JP2004166163A - Gray scale reproducing method, threshold matrix, image processing method, image forming apparatus, and printer driver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gray scale reproducing method, a threshold matrix, an image processing method, an image forming apparatus, and a printer driver, capable of acquiring a favorable image quality even in low resolution/high-speed recording. <P>SOLUTION: When a multi gradient image is threshold processed in a part of gradient by a dither matrix, gray scale image reproduction without an erasure and enhancement of line keynote is performed by forming the line base in predetermined direction and having high-pass filter characteristics in other portion except the line base, and by forming the line base in every 3 dots to 5 dots. <P>COPYRIGHT: (C)2004,JPO

Description

【０１２０】
図２９（ｂ）は上述したハイパスフィルタ特性を持たせたマトリクスのライン基調以外の部分の一例を示すものであり、このライン基調以外の部分の特性は同図（ｂ）に示すようなハイパスフィルター特性となっている。
【０１２１】
また、図３０は所定の方向のライン基調に形成され、かつ基調以外の部分においてハイパスフィルター特性を持つマトリクスによる階調グラデーション画像の例を示している。
【０１２２】
すなわち、前述した図２７（ａ）のＡ部の濃度部分においては、図３０（ａ）に示すライン基調の部分と同図（ｂ）に示すハイパスフィルター特性を持つライン基調以外の部分とを組み合わせた同図（ｃ）に示すような本発明に係るディザマトリクスを用いることで、同図（ｄ）に示すように濃度が連続した階調グラデーション画像が得られる。
【０１２３】
このように、所定方向のライン基調が形成されるだけでは階調の連続性が失われる一部の濃度において、所定のライン基調と基調以外の部分でハイパスフィルター特性を持つマトリクスとすることによって、低線数化（階調の連続性の喪失）を抑制することができ、ライン基調の連続性を高めることができて、特に低解像度／高速記録においても再現した画像の品質が向上する。
【０１２４】
また、所定方向のライン基調が形成されるだけでは階調の連続性が失われる一部の濃度における別の補完方法として、Ｂａｙｅｒ法型ディザ法、ブルーノイズ法で補完し、階調グラデーション画像を作成したところ、同様に濃度が連続した階調グラデーション画像が得られ、高線数（１０６線）の基調を維持したまま補完をすることができることを確認した。
【０１２５】
そこで、本発明者らは、上述したようにハイパスフィルター特性を持たせた上に、更にサブマトリクスのサイズを変えて、すなわち、基調の線数を変えて画像を作成し、その画像品質を評価した。このとき、画像の解像度は、３００ｄｐｉで画像を形成した。その評価結果を表１に示している。
【０１２６】
【表１】

【０１２７】
なお、同表中、×＝基調が見えず、階調の連続性喪失。
○＝基調が適度に見え、階調の連続性あり。
◎＝基調が極めて良好に見え、階調の連続性あり。
△＝基調が目ざわり。階調の連続性はあり。
の意味である。
【０１２８】
ここで、サブマトリクスのサイズについて、前述した図２５（ｆ）に示すように２×２のマトリクスも可能であるが、２×２のマトリクスではチェッカーフラグ様の基調が階調表現の過程で発生してしまう。また、６×６以上のサブマトリクスも可能であるが、基調が荒くなり、基調そのものが目につきやすく、汚く見えてしまう。また、マトリクスのサイズが大きくなると、それだけ処理時間が必要になるという不具合もある。
【０１２９】
そこで、本発明では、サブマトリクスの最小単位を３×３、最大単位を５×５、最適には４×４の斜め万線マスクとしている。
【０１３０】
所定方向のライン基調が形成されるだけでは階調の連続性が失われる一部の濃度において、所定のライン基調と基調以外の部分でハイパスフィルター特性を持つマトリクスとすることによって、低線数化（階調の連続性の喪失）を抑制することができ、ライン基調の連続性を高めることが可能となった。さらに、前述したような最小単位を３×３、最大単位を５×５、最適には４×４の斜め万線マスクを使用することで、３ドット〜５ドット毎のライン基調（１／３、１／４基調、１／５基調と称す。線数としては、８５〜１４１線）が実現でき、それにより全ての階調で、基調が細かすぎて見えなくなったり、荒すぎて目に付いてしまうことがない最適な基調表現を実現することとを両立し、再現した画像の品質を向上することができた。
【０１３１】
このように、本発明に係る階調再現方法或いは閾値マトリクスを用いることで、ディザマトリクスにより多階調画像が一部の濃度において閾値化されたときに、所定方向のライン基調に形成され、かつ基調以外の部分においてはハイパスフィルター特性を持つ構成としたので、低線数化を抑制することができて、ライン基調の連続性が高まり、特に低解像度／高速記録においても良好な画像品質が得られるようになる。
【０１３２】
それとともに、ライン基調を３ドット〜５ドット毎に形成する、或いは、ｍ×ｍ（ｍ＝３〜５の整数）のサイズの最小の閾値マトリクスを用いることで、基調が消失してしまったり、結果的に低線数となって、逆に目につきすぎてしまうということがない、より良好な画像品質を実現できる。また、ライン基調を斜め方向に形成することにより、斜め方向のライン基調が横スジ状のノイズを低減し、より高品質な多階調画像を得ることができる。
【０１３３】
さらに、前述したように吐出される滴の大きさを可変できることから、大滴、中滴、小滴のそれぞれに本発明のディザマトリクスを用いることで、より豊かな階調表現を行うことができる。
【０１３４】
例えば、図３４に示すように、大中小のそれぞれの滴サイズに対して、同図（ａ）に示すマスク（基準マスクＡとする。）、同図（ｂ）に示すマスク（基準マスクＢとする。）、同図（ｃ）に示すマスク（基準マスクＣとする。）を設ける。
【０１３５】
そして、これらの基準マスクＡ、Ｂ、Ｃと滴サイズとを図３４に示すように組み合わせて階調表現を行うことにより、基調を保ちつつ豊かな階調表現が可能となる。なお、同図の「Ａｓ」は基準マスクＡと小滴（滴サイズｓ）とを組み合わせたもの、同様に「Ａｍ＋Ｂｓ」は基準マスクＡと中滴（滴サイズｍ）との組み合わせと基準マスクＢと小滴（滴サイズｓ）との組み合わせを組み合わせたもの、「Ａｌ＋Ｂｌ＋Ｃｍ」は基準マスクＡと大滴（滴サイズｌ）との組み合わせと基準マスクＢと大滴（滴サイズｌ）の組合せと基準マスクＣと中滴（滴サイズｍ）との組み合わせを組み合わせたものの意味であり、その他の「Ａｍ」、「Ａｍ＋Ｂｍ＋Ｃｓ」等も同様の意味を表している。
【０１３６】
また、図３６に示すように、小滴（滴サイズｓ）のドットが主走査方向に連続して出現しない条件を加えたマスクとして階調表現を表すこともできる。このように、例えば記録ヘッドのインク噴射特性のうち苦手な印字（駆動）周波数がある場合などに、特定の周波数を避けて記録を行うことができる。これにより、噴射不安定による印字不良等を未然に防ぐことができ、階調表現のみならず、安定した高品質の画像を得られることになる。
【０１３７】
この図３６の例では、小滴（滴サイズｓ）のドットを主走査方向に連続して出現させないために、図３５の例と比較して、基本マスクＢと滴サイズｓの組み合わせ、及び、基本マスクＣと滴サイズｓの組み合わせを省く形をとっているが、これでも基調を保持した豊かな階調表現が可能となっている。なお、ここでの説明は、本発明の特徴の一部を示した例であり、基調を保ちつつ階調表現を行える範囲であれば上記と同様の効果が期待できる。
【０１３８】
このように、本発明に係る階調再現方法或いは閾値マトリクスを用いることで、ディザマトリクスにより多階調画像が一部の濃度において閾値化されたときに、所定方向のライン基調に形成され、かつｎ値のうち少なくとも１つの階調値のドットは主走査方向に対しある一定パターンを連続して出現させない構成とするので、記録ヘッドの特性を最大限に活かした印字を行うことができ、不安定なドット印字をなくせるため、画像品質を向上することができる。
【０１３９】
すなわち、各階調レベルにわたって変化しないような斜め基調を形成しても、基調の作り方によっては基調が維持できない、あるいは基調が判別できない、あるいは逆に基調が目立ちすぎてしまうとことが生じることから、本発明者らは基調の作成方法を変えた画像を作成、評価した結果、この問題を解決する基調の作成条件を見出し、本発明にいたった。すなわち、本発明の目的は、コストや記録速度のために高解像度化が困難な記録装置においても、基調を形成することで良好な画像品質を得ることができる中間調処理方法を提供したうえにさらに、各階調レベルで基調が維持でき、その基調がきちんと表現でき、しかも基調が目立ちすぎて目についてしまうことなく、かつ記録ヘッドの特性を最大限活かせる中
【０１４０】
この場合も、ディザマトリクスにより多階調画像が一部の濃度において閾値化されたときに、所定方向のライン基調に形成され、かつ基調以外の部分においてはハイパスフィルター特性を持つ構成することで、低線数化を抑制することができて、ライン基調の連続性が高まり、特に低解像度／高速記録においても良好な画像品質が得られるようになる。
【０１４１】
また、ライン基調を３ドット〜５ドット毎に形成する、或いは、ｍ×ｍ（ｍ＝３〜５の整数）のサイズの最小の閾値マトリクスを用いることで、基調が消失してしまったり、結果的に低線数となって、逆に目につきすぎてしまうということがない、より良好な画像品質を実現できる。また、ライン基調を斜め方向に形成することにより、斜め方向のライン基調が横スジ状のノイズを低減し、より高品質な多階調画像を得ることができる。
【０１４２】
なお、画像解像度としては、３００ｄｐｉに限ることではではなく、６００ｄｐｉ、１２００ｄｐｉの画像でも好適に用いることができるが、、高速化と高画質化を両立するうえで低解像度での画質向上という観点からみると、４００ｄｐｉ以下、特に３００ｄｐｉ以下の比較的低解像度で画像を形成、出力する場合に極めて好適に用いることができる。
【０１４３】
さらに、別の実施例として、基調以外を補完する方法を、ＶＴＦ法とＢａｙｅｒ法を階調レベルによって組み合わせて作成したディザマトリクスで作成した画像においても、基調が極めて良好で、階調の連続性が維持された階調グラデーション画像が実現できることが確認された。
【０１４４】
具体的には、例えば０〜２５５の階調レベルに対して、階調レベル５０まではＢａｙｅｒ、５０以上はＶＴＦ法で作成した。また、ブルーノイズ法とＢａｙｅｒ法、ブルーノイズ法とＶＴＦ法を組み合わせたディザマトリクスでも、同様に良好な画像が得られた。なお、適用する階調レベルも、階調レベル５０での切替に限られるものではなく、インクジェット記録装置で得られる変調ドット径のサイズ、それによって得られる階調グラデーション画像の品質に応じて決定すれば良い。
【０１４５】
このように、本発明にかかる階調再現方法によれば、ディザマトリクスにより多階調画像が一部の濃度において閾値化されたときに、所定方向のライン基調に形成され、かつ基調以外の部分においてはハイパスフィルター特性を持つ構成とするとき、そのハイパスフィルター特性は、人間の視覚の空間周波数特性を適用、あるいはＢａｙｅｒ型ディザ法で補完、あるいはブルーノイズ特性を持たせることで容易に実現可能であり、低線数化を抑制することができて、ライン基調の連続性が高まり、特に低解像度／高速記録においても良好な画像品質が得られるようになる。また、ライン基調を斜め方向に形成することにより、斜め方向のライン基調が横スジ状のノイズを低減し、より高品質な多階調画像を得ることができる。
【０１４６】
そして、これらの補完方法としての人間の視覚の空間周波数特性の適用、あるいはＢａｙｅｒ型ディザ法での補完、あるいはブルーノイズ特性を持たせるということを、階調レベルに応じ選択的或いは組み合わせて適用することにより、一層画像品質を向上することができる。
【０１４７】
ここで、上述したハイパスフィルター特性を持つ場合、一部の濃度間における双方のディザマトリクスを２値画像としたときに、両画像の差分画像においてハイパスフィルター特性を持つことになる。
【０１４８】
すなわち、ディザ法では、低い濃度側で設定された閾値ドットは、高い濃度側でも必ず存在する。上述のディザマトリクスの一部の濃度間における双方において、濃度が低い側をＡマスク、濃度が高い側をＢマスクとすると、ディザ法の性質上、Ａマスクに存在するライン基調は、Ｂマスクにも必ず存在する。したがって、Ｂマスクを２値化した画像から、Ａマスクを２値化した画像の差分を求めると、ライン基調が無い画像になる。
【０１４９】
つまり、図３４（ａ）に示すように、Ａマスクを用いるＡ部の濃度とＢマスクを用いるＢ部の濃度との間において、同図（ｂ）に示すＡマスクと同図（ｃ）に示すＢマスクとの差分（Ｂ−Ａ）を採ると、同図（ｄ）に示す差分画像（差分パターン）が得られ、この差分画像は同図（ｅ）に示すように前述したハイパスフィルタ特性を持つことになる。
【０１５０】
また、本発明に係るマトリクスを用いて多階調画像を一部の濃度において閾値化したときに、その２次元空間周波数による極座標上の所定方向のライン基調の角度以外のパワースペクトルの分布が一様に近似する。
【０１５１】
すなわち、画像の空間周波数のパワースペクトルは、次の（２）式で定義される。
【０１５２】
【数２】

【０１５３】
この値は空間周波数（ｕ，ｖ）の強さを表す。（ｕ，ｖ）から方向性を抽出するには、これを極座標であらわしてＰ（ｒ，θ）としたのち、次の（３）式で求める。
【０１５４】
【数３】

【０１５５】
上述したように、本発明に係るマトリクスは、所定の方向のライン基調に形成され、かつ基調以外の部分においてハイパスフィルター特性を持つ。ライン基調の部分においては、所定方向のみのライン基調があることから、極座標上のパワースペクトルは所定方向の角度のみ強い値を持つ。また、基調以外の部分においては、ハイパスフィルター特性を持つことから、極座標上のパワースペクトルの値は一様に近似することになる。
【０１５６】
図３５（ａ）は本発明に係るマトリクスの極座標上のパワースペクトルを、同図（ｂ）はライン角度４５度を除いた極座標上のパワースペクトルの分布を示したものである。同図（ａ）に示すように、マトリクスの極座標上のパワースペクトルでは４５度部分のみが強い値を持ち、同図（ｂ）に示すように、極座標上の４５度を除いたパワースペクトルでは、値は一様に近似している。
【０１５７】
したがって、マトリクスは４５度のライン基調方向に形成されており、それ以外の角度のパワースペクトルは、ハイパスフィルター特性により一様に近似していることになる。
【０１５８】
これに対して、従来の組織的ディザ法の代表的なマスク手法である、ベイヤー型ディザマトリクスおよび集中型ディザマトリクスによる図３５と同じ濃度によるパワースペクトル分布は、図３６及び図３７にそれぞれ示すようになる。これらは、それぞれ複数の角度で値が高くなっており、本発明に係るマトリクスの特徴が従来のマトリクスと異なることが分かる。
【０１５９】
このように、低線数化を抑制したマトリクスにより所定の方向のライン基調の連続性が高まり、連続した階調表現による所望の品質の多階調画像を再現することができる。また、誤差拡散法に比べ、マスク手法を使用することにより処理が単純になり処理速度（印刷速度、画像処理速度、或いは画像形成速度）の高速化がはかれる。この場合、所定の方向のライン基調を斜め方向とする、万線基調とすることにより、斜め方向のライン基調が画像形成装置による横スジ状のノイズを低減し、より高品質な多階調画像の出力を得ることができるようになる。
【０１６０】
そして、このような閾値マトリクス、階調再現方法を、多色画像を複数の色成分に分解し、少なくとも１つの色成分の原画像を入力画像とした多色画像の画像処理方法に適用することで、より高画質な多色画像を出力できる。
【０１６１】
多色画像形成装置、一般にカラー印刷装置では、等和色であるシアン、マゼンタ、イエローの３つの基本色が用いられる。また、インクジェットカラー印刷装置では、明度なども加味し、所望の色彩の外観を向上させるため、これらの３色にさらにブラックを加え、４色の原色を用いて印刷を行うものが主流になっている。さらに、最近では印刷画質の向上のため、基本色を組み合わせた多次色を予め用意して、さらに多くの色を用いた印刷を行うものも多くなってきている。
【０１６２】
これらの装置のように、複数の色成分に分解し、それぞれの色成分の原画像を入力画像とする多色画像形成方法は、色成分単位で擬似階調表現処理を行う。したがって、本発明を適用することで、各色成分単位でのライン基調の連続性が高まり、結果として単色画像、多色画像において、より高画質な画像を形成することができる。
【０１６３】
特に、マトリクスを用いて各ドット２値あるいは多値の画像データに変換する画像形成装置において、特におよそ３００ｄｐｉ以下の低解像度な精細度で出力する（画像形成する）場合に適用することで、画像形成速度、印刷速度の高速化と高画質化を図ることができる。
【０１６４】
すなわち、一般に擬似階調表現処理を使用する画像形成装置は、単位あたりのドット密度すなわち解像度を上げて画質を向上させている。しかし、解像度を上げることは同時に処理する画像データ量も面積単位で大きくなるため、処理時間も多く必要とする。３００ｄｐｉの解像度の画像形成装置においての擬似階調表現処理では、一般に１インチあたり最高で約１５０本の線を表現するのが限界とされ、高画質化が困難といわれている。
【０１６５】
上述した所定方向のライン基調を持つディザマトリクスによる擬似階調表現処理においては、一部の濃度において人間の視覚上では低線数化が発生し、特に３００ｄｐｉの解像度ではさらに線数が低くなることで、階調表現の連続性が失われ、画質低下が目立つことになる。このような画像形成装置における出力形態（低解像度での出力）で画像形成する場合に本発明を適用することで、低線数化を抑制し所定方向のライン基調の連続性が高まり、連続した階調表現による所望の品質の多階調画像を形成することができる。
【０１６６】
なお、上記実施形態では、本発明に係る階調再現方法、閾値マトリクス、画像処理を、ホスト１００のプリンタドライバ１０１に閾値マトリクステーブルとして保持し、ソフトウエア処理で行っている。
【０１６７】
この場合、ホスト１００のプリンタドライバ１０１によって入力画像データに基づいて、あるいは指定された出力設定情報に基づいて、「用紙の向き」や「横書き／縦書き」を判別し、基調の向きが常に一定となるように閾値マトリクスを回転させて適用する処理を行う。すなわち、用紙の向きが縦長となるポートレイトと用紙の向きが横長となるランドスケープとで基調の向きが同じになるように閾値マトリクステーブルを回転させて適用することが好ましい。
