JP3770802B2 - Image processing method, image processing apparatus, and image forming apparatus - Google Patents

Description

【００４３】
主要なＣＭＹ値の組合せをカラー画像出力装置３に与えてカラーパッチを出力させ、それをカラー画像入力装置１で読み込み、対応するＣＭＹ値とＲＧＢ値とを求める。これらの関係を満足する定数ａ１１〜ａ３３及び定数ｂ１〜ｂ３を最小二乗法で求める。より忠実な色再現を求める場合には、ＲＧＢの２次以上のより高次の項を含めれば良い。
【００４４】
ルックアップテーブルを用いる方法は、上述した変換行列を求めて、入力画像データのＲＧＢに対して出力されるＣＭＹの値を予め求めておき、ルックアップテーブルとして記憶しておく方法、対応するＣＭＹとＲＧＢとの関係を変換行列ではなく、ニューラルネットワークに学習させ、このニューラルネットワークを用いてルックアップテーブルを作成する方法等がある。
【００４５】
黒生成下色除去部（黒生成下色除去手段）１５は、与えられたＣＭＹの３信号から黒（Ｋ）信号を生成する黒生成処理を行い、元のＣＭＹ信号から、生成したＫ信号に基づく信号量を差し引いて新たなＣＭＹ信号を生成して、生成したＫ信号及び新たなＣＭＹ信号を下色追加部１６に与える。
図２は、黒生成下色除去部１５の内部構成を示すブロック図である。
黒生成下色除去部１５は、色補正部１４から入力されたＣＭＹの各信号Ｃ_in，Ｍ_in，Ｙ_inが、最小値検出部１５ａに与えられ、最小値検出部１５ａで検出された、各信号Ｃ_in，Ｍ_in，Ｙ_inの最小値であるＭＩＮ信号が黒生成部１５ｃに与えられる。黒生成部１５ｃは、与えられたＭＩＮ信号に基づいて黒Ｋの成分信号Ｋ_out を作成し、下色追加部１６を介して空間フィルタ処理部１７に与える。
【００４６】
ＣＭＹの各信号Ｃ_in，Ｍ_in，Ｙ_in及びＭＩＮ信号は、下色除去部１５ｂ及び黒生成下色除去部１５後段の下色追加部１６にも与えられ、下色除去部１５ｂは、与えられたＣＭＹの各信号Ｃ_in，Ｍ_in，Ｙ_in及びＭＩＮ信号に基づき、下色除去処理を行い、下色除去処理後のＣＭＹの各信号Ｃ_ucr ，Ｍ_ucr ，Ｙ_ucr 、及び下色除去量を示すＵＣＲ信号を下色追加部１６に与える。
下色追加部１６（下色追加処理手段）は、与えられたＣＭＹの各信号Ｃ_in，Ｍ_in，Ｙ_in、ＭＩＮ信号、下色除去処理後のＣＭＹの各信号Ｃ_ucr ，Ｍ_ucr ，Ｙ_ucr 及びＵＣＲ信号に基づいて、下色追加処理を行い、下色追加処理後のＣＭＹの各信号Ｃ_out ，Ｍ_out ，Ｙ_out を空間フィルタ処理部１７に与える。
【００４７】
空間フィルタ処理部１７は、与えられたＣＭＹＫ信号の画像データに対して、ディジタルフィルタによる空間フィルタ処理を行い、空間周波数特性を補正することによって、出力画像のぼやけや粒状性による劣化を防ぐ処理を行い、処理後のＣＭＹＫ信号を出力階調補正部１８に与える。
出力階調補正部１８は、与えられたＣＭＹＫ信号をカラー画像出力装置３の特性値である網点面積率に変換する出力階調補正処理を行った後、処理後のＣＭＹＫ信号を階調再現処理部１９に与える。
階調再現処理部１９は、与えられたＣＭＹＫ信号を最終的に画素に分離し、それぞれの階調を再現出来るように処理する階調再現処理（中間調生成）を行う。
【００４８】
文字及び写真混在原稿における特に黒文字又は色文字の再現性を高める為に、領域識別処理部１３において、黒文字（場合によっては色文字も含む）として識別された画像領域は、空間フィルタ処理部１７により、空間フィルタ処理における鮮鋭強調処理で高周波数の強調量が大きくされる。また、階調再現処理部１９では、高域周波数の再現に適した高解像度のスクリーンでの２値化又は多値化処理が選択出来るように構成されている。
【００４９】
一方、領域識別処理部１３において、網点として識別された画像領域は、空間フィルタ処理部１７により、入力網点成分を除去する為に、ローパスフィルタ処理が施される。また、階調再現処理部１９では、階調再現性を重視したスクリーンでの２値化又は多値化処理が行われる。
【００５０】
上述した各処理が施された画像データは、一旦、記憶部２０に記憶され、所定のタイミングで読出されてカラー画像出力装置３に与えられる。
この画像出力装置としては、画像データを記録媒体（例えば紙等）上に出力するもので、例えば、電子写真式やインクジェット方式を用いたカラー画像出力装置等を挙げることが出来るが、特に限定されるものではない。
【００５１】
以下に、このような構成の画像処理装置２及び画像形成装置の動作を説明する。
この画像形成装置では、カラー画像入力装置１において、原稿からの反射光像がＲ，Ｇ，Ｂの各アナログ信号として、ＣＣＤにより読み取られ、画像処理装置２を構成するＡ／Ｄ変換部１０に与えられる。
Ａ／Ｄ変換部１０は、与えられたＲＧＢの各アナログ信号をディジタル信号に変換してシェーディング補正部１１に与える。
シェーディング補正部１１は、与えられたＲＧＢのディジタル信号から、カラー画像入力装置１の照明系、結像系及び撮像系で生じた各種の歪みを取り除く補正を行い、補正したＲＧＢのディジタル信号を入力階調補正部１２に与える。
【００５２】
入力階調補正部１２は、与えられたＲＧＢのディジタル信号のカラーバランスを整えると共に、濃度信号等、画像処理装置２で採用されている画像処理システムが扱い易い信号に変換し、領域識別処理部１３に与える。
領域識別処理部１３は、文字領域、網点領域及び写真領域が混在する原稿に対して、各領域を識別する処理を行い、ＲＧＢのディジタル信号からなる注目画素が何れの領域に属しているかを示す領域識別信号を、黒生成下色除去部１５、下色追加部１６、空間フィルタ処理部１７及び階調再現処理部１９に与えると共に、入力階調補正部１２から与えられたＲＧＢのディジタル信号をそのまま色補正部１４に与える。
【００５３】
色補正部１４は、与えられたＲＧＢのディジタル信号から、色再現の忠実化実現の為に、不要吸収成分を含むＣＭＹ色材の分光特性に基づく色濁りを取除く補正処理を行い、補正後のＣＭＹの３信号を黒生成下色除去部１５に与える。
黒生成下色除去部１５は、与えられたＣＭＹの３信号から黒（Ｋ）信号を生成する黒生成処理を行い、元のＣＭＹ信号から、生成したＫ信号に基づく信号量を差し引いて新たなＣＭＹ信号を生成し、生成したＫ信号及び新たなＣＭＹ信号を下色追加部１６に与える。
下色追加部１６は、与えられたＣＭＹの各信号、ＭＩＮ信号、下色除去処理後のＣＭＹの各信号及びＵＣＲ信号に基づいて、下色追加処理を行い、下色追加処理後のＣＭＹの各信号を空間フィルタ処理部１７に与える。
【００５４】
空間フィルタ処理部１７は、与えられたＣＭＹＫ信号の画像データに対して、ディジタルフィルタによる空間フィルタ処理を行い、空間周波数特性を補正することによって、出力画像のぼやけや粒状性劣化を防ぐ処理を行い、処理後のＣＭＹＫ信号を出力階調補正部１８に与える。
出力階調補正部１８は、与えられたＣＭＹＫ信号をカラー画像出力装置３の特性値である網点面積率に変換する出力階調補正処理を行った後、処理後のＣＭＹＫ信号を階調再現処理部１９に与える。
【００５５】
階調再現処理部１９は、与えられたＣＭＹＫ信号を最終的に画素に分離し、それぞれの階調を再現出来るように処理する階調再現処理を行う。
階調再現処理部１９が、画素に分離し、階調再現処理を行ったＣＭＹＫ信号である画像データは、一旦、記憶部２０に記憶され、所定のタイミングで読出されてカラー画像出力装置３に与えられる。
【００５６】
ここで、黒生成下色除去部１５及びその後段の下色追加部１６では、ＣＭＹの各信号をＣ_ｉｎ，Ｍ_ｉｎ，Ｙ_ｉｎ、下色除去処理後のＣＭＹの各信号をＣ_ｕｃｒ，Ｍ_ｕｃｒ，Ｙ_ｕｃｒ、下色追加処理後のＣＭＹの各信号をＣ_ｏｕｔ，Ｍ_ｏｕｔ，Ｙ_ｏｕｔとすると、
Ｃ_ｏｕｔ＝Ｃ_ｕｃｒ＋β０_ｃＵ
Ｍ_ｏｕｔ＝Ｍ_ｕｃｒ＋β０_ｍＵ （２）
Ｙ_ｏｕｔ＝Ｙ_ｕｃｒ＋β０_ｙＵ
となるように演算を行う。ここで、β０_ｃ，β０_ｍ，β０_ｙ は、０＜β０_ｃ，β０_ｍ，β０_ｙ≦１を充たす係数であり、Ｕは、下色除去処理で減算される下色除去量（ＵＣＲ信号）である。
（２）式のように、下色追加処理において、下色除去量Ｕに比例した量を加算することにより、下色追加後の信号が出力画像データの範囲（８ビットの場合、０〜２５５）の上限値２５５を超えないように、係数β０ _ｃ ，β０ _ｍ ，β０ _ｙ を設定することが出来る。
【００５７】
例えば、入力されたＣＭＹ信号Ｃ_in，Ｍ_in，Ｙ_inの内、Ｃ_inが最小値を取るとすると、最小値検出部１５ａ（図２）で検出された最小値である信号Ｃ_inがＭＩＮ信号となって、下色除去部１５ｂに与えられる。
