JP3566357B2 - Color pseudo-halftone processing device - Google Patents

Description

【００３８】
図４は、周囲ドットを含めたイエローインクのドット印字状態を示している。図中斜線で示す矩形の領域は注目画素の理論面積Ｓａ＝１であり、円形の領域は実際の印字ドットの領域である。図から印字されるドットの配置により注目画素のドットと重なる領域が異なることが分かる。
【００３９】
表２は注目画素の周囲画素Ａ，Ｂ，Ｃ，Ｄがどのように印字されているかにより注目画素を印字するときの誤差調整係数αの例を示すもので、これは下記（２） 式により求め、図４の（ａ） 〜（Ｐ） に対応している。
【００４０】
α＝Ｄｍａｘ ・｛（Ｓｂ−Ｓｃ）−１｝ （２）
但し、Ｓｃは重複する領域の理論面積Ｓａに対する面積比である。
【００４１】
すなわち、周囲画素Ａ，Ｂ，Ｃ，Ｄがすべて印字されていない図４の（ａ） の場合は誤差調整係数αは１４５となる。また、図４の（ｃ） 、（ｉ） 、（ｋ） のように周囲画素が印字されていても注目画素と重なる部分が無い場合も誤差調整係数αは１４５となる。
【００４２】
また、図４の（ｂ） 、（ｄ） 、（ｅ） 、（ｇ） 、（ｊ） 、（ｌ） 、（ｍ） 、（ｏ） のように注目画素と周囲画素が重なる領域が１ヶ所の場合は誤差調整係数α＝７３となる。さらに、図４の（ｆ） 、（ｈ） 、（ｎ） 、（ｐ） のように注目画素と周囲画素が重なる領域が２ヶ所の場合は誤差調整係数α＝０となる。
【００４３】
このように重複する領域の面積を注目画素の印字ドット面積から引くことで過補正を防ぎ、周囲ドットの状態も考慮した補正ができるようになる。
【００４４】
【表２】

【００４５】
用紙上に最初に印字されるイエロー（Ｙ）のドットについてはこの方法で誤差調整係数αｙを設定することができる。すなわち、表２に示す内容でα設定用ＬＵＴ６ａを設定できる。
【００４６】
しかし、２色目以降のマゼンタ（Ｍ）、シアン（Ｃ）のドットはさらに補正を加える必要がある。
【００４７】
先ず、イエロー（Ｙ）の入力多階調データＤｉｎ０（ｙ）の２値化について述べる。入力多階調データＤｉｎ０（ｙ）が１画素入力されると、加算器１ａは、これに、この画素の周囲のすでに２値化された画素から拡散された注目画素累積誤差Ｅｓ（ｙ）を加算する。これにより平均濃度が保たれる。
【００４８】
次に、比較器３ａにより、この加算値Ｄｉｎ１（ｙ）を閾値Ｔｒｅｓ と比較する。そしてＤｉｎ１（ｙ）≧Ｔｒｅｓ のときには２値データＤｏｕｔ（ｙ）＝１を出力し、Ｄｉｎ１（ｙ）＜Ｔｒｅｓ のときには２値データＤｏｕｔ（ｙ）＝０を出力する。
【００４９】
こうして、注目画素のイエロー（Ｙ）の２値データが得られたことになる。 マゼンタ（Ｍ）及びシアン（Ｃ）の入力多階調データＤｉｎ０（ｍ）、Ｄｉｎ０（ｃ）の２値化についても同様の処理を行う。
【００５０】
次に誤差の計算を行う。イエロー（Ｙ）、マゼンタ（Ｍ）、シアン（Ｃ）の２値化データは、それぞれ２値データメモリ５ａ〜５ｃに記憶される。すでに２値化されている周囲４画素と今回２値化した注目画素の合計５画素の各色の２値データを色別α設定用ＬＵＴ６に入力し、各色に対応する誤差調整係数αｙ、αｍ、αｃを得る。
【００５１】
２値データＤｏｕｔ（ｙ）が１の場合は、条件付き加算器７ａにて最大入力階調値Ｄｍａｘ （２５５）に誤差拡散係数αｙを加算する。また、２値データＤｏｕｔ（ｙ）が０の場合は、条件付き加算器７ａから０を出力する。
【００５２】
２値データＤｏｕｔ（ｍ）が１の場合は、条件付き加算器７ｂにて最大入力階調値Ｄｍａｘ （２５５）に誤差拡散係数αｍを加算する。また、２値データＤｏｕｔ（ｍ）が０の場合は、条件付き加算器７ｂから０を出力する。
【００５３】
２値データＤｏｕｔ（ｃ）が１の場合は、条件付き加算器７ｃにて最大入力階調値Ｄｍａｘ （２５５）に誤差拡散係数αｃを加算する。また、２値データＤｏｕｔ（ｃ）が０の場合は、条件付き加算器７ｃから０を出力する。
【００５４】
そして、減算器８ａにおいて加算値Ｄｉｎ１（ｙ）から条件付き加算器７ａの出力を減算してイエローの２値化誤差Ｅｒｒ（ｙ） を得る。また、減算器８ｂにおいて加算値Ｄｉｎ１（ｍ）から条件付き加算器７ｂの出力を減算してマゼンタの２値化誤差Ｅｒｒ（ｍ） を得る。また、減算器８ｃにおいて加算値Ｄｉｎ１（ｃ）から条件付き加算器７ｃの出力を減算してシアンの２値化誤差Ｅｒｒ（ｃ） を得る。
【００５５】
これら求められた２値化誤差Ｅｒｒ（ｙ） 、Ｅｒｒ（ｍ） 、Ｅｒｒ（ｃ） から誤差拡散ブロック９ａ〜９ｃにて２値化していない周囲４画素に所定の拡散係数を乗算されて拡散誤差Ｅｉ（ｙ）、Ｅｉ（ｍ）、Ｅｉ（ｃ）が求められ、誤差累積メモリ２ａ〜２ｃの所定の位置に累積される。