【０１６８】
また、上記実施形態ではホスト側のプリンタドライバで閾値マトリクスをテーブルとして保持する例で説明しているが、図３８に示すように、プリンタドライバ１０１側にはホストコンピュータで実行されるアプリケーションソフトなどから画像データを処理するＣＭＭ処理部１０２及びＢＧ／ＵＣＲ，γ補正部１０３のみを有し、インクジェット記録装置２００側の制御部にズーミング処理部１０４及び本発明に係る閾値マトリクス１０５を格納したＲＯＭなどを備えて、ドット配置への変換はインクジェット記録装置側で行うようにすることもできる。何れの場合も、入力画像データに対して１対１の比較で処理を行うことができるので、高速かつ低コストで画像処理を行うことができるようになる。
【０１６９】
また、上記実施形態では主にインクジェット記録装置及びそのホストに本発明を適用する例で説明したが、ドットで画像を形成できる（ドット再現で画像を形成する）画像形成装置、例えば熱転写記録装置などにも同様に適用することができる。さらに、レーザープリンタ、ＬＥＤプリンタなどの電子写真方法を用いた画像形成装置にも適用することができる。
【０１７０】
熱転写記録装置は、図３９に示すように、発熱体３０１を有するサーマルヘッド３００と加圧ローラ３０３との間で用紙３０４とインクシート３０５とを挟んで送りながら、サーマルヘッド３００の発熱体３０１を駆動することによって、インクシート３０５のベース層３０６上の所定領域のワックス層３０７を用紙３０４に転写して画像を記録する。
【０１７１】
また、電子写真方法を用いた画像形成装置の一例について図４０及び図４１を参照して簡単に説明する。なお、図４０は画像形成装置の概略構成図、図４１は同画像形成装置を構成するプロセスカートリッジの概略構成図である。
【０１７２】
この画像形成装置４４０は、マゼンダ（Ｍ）、シアン（Ｃ）、イエロー（Ｙ）、ブラック（Ｂｋ）の４色でフルカラー画像を形成するレーザプリンタの一例であり、各色用の画像信号に応じたレーザビームを出射する４つの光書き込み装置４４２Ｍ、４４２Ｃ、４４２Ｙ、４４２Ｂｋと、作像用の４つのプロセスカートリッジ４４１Ｍ、４４１Ｃ、４４１Ｙ、４４１Ｂｋと、画像が転写される記録用紙を収納する給紙カセット４４３と、給紙カセット４４３から記録用紙を給紙する給紙ローラ４４４と、記録用紙を所定のタイミングで搬送するレジストローラ４４５と、記録用紙を各プロセスカートリッジの転写部に搬送する転写ベルト４４６と、記録用紙に転写された画像を定着する定着装置４４９と、定着後の記録用紙を排紙トレイ４５１に排紙する排紙ローラ４５０等を備えた構成となっている。
【０１７３】
４つのプロセスカートリッジ４４１Ｍ、４４１Ｃ、４４１Ｙ、４４１Ｂｋの構成は同じであり、図３７に示すように、各プロセスカートリッジ４４１は、ケース内に像担持体であるドラム状の感光体４５２と、帯電ローラ４５３と、現像器４５４と、クリーニングブレード４５９等を一体に備えている。
【０１７４】
また、現像器４５４内には、トナー供給ローラ、帯電ローラ、静電搬送基板４５７、トナー戻しローラ４５８が設けられており、各色のトナーが収納されている。また、プロセスカートリッジ４４１の背面側には、光書き込み装置からのレーザビームが入射される窓口となるスリット４６０が設けられている。
【０１７５】
各光書き込み装置４４２Ｍ、４４２Ｃ、４４２Ｙ、４４２Ｂｋは、半導体レーザ、コリメートレンズ、ポリゴンミラー等の光偏向器、走査結像用光学系等から構成され、装置外部のパーソナルコンピュータ等のホスト（画像処理装置）から入力される各色用の画像データに応じて変調されたレーザビームを出射し、各プロセスカートリッジ４４１Ｍ、４４１Ｃ、４４１Ｙ、４４１Ｂｋの感光体４５２上を走査し、静電荷像（静電潜像）を書き込む。
【０１７６】
そして、画像形成が開始されると、各プロセスカートリッジ４４１Ｍ、４４１Ｃ、４４１Ｙ、４４１Ｂｋの感光体４５２が帯電ローラ４５３で均一に帯電され、各光書き込み装置４４２Ｍ、４４２Ｃ、４４２Ｙ、４４２Ｂｋから画像データに応じたレーザビームが照射されて各感光体上に各色の静電潜像が形成される。感光体４５２上に形成された静電潜像は、現像器４５４の静電搬送基板４５７で静電搬送された各色のトナーにより現像され顕像化される。感光体４５２と静電搬送基板４５７の対向部間にパスル状の現像バイアスが印加され、感光体４５２上の静電潜像が、静電搬送基板４５７で搬送されて来たトナーによりホッピング現象される。また、現像に供されなかったトナーは静電搬送基板４５７で搬送されてトナー戻しローラ４５８に戻される。
【０１７７】
各プロセスカートリッジ４４１Ｂｋ、４４１Ｙ、４４１Ｃ、４４１Ｍの各色の画像形成に同期して、供給カセット４４３内の記録用紙が供給ローラ４４４で給紙され、レジストローラ４４５により所定のタイミングで転写ベルト４４６に向けて搬送される。そして、記録用紙は転写ベルト４４６に担持されて４つのプロセスカートリッジ４４１Ｂｋ、４４１Ｙ、４４１Ｃ、４４１Ｍの感光体に向けて順次搬送され、各感光体上のＢｋ、Ｙ、Ｃ、Ｍの各色のトナー像が順次重ね合わせて転写される。４色のトナー像が転写された記録用紙は、定着ベルト４４７と加圧ローラ４４８からなる定着装置４４９に搬送され、４色のトナー像からなるカラー画像が定着されて排紙トレイ４５１に排紙される。
【０１７８】
この画像形成装置においては、図４２（ａ）〜（ｃ）に示すように、各光書き込み装置４４２Ｍ、４４２Ｃ、４４２Ｙ、４４２Ｂｋから出射するレーザビームのオン時間又はオフ時間を変化させることによってドットサイズを変化させることができる。
【０１７９】
さらに、インクジェット記録装置のヘッド構成も上記実施形態のものに限らず、発熱抵抗体を用いるサーマル型インクジェットヘッドや振動板と電極を用いる静電型インクジェットヘッドを搭載する装置などでもよい。また、上記実施形態では画像形成装置について適用しているが、画像表示装置に画像データを出力する場合の画像処理、階調再現にも同様に適用することができる。
【０１８０】
【発明の効果】
以上説明したように、本発明にかかる階調再現方法、閾値マトリクス、画像処理方法、画像形成方法、プリンタドライバによれば、ライン基調の連続性を維持し、基調消失や基調強調が抑えられて、高い画像品質での階調再現、画像処理、画像形成を行うことができる。
【図面の簡単な説明】
【図１】画像形成装置としてのインクジェット記録装置の一例を示す機構部の概略斜視説明図
【図２】同機構部の側面説明図
【図３】同記録装置のヘッドの一例を示す分解斜視説明図
【図４】同ヘッドの液室長手方向に沿う断面説明図
【図５】図４の要部拡大説明図
【図６】同ヘッドの液室短手方向に沿う断面説明図
【図７】同ヘッドのノズル板の平面説明図
【図８】同記録装置の制御部の概要を説明するブロック図
【図９】同制御部のヘッド駆動制御に係わる部分のブロック説明図
【図１０】図９のヘッド駆動回路の一例を示すブロック図
【図１１】同ヘッド駆動制御に係わる部分の作用説明に供する説明図
【図１２】本発明に係る閾値マトリクスを含む本発明に係るプリンタドライバを搭載した本発明に係る画像処理装置であるホスト側の構成の一例を説明するブロック図
【図１３】入力画像に対するＢａｙｅｒ型ディザ処理、誤差拡散処理後のドットパターンを示す説明図
【図１４】入力画像に対するＢａｙｅｒ型ディザ処理後の画像データを示す説明図
【図１５】３００ｄｐｉのＢａｙｅｒ型ディザパターンと誤差拡散パターンの粒状度を１０％濃度おきに測定したときの説明図
【図１６】インクジェット記録装置における機械的な変動要因を説明するための説明図
【図１７】Ｂａｙｅｒ型ディザパターンと記録装置の機械的変動との干渉の説明に供する説明図
【図１８】本発明に係る閾値マトリクスにおける斜め基調のドット配置パターンを説明する説明図
【図１９】電子写真記録装置の万線ディザパターンのドットパターンと階調レベルの基調変化の説明に供する説明図
【図２０】同手法をインクジェット記録装置に適用した場合のドットパターンと階調レベルの基調変化の説明に供する説明図
【図２１】ディザマスクをタイリングすることで形成される基調についての説明に供する説明図
【図２２】マスクが１階調レベル当たり１ドットの場合のタイリングと基調についての説明に供する説明図
【図２３】マスクが１階調レベル当たり２ドットの場合のタイリングと基調についての説明に供する説明図
【図２４】マスクが１階調レベル当たり３ドットの場合のタイリングと基調についての説明に供する説明図
【図２５】基本マトリクスからサブマトリクスへの分割の説明に供する説明図
【図２６】所定方向のライン基調を有するマトリクスパターンの異なる例を説明する説明図
【図２７】階調グラデーション画像と一部の濃度において低線数化したマトリクスパターンの例を説明する説明図
【図２８】人間の視覚特性を説明する説明図
【図２９】マトリクスパターンのハイパスフィルタ特性を有する基調以外の部分の説明図
【図３０】所定のライン基調と基調以外の部分でハイパスフィルタ特性を有するマトリクスによる階調グラデーション画像の説明に供する説明図
【図３１】基本マスクの例を説明する説明図
【図３２】基本マスクと滴サイズの組合せの説明に供する説明図
【図３３】本発明に係る主走査方向に対しある一定のパターンが連続して出現しない基本マスクと滴サイズの組合せの例の説明に供する説明図
【図３４】一部濃度間のマトリクスの差分画像について説明する説明図
【図３５】本発明に係るマトリクスの極座標上のパワースペクトル分布の説明に供する説明図
【図３６】従来のベイヤー型ディザマトリクスの極座標上のパワースペクトル分布の説明に供する説明図
【図３７】従来の集中型ディザマトリクスの極座標上のパワースペクトル分布の説明に供する説明図
【図３８】本発明に係る画像形成装置の説明に供するブロック図
【図３９】熱転写記録装置の概略説明図
【図４０】電子写真方法を用いる画像形成装置の概略構成図
【図４１】同画像形成装置のプロセスカートリッジの概略構成図
【図４２】ドットサイズの変更の説明に供する説明図
【図４３】２値化および少値化により階調表現を行ったときに実際に配置されるドットの説明に供する説明図
【図４４】ディザ法による２値化処理の説明に供する説明図
【図４５】少値化処理におけるサイズ変調ドットとディザマスクの対応の説明に供する説明図
【図４６】各ドットサイズのディザマスクの一例を説明する説明図
【図４７】２値誤差拡散処理の手順について説明する説明図
【符号の説明】
１３…キャリッジ、１４…記録ヘッド、８７…駆動波形生成回路、８８…ヘッド駆動回路、９１…主制御部、１０１…プリンタドライバ、１０５…閾値マトリクス。[0001]
[Industrial application fields]
The present invention relates to a gradation reproduction method, a threshold matrix, an image processing method, an image forming apparatus, and a printer driver.
[0002]
[Prior art]
In conventional image forming apparatuses (image recording apparatuses) such as printers, facsimiles, and copiers, digital image output is mainly binary images composed of “1” and “0”, that is, “ON” and “OFF”. However, multi-valued images in which a plurality of gradations are expressed by one pixel are mainly used due to the progress of the image forming engine and the growing need for high-quality images.
[0003]
For example, in an inkjet recording apparatus that forms (records) an image by ejecting ink droplets, as a method for performing gradation recording, a dot size modulation method that uses dots of a different size from a density modulation method that changes the density of ink , And three of them are used mainly.
[0004]
In this case, the ink jet recording apparatus is a pressure generating means of the ink jet head, for example, a heating resistor for generating bubbles in the case of the thermal ink jet method, and a liquid chamber wall is deformed in the case of the piezo method. In the case of the electrostatic method, the dot size is changed by changing the drive voltage of the drive waveform applied to the electrode, the pulse width, the number of pulses, etc. Due to the spread of ink, etc., the dot size is limited to about four values of “large droplet, medium droplet, small droplet, no printing” at most.
[0005]
Here, FIG. 43 shows an example of a dot pattern when the binarization process and the multi-value process generally used are applied. FIG. 6A shows an example in the case of performing dot reproduction by performing binarization processing, FIG. 9B shows an example in the case of performing dot reproduction by performing density modulation, and FIG. This is an example of performing dot reproduction.
[0006]
In gradation expression with such dots, Basically, the amount of information is determined by the controllable dot size. The more steps you can control, the more information you have, A high-quality output image close to the original image data can be obtained. As described above, in an inkjet recording apparatus or the like, Most of them are capable of controlling only about 1 to 3 steps (4 steps including 0). Although some improvement can be achieved by combining with the density modulation method, That much As the proportion of colorants and recording units increases, Due to cost and device size constraints, It can only be improved about twice.
[0007]
In order to compensate for such a shortage of information amount per pixel, a pseudo gradation expression called halftone processing is generally used as a technique for performing gradation expression by controlling the number of dots per unit area. . In the pseudo gradation expression, the number of arranged dots is expressed as a density, and many gradations are expressed by changing the density of points.
[0008]
Dithering methods are widely used for pseudo-gradation representation, and representative examples include systematic dithering methods and random dithering methods. In the systematic dither method, a sub-matrix composed of n × n threshold values (this is called a dither matrix) is set, this dither matrix is overlaid on the input image, the gray level of each corresponding pixel is compared with the threshold value, and input. When the value of the image is larger, 1 (white) is displayed, and when it is smaller, 0 (black) is displayed in binary. When the processing of n × n pixels is completed, the dither matrix is sequentially moved to the position of the next n × n pixels, and the same processing is repeated to form an image.
[0009]
For example, with respect to multi-valued image data input as shown in FIG. 44A, a dither matrix that is an n × n threshold matrix created by a predetermined method as shown in FIG. This is a method of performing comparison and replacing only pixels that show values above (or below) the threshold value with dots, as shown in FIG.