下色除去部１５ｂは、与えられたＣＭＹの各信号Ｃ_in，Ｍ_in，Ｙ_inから、ＭＩＮ信号に基づく量を減算する。このとき、ＵＣＲ信号Ｕは、Ｕ＝Ｃ_in−Ｃ_ucr と表すことが出来る。ここで、Ｕ＝α×ｍｉｎ（この場合、Ｕ＝α×Ｃ_in）となっており、α（０≦α≦１）は係数であり、ｍｉｎはＭＩＮ信号値を表す。
【００５８】
黒生成部１５ｃは、ＭＩＮ信号を入力とし、黒Ｋの成分信号Ｋ_out を出力とするルックアップテーブルを有しており、ルックアップテーブルから読み出すことにより、黒Ｋの成分信号Ｋ_out を作成し、空間フィルタ処理部１７に与える。
また、下色除去部１５ｂは、ＭＩＮ信号を入力とし、下色除去量Ｕ（ＵＣＲ信号）を出力とするルックアップテーブルを有しており、ルックアップテーブルから読み出すことにより、下色除去を行う（黒Ｋの成分信号Ｋ_out 及び下色除去量Ｕは、別の関数で独立に定められる）。
ルックアップテーブルの数値は、様々な値でサンプルを出力評価する等の実験により、予め求めておく。
【００５９】
下色追加部１６は、黒生成処理及び下色除去処理により低下した彩度を補う為に、ＣＭＹの各信号から下色除去された量（Ｕ）の範囲内で、ＣＭＹの各信号に戻す処理を行う。（２）式のように、下色追加処理において、下色除去量Ｕに比例した量を加算することにより、下色追加後の信号が出力画像データの範囲（８ビットの場合、０〜２５５）の上限値２５５を超えないように、係数β０_c ，β０_m ，β０_y を設定することが出来る。即ち、係数β０_c ，β０_m ，β０_y は１以下であり、補正量を加算しても、下色除去処理前の信号値を超えることはない。
【００６０】
従って、下色追加部１６の後段に、下色追加部１６の出力値が上限値２５５を超えているか否かを確認する為の手段を設けることなく、下色追加処理による下色追加量が多くなり過ぎるのを防ぎながら、黒生成下色除去により生じる彩度低下を抑制することが出来る。（請求項１）
【００６１】
図３は、黒生成下色除去部１５及び下色追加部１６の動作を示すフローチャートである。黒生成下色除去部１５は、画素毎に、最小値検出部１５ａにおいて、最小値を検出し（Ｓ２）、下色除去部１５ｂにおいて、最小値を用いて下色除去部処理を行う（Ｓ４）。
次に、下色追加部１６は、与えられた下色除去部処理における下色除去量（ＵＣＲ量）が所定の閾値Ｔｈより大であるか否かを判定し（Ｓ６）、下色除去量が閾値Ｔｈより大であれば、その画素に下色追加処理を実行する（Ｓ８）。下色除去量が閾値Ｔｈより大でなければ、その画素に下色追加処理を実行しない。（請求項３）
尚、Ｓ６の判定は、下色追加部１６に備えられる、判定部１６ａ（第１判定手段）にて行われる。
【００６２】
上述した下色追加部１６における下色追加処理では、その下色追加量は、黒生成下色除去部１５における下色除去処理の下色除去量に比例している為、下色除去量が小さければ、下色追加量は微々たるものとなる。従って、下色除去量が閾値Ｔｈ（例えば、Ｔｈ＝１０）より大きい画素には、下色追加処理を実行して、下色追加処理後の信号Ｃ_out ，Ｍ_out ，Ｙ_out を出力し、下色除去量が閾値Ｔｈより小さい画素には、下色追加処理を実行せず、そのまま、下色除去処理後の信号Ｃ_ucr ，Ｍ_ucr ，Ｙ_ucr を、信号Ｃ_out ，Ｍ_out ，Ｙ_out として出力する。
これにより、余分な処理を省略し、処理速度を速くすることが出来る。
【００６３】
領域識別処理部１３は、与えられたＲＧＢ信号に基づき、画素毎に、その画素が含まれる領域が黒文字（エッジ）領域、網点領域及び写真（印画紙）領域の何れであるかを識別し、前述したように領域識別信号を出力する。
下色追加部１６に備えられる判定部１６ａ（第２判定手段）では、領域識別処理部１３の識別した結果である領域識別信号に基づいて下色追加処理の実行／不実行の切り換えを行う。例えば、領域識別信号が黒文字領域を表す信号であれば、その画素について下色追加処理は行わず、領域識別信号が網点領域又は写真（印画紙）領域を表す信号であれば、その画素について下色追加処理を行う。（請求項４）
【００６４】
黒文字（エッジ）領域等は、無彩色と判定される領域である為、下色追加処理を行う必要はない。
色にじみ領域のように有彩色の領域でも、本来は無彩色の領域と考えられる領域では、ＣＭＹ信号の大きさを揃えたり、下色除去量を大きくする等の無彩色に近づける補正処理が行われる為、下色追加処理を行う必要はない。
網点領域及び写真領域のような有彩色の画像の領域においては、下色追加処理を行う必要がある。従って、領域識別処理により判定された各画素の領域識別信号に応じて、下色追加処理の実行／不実行を切り換えることで、余分な処理を省略し、処理速度を速くすることが出来る。
【００６５】
実施の形態２．
本発明に係る画像処理方法、画像処理装置及び画像形成装置の実施の形態２における黒生成下色除去部１５及び下色追加部１６は、（２）式における係数β０_c ，β０_m ，β０_y を、
β０_c ＝β１_c （Ｃ_in−ｍｉｎ）／Ｃ_in
β０_m ＝β１_m （Ｍ_in−ｍｉｎ）／Ｍ_in （３）
β０_y ＝β１_y （Ｙ_in−ｍｉｎ）／Ｙ_in
とし、
Ｃ_out ＝Ｃ_ucr ＋β１_c Ｕ（Ｃ_in−ｍｉｎ）／Ｃ_in
Ｍ_out ＝Ｍ_ucr ＋β１_m Ｕ（Ｍ_in−ｍｉｎ）／Ｍ_in （４）
Ｙ_out ＝Ｙ_ucr ＋β１_y Ｕ（Ｙ_in−ｍｉｎ）／Ｙ_in
となるように演算を行う。
【００６６】
ここで、β１_ｃ，β１_ｍ，β１_ｙはＣＭＹの各色成分の信号量に基づいた係数であり、Ｕは、下色除去処理で減算される下色除去量（ＵＣＲ信号）であり、ｍｉｎはＭＩＮ信号を表す。また、β１ _ｃ ，β１ _ｍ ，β１ _ｙ は、０＜β０ _ｃ ，β０ _ｍ ，β０ _ｙ ≦１及び（３）式より、０＜β 1 _ｃ ，β 1 _ｍ ，β 1 _ｙ ≦１である。
ＭＩＮ信号はＣＭＹ信号のグレー成分を表しており、Ｃ_ｉｎ−ｍｉｎは、一種の彩度成分を表していると考えられる。即ち、（Ｃ_ｉｎ−ｍｉｎ）／Ｃ_ｉｎは、入力信号に対する彩度成分の割合を表している。
図７に示すように、下色除去処理後のＣＭＹ信号の彩度成分が大きければ、ＣＭＹ各色のトナーの量が多くなり、それに伴って、黒Ｋのトナーと重なる部分が大きくなることで、彩度低下も大きくなることが考えられる。これにより、入力信号に対する彩度成分の割合に比例した量を下色追加量として、下色追加処理を行う。
【００６７】
例えば、ＣＭＹ信号が２３０，２３０，２５０である場合と、５０、５０、７０である場合とを比較すると、暗い色と明るい色との彩度成分の差の絶対量は、同じように、２０（＝２５０−２３０＝７０−５０）である。しかし、下色追加処理においては、彩度成分ではなく、入力信号に対する彩度成分の割合に比例した量を追加するので、上述したように、相対量（彩度成分の差の絶対量の割合）が異なるような場合には、その補正量も異なり、例えば、Ｃ_ucr に対して、ＣＭＹ信号が２３０，２３０，２５０の場合は、２０／２３０に比例し、５０，５０，７０の場合は、２０／５０に比例する量が下色追加量となる。
【００６８】
この為、彩度の低下が目立つ明るい色、ここでは、５０，５０，７０の場合の下色追加量を大きくするような、適切な下色追加が可能となる。即ち、暗い色のような明度の低いところでは、再現可能な色域が小さい（再現可能な彩度が小さい）為、彩度低下は目立たず、大きく補正する必要がない。一方、明るい色から中濃度の色では、黒生成及び下色除去でのＣＭＹの信号量の減少による彩度の低下が目立ち易く、大きく信号量を補正する必要があり、上述したように、相対量（彩度成分の差の絶対量の割合）が異なるような場合でも、適切に下色追加処理を行うことが出来る。（請求項２）
本実施の形態２における構成及びその他の動作は、上述した実施の形態１における構成及び動作と同様であるので、説明を省略する。
【００６９】
【発明の効果】
本発明に係る画像処理方法によれば、下色追加後の信号量が出力画像データのレベル範囲の上限値を超えないように、比例係数を設定することが出来、下色追加量が大きくなり過ぎるのを防止すると共に、出力画像データの彩度の低下を抑制することが出来る。
【００７０】
また、本発明に係る画像処理方法によれば、元の入力画像データの特性に応じて彩度を強調して、彩度低下をより効果的に抑制することが出来る。
【００７１】
また、本発明に係る画像処理方法によれば、下色追加を行う必要がない場合、下色追加処理を省略して画像処理速度を速くすることが出来る。
【００７２】
また、本発明に係る画像処理方法によれば、下色追加を行う必要がない領域では、下色追加処理を省略して画像処理速度を速くすることが出来る。
【００７３】
