interpolation method is used. In this way, a correction curve to be actually used in the γ correction process is created by the processes in step B2 and step B3. In step B4, the halftone output tone processing unit 28 creates a γ correction table based on the created correction curve, and ends the process. In this way, the halftone output gradation processing unit 28 can create a correction curve by the processing described with reference to FIG.
[0122]
FIG. 13 is a correction curve created when the linear interpolation method is used for the interpolation processing in step B3 of FIG. FIG. 14 is a correction curve created when the spline interpolation method is used for the interpolation processing in step B3 of FIG. In FIG. 13 and FIG. 14, black circles indicate the correction values on the correction curve for the specified density value. When the linear interpolation method is used, the interpolation process can be performed very simply. When the spline interpolation process is used, a very smooth interpolation curve can be created, so that more appropriate γ correction can be performed. The interpolation method that is actually used in the interpolation processing in step B3 may be selected as appropriate according to the degree of γ correction desired by the user. When the correction amount corresponding to the designated density value is the sum of the first correction amount and the second correction amount, if the first procedure described with reference to FIG. 12 is used, data for gradation correction processing is stored. It is possible to further reduce the storage capacity required to do this.
[0123]
FIG. 15 is a flowchart showing a second procedure of step A2 of the main control process of the halftone output tone processing unit 28 of FIG. After the user inputs a document copy instruction from the operation panel 16, the process proceeds to step C2. In step C2, the halftone output gradation processing unit 28 obtains a correction amount corresponding to the remaining input density value other than the designated density value by an interpolation process using the correction quantity corresponding to the designated density value. In the correction amount interpolation processing, it is preferable to use a linear interpolation method. As a result, correction amounts corresponding to all input density values are obtained.
[0124]
In step C3, the halftone output tone processing unit 28 obtains the sum of the correction value on the reference correction curve for the input density value and the correction amount corresponding to the input density value, and the obtained sum is calculated for the input density value. Set the correction value on the correction curve. In this way, a correction curve to be actually used in the γ correction process is created by the processes in steps C2 and C3. In step C4, the halftone output tone processing unit 28 creates a γ correction table based on the created correction curve, and ends the process. When using the second procedure described in FIG. 15 and using the linear interpolation method, the halftone output tone processing unit 28 uses the first procedure described in FIG. 12 and using the linear interpolation method. A smooth correction curve can be created.
[0125]
As described above, the gradation correction process described above also serves as a dither process. When the tone correction process also serves as a dither process, it is preferable that the reference correction curve and the correction amount corresponding to the designated density value are set for each dither matrix size.
[0126]
[Table 2]

[0127]
FIG. 16 is a diagram illustrating combinations of gradation values of four pixels in the dither matrix when multi-value dither processing using a 2 × 2 dither matrix is performed. Table 2 shows a table of criteria assigned to each of the four pixels in the 2 × 2 dither matrix. The first table is used for the creation process of the correction table corresponding to the first pixel D1 in the dither matrix. The second table is used for creating the correction table corresponding to the second pixel D2. The third table is used for the creation process of the correction table corresponding to the third pixel D3. The fourth table is used for creating the correction table corresponding to the fourth pixel D4.
[0128]
FIG. 17 is a graph showing what the first to fourth tables have on the correction curve when multi-value dither processing using a 2 × 2 dither matrix is performed. In FIG. 17, the dither matrix size ms is 4 because the dither matrix is 2 × 2. The vertical axis of the correction curve graph is divided into four equal parts, and regions R1, R2, R3, and R4, which are division results, are sequentially assigned to the first table, the second table, the third table, and the fourth table. The correction value for the input density value in each table is determined according to a portion in the assigned area of the correction curve.
[0129]
For example, for the correction curve G34 to be used for the γ correction process, the correction values in each table are as follows. In the first table, the correction value for the input density value of 0 or more and less than a is equal to the correction value on the first part G34 (1) of the correction curve G34 for the input density value, and the input density of a or more and 255 or less. The correction value for the value is 255. In the second table, the correction value for an input density value of 0 or more and less than a is 0, and the correction value for an input density value of a or more and less than b is a correction on the second portion G34 (2) of the correction curve G34. It is equal to the value obtained by subtracting 255 from the value, and the correction value for the input density value between b and 255 is 255. In the third table, the correction value for the input density value of 0 or more and less than b is 0, and the correction value for the input density value of b or more and less than c is the correction on the third portion G34 (3) of the correction curve G34. It is equal to the value obtained by subtracting 510 from the value, and the correction value for the input density value between c and 255 is 255. In the fourth table, the correction value for the input density value from 0 to less than c is 0, and the correction value for the input density value from c to 255 is the correction on the fourth portion G34 (4) of the correction curve G34. Equal to the value obtained by subtracting 765 from the value. In FIG. 17, “a”, “b”, and “c” are input density values on the correction curve G34 for the correction values “255”, “510”, and “765”. In the second table, the correction value for the input density value of 0 or more and less than a is 0 because the input of 0 or more and less than a in the correction curve G34 is within the region R2 assigned to the second table. This is because there is no portion corresponding to the density value.
[0130]
Using the first to fourth tables based on the correction curve G34, for example, for the input density value 80, the correction value of the first pixel D1 is 255, and the correction value of the second pixel D2 is 100, It can be seen that the correction value of the third pixel D3 is 0 and the correction value of the fourth pixel D4 is 0. As described above, using the first to fourth tables, values obtained by distributing the correction values on the correction curve corresponding to the input density values to the first to fourth pixels are obtained.
[0131]
In the above description, a case where a 2 × 2 dither matrix is used has been described as an example. However, the dither matrix is not limited to this, and a 3 × 3 or more dither matrix may be used. When a 3 × 3 dither matrix is used, the vertical axis of the reference correction curve may be divided into nine to create a correction table for each pixel. When a 4 × 4 dither matrix is used, each pixel To create each table, the vertical axis of the correction curve may be divided into 16 equal parts. That is, the vertical axis of the correction curve may be equally divided so that the same number of regions as the number of pixels in the dither matrix can be obtained. When the reference correction curve and the correction amount are set for each dither matrix size, the halftone output tone processing unit 28 can perform appropriate tone correction according to the dither matrix size.
[0132]
FIG. 18 is a block diagram showing a configuration of an image processing apparatus 51 according to the second embodiment of the present invention. In the description of the second embodiment, the devices, processing units, and parameters already described in the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted. The image processing apparatus 51 according to the second embodiment includes an A / D conversion unit 21, a shading correction unit 22, an input tone correction unit 23, a color correction unit 24, an image area separation processing unit 25, and a black generation under color removal unit. 26, a spatial filter processing unit 27, a halftone output tone processing unit 53, a curve storage unit 31, a correction amount storage unit 32, and a correction coefficient storage unit (correction coefficient storage means) 54. The correction coefficient storage unit 54 is realized by at least a nonvolatile storage unit from which data can be read, for example, a ROM. The image forming apparatus 55 including the image processing apparatus 51 according to the second embodiment is the same as the image forming apparatus 11 according to the first embodiment, except that the image processing apparatus 14 according to the first embodiment is the second embodiment. The image processing apparatus 51 is replaced with the image processing apparatus 51 in the form.