[0010]
In the figure, only binary values of ON / OFF are shown, but for multi-values having more combinations, the reproducible gradation area is divided into small dots, medium dots, and large dots as shown in FIG. Then, as shown in FIGS. 46A to 46C, a threshold value matrix corresponding to each dot size is applied, and each is compared with the input image data to perform replacement with the corresponding dot. .
[0011]
The random dither method is a method in which random numbers are generated for each pixel of an input image and the value is used as a threshold value. However, in the case of the random dither method, generally formed images are rough images, and systematic. Compared to the dither method, it is not suitable for improving image quality.
[0012]
On the other hand, although there is an error diffusion method in the pseudo gradation expression, the error diffusion method is considerably more complicated than the dither method. FIG. 47 shows the procedure of binary error diffusion. Threshold processing is performed for each pixel, and the error at that time is held and reflected in a later calculation at a predetermined ratio. As a result, information that is forcibly discarded in the dither processing can be fed back to the output image, and quality superior to the dither image in terms of resolution and the like can be obtained.
[0013]
As for these dither method and error diffusion method, higher resolution is being promoted with the aim of higher quality output. This is because by increasing the resolution, the size of individual dots and the distance between them become smaller, and it becomes difficult to distinguish the dot pattern created by the dither method or the error diffusion method. If the pattern cannot be recognized by the human eye, it is synonymous with multi-value expression with one pixel, and some recent inkjet recording apparatuses have a resolution of 2880 dpi.
[0014]
[Problems to be solved by the invention]
By the way, although the image quality is improved by increasing the resolution, the disadvantage is that the cost of the recording unit increases and the recording speed decreases. In order to achieve high resolution, in addition to the technology for forming smaller dots than in the past, more advanced control is required in terms of dot position accuracy, which inevitably increases costs, and covers per dot. Since the area is small, recording with the same configuration takes longer time for recording with higher resolution.
[0015]
However, in fact, in addition to cases where high image quality is required over speed and cost, speed and cost may be prioritized if image quality of a certain level or higher can be obtained, and only high image quality is always required. It is not something.
[0016]
However, until now, everything has been considered as an extension of higher resolution, and hardware measures such as “increase the dot formation speed” and “increase the mounting density of the recording unit” have been taken while maintaining the high resolution. Yes. This is not an approach to improve the image quality even with an inexpensive apparatus, but merely to increase the speed of the high-quality image recording apparatus.
[0017]
In fact, in such a high-speed device, there is a limitation from the viewpoint of cost and mounting area, so it is not possible to realize a significant speed improvement unless the recording sequence itself is changed. If it is changed, image processing for high resolution cannot be applied as it is, and image processing according to a new recording sequence is required. However, in the conventional apparatus, only simple image processing is applied. There is no active improvement in image quality.
[0018]
The present invention has been made in view of the above problems, and provides a gradation reproduction method, a threshold matrix, an image processing method, an image forming apparatus, and a printer driver that can obtain good image quality even in low resolution / high speed recording. For the purpose.
[0019]
[Means for Solving the Problems]
In order to solve the above problems, the gradation reproduction method according to claim 1 of the present invention is formed in a line tone in a predetermined direction when a multi-tone image is thresholded at a certain density by a dither matrix. In addition, the portion other than the basic tone has a high-pass filter characteristic, and the line basic tone is formed every 3 to 5 dots.
[0020]
Here, the tone reproduction method according to claim 2 is configured to change the direction of the line tone according to the input image data or the specified output setting information in the tone reproduction method according to claim 1, The gradation reproduction method according to claim 3 is such that when a multi-gradation image is thresholded at a certain density by a dither matrix, power other than the angle of the line tone in a predetermined direction on the polar coordinate by the two-dimensional spatial frequency. In this configuration, the spectrum distribution is approximately approximated.
[0021]
The threshold value matrix according to claim 4 is formed in a line basic tone in a predetermined direction for at least a part of the density, and has a high-pass filter characteristic in a portion other than the basic tone, and the minimum threshold value matrix constituting the threshold value matrix is m. It is set as the structure which is the size of xm (m = integer of 3-5).
[0022]
Here, the threshold value matrix according to claim 5 is a line in a predetermined direction on a polar coordinate by a two-dimensional spatial frequency when the multi-tone image is thresholded for a part of the density in the threshold value matrix according to claim 4. In this configuration, the distribution of the power spectrum other than the angle of the basic tone is approximated uniformly.
[0023]
According to a sixth aspect of the present invention, there is provided an image processing method according to any one of the first to third aspects.
[0024]
An image forming apparatus according to a seventh aspect is configured to execute the gradation reproduction method according to any one of the first to third aspects.
[0025]
According to an eighth aspect of the present invention, there is provided a printer driver including means for executing the image processing method according to the sixth aspect.
[0026]
The image forming apparatus according to claim 9 is formed with a line tone in a predetermined direction when a multi-valued image is thresholded at a certain density by a threshold matrix, and the line tone is formed every 3 to 5 dots. It is set as the structure formed.
[0027]
The gradation reproduction method according to claim 10 is such that when a multi-gradation image is thresholded at a certain density by a dither matrix, the gradation is formed in a line tone in a predetermined direction, and a high-pass filter characteristic in a portion other than the tone. The portion other than the basic tone is formed by applying the spatial frequency characteristics of human vision by spatial frequency analysis.
[0028]
The gradation reproduction method according to claim 11 is such that when a multi-gradation image is thresholded at a certain density by a dither matrix, it is formed in a line tone in a predetermined direction and a high-pass filter characteristic in a portion other than the tone. The portion other than the basic tone is formed by the Bayer type dither method.
[0029]
The gradation reproduction method according to claim 12 is such that when a multi-gradation image is thresholded at a certain density by a dither matrix, a line tone in a predetermined direction is formed, and a portion other than the tone has a high-pass filter characteristic. The portion other than the basic tone is formed to have a blue noise characteristic.
[0030]
A gradation reproduction method according to a thirteenth aspect is configured to select and apply the gradation reproduction method according to any one of the tenth to twelfth aspects based on the gradation level.
[0031]
An image processing method according to a fourteenth aspect is configured to execute the gradation reproduction method according to any one of the tenth to thirteenth aspects.
[0032]
An image forming apparatus according to a fifteenth aspect is configured to execute the gradation reproduction method according to any one of the tenth to thirteenth aspects.