【００５６】
こうして１画素分の処理が終了する。次の入力画素に対しても同様の処理を行い、これを繰り返すことで入力多階調画像データの２値化を行う。
【００５７】
誤差調整係数αは、次のように定められる。先ず、イエロー（Ｙ）成分に関しては図４に示した現象を考慮し、各印字ドット配置における注目画素の印字ドット面積（Ｓｂ）及び重複する面積（Ｓｃ）を実測又はシュミレーションにより求め、上述した（２） 式より表２に示すようなデータを設定したα設定用ＬＵＴ６ａを求める。
【００５８】
次にマゼンタ（Ｍ）成分に関しても同様に考えてα設定用ＬＵＴ６ｂを求める。但し、マゼンタ（Ｍ）成分の場合には、周囲画素及び注目画素のイエロー（Ｙ）のドットの有無に応じてマゼンタ（Ｍ）のドットの面積が変化することを考慮して、イエロー（Ｙ）及びマゼンタ（Ｍ）のドットの配置に応じた値を実験又はシュミレーションにより求める。
【００５９】
マゼンタ（Ｍ）の誤差調整係数αｍを実測値に基づいて求める例を図５に示す。図５の（ａ） はイエロー（Ｙ）の印字ドット単独の場合で、面積比Ｓｂｙは１．５７となる。図５の（ｂ） はマゼンタ（Ｍ）の印字ドット単独の場合で、面積比Ｓｂｍは１．９０となる。このようにインクの色によりドット面積が異なることもある。
【００６０】
図５の（ｃ） はイエロー（Ｙ）の印字ドットの上にマゼンタ（Ｍ）の印字ドットが重なった場合で、イエロー（Ｙ）の印字ドットのためにマゼンタ（Ｍ）の印字ドットが大きくなり面積が増加する。すなわち、面積比Ｓｂｍは２．１０となる。
【００６１】
図５の（ｄ） はイエロー（Ｙ）の印字ドットが２つ隣接して印字された場合の一方のイエロー（Ｙ）の印字ドットの上にマゼンタ（Ｍ）の印字ドットが重なった場合で、この場合もイエロー（Ｙ）の印字ドットのためにマゼンタ（Ｍ）の印字ドットが大きくなるがイエロー（Ｙ）単独の場合よりもさらに面積が増加する。すなわち、面積比Ｓｂｍは２．１５となる。
【００６２】
このようにマゼンタ（Ｍ）の１ドット印字でも、イエロー（Ｙ）の印字ドットの配置によって印字面積が変化する。
【００６３】
このため個々に面積を測定して面積比Ｓｂｍを求め、上述した（２） 式よりイエロー（Ｙ）の印字ドットの配置毎に誤差調整係数αｍを求める。
【００６４】
また、図５の（ｅ） はマゼンタ（Ｍ）のドットを２つ隣接して印字した場合で、この場合、マゼンタ（Ｍ）の印字ドットの面積比Ｓｂｍは１．９０となり、また、重なった領域の面積比Ｓｃｍは０．４６となる。
【００６５】
また、図５の（ｆ） はマゼンタ（Ｍ）のドットを２つ隣接して印字し、しかも下にすでにイエロー（Ｙ）のドットがある場合で、この場合、マゼンタ（Ｍ）の印字ドットの面積比Ｓｂｍはさらに大きくなって２．１０となり、また、重なった領域の面積比Ｓｃｍも大きくなって０．７５となる。
【００６６】
この場合も同様に個々に面積を測定して面積比Ｓｂｍ、Ｓｃｍを求め、上述した（２） 式よりイエロー（Ｙ）の印字ドットの配置毎に誤差調整係数αｍを求める。
【００６７】
このように、イエロー（Ｙ）ドットの配置及びマゼンタ（Ｍ）ドットの配置に応じて印字面積が変化するため、全ての配置に対する誤差調整係数αを実測又はシュミレーションなどにより、（２） 式を用いて求める。これはシアン（Ｃ）の成分についても同様である。
【００６８】
このような方法により表３に示す内容でα設定用ＬＵＴ６ｂを設定でき、また表４に示す内容でα設定用ＬＵＴ６ｃを設定できる。
【００６９】
【表３】

【００７０】
【表４】

【００７１】
このように、マゼンタ（Ｍ）及びシアン（Ｃ）の誤差調整係数αｍ、αｃを定める際に、その下にあるすでに印字された他色のドット群の配置の影響を考慮することにより、今回印字する注目画素のドットのサイズ変化による画質への影響を精度よく補正できることになる。
【００７２】
以上の処理を行うことで、イエロー（Ｃ）、マゼンタ（Ｍ）、シアン（Ｃ）それぞれのドット形状に応じた補正ができ、ドット形状の理論値とのずれによる印字濃度の変化や色調のずれを防ぐことができる。
【００７３】
従って、このようなカラー疑似中間調処理を採用することにより、印字結果として高画質なカラー画像を安定して得ることができる。
【００７４】
なお、前記実施例ではイエロー（Ｃ）、マゼンタ（Ｍ）、シアン（Ｃ）それぞれ専用の処理ブロックを設けて並列処理を行う構成としたが、必ずしもこれに限定するものではなく、比較器３ａ〜３ｃ、条件付き加算器７ａ〜７ｃ及び誤差拡散ブロック９ａ〜９ｃは１つの比較器、条件付き加算器及び誤差拡散ブロックに共通化してもよい。この場合はイエロー（Ｃ）、マゼンタ（Ｍ）、シアン（Ｃ）について逐次処理することになるので処理速度は低下するが回路規模を小さくできる。
【００７５】