[0133]
The description of the processing of the halftone output gradation processing unit 28 of the image processing apparatus 14 according to the first embodiment described above is a case where there is almost no influence of variations in characteristics of the image input apparatus 13 and the background density of the document. It is. The effects of variations in characteristics of the image input device 13 and the background density of the document may not be ignored. For example, when an image is formed on a chromatic recording paper, or when an image is formed on a recording paper having a different background density even if it is a white recording paper, the influence of variations in characteristics of the image input device 13 is affected. It is difficult to ignore the influence of the background density of the document. In order to perform the tone correction processing in consideration of the influence of the characteristic variation of the image input device 13 and the influence of the background density of the document, the halftone output tone processing unit 53 of the second embodiment will be described below. It is necessary to perform gradation correction processing including processing.
[0134]
The processing in the halftone output gradation processing unit 53 of the second embodiment corresponds to (1) the designated density value as compared with the processing in the halftone output gradation processing unit 28 of the first embodiment. Comparing the output measurement value with a plurality of reference characteristic curves, selecting the reference characteristic curve closest to the output measurement value, and (2) obtaining the first correction amount corresponding to the designated density value In addition, since only the process of comparing the characteristic of the output measurement value with respect to the input density value and the predetermined target characteristic curve is different, and the other processes are the same, the above different (1) and (2) Only the process will be described.
[0135]
First, the process of selecting the reference characteristic curve closest to the output measurement value corresponding to the designated density value will be described. The halftone output gradation processing unit 53 first prints the test image on a recording sheet as a recording medium by the image output device 15 without performing gradation correction processing on the test image prepared in advance. Next, the image input device 13 is caused to read the printed test image. At this time, the halftone output tone processing unit 53 simultaneously reads the background density of the test image, and uses the read density value as the output measurement value of the patch whose input density value is 0. The measurement location of the background density of the test image may be any region as long as the test image is not formed on the recording paper surface. For example, in a recording medium on which a test image having the configuration shown in FIG. 19 is printed, an area 56 indicated by a broken line is used as a measurement point of the background density.
[0136]
The output measurement value of the test image read in this way is, for example, a black circle shown in FIG. FIG. 20 shows an example of the output measurement value of the test image, and the output measurement value varies depending on the characteristic variation of the image input device 13 and the background density. In this case, due to the characteristic variation of the image input device 13 and the influence of the background density, the measured output measurement value is observed to be high overall. At this time, the closest reference characteristic curve can be selected based on the output measurement value on the high density side, but in this case, the gradation characteristic on the low density side is deteriorated. Since the low density side is visually more noticeable than the high density side, it is not preferable that the gradation characteristics on the low density side deteriorate. Therefore, before performing the process of selecting the reference characteristic curve, it is necessary to pay attention to the low density side and to perform a correction process to remove the influence of the characteristic variation of the image input device 13 and the background density on the output measurement value. .
[0137]
The correction processing for the output measurement value uses a correction coefficient as shown in FIG. 21 and the output measurement value of the patch whose input density value of the test image is 0, that is, the density value of the recording medium read by the image input device 13. , By calculation. Specifically, the correction processing for the output measurement value uses a set of correction coefficients indicated by black circles in FIG. 21 to obtain a density correction amount that provides stronger correction as the density is lower, and outputs the density correction amount as an output measurement. Do this by subtracting from the value. In this specification, the density value obtained by measuring the density of the area of the recording paper on which the test image patch is not formed is referred to as “background density value”. When there are variations in the characteristics of the image input device 13 or when documents having different background densities are used, the factors are different, but in both cases, the background density values are shifted and observed.
[0138]
The correction coefficient is set corresponding to each designated density value. The correction coefficient corresponding to the designated density value represents the influence of the measured background density value on the corresponding designated density value. A value obtained by multiplying the background density value by the correction coefficient is the density correction amount used for the correction process of the output measurement value. The correction coefficient and the density correction amount are maximum when the input density value is 0, and minimum when the input density value is maximum. The maximum value acquired by the correction coefficient is 1, and the minimum value is 0. As the correction coefficient, an appropriate value is obtained in advance based on image reading test results using various image input devices 13 and various originals, and is stored in the correction coefficient storage unit 54.
[0139]
When the output measurement value and the background density value corresponding to each designated density value are measured, the correction coefficient stored in the correction coefficient storage unit 54 is read. First, a density correction amount corresponding to each designated density value is obtained by multiplying the background density value by a correction coefficient corresponding to each designated density value. Next, a corrected output density value corresponding to each designated density value is obtained by subtracting the density correction amount corresponding to each designated density value from the output measurement value corresponding to each designated density value.
[0140]
The corrected output density value thus obtained is used in place of the output measurement value, and the corrected output density value is compared with a plurality of reference characteristic curves according to the procedure described with reference to FIGS. A characteristic curve is selected. Then, a reference correction curve corresponding to the selected reference characteristic curve is selected. Since the reference correction curve is selected by the above procedure, the halftone output tone processing unit 53 of the second embodiment determines the characteristic variation of the image input device 13 and the background density of the recording medium on which the test image is printed. Excluding the influence, it is possible to select the most appropriate reference correction curve for the output characteristics unique to the image output device 15.
[0141]
The halftone output gradation processing unit 53 performs gradation correction processing on the test image using the selected reference correction curve for setting the first correction amount, and the processed test image is output to the image output device. 15 to print on the recording paper. The printed test image is read again by the image input device 13. As a result, an output measurement value as indicated by a black circle in FIG. 22 is obtained. Even when the test image subjected to the gradation correction process using the selected reference correction curve is printed on the recording paper and the density of the test image is read, the output measurement value is observed with a shift. Therefore, even when the first correction amount is set, the output measurement value is corrected using the same method as the output correction value correction process described when the reference correction curve is selected. At this time, as the background density value, a value newly read by the image input device 13 may be used. Alternatively, since the background density value to be read hardly changes if the types of recording paper are the same, the value for selecting the reference characteristic curve may be used.
[0142]
The corrected output density value thus obtained is used in place of the output measurement value, and the corrected output density value is compared with the target characteristic curve according to the procedure described with reference to FIGS. A quantity is required. The calculated first correction amount may be used as it is as the correction amount for the specified density value, and the sum of the calculated first correction amount and the second correction amount described above is used as the correction amount for the specified density value. May be. Since the correction amount is set by the above procedure, the halftone output tone processing unit 53 of the second embodiment is affected by the characteristic variation of the image input device 13 and the background density of the recording medium on which the test image is printed. The most appropriate correction amount can be calculated without receiving.
[0143]
As described above, even when the image input device 13 has characteristic variations or the background density of the document is different, output measurement is performed using a correction coefficient predetermined for each specified density value and the measured background density value. By correcting the value, the halftone output gradation processing unit 53 can eliminate these influences and perform appropriate gradation correction processing. The correction coefficient for each designated density value described above is preferably obtained for each color component of cyan (C), magenta (M), yellow (Y), and black (K). This makes it possible to obtain a correction amount suitable for each color component, so that the halftone output tone processing unit 53 can perform more appropriate tone correction processing.