[0033]
A printer driver according to a sixteenth aspect includes a means for executing the image processing method according to the fourteenth aspect.
[0034]
The gradation reproduction method according to claim 17 is a dot having at least one gradation value among n values formed when a multi-gradation image is thresholded at a certain density and formed in a line tone in a predetermined direction. Is a configuration in which a certain pattern does not appear continuously in the main scanning direction.
[0035]
Here, the gradation reproduction method according to claim 18 is the gradation reproduction method according to claim 17, wherein the gradation reproduction method has a high-pass filter characteristic in a portion other than the fundamental tone, and the line fundamental tone is formed every 3 to 5 dots. It is what.
[0036]
A gradation reproduction method according to a nineteenth aspect is the gradation reproduction method according to the seventeenth or eighteenth aspect, wherein dots having at least one gradation value among n values are continuously formed in a certain pattern in the main scanning direction. It is configured to selectively perform whether or not to appear.
[0037]
A gradation reproduction method according to a twentieth aspect is the gradation reproduction method according to the nineteenth aspect, in which the direction of the line tone is changed according to input image data or designated output setting information.
[0038]
A gradation reproduction method according to claim 21 is the gradation reproduction method according to any one of claims 17 to 20, wherein a two-dimensional space is obtained when a multi-gradation image is thresholded at a certain density by a dither matrix. The power spectrum distribution except the angle of the line tone in a predetermined direction on the polar coordinates according to the frequency is configured to uniformly approximate.
[0039]
The threshold value matrix according to claim 22 is formed with a line tone in a predetermined direction when a multi-tone image is thresholded at a certain density, and a dot of at least one tone value among n values is mainly In this configuration, a certain pattern does not appear continuously in the scanning direction.
[0040]
An image processing method according to a twenty-third aspect is configured to execute the gradation reproduction method according to any one of the seventeenth to nineteenth aspects.
[0041]
An image forming apparatus according to a twenty-fourth aspect is configured to execute the gradation reproduction method according to any one of the seventeenth to nineteenth aspects.
[0042]
A printer driver according to a twenty-fifth aspect includes a means for executing the image processing method according to the twenty-third aspect.
[0043]
A gradation reproduction method according to a twenty-sixth aspect is the gradation reproduction method according to any one of the seventeenth to nineteenth aspects, wherein a portion other than the basic tone is formed by applying a human visual spatial frequency characteristic by spatial frequency analysis. It is a configuration.
[0044]
A gradation reproduction method according to a twenty-seventh aspect is the gradation reproduction method according to any one of the seventeenth to nineteenth aspects, wherein a portion other than the basic tone is formed by a Bayer type dither method.
[0045]
A gradation reproduction method according to a twenty-eighth aspect is the gradation reproduction method according to any one of the seventeenth to nineteenth aspects, wherein a portion other than the basic tone is formed to have a blue noise characteristic.
[0046]
A gradation reproduction method according to a twenty-ninth aspect is configured such that the gradation reproduction method according to any one of the twenty-sixth to twenty-eighth aspects is selected and applied according to a gradation level.
[0047]
An image processing method according to a thirty-third aspect is configured to execute the gradation reproduction method according to any one of the twenty-sixth to twenty-eighth aspects.
[0048]
An image processing apparatus according to a thirty-first aspect is configured to execute the gradation reproduction method according to any one of the twenty-sixth to twenty-eighth aspects.
[0049]
A printer driver according to a thirty-second aspect includes a means for executing the image processing method according to the thirty-third aspect.
[0050]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a schematic perspective view of a mechanism portion of an ink jet recording apparatus as an image forming apparatus, and FIG. 2 is a side view of the mechanism portion.
[0051]
The ink jet recording apparatus includes a carriage that can move in the main scanning direction inside the recording apparatus main body 1, a recording head that includes an ink jet head mounted on the carriage, an ink cartridge that supplies ink to the recording head, and the like. The mechanism unit 2 and the like are accommodated, the paper 3 fed from the paper feed cassette 4 or the manual feed tray 5 is taken in, a required image is recorded by the printing mechanism unit 2, and then the paper discharge tray 6 mounted on the rear side is loaded. Eject paper.
[0052]
The printing mechanism unit 2 holds the carriage 13 slidably in the main scanning direction (vertical direction in FIG. 2) with a main guide rod 11 and a sub guide rod 12 which are guide members horizontally mounted on left and right side plates (not shown). The carriage 13 has a head 14 composed of an inkjet head for discharging ink droplets of yellow (Y), cyan (C), magenta (M), and black (Bk) with the ink droplet discharge direction directed downward. Each ink tank (ink cartridge) 15 for supplying ink of each color to the head 14 is replaceably mounted on the upper side of the carriage 13.
[0053]
The ink cartridge 15 has an air port that communicates with the atmosphere upward, a supply port that supplies ink to the inkjet head 14 below, and a porous body filled with ink inside. The ink supplied to the inkjet head 14 by the force is maintained at a slight negative pressure. Ink is supplied from the ink cartridge 15 into the head 14.
[0054]
Here, the carriage 13 is slidably fitted to the main guide rod 11 on the rear side (downstream side in the paper conveyance direction) and is slidably mounted on the sub guide rod 12 on the front side (upstream side in the paper conveyance direction). doing. In order to move and scan the carriage 13 in the main scanning direction, a timing belt 20 is stretched between a driving pulley 18 and a driven pulley 19 that are rotationally driven by a main scanning motor 17. The carriage 13 is reciprocally driven by forward and reverse rotation of the main scanning motor 17.
[0055]
In addition, although the heads 14 of the respective colors are used here as the recording heads, a single head having nozzles for ejecting ink droplets of the respective colors may be used. Further, as will be described later, a vibration plate that forms at least a part of the wall surface of the ink flow path and a piezo-type ink jet head that deforms the vibration plate with a piezoelectric element (piezoelectric element) are used as the head 14.
[0056]
However, the present invention is not limited to this. For example, a vibration plate that forms at least a part of the wall surface of the ink flow path and an electrode that faces the vibration plate, and an electrostatic force that deforms and displaces the vibration plate with an electrostatic force and pressurizes ink Ink droplets using an electric head, a piezoelectric element that uses buckling deformation of a diaphragm, or a pressure generated by generating bubbles by heating ink in an ink flow path using a heating resistor It is also possible to use a so-called thermal type that discharges water.
[0057]
On the other hand, in order to convey the paper 3 set in the paper feed cassette 4 to the lower side of the head 14, the paper feed roller 21 and the friction pad 22 for separating and feeding the paper 3 from the paper feed cassette 4 and the paper 3 are guided. A guide member 23, a transport roller 24 that reverses and transports the fed paper 3, a transport roller 25 that is pressed against the peripheral surface of the transport roller 24, and a tip that defines the feed angle of the paper 3 from the transport roller 24 A roller 26 is provided. The transport roller 24 is rotationally driven by a sub-scanning motor 27 through a gear train.
[0058]
A printing receiving member 29 is provided as a paper guide member for guiding the paper 3 fed from the transport roller 24 below the recording head 14 in accordance with the range of movement of the carriage 13 in the main scanning direction. A conveyance roller 31 and a spur 32 that are rotationally driven to send the paper 3 in the paper discharge direction are provided on the downstream side of the printing receiving member 29 in the paper conveyance direction, and the paper 3 is further delivered to the paper discharge tray 6. A roller 33 and a spur 34, and guide members 35 and 36 that form a paper discharge path are disposed.
[0059]
At the time of recording, the recording head 14 is driven according to the image signal while moving the carriage 13 to eject ink onto the stopped paper 3 to record one line, and after the paper 3 is conveyed by a predetermined amount, Record the line. Upon receiving a recording end signal or a signal that the trailing edge of the sheet 3 has reached the recording area, the recording operation is terminated and the sheet 3 is discharged.
[0060]
A recovery device 37 for recovering the ejection failure of the head 14 is disposed at a position outside the recording area on the right end side in the movement direction of the carriage 13. The recovery device 37 includes a cap unit, a suction unit, and a cleaning unit. While waiting for printing, the carriage 13 is moved to the recovery device 37 side and the head 14 is capped by the capping means, and the ejection port portion (nozzle hole) is kept in a wet state to prevent ejection failure due to ink drying. Further, by ejecting (purging) ink that is not related to recording during recording or the like, the ink viscosity of all the ejection ports is made constant and stable ejection performance is maintained.
[0061]
When a discharge failure occurs, the discharge port (nozzle) of the head 14 is sealed with a capping unit, and bubbles and the like are sucked out from the discharge port with a suction unit through the tube. Is removed by the cleaning means to recover the ejection failure. Further, the sucked ink is discharged to a waste ink reservoir (not shown) installed at the lower part of the main body and absorbed and held by an ink absorber inside the waste ink reservoir.
[0062]
Next, an ink jet head constituting the recording head 14 of the ink jet recording apparatus will be described with reference to FIGS. 3 is an exploded perspective view of the head, FIG. 4 is a cross-sectional view of the head along the longitudinal direction of the liquid chamber, FIG. 5 is an enlarged view of the main part of FIG. 4, and FIG. FIG. 7 is an explanatory plan view of a nozzle plate of the head.
[0063]
This ink jet head includes a flow path forming substrate (flow path forming member) 41 formed of a single crystal silicon substrate, a vibration plate 42 bonded to the lower surface of the flow path forming substrate 41, and bonded to the upper surface of the flow path forming substrate 41. A pressure plate 46 that is an ink flow path communicating with a nozzle 45 that discharges ink droplets that are droplets, and an ink supply path 47 that serves as a fluid resistance portion in the pressure chamber 46. A common liquid chamber 48 for supplying ink is formed, and a pressurizing chamber 46, an ink supply path 47, and a common liquid chamber 48 that are in contact with the ink of the flow path forming substrate 41 are formed on each wall surface with an organic resin film. A liquid-resistant thin film 50 made of is formed.
[0064]
Then, a laminated piezoelectric element 52 is bonded to the outer surface side (the surface opposite to the liquid chamber) of the vibration plate 42 so as to correspond to each pressurizing chamber 46, and this laminated piezoelectric element 42 is bonded and fixed to the base substrate 53. A spacer member 54 is bonded to the base substrate 53 around the row of the piezoelectric elements 52.
[0065]
As shown in FIG. 5, the piezoelectric element 52 is formed by alternately stacking piezoelectric materials 55 and internal electrodes 56. The pressurizing chamber 46 is contracted and expanded by expansion and contraction of the piezoelectric element 52 whose piezoelectric constant is d33. When a drive signal is applied to the piezoelectric element 52 and charging is performed, the piezoelectric element 52 expands in the direction of arrow A in FIG. 5, and when the charge charged in the piezoelectric element 52 is discharged, it contracts in the direction opposite to the direction of arrow A. It has become. The base substrate 53 and the spacer member 54 are formed with through holes for forming ink supply ports 49 for supplying ink to the common liquid chamber 48 from the outside.
[0066]
Also, the outer peripheral portion of the flow path forming substrate 41 and the lower outer edge portion of the vibration plate 42 are adhesively bonded to a head frame 57 formed by injection molding with epoxy resin or polyphenylene sulfite, and the head frame 57 and the base substrate 53 Are fixed to each other with an adhesive or the like at a portion not shown. Further, an FPC cable 58 is connected to the piezoelectric element 52 by solder bonding, ACF (anisotropic conductive film) bonding or wire bonding in order to give a drive signal, and the FPC cable 58 is selectively connected to each piezoelectric element 52. A drive circuit (driver IC) 59 for applying a drive waveform is mounted.
[0067]
Here, the flow path forming substrate 51 is formed by anisotropically etching a single crystal silicon substrate having a crystal plane orientation (110) using an alkaline etching solution such as an aqueous potassium hydroxide solution (KOH), thereby providing each pressure chamber 56. Are formed, a groove serving as an ink supply path 57, and a through hole serving as a common liquid chamber 58 are formed.
[0068]
The diaphragm 42 is formed of a nickel metal plate and is manufactured by an electroforming method. The vibration plate 42 has a thin portion 61 for facilitating deformation and a thick portion 62 for joining the piezoelectric element 52 in a portion corresponding to the pressurizing chamber 46 and a portion corresponding to the liquid chamber interval wall. In addition, the thick portion 23 is formed, the flat surface side is adhesively bonded to the flow path forming substrate 41, and the thick portion is adhesively bonded to the frame 17. A strut portion 64 is interposed between the thick wall portion 63 corresponding to the liquid chamber interval wall of the diaphragm 2 and the base substrate 53. This support portion 64 has the same configuration as the piezoelectric element 52.
[0069]
The nozzle plate 43 forms a nozzle 45 having a diameter of 10 to 30 μm corresponding to each pressurized liquid chamber 46 and is bonded to the flow path forming substrate 41 with an adhesive. Here, the plurality of nozzles 45 constitute a plurality of dot forming means, and as shown in FIG. 7, the rows of nozzles 45 (nozzle rows) are arranged orthogonal to the main scanning direction, The pitch between 45 is 2 × Pn. Further, two nozzle rows are arranged in one head at a distance L, and each nozzle row is arranged in a staggered manner by shifting the nozzle rows by a pitch Pn in the sub-scanning direction. Therefore, an image having a pitch Pn can be formed by one main scanning and sub-scanning.