また、前記実施例は２値カラーインクジェットプリンタのドット特性を使用して説明したが、この発明はインクリボンを使用した溶融転写型カラープリンタなどにも適用できる。
【００７６】
溶融転写型カラープリンタでは単色印字では図６に示すような転写特性になることが多い。図６の（ａ） は周囲画素Ａ，Ｂ，Ｃ，Ｄがすべて０のときの注目画素の印字ドットを示し、図６の（ｂ） は周囲画素Ａ，Ｃ，Ｄが０でＢが１のときの注目画素の印字ドットを示し、図６の（ｃ） は周囲画素Ｂ，Ｃ，Ｄが０でＡが１のときの注目画素の印字ドットを示し、図６の（ｄ） は周囲画素Ａ，Ｂ，Ｄが１でＣが０のときの注目画素の印字ドットを示している。
【００７７】
このように溶融転写型カラープリンタでも周囲画素の印字状態により、注目画素の転写面積が異なる特性を持つ。
【００７８】
また、２色以上の重ね合わせ印字が発生した場合には、例えば注目画素にすでにイエロー（Ｙ）の印字ドットがあり、その上にマゼンタ（Ｍ）を重ね印字する場合に、マゼンタ（Ｍ）を図６の（ａ） 〜（ｄ） に示すパターンと同じパターンに印字すると、図７の（ａ） 〜（ｄ） に示すようになる。この場合は、イエロー（Ｙ）ドットの境界部のインクの厚さ分の段差により、その上に転写されるマゼンタ（Ｍ）ドットは図６とは異なる転写面積となる。
【００７９】
また、図７はイエロー（Ｙ）ドットを注目画素のみ印字している場合を示しているが、イエロー（Ｙ）ドットが周囲画素にもある場合はイエロー（Ｙ）ドット自体が周囲画素の印字状況によりサイズが異なるため、その上に印字するマゼンタ（Ｍ）のドットサイズは図７とは異なったものとなる。
【００８０】
従って、溶融転写型カラープリンタの場合も前記実施例と同様の処理を行うことで、すでに印字した他色も含めた注目ドット及び周囲ドットの状態まで考慮して補正ができ、これにより平均濃度が正しく表わされ、高画質のカラー画像が得られる。
【００８１】
なお、この場合、誤差調整係数αはインクジェットプリンタの場合とは異なる値を実測などにより設定することになるのは当然である。また、誤差調整係数αは正の値のみでなく負の値の場合もある。
【００８２】
また、前記実施例は誤差を拡散する画素を４画素としたが必ずしもこれに限定するものでないのは勿論である。
【００８３】
また、前記実施例ではカラーの組み合わせをイエロー、マゼンタ、シアンの３色の場合について述べたが必ずしもこれに限定するものではなく、さらにブラックを加えるなど３色以外の組み合わせでもよく、また印字する各色の順序もイエロー、マゼンタ、シアンの順序に限定するものではない。
【００８４】
なお、本発明の装置は実施例に示すようなハードウエアでも、またマイクロコンピュータを使用したソフトウエアでも構成することができる。また、本発明は２値カラープリンタ一般に適用できるものである。
【００８５】
【発明の効果】
以上、請求項１対応の発明によれば、２値化処理済みの他色も含めた周囲画素と注目画素に対応する２値化処理済みの他色の画素のデータに基づいて各色毎の誤差調整係数αを決定し、この誤差調整係数αを利用して２値化誤差Ｅｒｒを決定して誤差拡散を行い、この拡散誤差を各画素毎に累積した注目画素累積誤差Ｅｓ を多階調画像データＤｉｎ０ に加算してから閾値と比較して２値データを得るようにしているので、濃度のずれや色調のずれが生じるのを防止でき高画質なカラー画像を安定して得ることができるカラー疑似中間調処理ができる。
【図面の簡単な説明】
【図１】本発明の実施例を示すブロック図。
【図２】同実施例の色別α設定用ＬＵＴの構成を示すブロック図。
【図３】２値カラーインクジェットプリンタで１ドットを印字した場合の各色のドットの大きさを示す図。
【図４】周囲ドットを含めたイエローインクのドット印字状態を示す図。
【図５】同実施例においてマゼンタの誤差調整係数を実測値に基づいて求める例を示す図。
【図６】本発明の他の実施例を示す溶融転写型カラープリンタの単色印字での転写特性を説明するための図。
【図７】同実施例の溶融転写型カラープリンタの２色重ね合わせ印字での転写特性を説明するための図。
【符号の説明】
１ａ〜１ｃ…加算器
２ａ〜２ｃ…誤差累積メモリ
３ａ〜３ｃ…比較器
４…閾値発生器
５ａ〜５ｃ…２値データメモリ
６…色別α設定用ＬＵＴ
７ａ〜７ｃ…条件付き加算器
８ａ〜８ｃ…減算器
９ａ〜９ｃ…誤差拡散ブロック[0001]
[Industrial applications]
The present invention relates to a color pseudo-halftone processing device that performs pseudo-halftone processing on multi-tone color image data to binarize it.