[0144]
In the above description, the correction coefficient is uniquely determined, and a case has been described in which the characteristic variation of the image input device 13 and the difference in the background density of the document are corrected using a set of correction coefficients. A digital color copying machine or the like may copy originals having various background colors.
[0145]
In such a case, if the first correction amount corresponding to each background density is used, more appropriate gradation correction processing can be performed. In this case, correction coefficients for a plurality of representative recording papers having different background densities are obtained in advance and stored in the correction coefficient storage unit 54, and correction coefficients corresponding to the first correction amount for each recording paper are obtained. The correction processing may be performed by reading from the correction coefficient storage unit 54. In this way, the first correction amount obtained corresponding to the recording paper having various background color densities is stored in the correction amount storage unit 32.
[0146]
For example, as shown in FIG. 23, the operation panel 56 used for selecting the first correction amount includes a display unit 62 for displaying a recording paper background sample 61 (background density) and a recording paper background sample 61. A configuration having an operation button 63 for sequentially displaying and a determination button 64 for setting the background of the recording paper can be mentioned. The operation buttons include a button for instructing display of the next sample and a button for instructing to return the displayed sample to the previous one. The user may display the samples sequentially while comparing the background of the document with the sample and select the closest sample. The halftone output gradation processing unit 53 reads and uses the first correction amount corresponding to the selected sample during the gradation correction process. Since the first correction amount suitable for the original is read from the correction amount storage unit 32 and used during the gradation correction processing, the halftone output gradation processing unit 53 can perform appropriate gradation correction processing.
[0147]
FIG. 24 is a block diagram showing a configuration of an image processing apparatus 71 according to the third embodiment of the present invention. In the description of the third embodiment, the devices, processing units, and parameters already described in the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted. The image processing apparatus 71 according to the third embodiment includes an A / D conversion unit 21, a shading correction unit 22, an input tone correction unit 23, a color correction unit 24, an image area separation processing unit 25, and a black generation under color removal unit. 26, a spatial filter processing unit 27, a halftone output tone processing unit 73, a curve storage unit 31, a correction amount storage unit 32, and a maximum correction value storage unit (maximum correction value storage means) 74. The maximum correction value storage unit 74 is realized by a non-volatile storage unit from which at least data can be read, for example, a ROM. The image forming apparatus 75 including the image processing apparatus 71 according to the third embodiment is the same as the image forming apparatus 11 according to the first embodiment, except that the image processing apparatus 14 according to the first embodiment is the third embodiment. The image processing apparatus 71 is replaced with the image processing apparatus 71 in the form.
[0148]
The above description of the processing of the halftone output gradation processing unit 28 of the image processing apparatus 14 according to the first embodiment is an explanation when the maximum output value of the image output apparatus 15 is stably controlled. In the image processing apparatus 14 according to the first embodiment, the plurality of designated density values may not include the maximum input density value 255. Even in this case, the output value (hereinafter referred to as “first output value”) of the image output device 15 corresponding to the designated density value having the highest density among the designated density values is corrected to a predetermined value. When 255 is not included in the plurality of designated density values, the highest input density value is equal to or higher than the designated density value with the highest density. The output value of the image output device 15 (hereinafter referred to as “second output value”) corresponding to the highest input density value is corrected depending on the maximum output value of the image output device 15. As a result, depending on the state of the image output device 15, the second output value does not become a constant value. Thereby, the density difference in the high density region, that is, the difference between the second output value and the first output value increases as the maximum output value of the image output device 15 increases, and as the maximum output value of the image output device 15 decreases. Get smaller.
[0149]
The image processing apparatus does not increase the density difference in the high density region even when the maximum output value is different due to the state of the image output apparatus 15, for example, the surface potential of the photoconductor or the development characteristics. It is desired to perform appropriate gradation correction processing. When the variation of the image output device 15 is large and the change in the maximum output value of the image output device 15 cannot be ignored, further correction processing such as limiting the correction value on the correction curve given to the image output device 15 is necessary. . For this purpose, the halftone output tone processing unit 73 of the third embodiment is used for (1) a step of limiting the correction value on the correction curve given to the image output device, and (2) a correction value limitation. The step of determining a maximum correction value that is a threshold value is further performed. The processing in the halftone output tone processing unit 73 of the third embodiment is compared with the whole process of the processing in the halftone output tone processing unit 28 of the first embodiment. However, only the above-described different steps (1) and (2) will be described. To do.
[0150]
FIG. 25 is a graph showing the relationship between the measured output value of each patch of the test image and the correction value used when outputting each patch of the test image in the step of selecting the reference correction curve. . In the reference correction curve selection step, the test image is output without gradation correction, so the correction value used for patch output is ms times the pixel density value of the patch portion of the test image. FIG. 25 shows an example of output measurement values when the maximum output value of the image output device 15 is normal and when the maximum output value becomes extremely high. If the density value of the pixel printed by the image output device 15 is measured by the image input device 13, the pixel printed by the image output device 15 if the image input device 13 has no characteristic variation and is not affected by the background density. Is equivalent to the output value of the image input device 13.
[0151]
  In the description of FIG. 25, the maximum output value of the image output device 15 in the normal case is Out-max1, the maximum output value of the image output device 15 in the case of extremely high is Out-max2, and the reference correction curve selection step.BestThe output value on the target characteristic curve for the specified density value xh having a high density is set as the target output value Out-z. Further, the output characteristic of the image output device 15 in the normal case is G41, the output characteristic of the image output device 15 in the case where the maximum output value is extremely high is G42, and the normal case with respect to the designated density value xh having the highest density. The correction value on the output characteristic G41 is Z1 (xh), and the correction value on the output characteristic G42 when the maximum output value is extremely high with respect to the designated density value xh having the highest density is Z2 (xh).
[0152]
Whether the maximum output value of the image output device 15 is normal or extremely high, the output value of the image output device 15 corresponding to the correction value on the correction curve at the specified density value xh having the highest density. Is approximately controlled to the target output value Out-z. That is, the correction value given to the image output device 15 is the correction value Z1 (xh) on the output characteristic G41 in the normal case with respect to the specified density value xh having the highest density, or the specified density value xh having the highest density. Regardless of the correction value Z2 (xh) on the output characteristic G42 when the maximum output value becomes extremely high, the output value of the image output device 15 is approximately equal to the target output value Out-z. The correction curve is constructed.
[0153]
However, the maximum output value of the image output device 15 is output after being given a correction value of 255 × ms when there is no restriction on the correction value. For this reason, if a correction value of 255 × ms is given in each of the normal case and the maximum output value becomes extremely high, the image output device 15 has the density of the maximum output values Out-max1 and Out-max2. Each pixel is output. Therefore, in the above two cases, the difference between the output value corresponding to the correction value for the specified density value xh having the highest density and the maximum output value is ΔOut1 and ΔOut2, and the output from the image output device 15 in the high density portion is output. It will be very different. The relationship between the input density value from the minimum input density value to the specified density value xh having the highest density and the output value is controlled so as to be substantially the same even if the image output device 15 varies, but it is the highest. Since an input density value equal to or higher than the specified density value xh of the density depends on the maximum output value of the image output device 15, a density difference occurs in a high density region. In order to prevent the occurrence of a density difference in the high density region, it is necessary to provide a restriction on the correction value given to the image output device 15. The maximum correction value Zlim for limiting the correction value is given in the step of selecting a reference correction curve.
