JP2004080065A - Tone reproducing method, threshold matrix, image processing method, image processing apparatus, image forming apparatus, and printer driver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem of image quality deteriorating with low resolution. <P>SOLUTION: Thresholds are provided, which perform dot reproduction, to always keep the same base tone at all halftone levels using only dot arrangement pattern. <P>COPYRIGHT: (C)2004,JPO

Description

【００９６】
図２９（ｂ）は上述したハイパスフィルタ特性を持たせたマトリクスのライン基調以外の部分の一例を示すものであり、このライン基調以外の部分の特性は同図（ｂ）に示すようなハイパスフィルター特性となっている。
【００９７】
また、図３０は所定の方向のライン基調に形成され、かつ基調以外の部分においてハイパスフィルター特性を持つマトリクスによる階調グラデーション画像の例を示している。
【００９８】
すなわち、前述した図２７（ａ）のＡ部の濃度部分においては、図３０（ａ）に示すライン基調の部分と同図（ｂ）に示すハイパスフィルター特性を持つライン基調以外の部分とを組み合わせた同図（ｃ）に示すような本発明に係るディザマトリクスを用いることで、同図（ｄ）に示すように濃度が連続した階調グラデーション画像が得られる。
【００９９】
このように、所定方向のライン基調が形成されるだけでは階調の連続性が失われる一部の濃度において、所定のライン基調と基調以外の部分でハイパスフィルター特性を持つマトリクスとすることによって、低線数化（階調の連続性の喪失）を抑制することができ、ライン基調の連続性を高めることができて、再現した画像の品質が向上する。
【０１００】
この場合、一部の濃度間における双方のディザマトリクスを２値画像としたときに、両画像の差分画像においてハイパスフィルター特性を持つことになる。
【０１０１】
すなわち、ディザ法では、低い濃度側で設定された閾値ドットは、高い濃度側でも必ず存在する。上述のディザマトリクスの一部の濃度間における双方において、濃度が低い側をＡマスク、濃度が高い側をＢマスクとすると、ディザ法の性質上、Ａマスクに存在するライン基調は、Ｂマスクにも必ず存在する。したがって、Ｂマスクを２値化した画像から、Ａマスクを２値化した画像の差分を求めると、ライン基調が無い画像になる。
【０１０２】
つまり、図３１（ａ）に示すように、Ａマスクを用いるＡ部の濃度とＢマスクを用いるＢ部の濃度との間において、同図（ｂ）に示すＡマスクと同図（ｃ）に示すＢマスクとの差分（Ｂ−Ａ）を採ると、同図（ｄ）に示す差分画像（差分パターン）が得られ、この差分画像は同図（ｅ）に示すように前述したハイパスフィルタ特性を持つことになる。
【０１０３】
また、本発明に係るマトリクスを用いて多階調画像を一部の濃度において閾値化したときに、その２次元空間周波数による極座標上の所定方向のライン基調の角度以外のパワースペクトルの分布が一様に近似する。
【０１０４】
すなわち、画像の空間周波数のパワースペクトルは、次の（２）式で定義される。
【０１０５】
【数２】

【０１０６】
この値は空間周波数（ｕ，ｖ）の強さを表す。（ｕ，ｖ）から方向性を抽出するには、これを極座標であらわしてＰ（ｒ，θ）としたのち、次の（３）式で求める。
【０１０７】
【数３】

【０１０８】
上述したように、本発明に係るマトリクスは、所定の方向のライン基調に形成され、かつ基調以外の部分においてハイパスフィルター特性を持つ。ライン基調の部分においては、所定方向のみのライン基調があることから、極座標上のパワースペクトルは所定方向の角度のみ強い値を持つ。また、基調以外の部分においては、ハイパスフィルター特性を持つことから、極座標上のパワースペクトルの値は一様に近似することになる。
【０１０９】
図３２（ａ）は本発明に係るマトリクスの極座標上のパワースペクトルを、同図（ｂ）はライン角度４５度を除いた極座標上のパワースペクトルの分布を示したものである。同図（ａ）に示すように、マトリクスの極座標上のパワースペクトルでは４５度部分のみが強い値を持ち、同図（ｂ）に示すように、極座標上の４５度を除いたパワースペクトルでは、値は一様に近似している。
【０１１０】
したがって、マトリクスは４５度のライン基調方向に形成されており、それ以外の角度のパワースペクトルは、ハイパスフィルター特性により一様に近似していることになる。
【０１１１】
これに対して、従来の組織的ディザ法の代表的なマスク手法である、ベイヤー型ディザマトリクスおよび集中型ディザマトリクスによる図３２と同じ濃度によるパワースペクトル分布は、図３３及び図３４にそれぞれ示すようになる。これらは、それぞれ複数の角度で値が高くなっており、本発明に係るマトリクスの特徴が従来のマトリクスと異なることが分かる。
【０１１２】
このように、低線数化を抑制したマトリクスにより所定の方向のライン基調の連続性が高まり、連続した階調表現による所望の品質の多階調画像を再現することができる。また、誤差拡散法に比べ、マスク手法を使用することにより処理が単純になり処理速度（印刷速度、画像処理速度、或いは画像形成速度）の高速化がはかれる。この場合、所定の方向のライン基調を斜め方向とする、万線基調とすることにより、斜め方向のライン基調が画像形成装置による横スジ状のノイズを低減し、より高品質な多階調画像の出力を得ることができるようになる。
【０１１３】
そして、このような閾値マトリクス、階調再現方法を、多色画像を複数の色成分に分解し、少なくとも１つの色成分の原画像を入力画像とした多色画像の画像処理方法に適用することで、より高画質な多色画像を出力できる。
【０１１４】
多色画像形成装置、一般にカラー印刷装置では、等和色であるシアン、マゼンタ、イエローの３つの基本色が用いられる。また、インクジェットカラー印刷装置では、明度なども加味し、所望の色彩の外観を向上させるため、これらの３色にさらにブラックを加え、４色の原色を用いて印刷を行うものが主流になっている。さらに、最近では印刷画質の向上のため、基本色を組み合わせた多次色を予め用意して、さらに多くの色を用いた印刷を行うものも多くなってきている。
【０１１５】
これらの装置のように、複数の色成分に分解し、それぞれの色成分の原画像を入力画像とする多色画像形成方法は、色成分単位で擬似階調表現処理を行う。したがって、本発明を適用することで、各色成分単位でのライン基調の連続性が高まり、結果として単色画像、多色画像において、より高画質な画像を形成することができる。
【０１１６】
特に、マトリクスを用いて各ドット２値あるいは多値の画像データに変換する画像形成装置において、特におよそ３００ｄｐｉ以下の低解像度な精細度で出力する（画像形成する）場合に適用することで、画像形成速度、印刷速度の高速化と高画質化を図ることができる。
【０１１７】
すなわち、一般に擬似階調表現処理を使用する画像形成装置は、単位あたりのドット密度すなわち解像度を上げて画質を向上させている。しかし、解像度を上げることは同時に処理する画像データ量も面積単位で大きくなるため、処理時間も多く必要とする。３００ｄｐｉの解像度の画像形成装置においての擬似階調表現処理では、一般に１インチあたり最高で約１５０本の線を表現するのが限界とされ、高画質化が困難といわれている。
【０１１８】
上述した所定方向のライン基調を持つディザマトリクスによる擬似階調表現処理においては、一部の濃度において人間の視覚上では低線数化が発生し、特に３００ｄｐｉの解像度ではさらに線数が低くなることで、階調表現の連続性が失われ、画質低下が目立つことになる。このような画像形成装置における出力形態（低解像度での出力）で画像形成する場合に本発明を適用することで、低線数化を抑制し所定方向のライン基調の連続性が高まり、連続した階調表現による所望の品質の多階調画像を形成することができる。
【０１１９】
また、上記実施形態ではホスト側のプリンタドライバで閾値マトリクスをテーブルとして保持する例で説明しているが、図３５に示すように、プリンタドライバ１０１側にはホストコンピュータで実行されるアプリケーションソフトなどから画像データを処理するＣＭＭ処理部１０２及びＢＧ／ＵＣＲ，γ補正部１０３のみを有し、インクジェット記録装置２００側の制御部にズーミング処理部１０４及び本発明に係る閾値マトリクス１０５を格納したＲＯＭなどを備えて、ドット配置への変換はインクジェット記録装置側で行うようにすることもできる。何れの場合も、入力画像データに対して１対１の比較で処理を行うことができるので、高速かつ低コストで画像処理を行うことができるようになる。
【０１２０】
また、上記実施形態では主にインクジェット記録装置及びそのホストに本発明を適用する例で説明したが、ドットで画像を形成できる（ドット再現で画像を形成する）画像形成装置、例えば熱転写記録装置などにも同様に適用することができる。さらに、レーザープリンタ、ＬＥＤプリンタなどの電子写真方法を用いた画像形成装置にも適用することができる。
【０１２１】
熱転写記録装置は、図３６に示すように、発熱体３０１を有するサーマルヘッド３００と加圧ローラ３０３との間で用紙３０４とインクシート３０５とを挟んで送りながら、サーマルヘッド３００の発熱体３０１を駆動することによって、インクシート３０５のベース層３０６上の所定領域のワックス層３０７を用紙３０４に転写して画像を記録する。
【０１２２】
また、電子写真方法を用いた画像形成装置の一例について図３７及び図３８を参照して簡単に説明する。なお、図３７は画像形成装置の概略構成図、図３８は同画像形成装置を構成するプロセスカートリッジの概略構成図である。
【０１２３】
この画像形成装置４４０は、マゼンダ（Ｍ）、シアン（Ｃ）、イエロー（Ｙ）、ブラック（Ｂｋ）の４色でフルカラー画像を形成するレーザプリンタの一例であり、各色用の画像信号に応じたレーザビームを出射する４つの光書き込み装置４４２Ｍ、４４２Ｃ、４４２Ｙ、４４２Ｂｋと、作像用の４つのプロセスカートリッジ４４１Ｍ、４４１Ｃ、４４１Ｙ、４４１Ｂｋと、画像が転写される記録用紙を収納する給紙カセット４４３と、給紙カセット４４３から記録用紙を給紙する給紙ローラ４４４と、記録用紙を所定のタイミングで搬送するレジストローラ４４５と、記録用紙を各プロセスカートリッジの転写部に搬送する転写ベルト４４６と、記録用紙に転写された画像を定着する定着装置４４９と、定着後の記録用紙を排紙トレイ４５１に排紙する排紙ローラ４５０等を備えた構成となっている。
【０１２４】
４つのプロセスカートリッジ４４１Ｍ、４４１Ｃ、４４１Ｙ、４４１Ｂｋの構成は同じであり、図３４に示すように、各プロセスカートリッジ４４１は、ケース内に像担持体であるドラム状の感光体４５２と、帯電ローラ４５３と、現像器４５４と、クリーニングブレード４５９等を一体に備えている。
【０１２５】
また、現像器４５４内には、トナー供給ローラ、帯電ローラ、静電搬送基板４５７、トナー戻しローラ４５８が設けられており、各色のトナーが収納されている。また、プロセスカートリッジ４４１の背面側には、光書き込み装置からのレーザビームが入射される窓口となるスリット４６０が設けられている。
【０１２６】
各光書き込み装置４４２Ｍ、４４２Ｃ、４４２Ｙ、４４２Ｂｋは、半導体レーザ、コリメートレンズ、ポリゴンミラー等の光偏向器、走査結像用光学系等から構成され、装置外部のパーソナルコンピュータ等のホスト（画像処理装置）から入力される各色用の画像データに応じて変調されたレーザビームを出射し、各プロセスカートリッジ４４１Ｍ、４４１Ｃ、４４１Ｙ、４４１Ｂｋの感光体４５２上を走査し、静電荷像（静電潜像）を書き込む。
【０１２７】
そして、画像形成が開始されると、各プロセスカートリッジ４４１Ｍ、４４１Ｃ、４４１Ｙ、４４１Ｂｋの感光体４５２が帯電ローラ４５３で均一に帯電され、各光書き込み装置４４２Ｍ、４４２Ｃ、４４２Ｙ、４４２Ｂｋから画像データに応じたレーザビームが照射されて各感光体上に各色の静電潜像が形成される。感光体４５２上に形成された静電潜像は、現像器４５４の静電搬送基板４５７で静電搬送された各色のトナーにより現像され顕像化される。感光体４５２と静電搬送基板４５７の対向部間にパスル状の現像バイアスが印加され、感光体４５２上の静電潜像が、静電搬送基板４５７で搬送されて来たトナーによりホッピング現象される。また、現像に供されなかったトナーは静電搬送基板４５７で搬送されてトナー戻しローラ４５８に戻される。