[0070]
The nozzle plate 43 may be made of a metal such as stainless steel or nickel, a combination of a metal and a resin such as a polyimide resin film, silicon, or a combination thereof. Further, a water repellent film is formed on the nozzle surface (surface in the ejection direction: ejection surface) by a known method such as a plating film or a water repellent coating in order to ensure water repellency with ink.
[0071]
In the ink jet head configured as described above, by selectively applying a drive pulse voltage of 20 to 50 V to the piezoelectric element 52, the piezoelectric element 52 to which the pulse voltage is applied is displaced in the stacking direction, and the diaphragm 42 is deformed in the direction of the nozzle 45, the ink in the pressurized liquid chamber 46 is pressurized by the volume / volume change of the pressurized liquid chamber 46, and ink droplets are ejected (jetted) from the nozzle 45.
[0072]
As the ink droplets are ejected, the liquid pressure in the pressurized liquid chamber 46 decreases, and a slight negative pressure is generated in the pressurized liquid chamber 46 due to the inertia of the ink flow at this time. Under this state, when the voltage application to the piezoelectric element 52 is turned off, the vibration plate 42 returns to the original position and the pressurized liquid chamber 46 becomes the original shape, so that further negative pressure is generated. To do. At this time, the ink is filled into the pressurized liquid chamber 46 from the ink supply port 49 through the common liquid chamber 48 and the ink supply path 47 which is a fluid resistance portion. Therefore, after the vibration of the ink meniscus surface of the nozzle 45 is attenuated and stabilized, a pulse voltage is applied to the piezoelectric element 52 for discharging the next ink droplet to discharge the ink droplet.
[0073]
Next, an outline of the control unit of the ink jet recording apparatus will be described with reference to FIG.
The control unit includes a microcomputer (hereinafter referred to as “CPU”) 80 that controls the entire recording apparatus, a ROM 81 that stores necessary fixed information, a RAM 82 that is used as a working memory, and the like from the host side. An image memory (lass data memory) 83 for storing image data to be transferred (referred to as dot data or dot pattern data), a parallel input / output (PIO) port 84, an input buffer 85, and a parallel input / output (PIO). A port 86, a waveform generation circuit 87, a head drive circuit 88, a driver 89, and the like are provided.
[0074]
Here, various information and data such as image data transferred from the printer driver 101 side of the host 100, detection signals from various sensors, and the like are input to the PIO port 84. Necessary information is sent to the operation panel side.
[0075]
The waveform generation circuit 87 generates and outputs a drive waveform to be applied to the piezoelectric element 52 of the recording head 14. As this waveform generation circuit 87, as will be described later, by using a D / A converter that D / A converts drive waveform data from the CPU 80, a required drive waveform can be generated and output with a simple configuration. .
[0076]
The head drive circuit 88 applies a drive waveform from the waveform generation circuit 87 to the piezoelectric element 52 of the selected channel of the recording head 14 based on various data and signals given through the PIO port 86. Further, the driver 89 drives and controls the main scanning motor 17 and the sub-scanning motor 27 in accordance with driving data given through the PIO port 86, thereby moving and scanning the carriage 13 in the main scanning direction. Is rotated to convey the paper 3 by a predetermined amount.
[0077]
A portion of the control unit related to head drive control will be described with reference to FIGS. FIG. 9 is a block diagram illustrating a portion related to the drive control, FIG. 10 is a block diagram illustrating an example of the head drive circuit, and FIG. 11 is an explanatory diagram for explaining the operation of the portion related to the head drive control.
[0078]
The main control unit 91 processes font data (dot data) as print data sent from the host side, performs vertical / horizontal conversion corresponding to the arrangement of the heads, and converts ink droplets into large droplets, small droplets, non-droplets. The 2-bit drive data SD necessary for distinguishing the three values for printing is generated and output to the head drive circuit (driver IC) 88. In addition, for the driver IC 88, a clock signal CLK, a latch signal LAT, a drive waveform corresponding to a dot (large droplet) having a size for forming an image dot as a drive waveform, and a drive waveform corresponding to a small droplet are selected. Drive waveform selection signals M1 to M3 are output. Further, the main control unit 91 reads out the drive waveform data stored in the internal ROM 81 and gives it to the drive waveform generation circuit 87.
[0079]
The drive waveform generation circuit 87 D / A converts drive waveform data supplied from the main controller 91 and outputs it as an analog signal, and the analog signal from the D / A converter 92 is converted into an actual drive voltage. 11 and a current amplifier 94 that amplifies the amplified output so that sufficient current can be supplied by driving the head. For example, driving including a plurality of driving pulses within one driving period as shown in FIG. A waveform Pv is generated and given to the driver IC 88.
[0080]
As shown in FIG. 10, the driver IC (head drive circuit) 88 is a shift register 95 that receives drive data SD by a clock signal CK from the main controller 91, and latches a register value of the shift register 95 by a latch signal LAT. A latch circuit 96 that selects the drive waveform selection signals M1 to M3 (logic signals) based on the 2-bit drive data latched by the latch circuit 96, and outputs (logic signals) from the data selector 97 as drive voltages. It comprises a level shifter 98 for converting to a level and a transmission gate 99 whose on / off is controlled by the output of the level shifter 98. The transmission gate 99 is supplied with the drive waveform Pv from the drive waveform generation circuit 87 and is connected to the piezoelectric element 52 corresponding to each nozzle of the recording head (inkjet head) 14.
[0081]
Therefore, in the head drive circuit 88, one of the drive waveform selection signals M1 to M3 is selected by the data selector 97 according to the drive data SD, and the selected drive waveform selection signals M1 to M3 which are logic signals are output by the level shifter 98. The voltage is converted to a drive voltage level and applied to the gate of the transmission gate 99.
[0082]
As a result, the transmission gate 99 is switched in accordance with the length of the selected drive waveform selection signals M1 to M3, so that the drive pulse constituting the drive waveform Pv for the channel in which the transmission gate 99 is open. Is applied.
[0083]
For example, when a drive waveform Pv including a plurality of drive pulses as shown in FIG. 11A is given, the transmission gate 99 which is opened only during the period T0 to T1 is shown in FIG. Thus, since one drive pulse is output and applied to the piezoelectric element 52, the size of the ejected droplet is a small droplet.
[0084]
Similarly, two drive pulses are output from the transmission gate 99 that is open only during the period T0 to T2 and applied to the piezoelectric element 52 as shown in FIG. The size of the medium drops. Further, similarly, five drive pulses are output from the transmission gate 99 which is open during the period T0 to T3 and applied to the piezoelectric element 52 as shown in FIG. The size of becomes a large drop. By adopting such a configuration, it is possible to vary the size of the droplets, and to improve the gradation reproducibility of the printed image.
[0085]
By generating a drive waveform including multiple drive pulses and selecting the number of drive pulses to be applied to the piezoelectric element, each waveform for small droplets, medium droplets, and large droplets is generated from one drive waveform. Therefore, only one circuit and signal line for supplying the drive waveform are required, and the cost can be reduced and the circuit board and the transmission line can be downsized.
[0086]
Next, according to the present invention, including a threshold matrix according to the present invention for transferring image data or the like to the inkjet recording apparatus, and means for executing the image processing method according to the present invention including the gradation reproduction method according to the present invention A host side which is an image processing apparatus according to the present invention equipped with a printer driver will be described with reference to FIG.
[0087]
As described above, the above-described recording apparatus does not have a function of generating a dot pattern to be actually recorded in response to an image drawing or character print command in the apparatus. The dot pattern data is generated by the printer driver using the threshold value matrix according to the present invention for executing the gradation reproduction method according to the present invention and transferred to the ink jet recording apparatus side.
[0088]
That is, the printer driver 101 processes image data from application software executed on the host computer by the CMM processing unit 102, the BG / UCR, the γ correction unit 103, and the zooming processing unit 104, and then the threshold matrix ( Table) 105 is used to replace the multivalued image data with a dot pattern and output.
[0089]
Therefore, first, a method of creating a threshold matrix for performing tone reproduction with a predetermined line tone will be described with reference to FIG.
In image processing, as long as the resolution of the image to be formed is higher than the resolution of the human eye, no matter what processing is used, there is no theoretical effect on the image quality. In such a resolution that can be discriminated by (1), there is a possibility that defects caused by the processing itself will be noticed.
[0090]
FIG. 13 shows an example of a dot pattern actually formed for a halftone processing method generally used in low resolution recording of about 300 dpi. For the input image data shown in FIG. The output image when the Bayer-type dither processing is performed is as shown in FIG. 5B, and the output image when the error diffusion processing is performed is as shown in FIG.
[0091]
In this way, in order to reproduce data that should originally be expressed in multiple values with one pixel with a recording device with less expressive power, the pseudo-scale is represented by the number of dots per unit area, that is, the dot area ratio as shown in FIG. Tonal expression will be performed.
[0092]
The two halftone processing methods given as examples above are not only simply matching the gradation level and area ratio, but also arranged almost evenly so that there is no bias in the arrangement of dots. It has been adjusted to have high-frequency characteristics that are not easily noticeable. When these processes are applied to high resolution recording such as 600 dpi and 1200 dpi, the dot arrangement pattern is hardly noticeable, and a very good image quality with no uneven dot distribution can be obtained.
[0093]
On the other hand, when low-resolution recording such as 150 dpi or 300 dpi is performed, the dot arrangement pattern itself can be noticed even with processing adjusted to have high-frequency characteristics. Originally, in the original image data, what is expressed by one pixel is expressed by using a plurality of pixels, so that a texture pattern that is not in the original original image is formed on the output image.
[0094]
The example shown in FIG. 13B is also the same, but the input image data as shown in FIG. 14A is output in a considerably coarse image of 72 dpi in actual size so that it becomes clearer when FIG. As shown in FIG. 5, a texture peculiar to Bayer type dither processing is generated and changed, or a portion where dots are neatly arranged and there is no texture is mixed, resulting in a very dirty image.
[0095]
On the other hand, in error diffusion processing, dots are formed in an arrangement that appears to be random at first glance. Since the randomness of this dot arrangement is maintained at all gradation levels, as shown in FIG. 13C, the texture does not change depending on the gradation level, and there is no fixed texture itself. Since there is no regular texture, interference with mechanical fluctuations in the recording apparatus hardly occurs, and a certain degree of freedom is obtained in the dot arrangement, so that higher resolution characteristics can be obtained compared to Bayer type dither or the like. .
[0096]
However, in error diffusion processing, as shown in FIG. 15, there may be a large difference in granularity when compared with Bayer type dither or the like. This figure is a comparison when recording at 300 dpi. In this way, randomness that should bring a number of advantages is easily perceived as a dirty noise component when it is noticed at low resolution, and a sensory evaluation produces an ordered texture like Bayer type dither. Tend to be better received.
[0097]
From these facts, it can be seen that the quality of the texture pattern formed by the arrangement of dots greatly affects the image quality. In order to obtain a good image quality at a low resolution from the above-mentioned two kinds of processing methods given as examples, a dot arrangement pattern with good alignment is formed and is not changed over each gradation level (or the change is made). It is necessary not to feel).
[0098]
Therefore, the threshold value matrix according to the present invention has a matrix configuration that performs dot reproduction that always maintains a predetermined line tone (a dot arrangement pattern having alignment properties) at all halftone levels with only a dot arrangement pattern. As a result, it is possible to improve the image quality when recording is performed with a recording apparatus that performs low-value representation of about 1 to 3 bits at a low resolution. In particular, it is possible to obtain print data suitable for the above-described ink jet recording apparatus capable of dot diameter modulation.
[0099]
Here, when considering a dot arrangement pattern having alignment (hereinafter referred to as “base tone”), it is necessary to always consider the correlation with the mechanical variation of the printing apparatus as described above. That is, as in the ink jet recording apparatus described above, as shown in FIG. 16, the recording unit including the small head 14 and the carriage 13 performs recording while moving in the main scanning direction as the paper 3 is fed. Go. At this time, if unevenness occurs in the paper feed accuracy in the sub-scanning direction or the head moving speed in the main scanning direction, there is a possibility that the dot may be recognized as vertical and horizontal stripes by interfering with the basic tone of the dots.
[0100]
For example, FIG. 17B shows interference when output is performed using the one-tone pattern of the Beyer-type dither shown in FIG. 17A. It turns out that it becomes easy to synchronize with the scanning fluctuations A and B. In particular, since the human eye has high sensitivity in the direction of 0 ° or 90 ° (180 ° or 270 °), it is better to avoid the keynote that tends to be aligned vertically and horizontally. However, as described in the error diffusion, the random type that hardly causes interference is not preferable because a noise component is emphasized and recognized at a low resolution.
[0101]
Therefore, in this example, as shown in FIG. In particular, the same effect can be obtained with respect to any variation in the main scanning direction and the sub-scanning direction by setting the line-based key tones such as the 45 ° oblique tone and the 135 ° oblique tone as shown in FIG. In addition, human vision is slightly less sensitive to the diagonal direction, making it less noticeable than vertical and horizontal keynotes. Here, the main point is to keep the keynote, and this characteristic is advantageous because it is not the main point to make the interference that is originally a defect stand out.