また、本発明に係る画像処理装置によれば、下色追加後の信号量が出力画像データのレベル範囲（８ビットの場合０〜２５５）の上限値を超えないように、比例係数を設定することが出来、下色追加量が大きくなり過ぎるのを防止すると共に、出力画像データの彩度の低下を抑制する画像処理装置を実現することが出来る。
【００７４】
また、本発明に係る画像処理装置によれば、元の入力画像データの特性に応じて彩度を強調して、彩度低下をより効果的に抑制する画像処理装置を実現することが出来る。
【００７５】
また、本発明に係る画像処理装置によれば、下色追加を行う必要がない場合、下色追加処理を省略して画像処理速度を速くすることが出来る画像処理装置を実現することが出来る。
【００７６】
また、本発明に係る画像処理装置によれば、下色追加を行う必要がない領域では、下色追加処理を省略して画像処理速度を速くすることが出来る。
【００７７】
また、本発明に係る画像形成装置によれば、下色追加後の信号量が出力画像データのレベル範囲の上限値を超えないように、比例係数を設定することが出来、下色追加量が大きくなり過ぎるのを防止すると共に、出力画像データの彩度の低下を抑制し品質の良い画像を出力する画像形成装置を実現することが出来る。
【図面の簡単な説明】
【図１】本発明に係る画像処理方法、画像処理装置及び画像形成装置の実施の形態の要部構成を示すブロック図である。
【図２】黒生成下色除去部及び下色追加部の内部構成を示すブロック図である。
【図３】黒生成下色除去部及び下色追加部の動作を示すフローチャートである。
【図４】下色除去処理を説明する為の説明図である。
【図５】下色追加処理を説明する為の説明図である。
【図６】下色除去処理を説明する為の説明図である。
【図７】下色除去処理の問題点を説明する為の説明図である。
【図８】ゾーベルフィルタのフィルタ係数を示す説明図である。
【図９】有彩色判定の例を示す説明図である。
【図１０】色にじみ判定における注目画素の近傍画素の例を示す説明図である。
【符号の説明】
１ カラー画像入力装置
２ 画像処理装置
３ カラー画像出力装置
１３ 領域識別処理部（領域識別手段）
１３ａ 有彩色判定部
１３ｂ エッジ処理部
１３ｃ 近傍画素判定部
１４ 色補正部
１５ 黒生成下色除去部（黒生成下色除去手段）
１５ａ 最小値検出部
１５ｂ 下色除去部
１５ｃ 黒生成部
１６ 下色追加部（下色追加処理手段）
１７ 空間フィルタ処理部
１８ 出力階調補正部
１９ 階調再現処理部[0001]
BACKGROUND OF THE INVENTION
The present invention provides a black generation and under color removal process for converting input image data including a plurality of color component signals read by a color image input device such as a scanner into output image data including a plurality of color component signals including black. The present invention relates to an image processing method for performing image processing, an image processing apparatus for executing the image processing method, and an image forming apparatus including the image processing apparatus.
[0002]
[Prior art]
In a color image forming apparatus such as a digital color copying machine and a color printer, a full color image is usually formed using four color materials of cyan C, magenta M, yellow Y and black K. In principle, it is possible to form a full-color image using color materials of C, M, and Y colors. However, since black expressed in three colors of CMY becomes imperfect black that is slightly colored, A black K color material is also used because high density black cannot be expressed, and when color misregistration occurs, color blur occurs around the black area.
[0003]
When a full color image is formed using four color materials of C, M, Y, and K, as shown in FIG. 4B, three colors of CMY are used for the original data as shown in FIG. Α × min (C, M, Y) obtained by multiplying the signal value (min (C, M, Y); MIN signal) by the coefficient α (<1) among the respective signal values of CMY for each color. By considering it as a gray (gray) color component, under color removal (UCR) is removed from each signal value of each CMY color, and black K corresponding to the signal value from which the under color is removed is added. Black generation and under color removal processing is performed to obtain the same color reproduction as when CMY three color materials are used.
[0004]
As the under color removal processing, there is a method of removing all but not a part of the MIN signal described above. In addition, as the black generation processing, an optimal black K generation amount is obtained in advance according to the MIN signal, and a table showing the correspondence between the MIN signal and the black K generation amount is created. A method for reading the amount of black K generated according to the MIN signal, a model indicating the characteristics of the image output device, such as a neural network, and the lightness and saturation of the target color to be reproduced are obtained. There is a way. When the amount of black K generated is determined from lightness and saturation, the amount of black K generated is reduced if the saturation is high, and the amount of black K generated is increased if the lightness is low.