【０１２８】
各４４１Ｂｋ、４４１Ｙ、４４１Ｃ、４４１Ｍの各色の画像形成に同期して、供給カセット４４３内の記録用紙が供給ローラ４４４で給紙され、レジストローラ４４５により所定のタイミングで転写ベルト４４６に向けて搬送される。そして、記録用紙は転写ベルト４４６に担持されて４つのプロセスカートリッジ４４１Ｂｋ、４４１Ｙ、４４１Ｃ、４４１Ｍの感光体に向けて順次搬送され、各感光体上のＢｋ、Ｙ、Ｃ、Ｍの各色のトナー像が順次重ね合わせて転写される。４色のトナー像が転写された記録用紙は、定着ベルト４４７と加圧ローラ４４８からなる定着装置４４９に搬送され、４色のトナー像からなるカラー画像が定着されて排紙トレイ４５１に排紙される。
【０１２９】
この画像形成装置においては、図３９（ａ）〜（ｃ）に示すように、各光書き込み装置４４２Ｍ、４４２Ｃ、４４２Ｙ、４４２Ｂｋから出射するレーザビームのオン時間又はオフ時間を変化させることによってドットサイズを変化させることができる。
【０１３０】
さらに、インクジェット記録装置のヘッド構成も上記実施形態のものに限らず、発熱抵抗体を用いるサーマル型インクジェットヘッドや振動板と電極を用いる静電型インクジェットヘッドを搭載する装置などでもよい。また、上記実施形態では画像形成装置について適用しているが、画像表示装置に画像データを出力する場合の画像処理、階調再現にも同様に適用することができる。
【０１３１】
【発明の効果】
以上説明したように、本発明にかかる階調再現方法によれば、ディザマトリクスにより多階調画像が一部の濃度において閾値化されたときに、所定方向のライン基調に形成され、かつ基調以外の部分においてはハイパスフィルター特性を持つ構成としたので、低線数化を抑制することができて、ライン基調の連続性が高まり、特に低解像度／高速記録においても良好な画像品質が得られるようになる。
【０１３２】
本発明に係る閾値マトリクスによれば、少なくとも一部の濃度について、所定方向のライン基調に形成され、かつ基調以外の部分においてはハイパスフィルター特性を持つ構成としたので、低線数化を抑制することができて、ライン基調の連続性が高まり、特に低解像度／高速記録においても良好な画像品質が得られる階調再現を行うことができるようになる。
【０１３３】
本発明に係る画像処理方法によれば、多色画像を複数の色成分に分解し、少なくとも１つの色成分の原画像を入力画像として、ディザマトリクスを用いて各ドット２値あるいは多値の画像データに変換処理するときに、本発明に係る階調再現方法を実行するので、良好な画像品質が得られる。
【０１３４】
本発明に係る画像処理装置によれば、複数のドットからなる画像を出力するための画像データを処理するときに、本発明に係る画像処理方法を実行するので、良好な画像品質が得られる。
【０１３５】
本発明に係るプリンタドライバによれば、複数のドットからなる画像を形成する画像形成装置用の画像データを処理するときに、本発明に係る画像処理方法を実行するので、低解像度／高速記録においても良好な画像品質が得られる。
【０１３６】
本発明に係る画像形成装置によれば、複数のドットからなる画像を形成するときに、本発明に係る画像処理方法を実行するので、低解像度／高速記録においても良好な画像品質が得られる。
【図面の簡単な説明】
【図１】画像形成装置としてのインクジェット記録装置の一例を示す機構部の概略斜視説明図
【図２】同機構部の側面説明図
【図３】同記録装置のヘッドの一例を示す分解斜視説明図
【図４】同ヘッドの液室長手方向に沿う断面説明図
【図５】図４の要部拡大説明図
【図６】同ヘッドの液室短手方向に沿う断面説明図
【図７】同ヘッドのノズル板の平面説明図
【図８】同記録装置の制御部の概要を説明するブロック図
【図９】同制御部のヘッド駆動制御に係わる部分のブロック説明図
【図１０】図９のヘッド駆動回路の一例を示すブロック図
【図１１】同ヘッド駆動制御に係わる部分の作用説明に供する説明図
【図１２】本発明に係る閾値マトリクスを含む本発明に係るプリンタドライバを搭載した本発明に係る画像処理装置であるホスト側の構成の一例を説明するブロック図
【図１３】入力画像に対するＢａｙｅｒ型ディザ処理、誤差拡散処理後のドットパターンを示す説明図
【図１４】入力画像に対するＢａｙｅｒ型ディザ処理後の画像データを示す説明図
【図１５】３００ｄｐｉのＢａｙｅｒ型ディザパターンと誤差拡散パターンの粒状度を１０％濃度おきに測定したときの説明図
【図１６】インクジェット記録装置における機械的な変動要因を説明するための説明図
【図１７】Ｂａｙｅｒ型ディザパターンと記録装置の機械的変動との干渉の説明に供する説明図
【図１８】本発明に係る閾値マトリクスにおける斜め基調のドット配置パターンを説明する説明図
【図１９】電子写真記録装置の万線ディザパターンのドットパターンと階調レベルの基調変化の説明に供する説明図
【図２０】同手法をインクジェット記録装置に適用した場合のドットパターンと階調レベルの基調変化の説明に供する説明図
【図２１】ディザマスクをタイリングすることで形成される基調についての説明に供する説明図
【図２２】マスクが１階調レベル当たり１ドットの場合のタイリングと基調についての説明に供する説明図
【図２３】マスクが１階調レベル当たり２ドットの場合のタイリングと基調についての説明に供する説明図
【図２４】マスクが１階調レベル当たり３ドットの場合のタイリングと基調についての説明に供する説明図
【図２５】基本マトリクスからサブマトリクスへの分割の説明に供する説明図
【図２６】所定方向のライン基調を有するマトリクスパターンの異なる例を説明する説明図
【図２７】階調グラデーション画像と一部の濃度において低線数化したマトリクスパターンの例を説明する説明図
【図２８】人間の視覚特性を説明する説明図
【図２９】マトリクスパターンのハイパスフィルタ特性を有する基調以外の部分の説明図
【図３０】所定のライン基調と基調以外の部分でハイパスフィルタ特性を有するマトリクスによる階調グラデーション画像の説明に供する説明図
【図３１】一部濃度間のマトリクスの差分画像について説明する説明図
【図３２】本発明に係るマトリクスの極座標上のパワースペクトル分布の説明に供する説明図
【図３３】従来のベイヤー型ディザマトリクスの極座標上のパワースペクトル分布の説明に供する説明図
【図３４】従来の集中型ディザマトリクスの極座標上のパワースペクトル分布の説明に供する説明図
【図３５】本発明に係る画像形成装置の説明に供するブロック図
【図３６】熱転写記録装置の概略説明図
【図３７】電子写真方法を用いる画像形成装置の概略構成図
【図３８】同画像形成装置のプロセスカートリッジの概略構成図
【図３９】ドットサイズの変更の説明に供する説明図
【図４０】２値化および少値化により階調表現を行ったときに実際に配置されるドットの説明に供する説明図
【図４１】ディザ法による２値化処理の説明に供する説明図
【図４２】少値化処理におけるサイズ変調ドットとディザマスクの対応の説明に供する説明図
【図４３】各ドットサイズのディザマスクの一例を説明する説明図
【図４４】２値誤差拡散処理の手順について説明する説明図
【符号の説明】
１３…キャリッジ、１４…記録ヘッド、８７…駆動波形生成回路、８８…ヘッド駆動回路、９１…主制御部、１０１…プリンタドライバ、１０５…閾値マトリクス。[0001]
[Industrial applications]
The present invention relates to a tone reproduction method, a threshold matrix, an image processing method, an image processing device, an image forming device, and a printer driver.
[0002]
[Prior art]
2. Description of the Related Art In a conventional image forming apparatus (image recording apparatus) such as a printer, a facsimile or a copying machine, a digital image output is mainly a binary image composed of "1" and "0", that is, "ON" and "OFF". However, with the progress of the imaging engine and the growing need for high-quality images, multi-valued images in which one pixel expresses a plurality of gradations are mainly used.
[0003]
For example, in an ink jet recording apparatus that forms (records) an image by ejecting ink droplets, as a method of performing gradation recording, a dot size modulation method using a dot of a different size from a density modulation method of changing the density of ink is used. And the combination of both are mainly used.
[0004]
In this case, in the ink jet recording apparatus, the pressure generating means of the ink jet head, for example, in the case of a thermal ink jet system, is a heating resistor for generating air bubbles, and in the case of a piezo system, deforms the liquid chamber wall. It is a piezo element that is an electromechanical conversion element for, in the case of the electrostatic method, the dot size is changed by changing the drive voltage of the drive waveform applied to the electrodes, the pulse width, the number of pulses, etc. Because of the spread of the ink, the dot size is limited to about four values of “large droplet, medium droplet, small droplet, and no printing” at most.
[0005]
Here, FIG. 40 shows an example of a dot pattern when commonly used binarization processing and multi-value processing are applied. FIG. 7A shows an example of performing dot reproduction by performing binarization processing, FIG. 7B shows an example of performing dot reproduction by performing density modulation, and FIG. This is an example of a case where dot reproduction is performed.
[0006]