[0102]
The line tone of FIG. 18 has been used in electrophotographic recording as “line dither”. In this electrophotographic recording, a latent image is formed with a laser on a charged optical semiconductor, and recording is performed by attaching and transferring toner, so that the dot size can be controlled in any number of steps by laser power modulation, Because toner adhesion and transfer defects are likely to occur, it is not suitable to express gradation with very small dots. Therefore, area modulation type dither (AM dither) that concentrates dots as much as possible to form large dots gradually. Is generally adopted.
[0103]
The line-type dither is a kind of AM dither and has directivity, but has an advantage that the recording density (number of lines) can be increased as compared with the “concentrated dither” in which dots are grown in a spiral shape.
[0104]
However, even if this line dither for electrophotography is applied as it is to an ink jet recording apparatus and other recording methods, the basic tone is not achieved. That is, in electrophotography, as shown in FIG. 19A, it is possible to shift not only the dot size but also the dot formation position. Therefore, no matter how many dots are arranged as shown in FIG. That is, even if the gradation level changes, the gradation can be expressed without breaking the oblique line shape.
[0105]
On the other hand, in the case of an ink jet recording apparatus, as shown in FIG. 20A, the dot formation position is fixed at a pitch determined by the recording resolution. For this reason, as shown in FIG. 4B, the basic tone is changed only by an increase of a few dots, and the processing method does not change the target basic tone (or is inconspicuous).
[0106]
In particular, in general dither processing, the same mask is tiled in a square shape for the purpose of simplifying the processing mechanism (for speeding up and cost reduction). However, it will be recognized as a pattern aligned vertically and horizontally in the tiling cycle.
[0107]
For example, when tiling is performed as shown in FIG. 21B using a 4 × 4 mask as shown in FIG. 21A, dots are aligned vertically and horizontally as a whole. As shown in FIG. 5C, the basic tone is a lattice-like basic tone.
[0108]
Therefore, in order to maintain the above-described line tone, first, 3 dots or more per gradation level are generated at the same time in order to avoid the change in the tone due to the tiling.
[0109]
That is, when reproduction is performed with a diagonal line tone, when a mask of 1 dot per gradation level is tiled as shown in FIG. 22B as shown in FIG. As shown, the vertical and horizontal grids are based. Further, when a mask of 2 dots per gradation level (dots are arranged obliquely as shown in FIG. 23A) is tiled as shown in FIG. As shown in c), although it becomes a diagonal keynote, it becomes a keynote where 45 ° and 135 ° intersect.
[0110]
On the other hand, as shown in FIG. 24A, by setting the number of dots per gradation level to 3 dots or more, even if tiling is performed as shown in FIG. As shown in (c), there is only a diagonal key in one direction.
[0111]
In this case, simultaneously forming 3 dots or more per gradation level means that the mask size is 3 × 3 = 9 times or more in order to obtain the same gradation reproduction capability. Although this value of 9 times or more seems to be large, it is insignificant compared to the buffer memory required for error diffusion processing, and unless the mask is extremely large, the processing speed is reduced and the cost is increased. However, in order to increase the speed, the vertical and horizontal sizes of the mask are set so as to be easy to process by the computer, that is, a multiple of 8 that does not generate a fraction when expanded on a memory.
[0112]
Next, enlargement of the mask size will be described. With reference to a reference mask having a diagonal line tone as shown in FIG. 25 (a), a mask for simultaneous occurrence of 4 dots is formed as shown in FIG. 25 (b), and further as shown in FIG. 25 (c). Then, one square and one square of the reference mask are divided into smaller sub-matrices so as to obtain the required number of gradations. At this time, the sub-matrix to be divided is an oblique line-like shape similar to the reference mask, so that it is possible to prevent the occurrence of a pattern that breaks the basic tone.
[0113]
For example, FIG. 4D shows that 36 gradations can be expressed with a 3 × 3 sub-matrix. FIG. 5E shows a sub-matrix 4 × 4 that can express 64 gradations.
[0114]
By making such a sub-matrix, it is possible to suppress the occurrence of another keynote that will break the diagonal keynote.
[0115]
By the way, even when a threshold value matrix (dither matrix) formed in a line tone in a predetermined direction such as the above-described line tone is used, the number of lines is reduced, that is, the gradation is continuous in some densities. In some cases, image quality may be degraded due to the interruption of the characteristics. That is, a part of the matrix pattern formed in a line tone in a predetermined direction is shown in FIGS. 26A and 26B, but there is a concentration that makes it difficult to maintain such a line tone. .
[0116]
For example, as in the gradation gradation image shown in FIG. 27 (a), the continuity of gradation is interrupted by reducing the number of lines in the matrix pattern at the density of part A as shown in FIG. 27 (b). As a result, image quality may be degraded.
[0117]
Therefore, in the present invention, in a threshold matrix (dither matrix) that is formed with a line tone in a predetermined direction, in order to solve the deterioration in image quality due to the reduction in the number of lines in a part of the density, A range portion is selected, and the dot arrangement of the matrix between both densities is newly arranged so as to have a high-pass filter characteristic and a line tone in a predetermined direction.
[0118]
For the high-pass filter characteristics, the spatial frequency characteristics of human vision by spatial frequency analysis are applied, and those having low spatial frequency characteristics are extracted. FIG. 28 shows a graph representing human visual characteristics. The spatial frequency characteristic of human vision is approximated by the following equation (1) from the spatial frequency f on the retina.
[0119]
[Expression 1]

[0120]
FIG. 29B shows an example of a portion other than the line tone of the matrix having the above-described high-pass filter characteristics, and the characteristics of the portion other than the line tone are high-pass filters as shown in FIG. It is a characteristic.
[0121]
FIG. 30 shows an example of a gradation gradation image formed by a matrix that is formed in a line tone in a predetermined direction and has a high-pass filter characteristic in a portion other than the tone.
[0122]
That is, in the density part of the A part in FIG. 27A described above, the part of the line tone shown in FIG. 30A is combined with the part other than the line tone having the high-pass filter characteristic shown in FIG. In addition, by using the dither matrix according to the present invention as shown in FIG. 6C, a gradation gradation image having a continuous density as shown in FIG.
[0123]
In this way, by forming a matrix having a high-pass filter characteristic in a portion other than the predetermined line basic tone and the basic tone at a certain density where the continuity of the gradation is lost only by forming the line basic tone in the predetermined direction. Reduction of the number of lines (loss of continuity of gradation) can be suppressed, and the continuity of the line tone can be increased, and the quality of the reproduced image is improved particularly in low resolution / high speed recording.
[0124]
In addition, as another complementing method for some densities where tone continuity is lost only by forming a line tone in a predetermined direction, the tone gradation image is complemented by the Bayer method type dither method or the blue noise method. As a result, it was confirmed that a gradation gradation image having a continuous density was obtained in the same manner, and it was possible to complement the image while maintaining the high tone number (106 lines).
[0125]
Therefore, the present inventors create an image by giving the high-pass filter characteristics as described above, further changing the size of the sub-matrix, that is, changing the number of lines of the basic tone, and evaluating the image quality. did. At this time, the image was formed with an image resolution of 300 dpi. The evaluation results are shown in Table 1.
[0126]
[Table 1]

[0127]
In the same table, x = the basic tone is not visible and the continuity of gradation is lost.
○ = The basic tone looks moderate and there is continuity of gradation.
A: Basic tone looks very good, and there is continuity of gradation.
Δ: The basic tone is noticeable. There is continuity of gradation.
Is the meaning of
[0128]
Here, with respect to the size of the sub-matrix, a 2 × 2 matrix is possible as shown in FIG. 25 (f) described above, but in the 2 × 2 matrix, a checker flag-like tone is generated in the process of gradation expression. Resulting in. Further, although a sub-matrix of 6 × 6 or more is possible, the basic tone becomes rough, and the basic tone itself is easily noticeable and looks dirty. Further, there is a problem that the processing time is required as the matrix size increases.
[0129]
Therefore, in the present invention, the oblique matrix mask is 3 × 3, the maximum unit is 5 × 5, and optimally 4 × 4.
[0130]
Reduced line count by creating a matrix with high-pass filter characteristics in the areas other than the predetermined line base tone and the part of the tone at some density where the continuity of gradation is lost only by forming the line base tone in the predetermined direction (Loss of gradation continuity) can be suppressed, and the continuity of the line tone can be increased. Furthermore, by using a diagonal line mask with a minimum unit of 3 × 3, a maximum unit of 5 × 5, and optimally 4 × 4 as described above, a line tone (1/3) every 3 to 5 dots is used. 1/4 tones and 1/5 tones.The number of lines is 85 to 141 lines), so that the tone is too fine to be visible or too rough for all tones. It was possible to improve the quality of the reproduced image while achieving the optimal keynote expression that would not occur.
[0131]
As described above, by using the gradation reproduction method or the threshold matrix according to the present invention, when a multi-tone image is thresholded at a certain density by the dither matrix, it is formed with a line tone in a predetermined direction, and Since it has a high-pass filter characteristic in parts other than the basic tone, the number of lines can be reduced and the continuity of the line basic tone is enhanced, and a good image quality can be obtained especially in low resolution / high speed recording. Be able to.
[0132]
At the same time, the line tone is formed every 3 to 5 dots, or the minimum tone matrix of m × m (m = integer of 3 to 5) size is used, so that the tone disappears. As a result, it is possible to realize a better image quality that is low in the number of lines and is not conspicuous. Further, by forming the line base tone in an oblique direction, the diagonal line base tone can reduce horizontal streak noise, and a higher quality multi-tone image can be obtained.
[0133]
Furthermore, since the size of the ejected droplets can be varied as described above, richer gradation expression can be achieved by using the dither matrix of the present invention for each of the large droplets, medium droplets, and small droplets. .
[0134]
For example, as shown in FIG. 34, for each of the large, medium, and small droplet sizes, the mask (referred to as reference mask A) shown in FIG. And a mask (referred to as a reference mask C) shown in FIG.
[0135]
Then, by combining these reference masks A, B, C and the droplet size as shown in FIG. 34 to perform gradation expression, rich gradation expression can be achieved while maintaining the basic tone. In the figure, “As” is a combination of the reference mask A and a small droplet (drop size s), and similarly “Am + Bs” is a combination of the reference mask A and a medium droplet (drop size m) and the reference mask B. "Al + Bl + Cm" is a combination of a reference mask A and a large drop (droplet size l), a combination of a reference mask B and a large drop (droplet size l), and a reference This means that the combination of the mask C and the medium droplet (droplet size m) is combined, and other “Am”, “Am + Bm + Cs”, and the like also have the same meaning.
[0136]
Further, as shown in FIG. 36, the gradation expression can be expressed as a mask to which a condition that small dots (drop size s) do not appear continuously in the main scanning direction is added. Thus, for example, when there is a printing (driving) frequency that is not good among the ink ejection characteristics of the recording head, recording can be performed while avoiding a specific frequency. As a result, it is possible to prevent printing defects due to unstable jetting, and to obtain not only gradation expression but also a stable high quality image.
[0137]
In the example of FIG. 36, in order to prevent dots of small droplets (droplet size s) from appearing continuously in the main scanning direction, the combination of the basic mask B and the droplet size s, as compared with the example of FIG. Although the basic mask C and the drop size s are not combined, it is possible to express a rich gradation while maintaining the basic tone. The description here is an example showing a part of the features of the present invention, and the same effect as described above can be expected as long as the gradation can be expressed while maintaining the basic tone.
[0138]
As described above, by using the gradation reproduction method or the threshold matrix according to the present invention, when a multi-tone image is thresholded at a certain density by the dither matrix, it is formed with a line tone in a predetermined direction, and The dot having at least one gradation value among the n values is configured so that a certain pattern does not continuously appear in the main scanning direction, so that printing that makes the best use of the characteristics of the recording head can be performed. Since stable dot printing can be eliminated, image quality can be improved.
[0139]
In other words, even if an oblique base tone that does not change over each gradation level is formed, depending on how the base tone is made, the base tone cannot be maintained, or the base tone cannot be determined, or conversely, the base tone is too conspicuous. As a result of creating and evaluating an image in which the basic tone creation method is changed, the present inventors have found a basic tone creation condition for solving this problem, and have arrived at the present invention. That is, an object of the present invention is to provide a halftone processing method capable of obtaining good image quality by forming a basic tone even in a recording apparatus in which high resolution is difficult due to cost and recording speed. In addition, the tone can be maintained at each gradation level, the tone can be expressed properly, and the tone is not so conspicuous that it makes the most of the printhead characteristics.
[0140]
Also in this case, when the multi-tone image is thresholded at a certain density by the dither matrix, it is formed in a line tone in a predetermined direction, and in a portion other than the tone, it has a high-pass filter characteristic. The reduction in the number of lines can be suppressed, the continuity of the line tone is increased, and a good image quality can be obtained even in the low resolution / high speed recording.
[0141]
In addition, the line tone is formed every 3 to 5 dots, or the minimum tone matrix of m × m (m = integer of 3 to 5) size is used, so that the tone disappears. Therefore, it is possible to realize a better image quality that does not become too conspicuous due to the low number of lines. Further, by forming the line base tone in an oblique direction, the diagonal line base tone can reduce horizontal streak noise, and a higher quality multi-tone image can be obtained.