[0002]
[Prior art]
An error diffusion method is known as a main method of the pseudo halftone processing. This is because the pixel data is binarized by comparing it with a threshold value, and then the difference between the value obtained by converting the binarized value into multiple gradations (0 and the maximum input gradation value 255) and the original pixel data is used as an error. In this method, the image data is diffused to surrounding pixels that have not been binarized, and the overall density is stored.
[0003]
Such an error diffusion method can achieve high-quality binarization because the resolution is high and the density is preserved.
[0004]
However, when actual printing is performed, dots to be printed have a shape different from a theoretically conceivable rectangle. For example, in an ink jet printer or the like, dots to be actually printed are circular, and a theoretical rectangle is included in the circle, so that the area is considerably larger than the theoretical area. On the other hand, if the dot shape is reduced, a gap is formed between adjacent dots, and the dot cannot be filled.
[0005]
On the other hand, in a fusion transfer printer or the like using an ink ribbon, a single dot is unlikely to be transferred, so the actual dot shape is smaller than the theoretical shape.
[0006]
Due to the phenomenon that the printed dot area is different from the theoretical value, the density that should be theoretically stored is not stored correctly, and the actual dot size becomes large on a large printer. There is a problem that a printer having a small shape has a low density.
[0007]
As a method for solving this problem, for example, a method described in Japanese Patent Laid-Open No. 4-150272 is known.
[0008]
This solves the above problem by correcting the error data using the difference between the dot shape actually printed and the theoretical shape.
[0009]
[Problems to be solved by the invention]
However, in a color printer, the method described in the above-mentioned publication does not sufficiently solve the problem.
[0010]
2. Description of the Related Art In a color binary printer, a plurality of color materials such as yellow (Y), magenta (M), and cyan (C) are generally superposed and arranged for each pixel in order to express a color. In this case, there is a problem of a dot size for each color. However, there is a further problem that the dot size changes due to an influence of a printing position and other color materials already printed around the printing position.
[0011]
For example, in a color ink jet printer, the ink becomes less likely to permeate where another ink has already been printed, so that when printing is performed on top of that, the dots become larger. For this reason, when printing in the order of yellow (Y), magenta (M), and cyan (C), the dot size of magenta (M) depends on whether or not a yellow (Y) dot is printed below it. Will change. The same applies to cyan (C), which changes depending on whether yellow (Y) or magenta (M) dots are printed below.
[0012]
Further, in the case of a fusion transfer type color printer, it is very difficult to make the positional deviation zero, and an upper dot is likely to be printed at the edge of the lower dot. In this case, a problem occurs in that the upper ink is not transferred to the periphery of this portion due to the effect of the step of the ink thickness at the edge portion of the lower dot, and the dot size is reduced. For this reason, the magenta (M) and cyan (C) dot sizes change depending on whether or not to perform overprinting.
[0013]
Further, in a fusion transfer type color printer, the dot size changes depending on the presence or absence of surrounding pixels. Therefore, when printing magenta (M), the presence or absence of dots of pixels around the target pixel of magenta (M) is of course required. The presence / absence of a yellow (Y) dot in the target pixel and the presence / absence of a yellow (Y) dot in surrounding pixels also have an effect.
[0014]
Accordingly, the present invention provides a color pseudo-halftone processing apparatus capable of performing color pseudo-halftone processing capable of preventing a shift in density and a shift in color tone and stably obtaining a high-quality color image.
[0015]
[Means for Solving the Problems]
According to a first aspect of the present invention, there is provided a color pseudo halftone processing apparatus for binarizing color multi-tone image data, wherein a target pixel accumulated error Es from surrounding pixels for each color is added to multi-tone image data Din0 of each color. Adding means for obtaining an added value Din1 by performing the above operation, and comparing means for comparing the added value Din1 of each color from the adding means with a threshold Tres, and outputting binary data Dout = 1 when Din1 ≧ Tres for each color; Binarization means for outputting binary data Dout = 0 when Din1 <Tres, and surrounding pixels including other binarized colors and pixels of the binarized other colors corresponding to the target pixel. Error adjustment coefficient determining means for determining an error adjustment coefficient α for each color based on data; and a maximum value Dmax of input multi-tone image data Din0 and an error for each color from the error adjustment coefficient determination means. When the binary data Dout from the binarizing means is 1 for each color based on the integer coefficient α, the binarization error Err is determined by the operation of Din1− (Dmax + α), and the binary data Dout is 0. At this time, a binarization error determining unit that determines a binarization error Err by setting Din1 = Err, and a binarization error Err determined by the binarization error determination unit is multiplied by a predetermined diffusion coefficient for each color. And error accumulation means for accumulating the diffusion error from the error diffusion means for each pixel, and a target pixel accumulation error Es from the error accumulation means. Is what you get.
[0016]
[Action]
In the invention corresponding to claim 1, the binarizing means compares an added value Din1 obtained by adding the pixel-of-interest cumulative error Es from surrounding pixels of each color to the multi-tone image data Din0 of each color with a threshold Tres. When Din1 ≧ Tres, binary data Dout = 1 is output for each color, and when Din1 <Tres, binary data Dout = 0 is output.
[0017]
On the other hand, the error adjustment coefficient determination means determines the error adjustment coefficient α for each color based on data of surrounding pixels including the binarized other color and pixels of the binarized other color corresponding to the target pixel. When the binary data Dout is 1 for each color based on the error adjustment coefficient α and the maximum value Dmax of the input multi-tone image data Din0, the binarization error determination means determines Din1− (Dmax + α). ), The binarization error Err is determined, and when the binary data Dout is 0, the binarization error Err is determined as Din1 = Err. Then, the error diffusion means multiplies the determined binarization error Err by a predetermined diffusion coefficient for each color, and then diffuses it to surrounding pixels that have not been binarized as a diffusion error, and accumulates the error accumulation means for each pixel.