In such gradation expression using dots, the amount of information is basically determined by a controllable dot size. As the number of controllable steps increases, the amount of information increases and a high-quality output image close to the original image data can be obtained. However, as described above, in an ink jet recording apparatus or the like, 1 to 3 steps (4 steps including 0) Most can only control the degree. Although some improvement can be achieved in combination with the density modulation method, the ratio of the colorant and the recording unit increases correspondingly, so that the improvement can be made only about twice due to restrictions due to the cost and the size of the apparatus.
[0007]
In order to compensate for such a shortage of information per pixel, as a method of performing tone expression by controlling the number of dots per unit area, pseudo tone expression generally called halftone processing is used. . In the pseudo gradation expression, the number of arranged dots is expressed as a density, and many gradations are expressed by changing the density of points.
[0008]
The dither method is widely used for the pseudo gradation expression, and there are a systematic dither method and a random dither method as typical ones. The systematic dither method sets a sub-matrix of n × n thresholds (this is called a dither matrix), superimposes this dither matrix on the input image, compares the gray level of each corresponding pixel with the threshold, If the value of the image is larger, it is displayed as 1 (white), and if it is smaller, it is displayed as 0 (black). After the processing of n × n pixels is completed, the dither matrix is sequentially moved to the position of the next n × n pixels, and the same processing is repeated to form an image.
[0009]
For example, the multi-valued image data input as shown in FIG. 41A is compared with a dither matrix which is an n × n threshold matrix created by a predetermined method as shown in FIG. In this method, a comparison is performed, and only pixels having a value equal to or greater than (or equal to or less than) the threshold are replaced with dots, as shown in FIG.
[0010]
Although FIG. 42 shows only two values of ON / OFF, for a multi-value having more combinations, the reproducible gradation area is divided into, for example, small dots, medium dots, and large dots as shown in FIG. Then, as shown in FIGS. 43 (a) to 43 (c), a threshold matrix according to each dot size is applied, and each is compared with input image data to perform replacement with corresponding dots. .
[0011]
In addition, the random dither method is a method in which a random number is generated for each pixel of an input image and the value is used as a threshold. In the case of the random dither method, an image formed generally becomes a rough image, and the It is not suitable for improving the image quality as compared with the dither method.
[0012]
On the other hand, although there is an error diffusion method in the pseudo gradation expression, the error diffusion method is a considerably complicated process as compared with the dither method. FIG. 44 shows the procedure of the binary error diffusion. Threshold processing is performed for each pixel, and the error at that time is reflected in a later calculation at a predetermined ratio while being retained. As a result, information that is forcibly discarded in the dither processing can be fed back to the output image, and a quality higher than that of the dither image in terms of resolution and the like can be obtained.
[0013]
With respect to these dither methods and error diffusion methods, higher resolution has been promoted with the aim of achieving higher image quality output. This is because the size of each dot and the distance between the dots are reduced by increasing the resolution, and it is difficult to distinguish a dot pattern created by the dither method or the error diffusion method. If the pattern becomes unrecognizable to the human eye, it is synonymous with multi-valued representation of one pixel, and some recent inkjet recording apparatuses have a resolution of 2880 dpi.
[0014]
[Problems to be solved by the invention]
By the way, although the image quality is improved by increasing the resolution, disadvantages thereof include an increase in the cost of the recording unit and a decrease in the recording speed. In order to achieve high resolution, in addition to the technology for forming smaller dots than before, higher control is also required in terms of dot position accuracy, which inevitably increases costs, and also increases the coverage per dot. Since the area is reduced, in a recording unit having the same configuration, recording at a higher resolution requires more time.
[0015]
However, in practice, in addition to cases where higher image quality is required than speed and cost, if image quality of a certain level or higher can be obtained, speed and cost may be given top priority. It is not done.
[0016]
However, to date, all resolutions have been considered as an extension of higher resolution, and hardware measures such as "increase the dot formation speed" and "increase the mounting density of recording units" have been adopted while maintaining high resolution. I have. This is intended only to increase the speed of a high-quality recording apparatus, but not to improve the image quality when a low-resolution and inexpensive apparatus is used.