[0154]
A curve G52 in FIG. 26 is a graph showing the relationship (correction curve) between the correction value and the input density value when a maximum correction value is provided as a limit to the correction value given to the image output device 15 and the correction value is limited. is there. When the maximum correction value is not limited, a correction value is given to the image output device 15 based on a correction curve G51 in which a correction value of 255 × ms corresponds to the maximum input density value 255. When a restriction is applied using the maximum correction value Zlim obtained in the step of selecting a reference correction curve, the image output device is based on the correction curve G52 that corresponds to the input correction value Zlim with respect to the input density value 255. A correction value is given to 15.
[0155]
  There are the following methods (1) to (3) for setting the maximum correction value. In method (1), a fixed maximum correction value is provided in advance. The correction value on the correction curve is the default maximum correction value.Less thanLimited to In the method (2), as the maximum correction value, a value of a predetermined ratio of the correction value on the correction curve with respect to the specified density value having the highest density is set..The correction value on the correction curve is the maximum correction value.Less thanLimited to In the method (3), a maximum correction value corresponding to the maximum output target value is obtained based on the predetermined maximum output target value and each output measurement value. The correction value on the correction curve is the maximum correction value.Less thanLimited to A procedure for determining the maximum correction value in these methods will be described.
[0156]
In the case of method (1), a maximum correction value that does not cause a problem even if the maximum output value of the image output device 15 fluctuates is obtained in advance, and this value is used as a fixed value. The maximum correction value obtained in this way is stored in a maximum correction value storage unit (maximum correction value storage means) 74 shown in FIG. When a correction curve is obtained and it is necessary to limit the correction value, the maximum correction value is read from the maximum correction value storage unit 74 and the correction curve is corrected.
[0157]
In the case of the method (2), the halftone output tone processing unit 73 calculates using the correction value Z (xh) on the correction curve for the specified density value xh having the highest density and the maximum correction value determination coefficient Klim. The maximum correction value Zlim is obtained based on the equation (2). The maximum correction value determination coefficient Klim is an optimum value obtained in advance based on various documents. In the case of the method (2), the maximum correction value Zlim obtained based on the equation (2) is stored in the maximum correction value storage unit 74.
Zlim = Klim × Z (xh) (2)
[0158]
Instead of the maximum correction value, the maximum correction value determination coefficient may be stored in the maximum correction value storage unit 74. In this case, the maximum correction value is the second correction amount for the specified density value Xh having the highest density obtained based on the adjustment amount input from the adjustment amount input means provided in the image forming apparatus 75. A correction amount corresponding to the designated density value Xh is obtained by adding to the first correction amount for the designated density value Xh having the highest density determined based on the output characteristics, and obtained based on the reference correction curve and the correction quantity. It is obtained by multiplying the correction value for the designated density value Xh having the highest density by the maximum correction value determination coefficient. If the maximum correction value determination coefficient is stored, when it is necessary to correct the obtained correction curve, the maximum correction value determination coefficient is read from the maximum correction value storage unit 74, and the read maximum correction value determination coefficient is read out. The maximum correction value is obtained by the above-described procedure using, and the correction value is limited using the obtained maximum correction value. When the adjustment amount is input in this way, the correction value on the correction curve varies depending on the second correction amount, so that the maximum correction value determination coefficient is stored in the maximum correction value storage unit 74 and the image is output. It is preferable to correct the correction curve by calculating the maximum correction value.
[0159]
In the case of the method (3), an image pattern having a maximum output value is output in advance using the image forming apparatus 75 that has been adjusted to an optimum state in advance, and the output image pattern is read by the image input apparatus 13 and its measured value is measured. Is defined as the maximum output target value Out-lim-target. The image pattern of the maximum output value is, for example, the same as the test image shown in FIG. 4, and is realized by a test image including a patch corresponding to the maximum output value. The test image is output and read a plurality of times using a plurality of image forming apparatuses 75. As a standard value of the output measurement value, an average value or a median value is used.
[0160]
The procedure for obtaining the maximum correction value based on the maximum output target value will be described with reference to FIG. First, (1) the output measurement value of the patch of the designated density value obtained in the process of selecting the reference correction curve and the value of ms times the pixel value of the patch of the designated density value, that is, the value of ms times the designated density value. Based on the correction value, a graph representing the relationship between the correction value ms times the specified density value and the output value of the image output device 15 is created using linear interpolation or spline interpolation. In this case, since the image forming apparatus 75 is adjusted to an optimum state, the relationship between the correction value and the output value is the output characteristic G41 of the image output apparatus in the normal case. Next, (2) the intersection (Zlim, Out-lim-target) between the created graph and the maximum output target value is obtained. Finally, (3) the correction value Zlim of the obtained intersection (Zlim, Out-lim-target) is set as the maximum correction value.
[0161]
In the procedure for obtaining the maximum correction value of the method (3), when the output measurement value cannot be stably obtained, a plurality of maximum correction values may be obtained. If the output measurement value cannot be obtained stably, for example, a test image in which the patch density monotonously decreases is output to a graph showing the relationship between the correction value and the output value. There is a case where there is a portion where the measured value increases. As a method for preventing a plurality of maximum correction values from being obtained, there is a method of providing a plurality of maximum output target values.
[0162]
The procedure for setting the maximum correction value when two maximum output target values are provided will be described with reference to FIG. Similarly to FIG. 25, a graph G44 in FIG. 27 graphs the relationship between the correction value that is ms times the specified density value and the output measurement value of the image output device 15 obtained in the step of selecting the reference correction curve. It is a thing. FIG. 27 shows an example in which the relationship between the output value and the correction value is linearly interpolated.
[0163]
First, {circle over (1)} intersections between a predetermined first maximum output target value Out-lim-target1 and second maximum output target value Out-lim-target2 and the graph G44 are obtained. Next, (2) the first intersection (Zlim2, Out-lim-target2) which is the intersection with the smallest correction value among the intersections between the obtained second maximum output target value Out-lim-target2 and the graph G44. Among the intersections between the first maximum output target value and the graph G44, the second intersection (Zlim1, which is the intersection with the largest correction value among the intersections whose correction value is equal to or smaller than the correction value Zlim2 of the first intersection) Out-lim-target 1). Finally, as shown in Expression (3), an average value of the obtained correction values of the first intersection and the second intersection is obtained, and the average value is set as a maximum correction value Zlim.
Zlim = (Zlim1 + Zlim2) ÷ 2 (3)
[0164]
When two maximum output target values are provided, the maximum output target value, the first maximum output target value, and the second maximum output target value are stored in the maximum correction value storage unit 74. The maximum correction value storage unit 74 stores the maximum output target value and all of the first maximum output target value and the second maximum output target value, and these values and the output measurement values in the image output device 15 What is necessary is just to obtain | require the intersection with the approximate curve showing the relationship with a correction value, and to determine which maximum output target value should be used according to the result. That is, when the number of intersections between the first maximum output target value and the second maximum output target value is one, the approximate curve is determined to have a monotonically increasing characteristic. To determine the maximum correction value. When there are two or more intersections between the first maximum output target value and the second maximum output target value, it is determined that the approximate curve does not show a monotonous increase characteristic, and therefore the first maximum output target value and the second maximum output target value are determined. What is necessary is just to obtain | require the maximum correction value based on Formula (3) using an output target value. When a plurality of maximum output target values are set, two maximum output target values are usually sufficient.