[0018]
The binarizing means obtains the pixel-of-interest cumulative error Es to be added to the multi-tone image data Din0 from the error accumulating means.
[0019]
【Example】
Hereinafter, embodiments of the present invention will be described with reference to the drawings. In this embodiment, a description will be given of an example in which three-color input multi-tone image data of yellow (Y), magenta (M), and cyan (C) are each subjected to pseudo halftone processing into binary data of each color.
[0020]
In FIG. 1, reference numerals 1a, 1b, and 1c denote adders, respectively. The adder 1a includes input multi-gradation data Din0 (y) for yellow (Y) and a target pixel cumulative error Es from a yellow (Y) error cumulative memory 2a. (Y) is added to output an addition value Din1 (y), and the adder 1b calculates the target pixel accumulation from the input multi-gradation data Din0 (m) of magenta (M) and the error accumulation memory 2b of magenta (M). The error Es (m) is added to output an added value Din1 (m). The adder 1c receives the input multi-gradation data Din0 (c) of cyan (C) and the error accumulation memory 2c of cyan (C). The pixel accumulated error Es (c) is added to output an added value Din1 (c).
[0021]
The addition values Din1 (y), Din1 (m), and Din1 (c) from the adders 1a to 1c are supplied to comparators 3a, 3b, and 3c, respectively. A threshold Tres is input from the threshold generator 4 to each of the comparators 3a to 3c.
[0022]
The threshold value generator 4 generates a threshold value for binarization, and is, for example, 値 of the maximum value Dmax of the input multi-tone data Din0.
[0023]
The comparators 3a to 3c compare the added values Din1 (y), Din1 (m), and Din1 (c) with the threshold Tres, respectively, and binary data Dout (y), Dout (m), and Dout (c). The comparator 3a outputs binary data Dout (y) = 1 when Din1 (y) ≧ Tres, and outputs binary data Dout (y) = 0 when Din1 (y) <Tres. The comparator 3b outputs binary data Dout (m) = 1 when Din1 (m) ≧ Tres, and outputs binary data Dout (m) = 0 when Din1 (m) <Tres. 3c outputs binary data Dout (c) = 1 when Din1 (c) ≧ Tres, and outputs binary data Dout (c) = 0 when Din1 (c) <Tres.
[0024]
The adders 1a to 1c, the comparators 3a to 3c and the threshold generator 4 constitute a binarizing means.
[0025]
Reference numerals 5a, 5b, and 5c denote binary data memories corresponding to the respective colors of yellow (Y), magenta (M), and cyan (C), and the binary data memories 5a to 5c store binary data for each color. The memory for the line is stored.
[0026]
Reference numeral 6 denotes a color-specific α setting LUT. The color-specific α setting LUT 6 is an error adjustment coefficient αi (i = y, i = y, i) for each color from the binarized peripheral pixels of each color of the target pixel and the binary data of the target pixel. m, c) is a look-up table.
[0027]
The binary data memories 5a to 5c and the color-specific α setting LUT 6 constitute error adjustment coefficient determining means.
[0028]
As shown in FIG. 2, the LUT 6 for α for each color includes LUTs 6a, 6b, and 6c for α of yellow (Y), magenta (M), and cyan (C), and the LUT 6a for α is In the 4-bit input look-up table for setting the error adjustment coefficient αy from the data of the surrounding four pixels that have been binarized for the yellow component, the α setting LUT 6b has the binarized 5-pixel data for the yellow component (surroundings). A 9-bit input look-up table for setting an error adjustment coefficient αm from the data of the four pixels surrounding the four pixels and the target pixel) and the magenta component. The α setting LUT 6c has a yellow component and a magenta component. A 14-bit input look that sets the error adjustment coefficient αc from the binarized 5-pixel data (surrounding 4 pixels and the target pixel) and the binarized surrounding 4-pixel data of the cyan component・ It is an up table. Although the pixel data from the binary data memories 5a to 5c are all five pixel data, the pixel data corresponding to the target pixel is not used for the same color as that of the pixel data. , That is, 4 bits.
[0029]
The binary data Dout (y), Dout (m) and Dout (c) from the comparators 3a to 3c are supplied to conditional adders 7a, 7b and 7c, respectively. The error adjustment coefficients αy, αm, and αc from the color-specific α setting LUT 6 are supplied to the conditional adders 7a, 7b, and 7c, respectively.
[0030]
When the binary data Dout (y), Dout (m), Dout (c) is "1", the conditional adders 7a, 7b, 7c provide a maximum value Dmax of input multi-tone data and an error adjustment coefficient. A value obtained by adding αy, αm, and αc is output, and when the binary data Dout (y), Dout (m), Dout (c) is “0”, a value “0” is output. The outputs are supplied to subtracters 8a, 8b, 8c, respectively.
[0031]
The subtracters 8a to 8c subtract output values from the conditional adders 7a, 7b and 7c from addition values Din1 (y), Din1 (m) and Din1 (c) from the adders 1a to 1c, respectively. Then, binarization errors Err (y), Err (m), and Err (c) are obtained, and are supplied to error diffusion blocks 9a, 9b, and 9c, respectively, which are error diffusion means.
[0032]
The conditional adders 7a to 7c and the subtractors 8a to 8c constitute a binarization error determination unit.