[0017]
When speeding up a high-quality recording device, there is a limit in terms of cost and mounting area, so it is not possible to achieve a significant speed increase unless the recording sequence itself is changed, and if the recording sequence is changed In this case, image processing for high resolution cannot be applied as it is, so image processing according to a new recording sequence is required.However, with conventional devices, simple image processing is only applied, No attempt has been made to improve image quality.
[0018]
The present invention has been made in view of the above problems, and has a tone reproduction method, a threshold matrix, an image processing method, an image processing apparatus, an image processing apparatus, an image forming apparatus, and a printer that can obtain good image quality even in low-resolution / high-speed printing. The purpose is to provide a driver.
[0019]
[Means for Solving the Problems]
In order to solve the above-described problems, the tone reproduction method according to the present invention is configured such that when a multi-tone image is thresholded at some densities by a dither matrix, the tone is formed in a line tone in a predetermined direction, and other than the tone. The portion has a high-pass filter characteristic.
[0020]
Here, when both dither matrices between a part of the density of the dither matrix are binary images, it is preferable that a difference image between the two images has a high-pass filter characteristic. Also, when the multi-tone image is thresholded at a part of the density by the dither matrix, the distribution of the power spectrum other than the angle of the line tone in a predetermined direction on the polar coordinate by the two-dimensional spatial frequency is uniformly approximated. Is preferred. Furthermore, it is preferable that the keynote is a line keynote.
[0021]
The threshold matrix according to the present invention has a configuration in which at least a part of the density is formed in a line tone in a predetermined direction, and a portion other than the tone has a high-pass filter characteristic.
[0022]
Here, when both matrices between some densities are binary images, it is preferable that a difference image between the two images has a high-pass filter characteristic. Further, it is preferable that the distribution of the power spectrum other than the angle of the line tone in a predetermined direction on the polar coordinate based on the two-dimensional spatial frequency is uniformly approximated for a part of the density of the multi-tone image.
[0023]
An image processing method according to the present invention decomposes a multicolor image into a plurality of color components, and converts each dot binary or multivalued image data using a dither matrix using an original image of at least one color component as an input image. When the conversion process is performed, the tone reproduction method according to the present invention is executed.
[0024]
An image processing method according to the present invention is configured to include means for executing the image processing method according to the present invention when processing image data for outputting an image composed of a plurality of dots.
[0025]
A printer driver according to the present invention is configured to include a unit that executes the image processing method according to the present invention when processing image data for an image forming apparatus that forms an image including a plurality of dots.
[0026]
An image forming apparatus according to the present invention has a configuration including means for executing the image processing method according to the present invention when forming an image including a plurality of dots. This image forming apparatus is preferably an ink jet recording apparatus or a thermal transfer recording apparatus.
[0027]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic perspective view illustrating a mechanism of an ink jet recording apparatus as an image forming apparatus, and FIG. 2 is a side view illustrating the mechanism.
[0028]
This ink jet recording apparatus includes a carriage that is movable in the main scanning direction inside the recording apparatus main body 1, a recording head including an inkjet head mounted on the carriage, an ink cartridge that supplies ink to the recording head, and the like. After storing the mechanism unit 2 and the like, taking in the paper 3 fed from the paper feed cassette 4 or the manual feed tray 5, recording the required image by the printing mechanism unit 2, the paper is transferred to the paper output tray 6 mounted on the rear side. Discharge paper.
[0029]
The printing mechanism 2 holds the carriage 13 slidably in the main scanning direction (the direction perpendicular to the paper surface in FIG. 2) by a main guide rod 11 and a sub guide rod 12, which are guide members that are laterally mounted on left and right side plates (not shown). The carriage 13 is provided with an ink jet head 14 for ejecting ink droplets of each color of yellow (Y), cyan (C), magenta (M), and black (Bk) with the ink droplet ejection direction directed downward. Each ink tank (ink cartridge) 15 for supplying each color ink to the head 14 is exchangeably mounted on the upper side of the carriage 13.
[0030]
The ink cartridge 15 has an upper air port that communicates with the atmosphere, a lower supply port for supplying ink to the inkjet head 14, and a porous body filled with ink inside. The ink supplied to the inkjet head 14 by the force is maintained at a slight negative pressure. The ink is supplied from the ink cartridge 15 into the head 14.
[0031]
Here, the carriage 13 is slidably fitted to the main guide rod 11 on the rear side (downstream side in the paper transport direction), and is slidably mounted on the front guide rod 12 (upstream side in the paper transport direction). are doing. In order to move and scan the carriage 13 in the main scanning direction, a timing belt 20 is stretched between a driving pulley 18 and a driven pulley 19 that are driven to rotate by a main scanning motor 17. , And the carriage 13 is reciprocated by the forward and reverse rotation of the main scanning motor 17.
[0032]
Although the heads 14 of each color are used here as the recording head, a single head having nozzles for ejecting ink droplets of each color may be used. Further, as described later, as described later, a diaphragm that forms at least a part of the wall surface of the ink flow path, and a piezo-type inkjet head that deforms the diaphragm with a piezoelectric element (piezoelectric element) are used.
[0033]
However, the present invention is not limited to this. For example, a diaphragm that forms at least a part of the wall surface of the ink flow path and an electrode facing the diaphragm is provided, and the diaphragm is deformed and displaced by electrostatic force to pressurize ink. An ink jet head that uses an electric head, a piezoelectric element, and uses buckling deformation of a diaphragm, or a pressure generated by heating ink in an ink flow path using a heating resistor to generate bubbles. A so-called thermal type that discharges the ink can also be used.
[0034]
On the other hand, in order to transport the paper 3 set in the paper cassette 4 to the lower side of the head 14, the paper 3 is guided by the paper feed roller 21 and the friction pad 22, which separate and feed the paper 3 from the paper cassette 4. A guide member 23, a transport roller 24 that reverses and transports the fed paper 3, a transport roller 25 pressed against the peripheral surface of the transport roller 24, and a leading end that defines an angle at which the paper 3 is sent out from the transport roller 24. A roller 26 is provided. The transport roller 24 is driven to rotate by a sub-scanning motor 27 via a gear train.
[0035]
Further, there is provided a printing receiving member 29 which is a paper guide member for guiding the paper 3 sent from the transport roller 24 below the recording head 14 in accordance with the moving range of the carriage 13 in the main scanning direction. On the downstream side of the printing receiving member 29 in the paper transport direction, there are provided a transport roller 31 and a spur 32 that are driven to rotate in order to transport the paper 3 in the paper discharge direction. Rollers 33 and spurs 34 and guide members 35 and 36 forming a paper discharge path are provided.
[0036]
At the time of recording, the recording head 14 is driven in accordance with an image signal while moving the carriage 13 to discharge ink on the stopped paper 3 to record one line, and after the paper 3 is conveyed by a predetermined amount, the next paper is transported. Record the line. Upon receiving a recording end signal or a signal indicating that the rear end of the sheet 3 has reached the recording area, the recording operation is terminated and the sheet 3 is discharged.
[0037]
Further, a recovery device 37 for recovering from the ejection failure of the head 14 is disposed at a position outside the recording area on the right end side in the moving direction of the carriage 13. The recovery device 37 has a cap unit, a suction unit, and a cleaning unit. The carriage 13 is moved to the recovery device 37 side during printing standby, the head 14 is capped by the capping means, and the ejection opening (nozzle hole) is kept in a wet state, thereby preventing ejection failure due to ink drying. In addition, by discharging (purging) ink that is not related to printing during printing or the like, the ink viscosity of all the discharge ports is made constant, and stable discharge performance is maintained.
[0038]
When a discharge failure occurs, the discharge port (nozzle) of the head 14 is sealed by capping means, bubbles are sucked out of the discharge port with ink by a suction means through a tube, and ink or dust adhered to the discharge port surface. Is removed by the cleaning means, and the ejection failure is recovered. The sucked ink is discharged to a waste ink reservoir (not shown) provided at a lower portion of the main body, and is absorbed and held by an ink absorber inside the waste ink reservoir.
[0039]
Next, an ink jet head constituting the recording head 14 of the ink jet recording apparatus will be described with reference to FIGS. 3 is an exploded perspective view of the head, FIG. 4 is a sectional view of the head along the longitudinal direction of the liquid chamber, FIG. 5 is an enlarged explanatory view of a main part of FIG. 4, and FIG. FIG. 7 is an explanatory cross-sectional view along the hand direction, and FIG.