[0005]
As described above, the use of black has advantages such that the achromatic color tone is stable, the color reproduction range in the low lightness portion is widened, and the consumption of color materials such as toner and ink can be suppressed. is there.
However, there is a problem that the saturation is lowered in the chromatic color portion only by performing the undercolor removal processing. As shown in FIG. 6, the under color removal process is to remove the gray color component from the CMY color components, and to obtain the target color with the CMYK four colors obtained by adding the black K component to each of the components after the removal. Reproduce. In the case of electrophotography or an ink jet printer, there is no problem if the dots created by toner or ink do not overlap as shown in FIG. 7A, but the dots that are actually output are shown in FIG. As shown in the figure, the black K dots and the CMY color dots are shifted and overlapped. For this reason, the black K dot overlaps with a part of the dots not including the gray components of the CMY colors, and the saturation is lowered.
[0006]
In order to solve such a problem, as shown in FIG. 5B, the components of each color of CMY are increased with respect to the data subjected to the black generation and under color removal processing as shown in FIG. As a result, an under color addition (UCA) process for increasing the saturation is performed.
Japanese Examined Patent Publication No. 6-44801 discloses subtracting an amount corresponding to the minimum value from each component of CMY to perform undercolor removal processing, and then black K component based on the minimum value of each component of CMY. Perform black generation processing by determining the amount, and add undercolor according to the generated black K component amount to each CMY color component that has undergone undercolor removal processing, thereby emphasizing and correcting each CMY color component An undercolor removal apparatus is disclosed.
[0007]
[Problems to be solved by the invention]
In the under color removal apparatus described above, under color is added to the component amounts of the CMY colors subjected to the under color removal processing in accordance with the generated black K component amount. The upper limit value 255 may be exceeded depending on the component amount or the component amount of each CMY color that has undergone undercolor removal processing.
[0008]
For example, when the signal level of the minimum value of each component of each CMY color is 10 or more, it is assumed that a lower color removal process for subtracting the minimum value from the component amount of each color of CMY is performed.
When the component amount of each CMY color is 250, 250, 100, the generated amount of black K is set to 0.8 × (100−10) = 72 based on the minimum value of each component of CMY, and CMY after removing the lower color When the under color addition process based on the black generation amount is performed so that the component amount of each color is multiplied by 1+ (K / 100) (here, K is the black generation amount), the component amount of each color of CMY The component amount of C or M taking the maximum value is (250−100) × (1+ (72/100)) = 258, which exceeds 255. Therefore, in order to prepare for such a case, a means for confirming whether or not the maximum value exceeds 255 is further required.
[0009]
In the undercolor removal apparatus described above, undercolor removal processing is performed to subtract 100% of the minimum value of each component of CMY. However, if the undercolor removal rate is less than 100%, the CMY colors If the lower color addition process is performed on this, the component amount of each CMY color after the lower color addition process exceeds the upper limit value 255, or the decrease amount of the component of each CMY color In some cases, the amount of undercolor addition becomes too large. Therefore, it is checked whether or not the maximum value exceeds the upper limit value 255. If a means for reducing the maximum value to be lower than the upper limit value is required or if the additional amount of the lower color becomes too large, the reproduced color and the original image There is a problem that the difference from the color becomes large and the quality of the reproduced image is lowered.
[0010]
The present invention has been made in view of the circumstances as described above, and prevents the image data value after the undercolor addition process from becoming too large, and suppresses the reduction in saturation caused by the black undercolor removal. An object of the present invention is to provide an image processing method that can be used.
It is another object of the present invention to provide an image processing apparatus that prevents the value of image data after under color addition processing from becoming excessively large and suppresses saturation reduction caused by black generation under color removal. Objective.
The present invention also provides an image forming apparatus provided with an image processing apparatus that can prevent the value of image data after the undercolor addition process from becoming too large and can suppress a reduction in saturation caused by the removal of black generation and undercolor. The purpose is to provide.
[0011]
[Means for Solving the Problems]
  An image processing method according to the present invention is an image processing method for performing black generation and under color removal processing for converting input image data including a plurality of color component signals into output image data including a plurality of color component signals including black.A signal amount obtained by multiplying the signal amount of the under color removal amount obtained by removing the under color in the black generation under color removal processing by each positive predetermined coefficient of 1 or less,For each color component signal of the output image data subjected to the black generation and under color removal processing, ThatIt is characterized in that a lower color addition process for each addition is performed.
[0012]
  In this image processing method,A signal amount obtained by multiplying the signal amount of the under color removal amount obtained by removing the under color in the black generation under color removal processing by each positive predetermined coefficient of 1 or less,For each color component signal of output image data that has undergone black generation and under color removal processing, ThatSince the under color addition process to be added is performed, the proportionality coefficient is set so that the signal amount after the under color addition does not exceed the upper limit value of the level range of output image data (0 to 255 in the case of 8 bits). It is possible to prevent the undercolor addition amount from becoming too large, and to suppress a decrease in the saturation of the output image data.
[0013]
  In addition, the image processing method according to the present invention is an image processing method for performing black generation and under color removal processing for converting input image data including a plurality of color component signals into output image data including a plurality of color component signals including black. InThe signal amount of each color component after the under color removal in the black generation under color removal processing and the signal amount representing each saturation component is in proportion to the signal amount of each color component of the input image data. A signal amount obtained by multiplying each positive predetermined coefficient of 1 or less based on the signal amount,For each color component signal of the output image data subjected to the black generation and under color removal processing, ThatIt is characterized in that a lower color addition process for each addition is performed.
[0014]
  In this image processing method,The signal amount of each color component after the under color removal processing in the black generation under color removal processing is the ratio of the signal amount representing each saturation component to the signal amount of each color component of the input image data, and the signal amount of each color component The signal amount obtained by multiplying each positive predetermined coefficient of 1 or less based onFor each color component signal of output image data that has undergone black generation and under color removal processing, ThatSince the undercolor addition process to be added is performed, saturation can be emphasized according to the characteristics of the original input image data, and saturation reduction can be more effectively suppressed.
[0015]
  In the image processing method according to the present invention, the under color is removed.Under color removal amountIt is characterized in that it is determined whether or not the lower color addition process should be performed based on the signal amount and the determined result is followed.
[0016]
  When the undercolor removal amount is small, the saturation correction amount is also small and has almost no effect. In this image processing method, the under color is removed.Under color removal amountBased on the signal amount, it is determined whether or not the under color addition process should be performed, and according to the determination result, if it is not necessary to add the lower color, the lower color addition process is omitted and the image processing speed is increased. I can do it.
[0017]
The image processing method according to the present invention should identify the input image data as a plurality of areas including a character area, a halftone dot area, and a photograph area, and perform the under color addition process based on the identified areas. It is characterized by determining whether or not and following the determined result.
[0018]
In areas identified as achromatic areas such as character areas and corrected color fringing areas, it is not necessary to add a lower color to enhance saturation. In this image processing method, the input image data is identified as a plurality of areas including a character area, a halftone dot area, and a photograph area, and based on the identified area, it is determined whether or not an undercolor addition process should be performed. Therefore, in the area where it is not necessary to add the lower color, the lower color addition process can be omitted to increase the image processing speed.
[0019]
  An image processing apparatus according to the present invention performs a black generation and under color removal process for converting input image data including a plurality of color component signals into output image data including a plurality of color component signals including black. InA signal amount obtained by multiplying the signal amount of the under color removal amount obtained by removing the under color in the black generation under color removal processing by each positive predetermined coefficient of 1 or less,For each color component signal of the output image data subjected to the black generation and under color removal processing, ThatUnder color addition processing means for adding each is provided.
[0020]
  This image processing apparatus performs black generation and under color removal processing for converting input image data including a plurality of color component signals into output image data including a plurality of color component signals including black. Under color addition processing meansA signal amount obtained by multiplying the signal amount of the under color removal amount obtained by removing the under color in the black generation under color removal processing by each positive predetermined coefficient of 1 or less,Each color component signal of the output image data that has undergone black generation and under color removal processingNisoAdd each one.