[0142]
Note that the image resolution is not limited to 300 dpi, and 600 dpi and 1200 dpi images can be used suitably. However, from the viewpoint of improving image quality at low resolution in order to achieve both high speed and high image quality. As a result, it can be used very suitably when an image is formed and output at a relatively low resolution of 400 dpi or less, particularly 300 dpi or less.
[0143]
Furthermore, as another example, even in an image created with a dither matrix created by combining the VTF method and the Bayer method according to the gradation level as a method of complementing other than the fundamental tone, the fundamental tone is extremely good and the gradation continuity is excellent. It was confirmed that a gradation gradation image maintaining the above can be realized.
[0144]
Specifically, for example, with respect to a gradation level of 0 to 255, it is created by Bayer up to gradation level 50, and by VTF method above 50. Similarly, a good image was obtained even with a dither matrix in which the blue noise method and the Bayer method, and the blue noise method and the VTF method were combined. Note that the gradation level to be applied is not limited to switching at the gradation level 50, but is determined according to the size of the modulation dot diameter obtained by the ink jet recording apparatus and the quality of the gradation gradation image obtained thereby. It ’s fine.
[0145]
As described above, according to the tone reproduction method of the present invention, when a multi-tone image is thresholded at a certain density by a dither matrix, a portion other than the key tone is formed with a line tone in a predetermined direction. In the case of a configuration having a high-pass filter characteristic, the high-pass filter characteristic can be easily realized by applying a spatial frequency characteristic of human vision, complementing with a Bayer-type dither method, or having a blue noise characteristic. In addition, the number of lines can be reduced, the continuity of the line tone is increased, and a good image quality can be obtained even at low resolution / high speed recording. Further, by forming the line base tone in an oblique direction, the diagonal line base tone can reduce horizontal streak noise, and a higher quality multi-tone image can be obtained.
[0146]
Then, applying the spatial frequency characteristics of human vision as these complementing methods, complementing with the Bayer-type dither method, or providing blue noise characteristics is applied selectively or in combination depending on the gradation level. As a result, the image quality can be further improved.
[0147]
Here, in the case of having the above-described high-pass filter characteristics, when both dither matrices between partial densities are binary images, the difference image between the two images has the high-pass filter characteristics.
[0148]
That is, in the dither method, threshold dots set on the low density side always exist on the high density side. Assuming that the low density side is the A mask and the high density side is the B mask in both of the above-mentioned dither matrices, the line tone existing in the A mask is the B mask because of the nature of the dither method. There must be. Therefore, when the difference between the images obtained by binarizing the A mask is obtained from the image obtained by binarizing the B mask, an image having no line tone is obtained.
[0149]
That is, as shown in FIG. 34A, between the density of the A portion using the A mask and the density of the B portion using the B mask, the A mask shown in FIG. When the difference (B−A) from the B mask shown is taken, the difference image (difference pattern) shown in FIG. 4D is obtained, and this difference image is the high-pass filter characteristic described above as shown in FIG. Will have.
[0150]
In addition, when the multi-tone image is thresholded at a certain density using the matrix according to the present invention, the distribution of the power spectrum other than the angle of the line tone in a predetermined direction on the polar coordinates by the two-dimensional spatial frequency is uniform. To approximate.
[0151]
That is, the power spectrum of the spatial frequency of the image is defined by the following equation (2).
[0152]
[Expression 2]

[0153]
This value represents the strength of the spatial frequency (u, v). In order to extract the directionality from (u, v), this is expressed in polar coordinates and set to P (r, θ), and then obtained by the following equation (3).
[0154]
[Equation 3]

[0155]
As described above, the matrix according to the present invention is formed in a line tone in a predetermined direction and has a high-pass filter characteristic in a portion other than the tone. In the line tone part, since there is a line tone only in a predetermined direction, the power spectrum on the polar coordinates has a strong value only in an angle in the predetermined direction. Further, since the portion other than the basic tone has a high-pass filter characteristic, the value of the power spectrum on the polar coordinates is approximated uniformly.
[0156]
35A shows the power spectrum on the polar coordinates of the matrix according to the present invention, and FIG. 35B shows the distribution of the power spectrum on the polar coordinates excluding the line angle of 45 degrees. As shown in the figure (a), only the 45 degree part has a strong value in the power spectrum on the polar coordinates of the matrix, and as shown in the figure (b), in the power spectrum excluding 45 degrees on the polar coordinates, The value approximates uniformly.
[0157]
Therefore, the matrix is formed in the line basic direction of 45 degrees, and the power spectrum at other angles is uniformly approximated by the high-pass filter characteristics.
[0158]
On the other hand, the power spectrum distribution with the same density as that of FIG. 35 by the Bayer type dither matrix and the concentrated dither matrix, which is a typical mask method of the conventional systematic dither method, is shown in FIGS. 36 and 37, respectively. become. These values are high at a plurality of angles, respectively, and it can be seen that the characteristics of the matrix according to the present invention are different from those of the conventional matrix.
[0159]
As described above, the continuity of the line tone in a predetermined direction is increased by the matrix in which the number of lines is suppressed, and a multi-tone image having a desired quality can be reproduced by continuous tone expression. Compared to the error diffusion method, the mask method is used to simplify the processing and increase the processing speed (printing speed, image processing speed, or image forming speed). In this case, the line tone in the predetermined direction is set to the diagonal direction, and the line tone in the diagonal direction reduces the horizontal streak noise caused by the image forming apparatus, and the multi-tone image with higher quality. Can be obtained.
[0160]
Such a threshold matrix and gradation reproduction method is applied to an image processing method for a multicolor image in which a multicolor image is decomposed into a plurality of color components and an original image of at least one color component is used as an input image. Thus, a higher-quality multicolor image can be output.
[0161]
In a multicolor image forming apparatus, generally a color printing apparatus, three basic colors of cyan, magenta and yellow are used. In addition, in order to improve the appearance of a desired color in consideration of lightness and the like, inkjet color printing apparatuses, in which black is added to these three colors and printing is performed using four primary colors, have become mainstream. Yes. Furthermore, recently, in order to improve the printing image quality, there have been an increasing number of printings using more colors by preparing in advance a multi-order color combining basic colors.
[0162]
Like these devices, a multicolor image forming method that decomposes into a plurality of color components and uses an original image of each color component as an input image performs pseudo-tone expression processing in units of color components. Therefore, by applying the present invention, the continuity of the line tone in each color component unit is increased, and as a result, a higher quality image can be formed in a single color image and a multicolor image.
[0163]
In particular, in an image forming apparatus that converts data into binary or multi-valued image data using a matrix, the image is applied particularly when outputting at a low resolution definition of about 300 dpi or less (image formation). The formation speed and printing speed can be increased and the image quality can be improved.
[0164]
That is, in general, an image forming apparatus that uses pseudo gradation expression processing improves the image quality by increasing the dot density per unit, that is, the resolution. However, increasing the resolution requires a large amount of processing time because the amount of image data to be processed simultaneously increases in area units. In the pseudo gradation expression processing in an image forming apparatus with a resolution of 300 dpi, it is generally limited to express about 150 lines at the maximum per inch, and it is said that high image quality is difficult.
[0165]
In the above-described pseudo gradation expression processing using a dither matrix having a line tone in a predetermined direction, a reduction in the number of lines occurs in human vision at a certain density, and the number of lines further decreases particularly at a resolution of 300 dpi. Therefore, the continuity of gradation expression is lost, and the deterioration of image quality is conspicuous. By applying the present invention when forming an image in an output form (output at a low resolution) in such an image forming apparatus, the number of lines is suppressed and the continuity of the line tone in a predetermined direction is increased, and the continuous It is possible to form a multi-tone image having a desired quality by gradation expression.
[0166]
In the above embodiment, the gradation reproduction method, threshold matrix, and image processing according to the present invention are held as a threshold matrix table in the printer driver 101 of the host 100 and are performed by software processing.
[0167]
In this case, the printer driver 101 of the host 100 determines “paper orientation” or “horizontal writing / vertical writing” based on input image data or based on designated output setting information, and the basic orientation is always constant. A process of rotating and applying the threshold matrix is performed so that In other words, the threshold matrix table is preferably applied by rotating the threshold matrix table so that the portrait orientation is the same between the portrait in which the orientation of the paper is portrait and the landscape in which the orientation of the paper is landscape.
[0168]
In the above-described embodiment, the threshold matrix is stored as a table in the host-side printer driver. However, as shown in FIG. 38, the printer driver 101 has application software executed on the host computer. Only a CMM processing unit 102 that processes image data and a BG / UCR / γ correction unit 103, a ROM that stores the zooming processing unit 104 and the threshold matrix 105 according to the present invention in the control unit on the ink jet recording apparatus 200 side, etc. In addition, the conversion to the dot arrangement can be performed on the ink jet recording apparatus side. In either case, since the input image data can be processed by a one-to-one comparison, the image processing can be performed at high speed and at low cost.
[0169]
In the above-described embodiment, an example in which the present invention is applied to an inkjet recording apparatus and its host has been described. However, an image forming apparatus capable of forming an image with dots (forming an image by dot reproduction), such as a thermal transfer recording apparatus. It can be similarly applied to. Furthermore, the present invention can be applied to an image forming apparatus using an electrophotographic method such as a laser printer or an LED printer.
[0170]
As shown in FIG. 39, the thermal transfer recording apparatus moves the heating element 301 of the thermal head 300 while feeding the sheet 304 and the ink sheet 305 between the thermal head 300 having the heating element 301 and the pressure roller 303. By driving, a predetermined area of the wax layer 307 on the base layer 306 of the ink sheet 305 is transferred to the paper 304 to record an image.
[0171]
An example of an image forming apparatus using an electrophotographic method will be briefly described with reference to FIGS. 40 is a schematic configuration diagram of the image forming apparatus, and FIG. 41 is a schematic configuration diagram of a process cartridge constituting the image forming apparatus.
[0172]
The image forming apparatus 440 is an example of a laser printer that forms a full color image with four colors of magenta (M), cyan (C), yellow (Y), and black (Bk), and corresponds to an image signal for each color. Four optical writing devices 442M, 442C, 442Y, 442Bk for emitting laser beams, four process cartridges 441M, 441C, 441Y, 441Bk for image formation, and a paper feed cassette 443 for storing recording paper onto which images are transferred A paper feed roller 444 that feeds recording paper from the paper feed cassette 443, a registration roller 445 that transports the recording paper at a predetermined timing, a transfer belt 446 that transports the recording paper to the transfer section of each process cartridge, A fixing device 449 for fixing an image transferred onto a recording sheet, and a recording sheet after fixing the sheet on a discharge tray 45 It has a configuration including a discharge roller 450 or the like for discharging the.
[0173]
The configuration of the four process cartridges 441M, 441C, 441Y, 441Bk is the same. As shown in FIG. 37, each process cartridge 441 includes a drum-shaped photosensitive member 452 that is an image carrier and a charging roller 453 in the case. And a developing unit 454, a cleaning blade 459, and the like.
[0174]
In the developing unit 454, a toner supply roller, a charging roller, an electrostatic transfer substrate 457, and a toner return roller 458 are provided, and each color toner is stored. Further, a slit 460 serving as a window through which the laser beam from the optical writing device is incident is provided on the back side of the process cartridge 441.
[0175]
Each of the optical writing devices 442M, 442C, 442Y, and 442Bk includes a semiconductor laser, a collimator lens, an optical deflector such as a polygon mirror, a scanning imaging optical system, and the like, and a host (image processing device) such as a personal computer outside the device. The laser beam modulated in accordance with the image data for each color inputted from () is emitted and scanned on the photosensitive member 452 of each process cartridge 441M, 441C, 441Y, 441Bk, and an electrostatic charge image (electrostatic latent image) Write.
[0176]
When image formation is started, the photosensitive members 452 of the process cartridges 441M, 441C, 441Y, and 441Bk are uniformly charged by the charging roller 453, and according to the image data from the optical writing devices 442M, 442C, 442Y, and 442Bk. The laser beam is irradiated to form an electrostatic latent image of each color on each photoconductor. The electrostatic latent image formed on the photoreceptor 452 is developed and visualized by the toner of each color electrostatically transported by the electrostatic transport substrate 457 of the developing unit 454. A pulse-like development bias is applied between the opposing portions of the photoreceptor 452 and the electrostatic transport substrate 457, and the electrostatic latent image on the photoreceptor 452 is hopped by the toner transported by the electrostatic transport substrate 457. The In addition, the toner that has not been developed is transported by the electrostatic transport substrate 457 and returned to the toner return roller 458.
[0177]
In synchronization with the image formation of each color of each process cartridge 441Bk, 441Y, 441C, 441M, the recording paper in the supply cassette 443 is fed by the supply roller 444, and directed to the transfer belt 446 by the registration roller 445 at a predetermined timing. Be transported. The recording paper is carried on the transfer belt 446 and is sequentially conveyed toward the photosensitive members of the four process cartridges 441Bk, 441Y, 441C, 441M, and the toner images of Bk, Y, C, and M on each photosensitive member. Are sequentially superimposed and transferred. The recording sheet on which the four color toner images are transferred is conveyed to a fixing device 449 including a fixing belt 447 and a pressure roller 448, and the color image including the four color toner images is fixed and discharged onto a discharge tray 451. Is done.