[0040]
This ink jet head includes a flow path forming substrate (flow path forming member) 41 formed of a single crystal silicon substrate, a vibration plate 42 bonded to the lower surface of the flow path forming substrate 41, and a flow path forming substrate 41 bonded to the upper surface of the flow path forming substrate 41. And a pressurizing chamber 46, which is an ink flow path communicating with a nozzle 45 that ejects ink droplets as droplets, and an ink supply path 47 that serves as a fluid resistance part to the pressurizing chamber 46. A common liquid chamber 48 for supplying ink through the intermediary is formed, and a pressurizing chamber 46, an ink supply path 47, and a common liquid chamber 48, which are surfaces of these flow path forming substrates 41 that are in contact with the ink, are formed of an organic resin film on each wall. The liquid-resistant thin film 50 made of is formed.
[0041]
Then, a laminated piezoelectric element 52 is joined to the outer surface side (opposite to the liquid chamber) of the vibration plate 42 corresponding to each pressurizing chamber 46, and the laminated piezoelectric element 42 is joined and fixed to a base substrate 53. A spacer member 54 is joined to the base substrate 53 around the row of the piezoelectric elements 52.
[0042]
As shown in FIG. 5, the piezoelectric element 52 is formed by alternately stacking piezoelectric materials 55 and internal electrodes 56. The pressurizing chamber 46 is contracted and expanded by the expansion and contraction of the piezoelectric element 52 whose piezoelectric constant is d33. When a drive signal is applied to the piezoelectric element 52 and charging is performed, the piezoelectric element 52 expands in the direction indicated by the arrow A in FIG. 5, and when the electric charge charged in the piezoelectric element 52 discharges, it contracts in the direction opposite to the direction indicated by the arrow A. Has become. The base substrate 53 and the spacer member 54 are formed with through holes for forming ink supply ports 49 for supplying ink to the common liquid chamber 48 from outside.
[0043]
Further, the outer peripheral portion of the flow path forming substrate 41 and the outer peripheral portion on the lower surface side of the vibration plate 42 are adhesively bonded to a head frame 57 formed by injection molding with epoxy resin or polyphenylene sulphite. Are fixed to each other by an adhesive or the like at a portion not shown. Further, an FPC cable 58 is connected to the piezoelectric element 52 by solder bonding, ACF (anisotropic conductive film) bonding or wire bonding in order to supply a drive signal, and the FPC cable 58 is selectively connected to each piezoelectric element 52. A drive circuit (driver IC) 59 for applying a drive waveform is mounted.
[0044]
Here, the flow path forming substrate 51 is formed by anisotropically etching a single crystal silicon substrate having a crystal plane orientation of (110) using an alkaline etching solution such as an aqueous solution of potassium hydroxide (KOH) so that each of the pressure chambers 56 is formed. , A groove serving as the ink supply path 57, and a through hole serving as the common liquid chamber 58.
[0045]
The vibration plate 42 is formed from a nickel metal plate, and is manufactured by an electroforming method. The vibration plate 42 has a thin portion 61 for facilitating deformation and a thick portion 62 for joining with the piezoelectric element 52 at a portion corresponding to the pressurizing chamber 46 and a portion corresponding to the liquid chamber spacing wall. Also, a thick portion 23 is formed, the flat surface side is bonded to the flow path forming substrate 41 with an adhesive, and the thick portion is bonded to the frame 17 with an adhesive. A support portion 64 is provided between the thick portion 63 corresponding to the liquid chamber spacing wall of the diaphragm 2 and the base substrate 53. The strut 64 has the same configuration as the piezoelectric element 52.
[0046]
The nozzle plate 43 forms a nozzle 45 having a diameter of 10 to 30 μm corresponding to each pressurized liquid chamber 46, and is bonded to the flow path forming substrate 41 with an adhesive. Here, the plurality of nozzles 45 constitute a plurality of dot forming means, and as shown in FIG. 7, the rows of nozzles 45 (nozzle rows) are arranged orthogonal to the main scanning direction. The pitch between 45 is 2 × Pn. Further, in one head, two nozzle rows are arranged at a distance L from each other, and the nozzle rows are arranged in a staggered manner while being shifted by a pitch Pn in the sub-scanning direction. Therefore, an image at the pitch Pn can be formed by one main scan and one sub-scan.
[0047]
As the nozzle plate 43, a metal such as stainless steel or nickel, a combination of a metal and a resin such as a polyimide resin film, silicon, or a combination thereof can be used. A water-repellent film is formed on the nozzle surface (surface in the discharge direction: discharge surface) by a well-known method such as a plating film or a water-repellent agent coating in order to ensure water repellency with ink.
[0048]
In the ink jet head configured as described above, by selectively applying a driving pulse voltage of 20 to 50 V to the piezoelectric element 52, the piezoelectric element 52 to which the pulse voltage is applied is displaced in the laminating direction, and 42 is deformed in the direction of the nozzle 45, the ink in the pressurized liquid chamber 46 is pressurized by the volume / volume change of the pressurized liquid chamber 46, and ink droplets are ejected (ejected) from the nozzle 45.
[0049]
Then, the liquid pressure in the pressurized liquid chamber 46 decreases as the ink droplets are ejected, and a slight negative pressure is generated in the pressurized liquid chamber 46 due to the inertia of the ink flow at this time. In this state, by turning off the application of the voltage to the piezoelectric element 52, the diaphragm 42 returns to the original position and the pressurized liquid chamber 46 returns to the original shape, so that a further negative pressure is generated. I do. At this time, ink is filled into the pressurized liquid chamber 46 from the ink supply port 49 through the common liquid chamber 48 and the ink supply path 47 which is a fluid resistance part. Then, after the vibration of the ink meniscus surface of the nozzle 45 is attenuated and stabilized, a pulse voltage is applied to the piezoelectric element 52 for discharging the next ink droplet, and the ink droplet is discharged.
[0050]
Next, an outline of a control unit of the inkjet recording apparatus will be described with reference to FIG.
The control unit includes a microcomputer (hereinafter, referred to as a “CPU”) 80 for controlling the entire recording apparatus, a ROM 81 storing required fixed information, a RAM 82 used as a working memory, and the like. An image memory (lass data memory) 83 for storing image data (referred to as dot data or dot pattern data) to be transferred, a parallel input / output (PIO) port 84, an input buffer 85, and a parallel input / output (PIO) A port 86, a waveform generation circuit 87, a head drive circuit 88, a driver 89, and the like are provided.
[0051]
Here, various information and data such as image data transferred from the printer driver 101 side of the host 100 and detection signals from various sensors are input to the PIO port 84. Required information is sent to the operation panel side.
[0052]
Further, the waveform generation circuit 87 generates and outputs a drive waveform to be applied to the piezoelectric element 52 of the recording head 14. As will be described later, the waveform generating circuit 87 can generate and output a required driving waveform with a simple configuration by using a D / A converter for D / A converting the driving waveform data from the CPU 80. .
[0053]
The head drive circuit 88 applies a drive waveform from the waveform generation circuit 87 to the piezoelectric element 52 of the selected channel of the recording head 14 based on various data and signals provided via the PIO port 86. Further, the driver 89 moves and scans the carriage 13 in the main scanning direction by controlling the driving of the main scanning motor 17 and the sub-scanning motor 27 in accordance with the driving data provided via the PIO port 86, respectively. Is rotated to convey the sheet 3 by a predetermined amount.
[0054]
The part of the control unit relating to the head drive control will be described with reference to FIGS. 9 is a block diagram of a portion related to the drive control, FIG. 10 is a block diagram showing an example of a head drive circuit, and FIG. 11 is an explanatory diagram for explaining an operation of a portion related to the head drive control.
[0055]
The main control unit 91 processes font data (dot data) as print data sent from the host side, performs vertical / horizontal conversion corresponding to the arrangement of the heads, and converts ink droplets into large droplets, small droplets, It generates 2-bit drive data SD necessary for separating the three values of printing and outputs it to the head drive circuit (driver IC) 88. In addition, for the driver IC 88, a clock signal CLK, a latch signal LAT, a driving waveform corresponding to a dot (large droplet) of a size forming an image dot as a driving waveform, and a driving waveform corresponding to a small droplet are selected. To output drive waveform selection signals M1 to M3 for performing the operation. Further, the main control section 91 reads out the drive waveform data stored in the internal ROM 81 and supplies it to the drive waveform generation circuit 87.
[0056]
The drive waveform generating circuit 87 D / A converts the drive waveform data supplied from the main controller 91 and outputs it as an analog signal, and converts the analog signal from the D / A converter 92 into an actual drive voltage. And a current amplifier 94 that amplifies the amplified output so as to supply a sufficient current by driving the head. For example, a drive including a plurality of drive pulses in one drive cycle as shown in FIG. A waveform Pv is generated and given to the driver IC 88.