  As a result, the proportional coefficient can be set so that the signal amount after adding the lower color does not exceed the upper limit of the level range of output image data (0 to 255 in the case of 8 bits), and the amount of added lower color is large. It is possible to realize an image processing apparatus that prevents the image from becoming too much and suppresses the decrease in the saturation of the output image data.
[0021]
  An image processing apparatus according to the present invention performs a black generation and under color removal process for converting input image data including a plurality of color component signals into output image data including a plurality of color component signals including black. InThe signal amount of each color component after the under color removal in the black generation under color removal processing and the signal amount representing each saturation component is in proportion to the signal amount of each color component of the input image data. A signal amount obtained by multiplying each positive predetermined coefficient of 1 or less based on the signal amount,For each color component signal of the output image data subjected to the black generation and under color removal processing, ThatUnder color addition processing means for adding each is provided.
[0022]
  This image processing apparatus performs black generation and under color removal processing for converting input image data including a plurality of color component signals into output image data including a plurality of color component signals including black. Under color addition processing meansThe signal amount of each color component after the under color removal processing in the black generation under color removal processing is the ratio of the signal amount representing each saturation component to the signal amount of each color component of the input image data, and the signal amount of each color component The signal amount obtained by multiplying each positive predetermined coefficient of 1 or less based onEach color component signal of the output image data that has undergone black generation and under color removal processingNisoAdd each one.
  As a result, it is possible to realize an image processing apparatus that enhances the saturation according to the characteristics of the original input image data and more effectively suppresses the decrease in saturation.
[0023]
  Further, the image processing apparatus according to the present invention has the undercolor removed.Under color removal amountAnd a first determination unit for determining whether or not to perform the under color addition process based on the signal amount of the first determination unit, wherein the first determination unit conforms to the determination result.
[0024]
  When the undercolor removal amount is small, the saturation correction amount is also small and has almost no effect. In this image processing apparatus, the first determination means has the undercolor removedUnder color removal amountBased on the signal amount, it is determined whether or not the under color addition process should be performed, and according to the determination result, if it is not necessary to add the lower color, the lower color addition process is omitted and the image processing speed is increased. It is possible to realize an image processing apparatus that can
[0025]
Further, the image processing apparatus according to the present invention, based on the result of the identification by the region identification unit for identifying the input image data into a plurality of regions including a character region, a halftone dot region, and a photo region, And a second determination unit that determines whether or not the undercolor addition process should be performed, and is configured to follow a result determined by the second determination unit.
[0026]
In areas identified as achromatic areas such as character areas and corrected color fringing areas, it is not necessary to add a lower color to enhance saturation. In this image processing apparatus, the area identifying means identifies input image data into a plurality of areas including a character area, a halftone dot area, and a photograph area, and the second determining means adds an under color based on the identified area. Since it is determined whether or not the process should be performed and the determined result is followed, in the area where it is not necessary to add the lower color, the lower color addition process can be omitted and the image processing speed can be increased.
[0027]
An image forming apparatus according to the present invention includes any one of the image processing apparatuses according to the present invention, and is configured to form an image based on image data processed by the image processing apparatus.
[0028]
Since this image forming apparatus includes any one of the image processing apparatuses according to the present invention and forms an image based on the image data processed by the image processing apparatus, the signal amount after the addition of the lower color is within the level range of the output image data. The proportionality coefficient can be set so as not to exceed the upper limit value (0 to 255 in the case of 8 bits), preventing the amount of additional undercolor from becoming too large and reducing the saturation of the output image data. An image forming apparatus that suppresses and outputs a high-quality image can be realized.
[0029]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described with reference to the drawings illustrating embodiments thereof.
Embodiment 1 FIG.
FIG. 1 is a block diagram showing the main configuration of Embodiment 1 of an image processing method, an image processing apparatus, and an image forming apparatus according to the present invention.
This image forming apparatus is a color image input apparatus 1 constituted by a scanner, and a reflected light image from an original as an analog signal of R, G, B (R; Red, G; Green, B; Blue). Read by Charge Coupled Device).
[0030]
The image data read by the CCD of the color image input device 1 includes an A / D conversion unit 10, a shading correction unit 11, an input tone correction unit 12, a region identification processing unit 13, and a color correction unit that constitute the image processing device 2. 14, after being sequentially processed by the black generation under color removal unit 15, the under color addition unit 16, the spatial filter processing unit 17, the output tone correction unit 18, and the tone reproduction processing unit 19 and once stored in the storage unit 20. , CMYK digital color signals are output to the color image output device 3.
[0031]
An A / D (analog / digital) conversion unit 10 converts an RGB analog signal into a digital signal and gives the digital signal to the shading correction unit 11. The shading correction unit 11 converts a color image input device from the given RGB digital signal. Correction for removing various distortions generated in one illumination system, imaging system, and imaging system is performed, and the corrected RGB digital signal is supplied to the input tone correction unit 12.
[0032]
The input tone correction unit 12 adjusts the color balance of the given RGB digital signal and converts it into a signal that can be handled easily by the image processing system employed in the image processing device 2 such as a density signal, and the region identification processing unit 13
An area identification processing unit (area identification means) 13 performs a process of identifying each area on a document in which character areas, halftone dot areas, and photo areas are mixed, and indicates which area the pixel of interest belongs to. The region identification signal is supplied to the black generation lower color removal unit 15, the lower color addition unit 16, the spatial filter processing unit 17, and the gradation reproduction processing unit 19, and the RGB digital signal supplied from the input gradation correction unit 12 is also received. The color correction unit 14 is provided as it is.
The region identification processing unit 13 includes a chromatic color determination unit 13a, an edge processing unit 13b, and a neighboring pixel determination unit 13c. The region is identified as a black character region with respect to the region determined to be a character region by the region identification processing. An identification process is performed to determine whether the area.
[0033]
As a method for identifying input image data into a character area, a halftone dot area, and a photographic area, for example, a method described in “Proceedings of the Institute of Image Electronics Engineers 90-06-04” can be used.
In this method, the following determination is performed within a block of M × N (M and N are natural numbers) pixels centered on the target pixel, and the determination result is used as a region identification signal of the target pixel.
Average signal level D for 9 pixels in the center of the block_aveAnd each pixel in the block is binarized using the average value. In addition, the maximum pixel signal level D_max, Minimum pixel signal level D_minAlso ask for.
[0034]
(Dot area)
In the halftone dot area, the halftone dot area is identified by utilizing the fact that the fluctuation of the image signal in the small area is large and the density is higher than the background. With respect to the binarized data, the number of change points from 0 to 1 and the number of change points from 1 to 0 are obtained in the main scanning direction and the sub-scanning direction, respectively._H, K_VAnd each predetermined threshold T_H, T_VIf both of them exceed the respective threshold values, a halftone dot region is set.
[0035]
In addition, in order to prevent erroneous determination with the background, the average value D previously obtained_ave, Maximum pixel signal level D_maxAnd the minimum pixel signal level D_minTo use a predetermined threshold B₁, B₂Compare with
D_max-D_ave> B₁  And D_ave-D_min> B₂  And K_H> T_H  And K_V> T_V  Is a halftone dot region, and if this condition is not satisfied, it is a non-halftone dot region.
An area identified as a non-halftone area is identified as a character area and a photograph area as follows.
[0036]
(Text area, Photo area)
In the character area, the maximum signal level D_maxAnd minimum signal level D_minTherefore, the character area is identified as follows.
In the previously identified non-halftone area, the maximum signal level D previously obtained_max, Minimum signal level D_minAnd their difference D_subFor each predetermined threshold P_A, P_B, P_CIf any one of them exceeds the character region, the character region is used.
D_max> P_A  Or D_min> P_B  Or D_sub> P_C  Is a character area, and if this condition is not satisfied, it is a photograph dot area.
[0037]
The area identified as a character area by the above-described method is further identified as a black character area or a color blur area (refer to Japanese Patent Application No. 11-342586 for details).
Since the color blur area occurs outside the edge of the black character, the area including the character area and its neighboring pixels (for example, about several pixels) is identified using the following conditions. That is, a color bleed region is defined as a region that satisfies the three conditions of a chromatic color of the target pixel, an edge of the target pixel, and a black character region pixel in any of the surrounding pixels of the target pixel. Identify. These three conditions are identified by the chromatic color determination unit 13a, the edge processing unit 13b, and the neighboring pixel determination unit 13c, respectively.