[0033]
The error diffusion blocks 9a, 9b and 9c respectively convert the binarization errors Err (y), Err (m) and Err (c) from the subtracters 8a to 8c into four pixels around the target pixel which are not binarized. Is multiplied by a predetermined diffusion coefficient to output diffusion errors Ei (y), Ei (m), and Ei (c). (However, i = 1, 2, 3, 4)
The error accumulation memories 2a to 2c accumulate diffusion errors Ei (y), Ei (m) and Ei (c) from the error diffusion blocks 9a, 9b and 9c at predetermined positions.
[0034]
FIG. 3 shows an example of the size of a dot of each color when one dot is printed by a binary color ink jet printer. A rectangular area indicated by oblique lines is an ideal shape of one dot. In an inkjet printer, the dots are circular. For complete filling, the dots need to be circular, containing the theoretical shape. Further, the dot shape differs for each color depending on the characteristics of the ink.
[0035]
As shown in Table 1, when the area Sa of the theoretical shape is 1, the area ratio Sb of the yellow print dots is 1.57, the area ratio Sb of the magenta print dots is 1.90, and the area of the cyan print dots is 1. The ratio Sb becomes 2.20. Then, when only one dot of each color is considered, the error adjustment coefficients αy, αm, and αc are αy = 145, αm = 230, and αc = 306, respectively.
[0036]
The error adjustment coefficients αy, αm, and αc are obtained from the following equation (1).
α = Dmax · (Sb−1) (1)
However, Dmax = 255
For example, when the measured print dot area ratio is Sb = 2.0, it indicates that printing of one dot results in printing of one dot extra, that is, printing of an area of two dots. The value of the error adjustment coefficient α in this case is Dmax · (Sb−1) = 255 · (2-1), which is 255.
[0037]
[Table 1]

[0038]
FIG. 4 shows a dot printing state of yellow ink including surrounding dots. A rectangular area indicated by oblique lines in the drawing has a theoretical area Sa = 1 of the target pixel, and a circular area is an actual print dot area. It can be seen from the figure that the area overlapping the dot of the target pixel differs depending on the arrangement of the dots to be printed.
[0039]
Table 2 shows an example of the error adjustment coefficient α when printing the target pixel according to how the surrounding pixels A, B, C, and D around the target pixel are printed. 4 (a) to (P) in FIG.
[0040]
α = Dmax · {(Sb−Sc) −1} (2)
Here, Sc is the area ratio of the overlapping area to the theoretical area Sa.
[0041]
That is, in the case of FIG. 4A where all the surrounding pixels A, B, C, and D are not printed, the error adjustment coefficient α is 145. In addition, the error adjustment coefficient α is 145 even when the surrounding pixels are printed and there is no overlapping portion with the target pixel as in (c), (i), and (k) of FIG.
[0042]
In addition, as shown in FIG. 4, (b), (d), (e), (g), (j), (l), (m), and (o), there is one region where the target pixel and surrounding pixels overlap. In this case, the error adjustment coefficient α = 73. Further, when there are two regions where the target pixel and the surrounding pixels overlap, as in (f), (h), (n), and (p) of FIG. 4, the error adjustment coefficient α = 0.
[0043]
By subtracting the area of the overlapping area from the print dot area of the target pixel in this way, overcorrection can be prevented, and correction can be performed in consideration of the state of surrounding dots.
[0044]
[Table 2]

[0045]
For a yellow (Y) dot to be printed first on paper, the error adjustment coefficient αy can be set by this method. That is, the α setting LUT 6a can be set with the contents shown in Table 2.
[0046]
However, magenta (M) and cyan (C) dots for the second and subsequent colors need to be further corrected.
[0047]
First, the binarization of the input multi-gradation data Din0 (y) for yellow (Y) will be described. When one pixel of the input multi-tone data Din0 (y) is input, the adder 1a adds the target pixel accumulated error Es (y) diffused from the already binarized pixels around this pixel to the adder 1a. to add. This keeps the average density.
[0048]
Next, the comparator 3a compares the sum Din1 (y) with a threshold Tres. When Din1 (y) ≧ Tres, the binary data Dout (y) = 1 is output, and when Din1 (y) <Tres, the binary data Dout (y) = 0 is output.
[0049]
Thus, binary data of yellow (Y) of the pixel of interest is obtained. Similar processing is performed for the binarization of the input multi-gradation data Din0 (m) and Din0 (c) for magenta (M) and cyan (C).
[0050]
Next, an error is calculated. The binary data of yellow (Y), magenta (M), and cyan (C) are stored in the binary data memories 5a to 5c, respectively. The binary data of each color of a total of 5 pixels, that is, the surrounding 4 pixels that have already been binarized and the target pixel that has been binarized this time, are input to the color-specific α setting LUT 6, and error adjustment coefficients αy, αm, Obtain αc.
[0051]
When the binary data Dout (y) is 1, the error diffusion coefficient αy is added to the maximum input tone value Dmax (255) by the conditional adder 7a. When the binary data Dout (y) is 0, the conditional adder 7a outputs 0.
[0052]
When the binary data Dout (m) is 1, the error diffusion coefficient αm is added to the maximum input gradation value Dmax (255) by the conditional adder 7b. If the binary data Dout (m) is 0, the conditional adder 7b outputs 0.
[0053]
When the binary data Dout (c) is 1, the error diffusion coefficient αc is added to the maximum input gradation value Dmax (255) by the conditional adder 7c. When the binary data Dout (c) is 0, the conditional adder 7c outputs 0.