[0057]
As shown in FIG. 10, the driver IC (head driving circuit) 88 latches a shift register 95 that captures drive data SD by a clock signal CK from the main control unit 91 and a register value of the shift register 95 with a latch signal LAT. Latch circuit 96, a data selector 97 for selecting drive waveform selection signals M1 to M3 (logic signals) based on the 2-bit drive data latched by the latch circuit 96, and an output (logic signal) of the data selector 97 as a drive voltage. It comprises a level shifter 98 for converting to a level, and a transmission gate 99 whose on / off is controlled by the output of the level shifter 98. The transmission gate 99 is supplied with a drive waveform Pv from the drive waveform generation circuit 87 and is connected to the piezoelectric element 52 corresponding to each nozzle of the recording head (inkjet head) 14.
[0058]
Therefore, in the head drive circuit 88, one of the drive waveform selection signals M1 to M3 is selected by the data selector 97 in accordance with the drive data SD, and the selected drive waveform selection signals M1 to M3, which are logic signals, are transmitted by the level shifter 98. The signal is converted into a drive voltage level and applied to the gate of the transmission gate 99.
[0059]
As a result, the transmission gate 99 is switched according to the length of the selected drive waveform selection signals M1 to M3. Therefore, the drive pulse constituting the drive waveform Pv for the channel in which the transmission gate 99 is open. Is applied.
[0060]
For example, when a drive waveform Pv including a plurality of drive pulses as shown in FIG. 11A is given, the transmission gate 99 which is open only during the period T0 to T1 is shown in FIG. As described above, since one driving pulse is output and applied to the piezoelectric element 52, the size of the ejected droplet becomes a small droplet. Similarly, two drive pulses are output from the transmission gate 99 which is open only during the period T0 to T2 and applied to the piezoelectric element 52 as shown in FIG. Is a medium drop. Similarly, five drive pulses are output from the transmission gate 99 that is open during the periods T0 to T3 and applied to the piezoelectric element 52 as shown in FIG. Is a large drop.
[0061]
By generating a drive waveform including a plurality of drive pulses and selecting the number of drive pulses to be applied to the piezoelectric element, waveforms for small droplets, medium droplets, and large droplets are generated from one drive waveform. Therefore, only one circuit and signal line are required to supply the driving waveform, and the cost can be reduced, and the circuit board and the transmission line can be reduced in size.
[0062]
Next, according to the present invention, a threshold matrix according to the present invention for transferring image data or the like to the inkjet recording apparatus is included, and a means for executing the image processing method according to the present invention including the gradation reproducing method according to the present invention is provided. The host side, which is an image processing apparatus according to the present invention equipped with a printer driver, will be described with reference to FIG.
[0063]
As described above, the above-described recording apparatus does not have a function of generating a dot pattern to be actually recorded in response to a command to draw an image or print a character in the apparatus. The dot pattern data is created by the printer driver using the threshold matrix according to the present invention that executes the tone reproduction method according to the present invention, and is transferred to the ink jet recording apparatus.
[0064]
That is, the printer driver 101 processes the image data from the application software or the like executed by the host computer in the CMM processing unit 102, the BG / UCR, the γ correction unit 103, and the zooming processing unit 104, and then processes the threshold matrix ( The multi-valued image data is replaced with a dot pattern by using the table 105 and output.
[0065]
Therefore, first, a method of creating a threshold matrix for performing tone reproduction with a predetermined line tone will be described with reference to FIG.
In image processing, as long as the resolution of the formed image exceeds the resolution of the human eye, high resolution can be realized, no matter what processing is used, there is no theoretical effect on the image quality. With a resolution that can be determined by the above, there is a possibility that a defect caused by the processing itself may be noticeable.
[0066]
FIG. 13 shows an example of a dot pattern actually formed in the halftone processing method generally used in low-resolution printing of about 300 dpi. The input image data shown in FIG. The output image when the Bayer-type dither processing is performed is as shown in FIG. 3B, and the output image when the error diffusion processing is performed is as shown in FIG.
[0067]
As described above, in order to reproduce data that should be multivalued with one pixel by using a recording apparatus with less expressive power, as shown in FIG. Key expression will be performed.
[0068]
The two types of halftone processing methods cited as examples above not only match the gradation level and the area ratio, but also arrange the dots almost uniformly so as not to cause a bias in the arrangement of the dots. It is also adjusted to have high frequency characteristics that are hardly noticeable. When these processes are applied to high-resolution printing such as 600 dpi and 1200 dpi, it is possible to obtain a very good image quality in which the dot arrangement pattern is hardly noticeable and the dot distribution is not uneven.
[0069]
On the other hand, when low-resolution printing such as 150 dpi or 300 dpi is performed, even if the processing is adjusted to have high-frequency characteristics, the dot arrangement pattern itself becomes truly noticeable. Originally, the original image data is expressed by one pixel, but is expressed by using a plurality of pixels. Therefore, a texture pattern not present in the original original image is formed on the output image.
[0070]
The example shown in FIG. 13B is also the same, but the input image data as shown in FIG. 14A is output as a considerably coarse image of 72 dpi in actual size, so that it becomes clearer. As shown in (1), a part where the texture peculiar to the Bayer type dither processing is generated and changed, and a part where the dots are arranged neatly and there is no texture at all intermingle, resulting in a very dirty image.
[0071]
On the other hand, in the error diffusion process, dots are formed in an arrangement that appears to be seemingly random. Since the randomness of the dot arrangement is maintained at all the gradation levels, as shown in FIG. 13C, the texture does not change depending on the gradation level, and there is no standard texture itself. Since there is no fixed texture, interference with mechanical fluctuations in the printing apparatus hardly occurs, and a certain degree of freedom can be obtained in dot arrangement, so that higher resolution characteristics can be obtained as compared with Bayer type dither and the like. .
[0072]
However, in the error diffusion processing, as shown in FIG. 15, there may be a large difference in granularity when compared with a Bayer type dither or the like. FIG. 7 is a comparison in the case of recording at 300 dpi. In this way, the randomness that should bring a number of advantages is easily recognized as a dirty noise component when it is noticeable at a low resolution, and an ordered texture like a Bayer type dither is generated in a sensory evaluation. Tend to be better received.
[0073]
From these facts, it can be understood that the quality of the texture pattern formed by the arrangement of the dots greatly affects the image quality. To obtain good image quality at low resolution from the above two types of processing methods described above, a dot arrangement pattern with good alignment is formed and is not changed over each gradation level (or the change is not changed). It is necessary not to make them feel).
[0074]
Therefore, the threshold value matrix according to the present invention has a matrix configuration that performs dot reproduction while maintaining a predetermined line tone (dot arrangement pattern with alignment) at all halftone levels using only the dot arrangement pattern. As a result, it is possible to improve the image quality when recording is performed by a recording device that performs low-value representation of about 1 to 3 bits at low resolution. In particular, it is possible to obtain print data suitable for an ink jet recording apparatus capable of performing the dot diameter modulation described above.
[0075]
Here, when considering a dot arrangement pattern having an alignment property (hereinafter referred to as a “keynote”), it is necessary to always consider the correlation with the mechanical fluctuation of the printing apparatus as described above. That is, as is the case with the above-described ink jet recording apparatus, as shown in FIG. 16, a recording unit including a small head 14 and a carriage 13 moves in the main scanning direction to perform recording while feeding the paper 3. Go. At this time, if unevenness occurs in the paper feeding accuracy in the sub-scanning direction or the head moving speed in the main scanning direction, it may interfere with the basic tone of the dot and be recognized as a vertical or horizontal stripe.
[0076]
For example, FIG. 17B shows interference when output is performed using one tone pattern of the Beyer-type dither shown in FIG. 17A. It can be seen that it becomes easy to synchronize with the scanning fluctuations A and B. In particular, since the human eye has high sensitivity in the 0 ° and 90 ° (180 ° and 270 °) directions, it is better to avoid a keynote that is easily aligned vertically and horizontally. However, as described in the error diffusion, the random type that causes the least interference is not preferable because the noise component is emphasized and recognized at low resolution.
[0077]
Therefore, here, the dots are arranged in an oblique basic tone as shown in FIG. In particular, by using a parallel line tone such as a 45.degree. Oblique tone or a 135.degree. Oblique tone as shown in FIG. 9A, the same effect can be obtained with respect to fluctuations in both the main scanning direction and the sub-scanning direction. Furthermore, since human vision is slightly less sensitive to oblique directions, it has a feature that it is less noticeable than vertical and horizontal tones. Here, this characteristic is an advantage since the main purpose is to make the basic tone consistent, and not to make the interference which is originally a defect inconspicuous.