[0165]
Any of the above three methods may be used as a method for obtaining the maximum correction value. In the case of the method (3), since the maximum correction value is determined using the image forming apparatus adjusted to the optimum state, more appropriate gradation compared to the case of the method (1) and the method (2). This is preferable because corrective measures can be taken.
[0166]
The correction curve limiting step using the maximum correction value is performed after an image is given to the halftone output tone processing unit 73 and the correction curve G51 is created. As shown in FIG. 26, the corrected correction curve G52 is equal to the correction curve G51 that is not restricted within the first range from the minimum input density value 0 to the predetermined input density value xα. The correction value Z (α) on the correction curve G51 before the restriction with respect to the predetermined input density value xα is less than the maximum correction value Zlim. In the second range from the predetermined input density value xα to the maximum input density value 255, the correction value on the corrected correction curve G52 for each input density value is kept below the maximum correction value Zlim. When the correction value on the correction curve G51 before the restriction in the second range monotonically increases, the correction value is such that the correction value on the correction curve G52 after the restriction within the second range also monotonously increases. Limited. The correction values on the corrected correction curve G52 for each input density value in the second range are, for example, the maximum output value and the maximum correction value from the correction values on the correction curve G51 before the limit for each input density value. Is obtained by subtracting a product obtained by multiplying the difference between and a predetermined coefficient corresponding to each input density value.
[0167]
The maximum correction value described above is preferably determined for each color component. When using the maximum correction value determination coefficient as in the method (2), when using the maximum output target value, the first maximum output target value, the second maximum output target value, etc. as in the method (3), these values are used. Are determined for each color component, and a maximum correction value is obtained for each color component. In this way, by determining the maximum correction value for each color component, the correction curve can be corrected for each color component, so that more appropriate gradation processing can be performed.
[0168]
Depending on the type of document to be copied and the user's preference, it may or may not be preferable to limit the correction value using the maximum correction value set in the above procedure. When copying originals with smooth gradation, such as photographic paper photographs and printed photographs, it is often better to limit the correction value. For text documents, it is better not to limit the correction value. Often good. For this reason, it is preferable to automatically switch whether or not to limit the correction value according to the type of document to be copied, or to enable manual switching by the user. As a result, a more preferable image can be obtained.
[0169]
As a method of automatically switching whether or not to limit the correction value, there is a method of controlling based on a copy mode from the operation panel 16 provided in the image forming apparatus 75. For example, when a photo mode or a color mode for outputting a color image is selected from the operation panel 16, a process for limiting the correction value on the correction curve is performed, and a character mode or a monochrome image mode is input. In this case, the process for limiting the correction value may not be performed. In addition, in order for the user to arbitrarily switch the process, the operation panel 16 provided in the image forming apparatus 75 can be used to arbitrarily input an instruction as to whether or not to limit the correction value. That's fine. For example, there is a method of setting the manual input mode on the operation panel 16 and displaying a message such as “Do you want to make a smooth image?” On the display unit composed of a liquid crystal display or the like to prompt the user to input. .
[0170]
Specific values of the various parameters used in the description of the first to third embodiments are merely examples, and other values may be used. For example, the number of reference correction curves is not limited to five but may be at least one. The designated density value is not limited to 16 and may be plural. The upper limit value of the input density value is not limited to 255. The lower limit value of the input density value is not limited to zero. When the gradation correction process also serves as part of the dither process, the upper limit value of the reference correction value may be ms times the upper limit value of the input density value. If not, the upper limit value of the reference correction value is the input density. It only needs to be equal to the upper limit value.
[0171]
In the first to third embodiments, the image processing apparatuses 14, 51, 71 include processing units other than the halftone output gradation processing units 28, 53, 73. The image processing apparatus 14 of the first embodiment only needs to include at least the halftone output tone processing unit 28, the curve storage unit 31, and the correction value storage unit 32, and other processing units are omitted as appropriate. Also good. The image processing apparatus 51 according to the second embodiment only needs to include at least the halftone output gradation processing unit 53, the curve storage unit 31, the correction value storage unit 32, and the correction coefficient storage unit 54. The part may be omitted as appropriate. The image processing apparatus 71 according to the third embodiment only needs to include at least a halftone output gradation processing unit 73, a curve storage unit 31, a correction value storage unit 32, and a maximum correction value storage unit 74. The processing unit may be omitted as appropriate. The gradation correction processing of the first to third embodiments also serves as halftone generation processing. The tone correction processing and halftone generation processing of the first to third embodiments may be executed individually. Further, the gradation correction processing of the first to third embodiments may be applied when performing input gradation correction processing.
[0172]
The correction value limiting step and the maximum correction value setting step for removing the influence of the fluctuation of the maximum output value of the image output device 15 as described in the third embodiment are the same as those in the second embodiment. You may add to the gradation correction process which has the correction process of the output measurement value for removing the influence of the characteristic variation of the image input device 13 and background density which was demonstrated. More preferably, the image processing apparatus performs both the output measurement value correction process and the correction value limiting step in accordance with the gradation correction process. As a result, it is possible to remove both the influence of the characteristic variation of the image input device 13 and the background density and the influence of the fluctuation of the maximum output value of the image output device 15 from the gradation correction processing. It becomes possible.
[0173]
Furthermore, the processing performed by the halftone output gradation processing units 28, 53, 73 of the first to third embodiments may be performed by a computer. For this purpose, a control program for causing the central processing circuit to perform processing of the halftone output gradation processing units 28, 53, and 73, and data stored in the curve storage unit 21 and the correction amount storage unit 22, It is stored in a computer readable storage medium. The control program and data constitute control software for gradation correction processing. After the control software in the storage medium is installed in the computer, when the central processing circuit of the computer executes the control program, the computer can perform the same processing as the halftone output gradation processing units 28, 53, and 73. .
[0174]
【The invention's effect】
  As described above, according to the present invention, in the image processing apparatus, only the correction amount and the reference correction curve corresponding to a plurality of designated density values are always stored, and the gradation correction processing means performs gradation correction processing. A correction curve to be used is created every time an image is given. As a result, when the correction curve used for the gradation correction process can be changed, the image processing apparatus only needs to store a plurality of sets of correction amounts and reference correction curves, so that the storage capacity of the storage unit can be reduced. it can.
  Further, according to the present invention, the correction amount and the reference correction curve corresponding to the designated density value are set for each dither matrix size. Thus, the gradation correction processing means can perform appropriate gradation correction processing for each dither matrix size.
[0175]
According to the present invention, when a plurality of reference correction curves are stored, the reference correction curve used for creating the correction curve is selected according to the output characteristics unique to the image output means. As a result, the gradation correction processing unit can create a correction curve that is more suitable for the output characteristics unique to the image output unit. Furthermore, according to the present invention, the reference correction curve is selected based on the result of outputting the image pattern of the designated density value as it is to the image output device. Thereby, the gradation correction processing means can select the most appropriate reference correction curve for the output characteristics unique to the image output means.