[0038]
As shown in FIG. 9, the chromatic color determination unit 13a determines that the color is a chromatic color when the difference between the maximum value MAX and the minimum value MIN of the CMY signal exceeds a predetermined threshold Δ, and does not exceed the threshold Δ. In this case, it is determined that the color is achromatic.
The edge processing unit 13b uses, for example, a 3 × 3 area Sobel filter centered on the target pixel as shown in FIG. FIG. 8A is for detecting an edge in the vertical direction, and FIG. 8B is for detecting an edge in the horizontal direction. The sum of values obtained by applying these Sobel filters to the target pixel is predetermined. When the threshold value is exceeded, it is determined that the edge.
[0039]
In the neighboring pixel determination unit 13c, the same chromatic / achromatic color as the chromatic color determination unit 13a is used to determine whether or not pixels in the black character area exist in the eight pixels near the target pixel as shown in FIG. This determination is performed on the neighboring 8 pixels, and if any one of the pixels is an achromatic pixel, it is determined that the target pixel is a color blur.
[0040]
The color correction unit 14 changes the spectral characteristics of the CMY (C: cyan, M: magenta, Y: yellow) color material including unnecessary absorption components from a given RGB digital signal to realize faithful color reproduction. A correction process for removing the color turbidity is performed, and the corrected three CMY signals are supplied to the black generation and under color removal unit 15.
As correction processing to remove color turbidity, a method for creating a transformation matrix, a model that describes the relationship between RGB and CMY using a neural network, and a CMY value for each RGB value are look-up table (reference table). There is a method to have as.
[0041]
In the method of creating the transformation matrix, the transformation from RGB to CMY is realized using a matrix operation such as the equation (1).
[0042]
[Expression 1]

[0043]
A combination of main CMY values is given to the color image output device 3 to output a color patch, which is read by the color image input device 1, and corresponding CMY values and RGB values are obtained. Constants a11 to a33 and constants b1 to b3 satisfying these relationships are obtained by the method of least squares. In order to obtain more faithful color reproduction, it is sufficient to include higher-order terms of the second or higher order of RGB.
[0044]
The method using the look-up table is to obtain the conversion matrix described above, obtain the CMY values output for RGB of the input image data in advance, and store them as a look-up table, and the corresponding CMY and There is a method in which a relationship with RGB is learned by a neural network instead of a conversion matrix, and a lookup table is created using this neural network.
[0045]
The black generation and under color removal unit (black generation and under color removal means) 15 performs black generation processing for generating a black (K) signal from the given three CMY signals, and converts the generated C signal from the original CMY signal. A new CMY signal is generated by subtracting the signal amount based on the generated signal, and the generated K signal and the new CMY signal are provided to the lower color adding unit 16.
FIG. 2 is a block diagram showing the internal configuration of the black generation and under color removal unit 15.
The black generation and under color removal unit 15 receives the CMY signals C input from the color correction unit 14._in, M_in, Y_inIs supplied to the minimum value detection unit 15a and each signal C detected by the minimum value detection unit 15a_in, M_in, Y_inThe MIN signal, which is the minimum value of, is supplied to the black generator 15c. The black generation unit 15c generates a black K component signal K based on the given MIN signal._outIs provided to the spatial filter processing unit 17 via the under color adding unit 16.
[0046]
CMY signals C_in, M_in, Y_inThe MIN signal and the MIN signal are also provided to the lower color removing unit 15b and the lower color adding unit 16 subsequent to the black generation lower color removing unit 15, and the lower color removing unit 15b receives each signal C of the given CMY._in, M_in, Y_inAnd undercolor removal processing based on the MIN signal and CMY signals C after the undercolor removal processing._ucr, M_ucr, Y_ucr, And a UCR signal indicating the under color removal amount is given to the under color adding unit 16.
The under color addition unit 16 (under color addition processing means) outputs each signal C of the given CMY._in, M_in, Y_inMIN signal, CMY signal C after under color removal processing_ucr, M_ucr, Y_ucrThe undercolor addition process is performed based on the UCR signal and each CMY signal C after the undercolor addition process is performed._out, M_out, Y_outIs given to the spatial filter processing unit 17.
[0047]
The spatial filter processing unit 17 performs a spatial filter process using a digital filter on the image data of the given CMYK signal, and corrects the spatial frequency characteristics, thereby preventing a deterioration due to blurring or graininess of the output image. Then, the processed CMYK signal is supplied to the output tone correction unit 18.
The output tone correction unit 18 performs output tone correction processing for converting the given CMYK signal into a halftone dot area ratio that is a characteristic value of the color image output device 3, and then performs tone reproduction of the processed CMYK signal. This is given to the processing unit 19.
The gradation reproduction processing unit 19 performs gradation reproduction processing (halftone generation) for finally separating the given CMYK signal into pixels and processing so that each gradation can be reproduced.
[0048]
In order to improve the reproducibility of black characters or color characters especially in a mixed document of characters and photographs, an image region identified as black characters (including color characters in some cases) by the region identification processing unit 13 is processed by the spatial filter processing unit 17. The sharpness enhancement processing in the spatial filter processing increases the amount of high frequency enhancement. In addition, the gradation reproduction processing unit 19 is configured so that binarization or multi-value processing on a high-resolution screen suitable for high frequency reproduction can be selected.
[0049]
On the other hand, the image region identified as a halftone dot by the region identification processing unit 13 is subjected to low-pass filter processing by the spatial filter processing unit 17 in order to remove the input halftone dot component. Further, the gradation reproduction processing unit 19 performs binarization or multi-value processing on the screen with an emphasis on gradation reproducibility.
[0050]
The image data subjected to the above-described processes is temporarily stored in the storage unit 20, read at a predetermined timing, and given to the color image output device 3.
The image output device outputs image data on a recording medium (for example, paper). Examples of the image output device include a color image output device using an electrophotographic method or an inkjet method, but are not particularly limited. It is not something.
[0051]
Hereinafter, operations of the image processing apparatus 2 and the image forming apparatus having such a configuration will be described.
In this image forming apparatus, in the color image input apparatus 1, the reflected light image from the original is read as an R, G, B analog signal by the CCD, and is sent to the A / D converter 10 constituting the image processing apparatus 2. Given.
The A / D conversion unit 10 converts each received RGB analog signal into a digital signal and supplies the digital signal to the shading correction unit 11.
The shading correction unit 11 performs correction for removing various distortions generated in the illumination system, imaging system, and imaging system of the color image input apparatus 1 from the given RGB digital signal, and inputs the corrected RGB digital signal. This is given to the gradation correction unit 12.
[0052]
The input tone correction unit 12 adjusts the color balance of the given RGB digital signal and converts it into a signal that can be handled easily by the image processing system employed in the image processing device 2 such as a density signal, and the region identification processing unit 13
The area identification processing unit 13 performs a process of identifying each area on a document in which a character area, a halftone dot area, and a photographic area are mixed, and to which area the pixel of interest consisting of RGB digital signals belongs. The region identification signal shown is given to the black generation under color removal unit 15, the under color addition unit 16, the spatial filter processing unit 17, and the tone reproduction processing unit 19, and also the RGB digital signal given from the input tone correction unit 12 To the color correction unit 14 as it is.
[0053]
The color correction unit 14 performs correction processing for removing color turbidity based on the spectral characteristics of the CMY color material including unnecessary absorption components from the given RGB digital signal in order to realize faithful color reproduction. The three CMY signals are supplied to the black generation and under color removal unit 15.
The black generation and under color removal unit 15 performs black generation processing for generating a black (K) signal from the given three CMY signals, and subtracts a signal amount based on the generated K signal from the original CMY signal to generate a new signal. A CMY signal is generated, and the generated K signal and new CMY signal are supplied to the lower color adding unit 16.
The under color addition unit 16 performs under color addition processing based on the given CMY signals, MIN signal, CMY signals after under color removal processing, and UCR signals, and the CMY after under color addition processing. Each signal is given to the spatial filter processing unit 17.