[0078]
It should be noted that the parallel line tone shown in FIG. 18 has been used in electrophotographic recording as a "linear line dither". In this electrophotographic recording, a latent image is formed with a laser on a charged optical semiconductor and recording is performed by attaching and transferring toner, so that the dot size can be controlled in any number of steps by laser power modulation, Since toner adhesion and transfer failure are likely to occur, gradation expression using very small dots is not suitable. Therefore, area modulation dither (AM dither) in which dots are concentrated as much as possible to form gradually larger dots. Is generally adopted.
[0079]
The linear dither is a kind of the AM dither, and has the directivity, but has an advantage that the recording density (the number of lines) can be increased as compared with the "centralized dither" in which dots are spirally grown.
[0080]
However, even if this line dither for electrophotography is applied as it is to an ink jet recording apparatus or another recording method, the basic tone will not be uniform. That is, in the electrophotography, as shown in FIG. 19A, not only the dot size but also the dot formation position can be shifted, and therefore, no matter how many dots are arranged as shown in FIG. That is, even if the gradation level changes, the gradation can be expressed without breaking the oblique line shape.
[0081]
On the other hand, in the case of the ink jet recording apparatus, as shown in FIG. 20A, the dot formation position is fixed to a pitch determined by the recording resolution. For this reason, as shown in FIG. 3B, the basic tone changes even if the number of dots is increased by only a few dots, which deviates from the target processing method in which the basic tone does not change (or is not noticeable).
[0082]
In particular, in general dither processing, the same mask is used by being tiled in a square shape in order to simplify the processing mechanism (to increase the speed and reduce the cost). However, the pattern is recognized as a vertically and horizontally aligned pattern at the tiling cycle.
[0083]
For example, if tiling is performed as shown in FIG. 21B using a 4 × 4 mask as shown in FIG. 21A, dots are aligned vertically and horizontally as a whole. However, as shown in FIG. 3C, the base tone is a lattice-like base tone.
[0084]
Therefore, in order to maintain the above-mentioned parallel line tone, first, in order to avoid the tone change due to the tiling, three or more dots are simultaneously generated per one gradation level.
[0085]
That is, when the reproduction is performed in the oblique line basis, if a mask of one dot per gradation level is tiled as shown in FIG. 22B as shown in FIG. As shown, vertical and horizontal grid tones are used. Also, as shown in FIG. 23A, when a mask of 2 dots per gradation level (dots are arranged diagonally) is tiled as shown in FIG. As shown in (c), the tone is oblique, but the tone intersects 45 ° and 135 °.
[0086]
On the other hand, by setting the number of dots per gradation level to 3 or more as shown in FIG. 24A, even if tiling is performed as shown in FIG. As shown in (c), there is only a diagonal tone in one direction.
[0087]
In this case, the simultaneous formation of three or more dots per gradation level requires a mask size of 3 × 3 = 9 times or more in order to obtain the same gradation reproducibility. Although the value of 9 times or more seems to be large, it is insignificant in comparison with a buffer memory or the like required for error diffusion processing. Unless an extremely large mask is used as a reference, the processing speed decreases and the cost increases. However, in order to increase the speed, the vertical and horizontal sizes of the mask are set so that they can be easily processed by the computer, that is, are set to a multiple of 8 that does not generate a fraction when expanded on a memory.
[0088]
Next, the enlargement of the mask size will be described. Based on a reference mask having a diagonal parallel line as shown in FIG. 25A, a mask at the time of simultaneous generation of four dots is formed as shown in FIG. 25B, and further as shown in FIG. Each of the reference masks is divided into smaller sub-matrices so that the required number of gradations is obtained. At this time, by forming the sub-matrix to be divided into oblique parallel lines similar to the reference mask, it is possible to prevent the occurrence of a pattern that breaks the basic tone.
[0089]
For example, FIG. 6D shows that 36 sub-matrices can be expressed by using a 3 × 3 sub-matrix. FIG. 3E shows a sub-matrix of 4.times.4, which can express 64 gradations. Note that a 2 × 2 matrix is also possible as shown in FIG. 1F, but in a 2 × 2 matrix, a checker flag-like tone occurs in the process of gradation expression. Therefore, in the present invention, the minimum unit of the sub-matrix is a 3 × 3 oblique line mask.
[0090]
By using such a sub-matrix, it is possible to suppress the occurrence of another key tone that breaks the oblique line key tone.
[0091]
By the way, even when using a threshold matrix (dither matrix) formed in a line tone in a predetermined direction such as a line tone as described above, a low number of lines, that is, a continuous tone There is a case where the image quality is deteriorated due to the interruption of the performance. That is, FIGS. 26A and 26B show a part of a matrix pattern formed in a line tone in a predetermined direction. However, there is a density at which it is difficult to maintain such a line tone. .
[0092]
For example, like the gradation gradation image shown in FIG. 27A, the matrix pattern is reduced in the number of lines as shown in FIG. Image quality may deteriorate.
[0093]
Therefore, in the present invention, in a threshold matrix (dither matrix) formed based on a line tone in a predetermined direction, in order to solve the deterioration of the image quality due to the lowering of the number of lines at a part of the density, the density of the lower number of lines is reduced. A range portion is selected and the dot arrangement of the matrix between the two densities is newly arranged so as to have a high-pass filter characteristic and a line tone in a predetermined direction.
[0094]
As the high-pass filter characteristics, the spatial frequency characteristics of human vision based on the spatial frequency analysis are applied, and those with low spatial frequency characteristics are extracted. FIG. 28 is a graph showing human visual characteristics. The spatial frequency characteristic of human vision is approximated by the following equation (1) from the spatial frequency f on the retina.
[0095]
(Equation 1)

[0096]
FIG. 29B shows an example of a part other than the line tone of the matrix having the above-described high-pass filter characteristic. The characteristics of the part other than the line tone are the characteristics of the high-pass filter as shown in FIG. It is a characteristic.
[0097]
FIG. 30 shows an example of a gradation gradation image formed by a matrix having a high-pass filter characteristic in a portion other than the base tone formed in a line tone in a predetermined direction.
[0098]
That is, in the density portion of the portion A in FIG. 27A described above, the portion of the line tone shown in FIG. 30A is combined with the portion other than the line tone having the high-pass filter characteristic shown in FIG. By using the dither matrix according to the present invention as shown in FIG. 3C, a gradation gradation image having a continuous density as shown in FIG.
[0099]
In this way, by forming a matrix having a high-pass filter characteristic in a portion other than the predetermined line tone and the tone at a part of the density where the continuity of the gradation is lost only by forming the line tone in the predetermined direction, The reduction in the number of lines (loss of continuity of gradation) can be suppressed, the continuity of the line tone can be increased, and the quality of the reproduced image can be improved.
[0100]
In this case, when both dither matrices for some densities are binary images, the difference image between the two images has a high-pass filter characteristic.
[0101]
That is, in the dither method, a threshold dot set on the lower density side always exists on the higher density side. Assuming that the lower density side is the A mask and the higher density side is the B mask in both of the above-mentioned portions of the dither matrix, the line tone existing in the A mask is Always exists. Therefore, when the difference between the image obtained by binarizing the A mask and the image obtained by binarizing the B mask is obtained from the image obtained by binarizing the B mask, the image has no line tone.
[0102]
That is, as shown in FIG. 31A, between the density of the part A using the A mask and the density of the part B using the B mask, the density of the A mask shown in FIG. When the difference (BA) from the B mask shown is taken, a difference image (difference pattern) shown in FIG. 11D is obtained, and the difference image is obtained by the above-described high-pass filter characteristic as shown in FIG. Will have.
[0103]
Further, when the multi-tone image is thresholded at some densities using the matrix according to the present invention, the distribution of the power spectrum other than the angle of the line tone in a predetermined direction on the polar coordinate by the two-dimensional spatial frequency is one. Approximation.
[0104]
That is, the power spectrum of the spatial frequency of the image is defined by the following equation (2).
[0105]
(Equation 2)

[0106]
This value indicates the intensity of the spatial frequency (u, v). In order to extract the directionality from (u, v), this is expressed in polar coordinates and set as P (r, θ), and is then obtained by the following equation (3).
[0107]
[Equation 3]

[0108]
As described above, the matrix according to the present invention is formed in a line tone in a predetermined direction, and has a high-pass filter characteristic in a portion other than the tone. In the line tone portion, since there is a line tone only in a predetermined direction, the power spectrum on the polar coordinates has a strong value only in the angle in the predetermined direction. In addition, since the portion other than the base tone has a high-pass filter characteristic, the value of the power spectrum on the polar coordinates is uniformly approximated.
[0109]
FIG. 32A shows the power spectrum on the polar coordinates of the matrix according to the present invention, and FIG. 32B shows the distribution of the power spectrum on the polar coordinates excluding the line angle of 45 degrees. As shown in FIG. 7A, only the 45-degree portion has a strong value in the power spectrum on the polar coordinates of the matrix, and as shown in FIG. The values are uniformly approximated.