[0176]
According to the invention, the correction amount corresponding to the designated density value is determined based on the output characteristics unique to the image output means. As a result, the gradation correction processing means can perform appropriate γ correction processing based on the output characteristics unique to the image output means. Furthermore, according to the present invention, the correction amount corresponding to the designated density value is determined based on the adjustment amount. As a result, the gradation correction processing means can perform density value correction processing based on the adjustment amount simultaneously with the γ correction processing. According to the present invention, the correction amount corresponding to the designated density value is determined based on the output characteristics specific to the image output means and the adjustment amount. Thus, the gradation correction processing unit can perform appropriate γ correction processing based on output characteristics unique to the image output unit, and can perform correction processing based on the adjustment amount.
[0177]
Furthermore, according to the present invention, the correction amount based on the output characteristic unique to the image output means is set according to the output result of the image pattern of the designated density value that has been subjected to the gradation correction process using the reference correction curve. . As a result, the gradation correction processing means can set the most appropriate first correction amount for the output characteristics unique to the image output means. According to the present invention, when a plurality of adjustment amounts are input, the correction amount determined based on the adjustment amount is the sum of the correction amounts based on the adjustment amounts. Accordingly, the gradation correction processing means can easily calculate the correction amount based on the plurality of adjustment amounts when a plurality of adjustment amounts are input.
[0178]
Furthermore, according to the present invention, the correction value on the correction curve for the specified density value is obtained by adding the correction amount for the specified density value to the correction value on the reference correction curve for the specified density value. The correction value on the correction curve for the input density value is obtained by interpolation processing using the correction value on the correction curve for the specified density value. Thereby, the gradation correction processing means can create a correction curve using only one set of correction amount and the reference correction curve. According to the present invention, the interpolation process used when creating the correction curve is a linear interpolation process. Accordingly, the gradation correction processing means can easily create a correction curve. Furthermore, according to the present invention, the interpolation process used when creating the correction curve is a spline interpolation process. Thereby, the gradation correction processing means can create a smooth correction curve.
[0179]
According to the invention, the correction amount corresponding to the input density value other than the specified density value is obtained by linear interpolation using the correction amount corresponding to the specified density value, and the specified density value and the input other than the specified density value are input. The correction value on the correction curve for the density value is obtained by adding the correction amount of these density values to the correction value on the reference correction curve for these density values. Thereby, the gradation correction processing means can easily create a smoother correction curve.
[0180]
  Furthermore, according to the present invention, the correction amount and the reference correction curve corresponding to the designated density value are set for each color component. Thereby, the gradation correction processing means can perform appropriate gradation correction processing for each density value of each color component..
[0181]
As described above, according to the present invention, in addition to the correction amount corresponding to a plurality of designated density values and the reference correction curve, an appropriate correction coefficient used for setting the correction amount is obtained in advance in the image processing apparatus. It is remembered. The gradation correction processing means creates a correction curve used for the gradation correction processing every time an image is given. As a result, the gradation correction processing means can create a correction curve by eliminating these effects even when the image input means has characteristic variations or when a document having a different background density is copied. Tone correction processing can be performed.
[0182]
According to the present invention, when a plurality of reference correction curves are stored, the reference correction curve has no image pattern formed as a result of outputting the image pattern of the designated density value as it is to the image output means. It is selected based on the result of correction based on the density value of the recording medium in the area and the correction coefficient. As a result, the gradation correction processing means can select the most appropriate reference correction curve for the output characteristics unique to the image output means, excluding the influence of variations in the characteristics of the image input means and the background density of the original. Furthermore, according to the present invention, the correction amount based on the output characteristics unique to the image output means is relative to the read density value of the image pattern having the designated density value subjected to the gradation correction process using the reference correction curve. It is set in accordance with the result of correction based on the density value and correction coefficient of the recording medium in the area where no image pattern is formed. As a result, the gradation correction processing means can calculate the most appropriate correction amount without being affected by variations in the characteristics of the image input means and the background density of the recording medium.
[0183]
According to the present invention, the correction coefficient is stored according to the type of the recording medium, and is arbitrarily selected by manual operation with reference to the display on the operation panel. Thus, the gradation correction processing means can perform appropriate gradation correction processing using an appropriate correction amount even for recording media having different background densities. When a plurality of correction amounts are provided, a correction amount that is paired with the selected correction coefficient is selected, so that the gradation correction processing unit can appropriately correct even a recording medium having a different background density. An appropriate gradation correction process can be performed using the amount. Furthermore, according to the present invention, since the correction coefficient is obtained for each color component, the correction amount can be obtained for each color component, and the gradation correction processing means performs more appropriate gradation correction processing. It can be carried out.
[0184]
In addition, as described above, according to the present invention, even when the change width of the maximum output value of the image output unit varies to a degree that cannot be ignored, the gradation correction processing unit of the image processing apparatus provides the correction value to be given to the image output unit. By limiting the above, it is possible to suppress the density difference in the high density region and perform appropriate gradation correction processing. Furthermore, according to the present invention, the maximum correction value is determined by multiplying the correction value on the correction curve corresponding to the darkest specified density value by the maximum correction value determination coefficient. Therefore, a preferable maximum correction value determination coefficient is determined in advance. The determined maximum correction value determination coefficient may be stored in the maximum correction value storage means. Since the maximum correction value can be calculated prior to output each time an image is output, the gradation correction processing means can perform appropriate gradation correction processing.
[0185]
According to the invention, the gradation correction processing means outputs a test image, and obtains a maximum correction value based on an output measurement value obtained by reading the test image by the image input means. Thus, the maximum correction value is obtained every time the output test image is read, so that the gradation correction processing means can perform more appropriate gradation correction processing. Furthermore, according to the present invention, even when there is a possibility that the output measurement value may not be stably obtained, the gradation correction processing means can provide the maximum correction target value by providing a plurality of maximum output target values used for calculating the maximum correction value. The correction value can be obtained stably. Thereby, the gradation correction processing means can perform more appropriate gradation correction processing.
[0186]
Further, according to the present invention, when adjustments such as image density and contrast are performed, the maximum correction value is obtained in accordance with these adjustment amounts, so that the gradation correction processing means performs more appropriate gradation correction processing. be able to. Furthermore, according to the present invention, since the maximum correction value is obtained for each color component, the gradation correction processing means can perform appropriate gradation correction processing for each color component. Further, according to the present invention, since it is possible to select whether or not to use the maximum correction value, the gradation correction processing means can perform more appropriate gradation correction processing according to the type of document and the user's preference. It can be performed.
[Brief description of the drawings]
FIG. 1 is a front sectional view showing a configuration of an image forming apparatus provided with an image processing apparatus according to an embodiment of the present invention.
FIG. 2 is a block diagram showing a configuration of an image processing apparatus according to the first embodiment of the present invention.
FIG. 3 is a graph showing a reference correction curve used for gradation correction processing of the present invention.