[0054]
The spatial filter processing unit 17 performs a spatial filter process using a digital filter on the image data of the given CMYK signal, and corrects the spatial frequency characteristics, thereby performing a process for preventing blurring of the output image and deterioration of graininess. Then, the processed CMYK signal is supplied to the output tone correction unit 18.
The output tone correction unit 18 performs output tone correction processing for converting the given CMYK signal into a halftone dot area ratio that is a characteristic value of the color image output device 3, and then performs tone reproduction of the processed CMYK signal. This is given to the processing unit 19.
[0055]
The gradation reproduction processing unit 19 performs gradation reproduction processing for finally separating the given CMYK signal into pixels and processing so that each gradation can be reproduced.
Image data, which is a CMYK signal separated by the gradation reproduction processing unit 19 and subjected to gradation reproduction processing, is temporarily stored in the storage unit 20 and read out at a predetermined timing to the color image output device 3. Given.
[0056]
  Here, the black generation lower color removal unit 15 and the subsequent lower color addition unit 16 process the CMY signals as C_in, M_in, Y_in, CMY signals after under color removal processing_ucr, M_ucr, Y_ucr, CMY signals after under color addition processing_out, M_out, Y_outThen,
        C_out= C_ucr+ Β0_cU
        M_out= M_ucr+ Β0_mU (2)
        Y_out= Y_ucr+ Β0_yU
The operation is performed so that Where β0_c, Β0_m, Β0_y Is 0<Β0_c, Β0_m, Β0_y≦ 1Person in chargeU is a lower color removal amount (UCR signal) to be subtracted in the lower color removal process.
  As in equation (2), by adding an amount proportional to the under color removal amount U in the under color addition process, the signal after the under color addition is within the range of output image data (0 to 255 in the case of 8 bits). ) To avoid exceeding the upper limit 255 of _c , Β0 _m , Β0 _y Can be set.
[0057]
For example, the input CMY signal C_in, M_in, Y_inOf which C_inIs a minimum value, the signal C which is the minimum value detected by the minimum value detector 15a (FIG. 2)._inBecomes a MIN signal and is given to the under color removal unit 15b.
The under color removal unit 15b performs the given CMY signals C_in, M_in, Y_inFrom this, the amount based on the MIN signal is subtracted. At this time, the UCR signal U is U = C_in-C_ucrCan be expressed as Here, U = α × min (in this case, U = α × C_inΑ (0 ≦ α ≦ 1) is a coefficient, and min represents a MIN signal value.
[0058]
The black generation unit 15c receives the MIN signal and inputs the black K component signal K._outAnd a black K component signal K by reading from the lookup table._outIs provided to the spatial filter processing unit 17.
The under color removal unit 15b has a lookup table that receives the MIN signal and outputs the under color removal amount U (UCR signal), and performs under color removal by reading from the lookup table. (Black K component signal K_outAnd the undercolor removal amount U is independently determined by another function).
The numerical values in the lookup table are obtained in advance by experiments such as output evaluation of samples with various values.
[0059]
The under color adding unit 16 returns the CMY signals to the CMY signals within the range of the amount (U) from which the under color is removed from the CMY signals in order to compensate for the saturation that has decreased due to the black generation process and the under color removal process. Process. As in equation (2), by adding an amount proportional to the under color removal amount U in the under color addition process, the signal after the under color addition is within the range of output image data (0 to 255 in the case of 8 bits). ) To avoid exceeding the upper limit 255 of_c, Β0_m, Β0_yCan be set. That is, the coefficient β0_c, Β0_m, Β0_yIs 1 or less, and even if the correction amount is added, the signal value before the undercolor removal process does not exceed.
[0060]
Therefore, the lower color addition amount by the lower color addition process is not provided in the subsequent stage of the lower color addition unit 16 without providing a means for confirming whether or not the output value of the lower color addition unit 16 exceeds the upper limit value 255. While preventing an excessive increase, it is possible to suppress a decrease in saturation caused by the removal of the black generation and under color. (Claim 1)
[0061]
FIG. 3 is a flowchart showing the operations of the black generation and under color removal unit 15 and the under color addition unit 16. For each pixel, the black generation and under color removal unit 15 detects the minimum value in the minimum value detection unit 15a (S2), and the under color removal unit 15b performs the under color removal unit process using the minimum value (S4). ).
Next, the under color addition unit 16 determines whether or not the under color removal amount (UCR amount) in the given under color removal unit processing is greater than a predetermined threshold Th (S6), and the under color removal amount. Is larger than the threshold value Th, the under color addition process is executed for the pixel (S8). If the under color removal amount is not larger than the threshold Th, the under color addition process is not executed for the pixel. (Claim 3)
Note that the determination in S6 is performed by the determination unit 16a (first determination unit) provided in the under color addition unit 16.
[0062]
In the under color addition process in the under color addition unit 16 described above, the under color addition amount is proportional to the under color removal amount of the under color removal process in the black generation under color removal unit 15, and therefore the under color removal amount is If it is smaller, the additional amount of the under color is insignificant. Therefore, for a pixel whose undercolor removal amount is larger than a threshold Th (for example, Th = 10), the undercolor addition process is executed, and the signal C after the undercolor addition process is executed._out, M_out, Y_outFor the pixel whose undercolor removal amount is smaller than the threshold value Th, the undercolor removal process is not performed, and the signal C after the undercolor removal process is performed as it is._ucr, M_ucr, Y_ucrTo signal C_out, M_out, Y_outOutput as.
Thereby, extra processing can be omitted and the processing speed can be increased.
[0063]
The area identification processing unit 13 identifies, for each pixel, whether the area including the pixel is a black character (edge) area, a halftone dot area, or a photograph (printing paper) area based on the given RGB signal. As described above, the area identification signal is output.
The determination unit 16 a (second determination unit) provided in the under color addition unit 16 switches execution / non-execution of the under color addition process based on the region identification signal that is the result of identification by the region identification processing unit 13. For example, if the area identification signal is a signal representing a black character area, the lower color addition process is not performed on the pixel, and if the area identification signal is a signal representing a halftone area or a photograph (printing paper) area, Under color addition processing is performed. (Claim 4)
[0064]
Since the black character (edge) region or the like is a region determined to be an achromatic color, it is not necessary to perform the under color addition process.
Even in chromatic areas such as color fringing areas, in areas that are originally considered to be achromatic areas, correction processing is performed to bring the colors closer to achromatic colors, such as aligning the size of CMY signals and increasing the amount of undercolor removal. Therefore, it is not necessary to perform the under color addition process.
In a chromatic image area such as a halftone dot area and a photographic area, it is necessary to perform a lower color addition process. Therefore, by switching execution / non-execution of the under color addition process in accordance with the area identification signal of each pixel determined by the area identification process, it is possible to omit unnecessary processes and increase the processing speed.
[0065]
Embodiment 2. FIG.
In the image processing method, the image processing apparatus, and the image forming apparatus according to the second embodiment of the present invention, the black generation lower color removal unit 15 and the lower color addition unit 16 have a coefficient β0 in the equation (2)._c, Β0_m, Β0_yThe
β0_c= Β1_c(C_in-Min) / C_in
β0_m= Β1_m(M_in-Min) / M_in      (3)
β0_y= Β1_y(Y_in-Min) / Y_in
age,
C_out= C_ucr+ Β1_cU (C_in-Min) / C_in
M_out= M_ucr+ Β1_mU (M_in-Min) / M_in    (4)
Y_out= Y_ucr+ Β1_yU (Y_in-Min) / Y_in
The operation is performed so that
[0066]
  Where β1_c, Β1_m, Β1_yIsBased on signal amount of each color component of CMYThis is a coefficient, U is an under color removal amount (UCR signal) to be subtracted in the under color removal process, and min represents a MIN signal.Β1 _c , Β1 _m , Β1 _y Is 0 <β0 _c , Β0 _m , Β0 _y ≦ 1 and (3), 0 <β 1 _c , Β 1 _m , Β 1 _y ≦ 1.
  The MIN signal represents the gray component of the CMY signal, and C_in-Min is considered to represent a kind of saturation component. That is, (C_in-Min) / C_inRepresents the ratio of the saturation component to the input signal.