[0178]
In this image forming apparatus, as shown in FIGS. 42A to 42C, the dot size is changed by changing the on time or the off time of the laser beam emitted from each of the optical writing devices 442M, 442C, 442Y, and 442Bk. Can be changed.
[0179]
Furthermore, the head configuration of the ink jet recording apparatus is not limited to that of the above-described embodiment, and a thermal ink jet head using a heating resistor or an apparatus mounting an electrostatic ink jet head using a diaphragm and an electrode may be used. In the above-described embodiment, the present invention is applied to the image forming apparatus. However, the present invention can be similarly applied to image processing and gradation reproduction when image data is output to the image display apparatus.
[0180]
【The invention's effect】
As described above, according to the gradation reproduction method, threshold matrix, image processing method, image forming method, and printer driver according to the present invention, the continuity of the line tone is maintained, and the disappearance of the tone and the tone emphasis are suppressed. It is possible to perform gradation reproduction, image processing, and image formation with high image quality.
[Brief description of the drawings]
FIG. 1 is a schematic perspective explanatory view of a mechanism unit showing an example of an ink jet recording apparatus as an image forming apparatus.
FIG. 2 is an explanatory side view of the mechanism part.
FIG. 3 is an exploded perspective view showing an example of the head of the recording apparatus.
FIG. 4 is a cross-sectional explanatory diagram along the longitudinal direction of the liquid chamber of the head.
5 is an enlarged explanatory view of the main part of FIG.
FIG. 6 is an explanatory cross-sectional view of the head along the lateral direction of the liquid chamber.
FIG. 7 is an explanatory plan view of the nozzle plate of the head.
FIG. 8 is a block diagram illustrating an outline of a control unit of the recording apparatus
FIG. 9 is a block explanatory diagram of a portion related to head drive control of the control unit.
10 is a block diagram showing an example of the head drive circuit of FIG. 9;
FIG. 11 is an explanatory diagram for explaining the operation of a portion related to the head drive control.
FIG. 12 is a block diagram for explaining an example of a configuration on the host side that is an image processing apparatus according to the present invention equipped with a printer driver according to the present invention including a threshold matrix according to the present invention;
FIG. 13 is an explanatory diagram showing a dot pattern after Bayer-type dither processing and error diffusion processing for an input image;
FIG. 14 is an explanatory diagram showing image data after Bayer-type dither processing for an input image;
FIG. 15 is an explanatory diagram when the granularity of a 300 dpi Bayer-type dither pattern and an error diffusion pattern is measured every 10% density.
FIG. 16 is an explanatory diagram for explaining a mechanical variation factor in the ink jet recording apparatus.
FIG. 17 is an explanatory diagram for explaining interference between a Bayer-type dither pattern and mechanical fluctuations of a recording apparatus.
FIG. 18 is an explanatory diagram for explaining an oblique dot arrangement pattern in a threshold matrix according to the present invention;
FIG. 19 is an explanatory diagram for explaining a dot pattern of a line dither pattern and a gradation change in gradation level of an electrophotographic recording apparatus.
FIG. 20 is an explanatory diagram for explaining a change in tone of a dot pattern and a gradation level when the method is applied to an ink jet recording apparatus.
FIG. 21 is an explanatory diagram for explaining a basic tone formed by tiling a dither mask.
FIG. 22 is an explanatory diagram for explaining tiling and keynote when a mask has one dot per gradation level.
FIG. 23 is an explanatory diagram for explaining tiling and keynote when the mask is 2 dots per gradation level;
FIG. 24 is an explanatory diagram for explaining tiling and keynote when the mask is 3 dots per gradation level;
FIG. 25 is an explanatory diagram for explaining division from a basic matrix to a sub-matrix.
FIG. 26 is an explanatory diagram for explaining different examples of a matrix pattern having a line tone in a predetermined direction.
FIG. 27 is an explanatory diagram for explaining an example of a gradation pattern and a matrix pattern in which the number of lines is reduced in some densities
FIG. 28 is an explanatory diagram for explaining human visual characteristics.
FIG. 29 is an explanatory diagram of a part other than the basic tone having a high-pass filter characteristic of a matrix pattern
FIG. 30 is an explanatory diagram for explaining a gradation gradation image by a matrix having a high-pass filter characteristic in a portion other than a predetermined line tone and a key tone.
FIG. 31 is an explanatory diagram illustrating an example of a basic mask
FIG. 32 is an explanatory diagram for explaining a combination of a basic mask and a droplet size.
FIG. 33 is an explanatory diagram for explaining an example of a combination of a basic mask and a droplet size in which a certain pattern does not continuously appear in the main scanning direction according to the present invention.
FIG. 34 is an explanatory diagram for explaining a matrix difference image between partial densities;
FIG. 35 is an explanatory diagram for explaining a power spectrum distribution on polar coordinates of a matrix according to the present invention.
FIG. 36 is an explanatory diagram for explaining a power spectrum distribution on polar coordinates of a conventional Bayer-type dither matrix.
FIG. 37 is an explanatory diagram for explaining a power spectrum distribution on polar coordinates of a conventional concentrated dither matrix.
FIG. 38 is a block diagram for explaining an image forming apparatus according to the present invention.
FIG. 39 is a schematic explanatory diagram of a thermal transfer recording apparatus.
FIG. 40 is a schematic configuration diagram of an image forming apparatus using an electrophotographic method.
FIG. 41 is a schematic configuration diagram of a process cartridge of the image forming apparatus.
FIG. 42 is an explanatory diagram for explaining the dot size change.
FIG. 43 is an explanatory diagram for explaining dots that are actually arranged when gradation representation is performed by binarization and subtraction.
FIG. 44 is an explanatory diagram for explaining binarization processing by a dither method.
FIG. 45 is an explanatory diagram for explaining the correspondence between size-modulated dots and dither masks in the reduction processing.
FIG. 46 is an explanatory diagram for explaining an example of a dither mask for each dot size.
FIG. 47 is an explanatory diagram for explaining the procedure of binary error diffusion processing;
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 13 ... Carriage, 14 ... Recording head, 87 ... Drive waveform generation circuit, 88 ... Head drive circuit, 91 ... Main control part, 101 ... Printer driver, 105 ... Threshold matrix.

Claims (32)

In a gradation reproduction method for converting a multi-tone image into binary image data or multi-value image data using a dither matrix, when the multi-tone image is thresholded at a certain density by the dither matrix, A gradation reproduction method characterized by being formed in a line tone in a predetermined direction and having a high-pass filter characteristic in a portion other than the tone, and the line tone is formed every 3 to 5 dots. 2. The gradation reproduction method according to claim 1, wherein the direction of the line tone is changed in accordance with input image data or designated output setting information. 3. The gradation reproduction method according to claim 1, wherein when a multi-gradation image is thresholded at a certain density by the dither matrix, a line tone of a predetermined direction on a polar coordinate by the two-dimensional spatial frequency is determined. A gradation reproduction method characterized in that the distribution of the power spectrum other than the angle approximates uniformly. In a threshold matrix for converting a multi-tone image into binary data or multi-value image data, at least a part of the density is formed with a line tone in a predetermined direction, and a portion other than the tone has a high-pass filter characteristic. And the minimum threshold value matrix constituting the threshold value matrix has a size of m × m (m = integer of 3 to 5). 5. The threshold matrix according to claim 4, wherein when the multi-tone image is thresholded with respect to a part of density, a power spectrum distribution other than a line tone angle in a predetermined direction on a polar coordinate by a two-dimensional spatial frequency is present. A threshold matrix characterized by uniform approximation. In the image processing method for decomposing a multi-color image into a plurality of color components, converting an original image of at least one color component into an input image and converting each dot binary or multi-value image data using a dither matrix, An image processing method, wherein the gradation reproduction method according to claim 1 is executed. 3. An image forming apparatus for forming an image composed of a plurality of dots, wherein the gradation reproducing method according to claim 1 or 2 is executed. 7. A printer driver for processing image data for an image forming apparatus for forming an image composed of a plurality of dots, comprising: means for executing the image processing method according to claim 6. In an image forming apparatus that converts image data input in multiple values into image data having a smaller information level using a threshold matrix and forms an image with a resolution of 300 dpi or less, the threshold matrix is used to generate a multi-valued image. An image formation characterized by executing a gradation reproduction method in which a line tone in a predetermined direction is formed when a threshold value is set at a certain density and the line tone is formed every 3 to 5 dots. apparatus. In a gradation reproduction method for converting a multi-tone image into binary image data or multi-value image data using a dither matrix, when the multi-tone image is thresholded at a certain density by the dither matrix, It is formed in a line tone of a predetermined direction and has a high-pass filter characteristic in a portion other than the key tone, and the portion other than the key tone is formed by applying a human visual spatial frequency characteristic by spatial frequency analysis. Tone reproduction method. In a gradation reproduction method for converting a multi-tone image into binary image data or multi-value image data using a dither matrix, when the multi-tone image is thresholded at a certain density by the dither matrix, A gradation reproduction method characterized by forming a line tone in a predetermined direction and having a high-pass filter characteristic in a portion other than the key tone, and forming a portion other than the key tone by a Bayer-type dither method. In a gradation reproduction method for converting a multi-tone image into binary image data or multi-value image data using a dither matrix, when the multi-tone image is thresholded at a certain density by the dither matrix, A gradation reproduction method characterized by forming a line in a predetermined direction and having a high-pass filter characteristic in a portion other than the basic tone and having a blue noise characteristic in a portion other than the basic tone. 13. The gradation reproduction method according to claim 10, wherein the gradation reproduction method converts a multi-gradation image into each dot binary or multi-value image data using a dither matrix, based on the gradation level. A gradation reproduction method characterized by selecting and applying a method. In the image processing method for decomposing a multi-color image into a plurality of color components, converting an original image of at least one color component into an input image and converting each dot binary or multi-value image data using a dither matrix, 14. An image processing method, wherein the gradation reproduction method according to claim 10 is executed. 14. An image forming apparatus for forming an image composed of a plurality of dots, wherein the gradation reproducing method according to claim 10 is executed. 15. A printer driver for processing image data for an image forming apparatus for forming an image composed of a plurality of dots, comprising: means for executing the image processing method according to claim 14. In a gradation reproduction method in which a multi-tone image is converted into image data of each dot binary value or n value (n = an integer of 3 or more) using a dither matrix, the multi-tone image is thresholded at a certain density. The gradation reproduction is characterized in that dots having at least one gradation value among the n values do not continuously appear in a certain pattern in the main scanning direction. Method. 18. The gradation reproduction method according to claim 17, wherein a portion other than the basic tone has a high-pass filter characteristic, and the line basic tone is formed every 3 to 5 dots. 19. The gradation reproduction method according to claim 17 or 18, wherein at least one gradation value dot among n values is selectively made to appear in a certain pattern in the main scanning direction. A gradation reproduction method characterized by being performed. 20. The gradation reproduction method according to claim 19, wherein the direction of the line tone is changed in accordance with input image data or designated output setting information. 21. The gradation reproduction method according to claim 17, wherein when a multi-gradation image is thresholded at a certain density by the dither matrix, a predetermined direction on a polar coordinate by the two-dimensional spatial frequency is obtained. A gradation reproduction method characterized in that the distribution of the power spectrum other than the angle of the line fundamental tone is uniformly approximated. In a threshold value matrix for converting a multi-tone image into image data of each dot binary value or n value (n = an integer of 3 or more), a predetermined value is set when the multi-tone image is thresholded at a certain density. A threshold value matrix, wherein dots having at least one gradation value among the n values are formed so as not to continuously appear in a certain pattern in the main scanning direction. A multicolor image is decomposed into a plurality of color components, and an original image of at least one color component is used as an input image and converted to image data of each dot binary value or n value (n = integer of 3 or more) using a dither matrix. 20. An image processing method for processing, wherein the gradation reproduction method according to claim 17 is executed. 20. An image forming apparatus for forming an image composed of a plurality of dots, wherein the gradation reproducing method according to claim 17 is executed. 24. A printer driver for processing image data for an image forming apparatus for forming an image composed of a plurality of dots, comprising: means for executing the image processing method according to claim 23. 20. The gradation reproduction method according to claim 17, wherein a portion other than the basic tone is formed by applying a human visual spatial frequency characteristic by spatial frequency analysis. Method. 20. The gradation reproduction method according to claim 17, wherein a portion other than the basic tone is formed by a Bayer type dither method. 20. The gradation reproduction method according to claim 17, wherein a portion other than the basic tone is formed so as to have a blue noise characteristic. 29. The gradation reproduction method for converting a multi-gradation image into image data of each dot binary value or n value (n = integer of 3 or more) using a dither matrix, according to the gradation level. A gradation reproduction method characterized by selecting and applying any one of the gradation reproduction methods. A multicolor image is decomposed into a plurality of color components, and the current image of at least one color component is used as an input image and converted into image data of each dot binary value or n value (n = integer of 3 or more) using a dither matrix. 29. An image processing method according to claim 26, wherein the gradation reproduction method according to claim 26 is executed. 29. An image forming apparatus for forming an image composed of a plurality of dots, wherein the gradation reproducing method according to claim 26 is executed. 31. A printer driver for processing image data for an image forming apparatus for forming an image composed of a plurality of dots, comprising: means for executing the image processing method according to claim 30.

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