[0054]
Then, the subtracter 8a subtracts the output of the conditional adder 7a from the addition value Din1 (y) to obtain a binary error Err (y) of yellow. The subtracter 8b subtracts the output of the conditional adder 7b from the addition value Din1 (m) to obtain a magenta binarization error Err (m). Further, the subtracter 8c subtracts the output of the conditional adder 7c from the addition value Din1 (c) to obtain the binarization error Err (c) of cyan.
[0055]
From the obtained binarization errors Err (y), Err (m), and Err (c), the surrounding four pixels that have not been binarized by the error diffusion blocks 9a to 9c are multiplied by a predetermined diffusion coefficient to obtain a diffusion error. Ei (y), Ei (m), and Ei (c) are obtained and accumulated at predetermined positions in the error accumulation memories 2a to 2c.
[0056]
Thus, the processing for one pixel is completed. The same processing is performed on the next input pixel, and by repeating this processing, binarization of the input multi-tone image data is performed.
[0057]
The error adjustment coefficient α is determined as follows. First, regarding the yellow (Y) component, the print dot area (Sb) and the overlapping area (Sc) of the target pixel in each print dot arrangement are obtained by actual measurement or simulation in consideration of the phenomenon shown in FIG. 2) The α setting LUT 6a in which data as shown in Table 2 are set is obtained from the equation.
[0058]
Next, the LUT 6b for α setting is obtained in the same manner for the magenta (M) component. However, in the case of the magenta (M) component, considering that the area of the magenta (M) dot changes according to the presence or absence of the yellow (Y) dot of the surrounding pixel and the target pixel, the yellow (Y) is considered. And a value corresponding to the arrangement of magenta (M) dots is obtained by experiment or simulation.
[0059]
FIG. 5 shows an example of calculating the error adjustment coefficient αm of magenta (M) based on the actually measured value. FIG. 5A shows the case of yellow (Y) print dots alone, and the area ratio Sby is 1.57. FIG. 5B shows the case of printing dots of magenta (M) alone, and the area ratio Sbm is 1.90. As described above, the dot area may be different depending on the color of the ink.
[0060]
FIG. 5C shows a case in which the magenta (M) print dot overlaps the yellow (Y) print dot, and the magenta (M) print dot becomes large because of the yellow (Y) print dot. The area increases. That is, the area ratio Sbm is 2.10.
[0061]
FIG. 5D shows a case where a magenta (M) print dot overlaps one yellow (Y) print dot when two yellow (Y) print dots are printed adjacent to each other. In this case as well, the print dots of magenta (M) become large due to the print dots of yellow (Y), but the area is further increased as compared with the case of yellow (Y) alone. That is, the area ratio Sbm is 2.15.
[0062]
As described above, even in the printing of one dot of magenta (M), the printing area changes depending on the arrangement of the printing dots of yellow (Y).
[0063]
For this reason, the areas are individually measured to determine the area ratio Sbm, and the error adjustment coefficient αm is determined for each arrangement of the yellow (Y) print dots from the above equation (2).
[0064]
FIG. 5E shows a case in which two magenta (M) dots are printed adjacent to each other. In this case, the area ratio Sbm of the magenta (M) print dots is 1.90, and the dots overlap. The area ratio Scm of the region is 0.46.
[0065]
FIG. 5F shows a case in which two magenta (M) dots are printed adjacent to each other, and there is already a yellow (Y) dot below. In this case, the magenta (M) print dot The area ratio Sbm further increases to 2.10, and the area ratio Scm of the overlapping region also increases to 0.75.
[0066]
Also in this case, similarly, the areas are individually measured to determine the area ratios Sbm and Scm, and the error adjustment coefficient αm is determined for each arrangement of the yellow (Y) print dots from the above equation (2).
[0067]
As described above, since the printing area changes in accordance with the arrangement of the yellow (Y) dots and the arrangement of the magenta (M) dots, the error adjustment coefficient α for all the arrangements is obtained by actually measuring or simulating the equation (2). Ask. This is the same for the cyan (C) component.
[0068]
With this method, the α setting LUT 6b can be set with the contents shown in Table 3, and the α setting LUT 6c can be set with the contents shown in Table 4.
[0069]
[Table 3]

[0070]
[Table 4]

[0071]
As described above, when determining the error adjustment coefficients αm and αc of the magenta (M) and cyan (C), the influence of the arrangement of the dot group of another color already printed under the magenta (M) and the cyan (C) is taken into consideration. The effect on the image quality due to the change in the dot size of the pixel of interest can be accurately corrected.
[0072]
By performing the above-described processing, it is possible to perform correction in accordance with the dot shapes of yellow (C), magenta (M), and cyan (C), and change in print density and color tone due to deviation from the theoretical value of the dot shape. Can be prevented.
[0073]
Therefore, by employing such color pseudo halftone processing, a high-quality color image can be stably obtained as a printing result.
[0074]
In the above-described embodiment, a configuration is adopted in which dedicated processing blocks are provided for each of yellow (C), magenta (M), and cyan (C) to perform parallel processing. However, the present invention is not necessarily limited to this configuration. 3c, the conditional adders 7a to 7c and the error diffusion blocks 9a to 9c may be shared by one comparator, the conditional adder and the error diffusion block. In this case, yellow (C), magenta (M), and cyan (C) are sequentially processed, so that the processing speed is reduced, but the circuit scale can be reduced.
[0075]
Although the above embodiment has been described using the dot characteristics of a binary color ink jet printer, the present invention can also be applied to a fusion transfer type color printer using an ink ribbon.