FIG. 4 is a graph showing a reference characteristic curve used for selecting a reference correction curve necessary for the gradation correction processing of the present invention.
FIG. 5 is a diagram showing a test image used for gradation correction processing of the present invention.
FIG. 6 is a graph for explaining a technique for obtaining a difference between an output measurement value corresponding to a designated density value and a reference characteristic curve in selecting a reference characteristic curve.
FIG. 7 is a graph for explaining a method of selecting a closest reference characteristic curve by comparing an output measurement value corresponding to a specified density value with a reference characteristic curve.
FIG. 8 is a graph showing an output measurement value and a target characteristic curve when a patch having a specified density value is output based on a reference correction curve and read by a scanner.
FIG. 9 is a graph for explaining a method of obtaining an input density value on a target characteristic curve with respect to an output measurement value.
FIG. 10 is a graph for explaining a method of setting a correction value for an input density value on a target characteristic curve for an output measurement value.
11 is a flowchart showing main control processing of a halftone output gradation processing unit in the image processing apparatus of FIG. 2;
FIG. 12 is a flowchart showing a first method for creating a γ correction table used in the gradation correction processing of the present invention.
FIG. 13 shows a correction when the correction value on the correction curve for the input density value other than the specified density value is obtained by linearly interpolating the correction value on the correction curve for the specified density value in the gradation correction processing of the present invention. It is a graph which shows a curve.
FIG. 14 shows a correction when the correction value on the correction curve for the input density value other than the specified density value is obtained by spline interpolation of the correction value on the correction curve for the specified density value in the gradation correction processing of the present invention. It is a graph which shows a curve.
FIG. 15 is a flowchart showing a second method for creating a γ correction table used in the gradation correction processing of the present invention.
FIG. 16 shows a 2 × 2 method for applying a gradation correction process of the present invention to a dither process and setting a reference correction curve and a correction amount corresponding to a specified density value for each dither matrix size. It is a figure which shows the combination of the gradation value in a dither matrix.
FIG. 17 shows a case where a 2 × 2 dither matrix is used to show a technique of applying the tone correction processing of the present invention to dither processing and setting a correction table for each pixel in the dither matrix from the correction curve. It is a graph which shows the relationship between the table of each pixel, and a correction curve.
FIG. 18 is a block diagram showing a configuration of an image processing apparatus according to a second embodiment of the present invention.
FIG. 19 is a diagram showing a test image used for gradation correction processing in the second embodiment of the present invention.
FIG. 20 is a graph showing a relationship between an output measurement value corresponding to a measured designated density value and a reference characteristic curve when there is a characteristic variation in the image input apparatus or when the background density of the original is different.
FIG. 21 is a graph of correction coefficients used to correct the influence when there is a characteristic variation in the image input device or when the background density of the original is different.
FIG. 22 shows output measured values when a specified density value patch is output based on a reference correction curve and read by the image input device when there is a characteristic variation in the image input device or when the background density of the document is different. It is a graph which shows the relationship with a target characteristic curve.
FIG. 23 is a diagram illustrating a configuration example of an operation panel for displaying a sample and arbitrarily selecting a correction amount corresponding to a sample when the background density of the document is different.
FIG. 24 is a block diagram showing a configuration of an image processing apparatus according to a third embodiment of the present invention.
FIG. 25 is a graph for explaining a factor that causes a density difference in a high density region when there is a change in the image output apparatus;
FIG. 26 is a graph showing a correction curve when the correction value is limited and a correction curve when the correction value is not provided.
FIG. 27 is a graph for explaining a method of obtaining a maximum correction value when the output value does not exhibit a monotonous increase characteristic with respect to the correction value.
FIG. 28 is a diagram showing an image made up of patches of each density of light and shade tones used in a conventional image creating apparatus.
FIG. 29 is a graph for explaining a state of γ correction processing in a conventional image creating apparatus;
[Explanation of symbols]
11, 55, 75 Image forming apparatus
13 Image input device
14, 51, 71 Image processing apparatus
15 Image output device
16, 56 Operation panel
28, 53, 73 Halftone output gradation processing section (gradation correction processing means / halftone correction processing means)
31 Reference correction curve storage unit (reference correction curve storage means)
32 Correction amount storage unit (correction amount storage means)
54 Correction coefficient storage (correction coefficient storage means)
74 Maximum correction value storage unit (maximum correction value storage means)

Claims (25)

An input density value of pixels constituting an image is given from an image input means for inputting an image, and gradation correction processing means for applying a gradation correction process using a correction curve to the input image is provided.
In the image processing apparatus, the gradation correction processing unit is configured to provide a correction value on the correction curve for the input density value, and to provide an image output unit with an image subjected to gradation correction processing.
A reference correction curve storage means for storing a reference correction curve indicating a predetermined change in the correction value with respect to the input density value;
A correction value on the reference correction curve for the specified density value and the specified density corresponding to a plurality of specified density values, which are input density values specified with an interval between them in the range of the input density value. A correction amount storage means for storing a correction amount set in advance based on the reference correction curve, the difference between the correction value on the correction curve with respect to the value,
Halftone correction processing means for applying a dither halftone correction process to the image,
The gradation correction processing means creates the correction curve used for gradation correction processing based on the reference correction curve and the correction amount corresponding to the specified density value each time an image is output.