  As shown in FIG. 7, if the saturation component of the CMY signal after the undercolor removal process is large, the amount of toner of each color of CMY increases, and accordingly, the portion overlapping with the black K toner increases. It is conceivable that the desaturation increases. Thus, the lower color addition process is performed with the amount proportional to the ratio of the saturation component to the input signal as the lower color addition amount.
[0067]
For example, comparing the case where the CMY signals are 230, 230 and 250 with the case where the CMY signals are 50, 50 and 70, the absolute amount of the difference in the saturation component between the dark color and the light color is 20 (= 250-230 = 70-50). However, in the undercolor addition process, an amount proportional to the ratio of the saturation component to the input signal is added instead of the saturation component, so as described above, the relative amount (the ratio of the absolute amount of the difference between the saturation components) ) Are different, the correction amount is also different. For example, C_ucrOn the other hand, when the CMY signals are 230, 230, and 250, the amount is 20/230, and when 50, 50, and 70, the amount that is proportional to 20/50 is the additional amount of the under color.
[0068]
For this reason, it is possible to add an appropriate lower color so as to increase the amount of the lower color added in the case of a bright color in which the saturation is conspicuous, here 50, 50, and 70. That is, in low-lightness areas such as dark colors, the reproducible color gamut is small (the reproducible saturation is small), so the saturation reduction is not noticeable and does not need to be greatly corrected. On the other hand, for light to medium density colors, the decrease in saturation due to the decrease in the CMY signal amount due to black generation and under color removal is conspicuous, and it is necessary to greatly correct the signal amount. Even when the amount (the ratio of the absolute amount of the difference between the saturation components) is different, the undercolor addition process can be appropriately performed. (Claim 2)
Since the configuration and other operations in the second embodiment are the same as those in the first embodiment described above, the description thereof is omitted.
[0069]
【The invention's effect】
According to the image processing method of the present invention, the proportional coefficient can be set so that the signal amount after adding the lower color does not exceed the upper limit of the level range of the output image data, and the additional amount of the lower color is increased. It is possible to prevent the saturation of the output image data from being reduced.
[0070]
Further, according to the image processing method of the present invention, the saturation can be emphasized according to the characteristics of the original input image data, and the saturation reduction can be more effectively suppressed.
[0071]
Also, according to the image processing method of the present invention, when it is not necessary to add a lower color, the lower color addition process can be omitted to increase the image processing speed.
[0072]
Further, according to the image processing method of the present invention, in an area where it is not necessary to add a lower color, it is possible to increase the image processing speed by omitting the lower color addition process.
[0073]
In addition, according to the image processing apparatus of the present invention, the proportionality coefficient is set so that the signal amount after addition of the lower color does not exceed the upper limit value of the level range of output image data (0 to 255 in the case of 8 bits). Thus, it is possible to realize an image processing apparatus that prevents an additional amount of under color from becoming too large and suppresses a decrease in the saturation of output image data.
[0074]
Further, according to the image processing apparatus of the present invention, it is possible to realize an image processing apparatus that enhances the saturation according to the characteristics of the original input image data and more effectively suppresses the decrease in saturation.
[0075]
Further, according to the image processing apparatus of the present invention, when it is not necessary to add a lower color, it is possible to realize an image processing apparatus capable of increasing the image processing speed by omitting the lower color addition process.
[0076]
Further, according to the image processing apparatus of the present invention, in an area where it is not necessary to add a lower color, the lower color addition process can be omitted to increase the image processing speed.
[0077]
Further, according to the image forming apparatus of the present invention, the proportional coefficient can be set so that the signal amount after the under color addition does not exceed the upper limit value of the level range of the output image data, and the under color addition amount is It is possible to realize an image forming apparatus that prevents an excessive increase in size and suppresses a decrease in the saturation of output image data and outputs a high-quality image.
[Brief description of the drawings]
FIG. 1 is a block diagram illustrating a main configuration of an embodiment of an image processing method, an image processing apparatus, and an image forming apparatus according to the present invention.
FIG. 2 is a block diagram showing an internal configuration of a black generation and under color removal unit and an under color addition unit.
FIG. 3 is a flowchart illustrating operations of a black generation under color removal unit and an under color addition unit.
FIG. 4 is an explanatory diagram for explaining an undercolor removal process.
FIG. 5 is an explanatory diagram for explaining a lower color addition process;
FIG. 6 is an explanatory diagram for explaining undercolor removal processing;
FIG. 7 is an explanatory diagram for explaining a problem in the undercolor removal process.
FIG. 8 is an explanatory diagram showing filter coefficients of a Sobel filter.
FIG. 9 is an explanatory diagram illustrating an example of chromatic color determination.
FIG. 10 is an explanatory diagram illustrating an example of a pixel in the vicinity of a target pixel in color blur determination.
[Explanation of symbols]
1 Color image input device
2 Image processing device
3 Color image output device
13 Area identification processing unit (area identification means)
13a Chromatic color determination unit
13b Edge processing unit
13c Neighboring pixel determination unit
14 color correction unit
15 Black generation under color removal unit (black generation under color removal means)
15a Minimum value detector
15b Under color removal part
15c Black generator
16 Under color addition part (under color addition processing means)
17 Spatial filter processing section
18 Output tone correction unit
19 Tone reproduction processing section

Claims (9)

In an image processing method for performing black generation and under color removal processing for converting input image data including a plurality of color component signals into output image data including a plurality of color component signals including black,
An output obtained by performing the black generation and under color removal processing on a signal amount obtained by multiplying the signal amount of the under color removal amount obtained by removing the under color in the black generation and under color removal processing by each positive predetermined coefficient equal to or less than 1. to each color component signals of the image data, the image processing method characterized by performing under color addition process for adding, respectively Re it. In an image processing method for performing black generation and under color removal processing for converting input image data including a plurality of color component signals into output image data including a plurality of color component signals including black,
The signal amount of each color component after the under color removal in the black generation under color removal processing and the signal amount representing each saturation component is in proportion to the signal amount of each color component of the input image data. the amount of signal to the multiplying each predetermined coefficient of 1 or less positive based on the signal amount, the respective color component signals of the output image data in which the black generation and under color removal process is performed, additional under color add, respectively Re their processing And an image processing method. 3. The image processing method according to claim 1, wherein it is determined whether or not the lower color addition processing should be performed based on a signal amount of the lower color removal amount from which the lower color has been removed , and the image processing method according to the determination result. The input image data is identified as a plurality of areas including a character area, a halftone dot area, and a photograph area, and based on the identified area, it is determined whether or not the under color addition process is to be performed, and the result is determined. the image processing method according to any one of items 1-3. In an image processing apparatus that performs black generation and under color removal processing for converting input image data including a plurality of color component signals into output image data including a plurality of color component signals including black,
An output obtained by performing the black generation and under color removal processing on a signal amount obtained by multiplying the signal amount of the under color removal amount obtained by removing the under color in the black generation and under color removal processing by each positive predetermined coefficient equal to or less than 1. to each color component signals of the image data, the image processing apparatus, characterized in that it comprises a lower color addition processing means for adding, respectively Re it. In an image processing apparatus that performs black generation and under color removal processing for converting input image data including a plurality of color component signals into output image data including a plurality of color component signals including black,
The signal amount of each color component after the under color removal in the black generation under color removal processing and the signal amount representing each saturation component is in proportion to the signal amount of each color component of the input image data. the amount of signal to the multiplying each predetermined coefficient of 1 or less positive based on the signal amount, the respective color component signals of the output image data in which the black generation and under color removal process is performed, additional under color add, respectively Re their processing An image processing apparatus comprising: means. The apparatus further comprises first determination means for determining whether or not the lower color addition processing should be performed based on the signal amount of the lower color removal amount from which the lower color has been removed, and to follow the result determined by the first determination means. The image processing apparatus according to claim 5 or 6. An area identifying unit that identifies the input image data as a plurality of areas including a character area, a halftone dot area, and a photograph area, and whether or not to perform the lower color addition process based on a result identified by the area identifying unit Furthermore a second determination means for determining comprises image processing apparatus according to any one of claims 5-7 in which said second judging means are no to follow the result of the determination. An image processing apparatus according to any one of claims 5-8, based on the image data which the image processing device has processed the image forming apparatus characterized by are none so as to form an image.

Images