[0076]
In the case of a fusion transfer type color printer, transfer characteristics as shown in FIG. FIG. 6A shows a print dot of the target pixel when all the surrounding pixels A, B, C, and D are 0, and FIG. 6B shows a case where the surrounding pixels A, C, and D are 0 and B is 1. FIG. 6C shows the print dot of the target pixel when the surrounding pixels B, C, and D are 0 and A is 1, and FIG. The print dots of the target pixel when pixels A, B, and D are 1 and C is 0 are shown.
[0077]
As described above, even in the fusion transfer type color printer, the transfer area of the target pixel is different depending on the printing state of the surrounding pixels.
[0078]
Further, when superimposition printing of two or more colors occurs, for example, the target pixel already has a printing dot of yellow (Y), and when magenta (M) is superimposed and printed thereon, magenta (M) is displayed. When printing is performed in the same pattern as the patterns shown in FIGS. 6A to 6D, the patterns shown in FIGS. 7A to 7D are obtained. In this case, the magenta (M) dot transferred thereon has a transfer area different from that in FIG. 6 due to the step corresponding to the thickness of the ink at the boundary between the yellow (Y) dots.
[0079]
FIG. 7 shows the case where only the target pixel is printed with the yellow (Y) dot. However, when the yellow (Y) dot is also present in the surrounding pixels, the yellow (Y) dot itself is printed in the surrounding pixels. , The dot size of magenta (M) printed thereon is different from that of FIG.
[0080]
Therefore, even in the case of a fusion transfer type color printer, by performing the same processing as in the above-described embodiment, it is possible to perform a correction in consideration of the state of the attention dot including the other colors already printed and the surrounding dots, thereby reducing the average density. A correctly expressed and high quality color image is obtained.
[0081]
In this case, it is natural that the error adjustment coefficient α is set to a value different from that of the ink jet printer by actual measurement or the like. Further, the error adjustment coefficient α may be not only a positive value but also a negative value.
[0082]
In the above-described embodiment, the number of pixels for diffusing an error is four, but it is a matter of course that the present invention is not limited to this.
[0083]
Further, in the above-described embodiment, the case where the color combination is three colors of yellow, magenta, and cyan is described. However, the present invention is not necessarily limited to this, and a combination other than three colors such as adding black may be used. Is not limited to the order of yellow, magenta, and cyan.
[0084]
The device of the present invention can be constituted by hardware as shown in the embodiment or by software using a microcomputer. The present invention can be applied to a binary color printer in general.
[0085]
【The invention's effect】
As described above, according to the first aspect of the present invention, the error for each color is determined based on the data of the surrounding pixels including the binarized other color and the data of the binarized other color pixel corresponding to the target pixel. An adjustment coefficient α is determined, a binarization error Err is determined using the error adjustment coefficient α, error diffusion is performed, and a target pixel accumulated error Es obtained by accumulating the diffusion errors for each pixel is represented by a multi-tone image. Since binary data is obtained by adding the data to the data Din0 and comparing with a threshold value, it is possible to prevent a shift in density and a shift in color tone from occurring and to stably obtain a high-quality color image. Pseudo halftone processing can be performed.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an embodiment of the present invention.
FIG. 2 is a block diagram showing a configuration of a color-specific α setting LUT of the embodiment.
FIG. 3 is a diagram illustrating the size of dots of each color when one dot is printed by a binary color inkjet printer.
FIG. 4 is a diagram illustrating a dot printing state of yellow ink including surrounding dots.
FIG. 5 is a diagram showing an example in which a magenta error adjustment coefficient is obtained based on an actually measured value in the embodiment.
FIG. 6 is a view for explaining transfer characteristics in single-color printing of a fusion transfer type color printer showing another embodiment of the present invention.
FIG. 7 is a view for explaining transfer characteristics in two-color superposition printing of the fusion transfer type color printer of the embodiment.
[Explanation of symbols]
1a to 1c ... adder
2a to 2c: Error accumulation memory
3a-3c ... Comparator
4: Threshold generator
5a to 5c ... binary data memory
6 ... LUT for alpha setting for each color
7a to 7c: Conditional adder
8a to 8c: subtractor
9a to 9c: error diffusion block

Claims (1)

In a color pseudo halftone processing device for binarizing color multi-tone image data,
Adding means for adding the pixel-of-interest error Es from the surrounding pixels of each color to the multi-tone image data Din0 of each color to obtain an added value Din1, and comparing the added value Din1 of each color from the adding means with a threshold Tres A binarizing unit that includes a comparing unit, outputs binary data Dout = 1 when Din1 ≧ Tres for each color, and outputs binary data Dout = 0 when Din1 <Tres; Error adjustment coefficient determination means for determining an error adjustment coefficient α for each color based on data of pixels of other colors subjected to binarization processing corresponding to surrounding pixels including the color and the pixel of interest, and input multi-tone image data Based on the maximum value Dmax of Din0 and the error adjustment coefficient α for each color from the error adjustment coefficient determination means, when the binary data Dout from the binarization means is 1 for each color, Din A binarization error Err is determined by an operation of-(Dmax + α), and when the binary data Dout is 0, a binarization error determination unit that determines the binarization error Err as Din1 = Err, and the binarization error Error diffusion means for multiplying the binarization error Err determined by the error determination means for each color by a predetermined diffusion coefficient, and then diffusing it as a diffusion error to surrounding pixels which have not been binarized; and diffusion from the error diffusion means. An error accumulating means for accumulating an error for each pixel, and the target pixel accumulated error Es is obtained from the error accumulating means.

Images