An image processing apparatus, wherein a correction amount corresponding to the reference correction curve and the designated density value is set for each dither matrix size that can be used by the halftone correction processing means.   When the reference correction curve storage unit stores a plurality of the reference correction curves, the gradation correction processing unit selects any one of the plurality of reference correction curves according to output characteristics unique to the image output unit. The image processing apparatus according to claim 1, wherein the selected reference correction curve is used to create the correction curve. The gradation correction processing means, when selecting the reference correction curve,
The image pattern of the plurality of designated density values is output by the image output means without performing gradation correction processing,
The density value of the output image pattern is read by the image input means,
The read density value is compared with a predetermined reference characteristic curve associated with each of the reference correction curves,
3. The image processing apparatus according to claim 2, wherein a reference correction curve associated with a reference characteristic curve closest to the read density value is selected.   The image processing apparatus according to claim 1, wherein the correction amount corresponding to the specified density value is a first correction amount that is a correction amount based on an inherent output characteristic of the image output unit. It further includes an adjustment amount input means for inputting an adjustment amount set for the gradation correction processing of the image,
The image processing apparatus according to claim 1, wherein the correction amount corresponding to the designated density value is a second correction amount that is a correction amount based on the input adjustment amount. It further includes an adjustment amount input means for inputting an adjustment amount set for the gradation correction processing of the image,
The correction amount corresponding to the specified density value is a first correction amount that is a correction amount based on the unique output characteristics of the image output means, and a second correction amount that is a correction amount based on the input adjustment amount. The image processing apparatus according to claim 1, wherein the image processing apparatus is a sum with a quantity. The gradation correction processing means determines the first correction amount when
The image pattern of the specified density value is subjected to gradation correction processing using the reference correction curve,
An image pattern subjected to gradation correction processing is output by the image output means,
The density value of the output image pattern is read by the image input means,
7. The image processing apparatus according to claim 4, wherein the read density value is compared with a predetermined target characteristic curve, and the first correction amount is determined based on the comparison result.   7. The image according to claim 5, wherein when a plurality of adjustment amounts are input from the adjustment amount input means, the second correction amount is a sum of correction amounts determined for each adjustment amount. Processing equipment. The gradation correction processing means, at the time of creating the correction curve,
Obtaining a correction value on the correction curve for the specified density value by adding a correction value on the reference correction curve for the specified density value and a correction amount corresponding to the specified density value;
2. The image processing according to claim 1, wherein a correction value on the correction curve for an input density value other than the specified density value is obtained by an interpolation process using a correction value on the correction curve for the specified density value. apparatus.   The image processing apparatus according to claim 9, wherein the interpolation process is a linear interpolation process.   The image processing apparatus according to claim 9, wherein the interpolation processing is spline interpolation processing. The gradation correction processing means, at the time of creating the correction curve,
A correction amount corresponding to an input density value other than the specified density value is obtained by linear interpolation using a correction amount corresponding to the specified density value,
Obtaining a correction value on the correction curve for the specified density value by adding a correction value on the reference correction curve for the specified density value and a correction amount corresponding to the specified density value;
A correction value on the correction curve for an input density value other than the designated density value is obtained by adding a correction value on the reference correction curve for the input density value and a correction amount obtained by the linear interpolation process. The image processing apparatus according to claim 1, wherein:   The image processing apparatus according to claim 1, wherein when the image is a color image, the correction amount corresponding to the reference correction curve and the designated density value is set for each of a plurality of color components. Gradation correction processing means for applying gradation correction processing using a correction curve to an image input from the image input means;
A reference correction curve storage means for storing a plurality of reference correction curves determined in accordance with output characteristics specific to the image output means, showing a predetermined change in the correction value for the input density value of the pixels constituting the image;
A correction amount set in advance based on the reference correction curve is stored in correspondence with each of a plurality of specified density values which are input density values which are specified with an interval between them in the range of the input density value. Correction amount storage means,
The gradation correction processing means creates a correction curve used for gradation correction processing based on the reference correction curve and the correction amount corresponding to the designated density value every time an image is output, and the input image In the image processing apparatus having a configuration in which the correction value on the correction curve corresponding to the input density value of the constituent pixels is provided to the image output unit,
An area in which the image pattern having a plurality of designated density values is output by the image output means on the recording medium without performing gradation correction processing, and the density value of the output image pattern and the image pattern are not formed. The density value of the recording medium is read by the image input means, and is multiplied by the background density value obtained by measuring the density of the area where the image pattern is not formed. Correction coefficient storage means for storing a correction coefficient that is a coefficient for obtaining a density correction amount for correcting an output measurement value that is a density value of the image pattern;
Halftone correction processing means for applying a dither halftone correction process to the image,
The gradation correction processing means subtracts each density correction amount corresponding to each designated density value from an output measurement value corresponding to each designated density value, thereby obtaining a corrected output density value corresponding to each designated density value. The corrected output density value is compared with a predetermined reference characteristic curve associated with each of the reference correction curves, and a reference correction curve associated with the reference characteristic curve closest to the density value of the corrected image pattern is obtained. Selected,
An image processing apparatus, wherein a correction amount corresponding to the reference correction curve and the designated density value is set for each dither matrix size that can be used by the halftone correction processing means. The correction amount corresponding to the specified density value is a first correction amount that is a correction amount based on an inherent output characteristic of the image output means,
When the gradation correction processing means determines the first correction amount,
The image pattern of the specified density value is subjected to gradation correction processing using the reference correction curve,
The image pattern subjected to the gradation correction process is output on the recording medium by the image output means,
The image input means reads the density value of the output image pattern and the density value of the recording medium in the area where the image pattern is not formed,
Using the density value of the read recording medium and the correction coefficient, the density value of the read image pattern is corrected to obtain the corrected output density value,
15. The image processing apparatus according to claim 14, wherein the corrected output density value is compared with a predetermined target characteristic curve, and the first correction amount is determined based on the comparison result. The correction coefficient is determined according to each of a plurality of types of recording media,
The image processing apparatus according to claim 14, wherein the gradation correction processing unit is configured to be able to select one of correction coefficients corresponding to a plurality of types of recording media.   The image processing apparatus according to claim 14, wherein when the image is a color image, the correction coefficient is determined for each of a plurality of color components. Gradation correction processing means for applying gradation correction processing using a correction curve to an image input from the image input means;
A reference correction curve storage means for storing a reference correction curve indicating a predetermined change of a correction value with respect to an input density value of pixels constituting an image;
A correction amount set in advance based on the reference correction curve is stored in correspondence with each of a plurality of specified density values which are input density values which are specified with an interval between them in the range of the input density value. Correction amount storage means,
The gradation correction processing means creates the correction curve used for the gradation correction processing based on the reference correction curve and the correction amount corresponding to the designated density value every time an image is output, and the input image In the image processing apparatus configured to provide the image output means with a correction value on the correction curve corresponding to the input density value of the pixels constituting
Maximum correction value storage means for storing a maximum correction value that is an upper limit value of the correction value on the correction curve set based on the maximum output value of the image output means used when the correction curve is created With
The gradation correction processing means, have rows the gradation correction by using the correction curve correction values on the correction curve is restricted to be less than the maximum correction value,
The image further includes halftone correction processing means for performing dither halftone correction processing on the image,
An image processing apparatus , wherein a correction amount corresponding to the reference correction curve and the designated density value is set for each dither matrix size that can be used by the halftone correction processing means .   The maximum correction value is determined based on the correction value on the correction curve corresponding to the specified density value of the darkest density and a maximum correction value determination coefficient indicating a predetermined ratio with respect to the correction value. The image processing apparatus according to claim 18. The reference correction curve storage means stores a plurality of the reference correction curves;
When the gradation correction processing unit selects one of the plurality of reference correction curves to be used for processing according to output characteristics specific to the image output unit,
The image pattern of the plurality of designated density values is output by the image output means without performing gradation correction processing,
The density value of the output image pattern is read by the image input means,
A reference correction curve that compares the density value of the read image pattern with a predetermined reference characteristic curve that is associated with each of the reference correction curves and that is associated with the reference characteristic curve that is closest to the density value of the read image pattern Select
A maximum output target value which is a standard measurement value of an image pattern indicating a maximum output value among the image patterns output from the image output unit adjusted to an optimum state and read by the image input unit, and the reading 19. The image processing apparatus according to claim 18, wherein the maximum correction value is obtained based on the density value of the obtained image pattern.   21. The image processing apparatus according to claim 20, wherein a plurality of maximum output target values are determined. An adjustment amount input means for inputting an adjustment amount set for the gradation correction processing of the image is further provided.
The image processing apparatus according to claim 18, wherein the maximum correction value is obtained according to a second correction amount that is a correction amount based on the input adjustment amount.   19. The image processing apparatus according to claim 18, wherein when the image is a color image, the maximum correction value is determined for each of a plurality of color components.   The image processing apparatus according to claim 18, wherein the gradation correction processing unit is configured to be able to switch whether or not to use the maximum correction value. An image forming apparatus comprising an image processing apparatus according to one of claims 1 to 2 4.

Images