[0110]
Therefore, the matrix is formed in the 45-degree line tone direction, and the power spectra at other angles are more uniformly approximated by the high-pass filter characteristics.
[0111]
On the other hand, power spectrum distributions at the same density as in FIG. 32 by the Bayer-type dither matrix and the lumped-type dither matrix, which are typical mask methods of the conventional systematic dither method, are shown in FIGS. 33 and 34, respectively. become. These have higher values at a plurality of angles, respectively, and it can be seen that the characteristics of the matrix according to the present invention are different from those of the conventional matrix.
[0112]
As described above, the continuity of the line tone in a predetermined direction is enhanced by the matrix in which the number of lines is suppressed, and a multi-tone image of desired quality can be reproduced by continuous tone expression. In addition, compared to the error diffusion method, the use of the mask method simplifies the processing and increases the processing speed (printing speed, image processing speed, or image forming speed). In this case, the line tone in the predetermined direction is oblique, and the line tone is oblique, so that the line tone in the oblique direction reduces horizontal streak-like noise caused by the image forming apparatus, and a higher-quality multi-tone image is obtained. Output can be obtained.
[0113]
The threshold matrix and the tone reproduction method are applied to an image processing method for a multicolor image in which a multicolor image is decomposed into a plurality of color components and an original image of at least one color component is used as an input image. Thus, a higher-quality multicolor image can be output.
[0114]
In a multicolor image forming apparatus, generally a color printing apparatus, three basic colors of cyan, magenta and yellow, which are equal colors, are used. In addition, in order to improve the appearance of a desired color in consideration of lightness and the like, an ink jet color printing apparatus that performs printing using four primary colors by adding black to these three colors has become mainstream. I have. Further, recently, in order to improve the print quality, there has been an increasing number of printers that prepare multi-order colors in which basic colors are combined in advance and perform printing using more colors.
[0115]
As in these apparatuses, a multicolor image forming method in which an original image of each color component is decomposed into a plurality of color components and an input image is used, a pseudo gradation expression process is performed for each color component. Therefore, by applying the present invention, the continuity of the line tone in each color component unit is increased, and as a result, a higher quality image can be formed in a single color image and a multicolor image.
[0116]
In particular, in an image forming apparatus that converts each dot into binary or multi-valued image data using a matrix, it is particularly applied to a case of outputting (forming an image) with a low-resolution definition of about 300 dpi or less. Higher forming speed and printing speed and higher image quality can be achieved.
[0117]
That is, in general, an image forming apparatus that uses the pseudo gradation expression processing improves the image quality by increasing the dot density per unit, that is, the resolution. However, increasing the resolution also increases the amount of image data to be processed at the same time per area, and therefore requires a long processing time. In a pseudo-gradation expression process in an image forming apparatus having a resolution of 300 dpi, generally, a maximum of approximately 150 lines per inch is limited, and it is said that high image quality is difficult to achieve.
[0118]
In the above-described pseudo-gradation expression processing using a dither matrix having a line tone in a predetermined direction, the number of lines is reduced on human vision at some densities, and the number of lines is further reduced particularly at a resolution of 300 dpi. As a result, the continuity of the gradation expression is lost, and the image quality is noticeably reduced. By applying the present invention when forming an image in an output form (output at a low resolution) in such an image forming apparatus, the number of lines is reduced, the continuity of the line tone in a predetermined direction is increased, and It is possible to form a multi-tone image of desired quality by gradation expression.
[0119]
In the above-described embodiment, an example is described in which the host-side printer driver holds the threshold matrix as a table. However, as shown in FIG. 35, the printer driver 101 side receives application software or the like executed by the host computer. It has only a CMM processing unit 102 for processing image data, a BG / UCR, and a γ correction unit 103, and a control unit on the side of the ink jet printing apparatus 200 includes a zoom processing unit 104 and a ROM storing a threshold matrix 105 according to the present invention. In addition, the conversion into the dot arrangement can be performed on the ink jet recording apparatus side. In any case, since the processing can be performed by one-to-one comparison on the input image data, image processing can be performed at high speed and at low cost.
[0120]
In the above embodiment, an example in which the present invention is applied to an ink jet recording apparatus and its host has been mainly described. However, an image forming apparatus capable of forming an image by dots (forming an image by dot reproduction), such as a thermal transfer recording apparatus The same can be applied to. Further, the present invention can be applied to an image forming apparatus using an electrophotographic method such as a laser printer and an LED printer.
[0121]
As shown in FIG. 36, the thermal transfer recording apparatus feeds the heating element 301 of the thermal head 300 while sandwiching the paper 304 and the ink sheet 305 between the thermal head 300 having the heating element 301 and the pressure roller 303. By driving, the wax layer 307 in a predetermined area on the base layer 306 of the ink sheet 305 is transferred to the sheet 304 to record an image.
[0122]
An example of an image forming apparatus using an electrophotographic method will be briefly described with reference to FIGS. FIG. 37 is a schematic configuration diagram of the image forming apparatus, and FIG. 38 is a schematic configuration diagram of a process cartridge constituting the image forming apparatus.
[0123]
The image forming apparatus 440 is an example of a laser printer that forms a full-color image with four colors of magenta (M), cyan (C), yellow (Y), and black (Bk), and responds to an image signal for each color. Four optical writing devices 442M, 442C, 442Y, 442Bk for emitting a laser beam, four process cartridges 441M, 441C, 441Y, 441Bk for image formation, and a paper feed cassette 443 for storing recording paper onto which an image is transferred. A paper feed roller 444 that feeds recording paper from a paper feed cassette 443, a registration roller 445 that transports the recording paper at a predetermined timing, a transfer belt 446 that transports the recording paper to a transfer unit of each process cartridge, A fixing device 449 for fixing the image transferred to the recording paper; It has a configuration including a discharge roller 450 or the like for discharging the.
[0124]
The configuration of the four process cartridges 441M, 441C, 441Y, and 441Bk is the same. As shown in FIG. 34, each process cartridge 441 includes a drum-shaped photosensitive member 452, which is an image carrier, and a charging roller 453 in a case. , A developing unit 454, a cleaning blade 459, and the like.
[0125]
Further, in the developing device 454, a toner supply roller, a charging roller, an electrostatic transport substrate 457, and a toner return roller 458 are provided, and each color toner is stored. On the back side of the process cartridge 441, a slit 460 serving as a window into which a laser beam from the optical writing device is incident is provided.
[0126]
Each of the optical writing devices 442M, 442C, 442Y, and 442Bk includes a semiconductor laser, a collimator lens, an optical deflector such as a polygon mirror, a scanning imaging optical system, and the like, and a host (image processing device) such as a personal computer outside the device. ), A laser beam modulated in accordance with the image data for each color input from each of the process cartridges 441M, 441C, 441Y, and 441Bk is scanned on the photosensitive member 452 to form an electrostatic image (electrostatic latent image). Write.
[0127]
Then, when the image formation is started, the photosensitive members 452 of the respective process cartridges 441M, 441C, 441Y, and 441Bk are uniformly charged by the charging roller 453, and each of the optical writing devices 442M, 442C, 442Y, 442Bk responds to the image data. The irradiated laser beam forms an electrostatic latent image of each color on each photoconductor. The electrostatic latent image formed on the photoconductor 452 is developed and visualized by the toner of each color electrostatically transported on the electrostatic transport substrate 457 of the developing device 454. A pulse-like developing bias is applied between the opposing portion of the photoconductor 452 and the electrostatic transport substrate 457, and the electrostatic latent image on the photoconductor 452 is hopped by the toner transported by the electrostatic transport substrate 457. You. Further, the toner not subjected to the development is transported by the electrostatic transport substrate 457 and returned to the toner return roller 458.
[0128]
The recording paper in the supply cassette 443 is fed by the supply roller 444 in synchronization with the image formation of each color of 441Bk, 441Y, 441C, and 441M, and is conveyed toward the transfer belt 446 at a predetermined timing by the registration roller 445. You. Then, the recording paper is carried on the transfer belt 446 and is sequentially conveyed toward the photoconductors of the four process cartridges 441Bk, 441Y, 441C, and 441M, and the toner images of the respective colors Bk, Y, C, and M on the photoconductors. Are sequentially superimposed and transferred. The recording paper onto which the four color toner images have been transferred is conveyed to a fixing device 449 including a fixing belt 447 and a pressure roller 448, where the color image composed of the four color toner images is fixed, and is discharged onto a discharge tray 451. Is done.
[0129]
In this image forming apparatus, as shown in FIGS. 39A to 39C, the dot size is changed by changing the on-time or off-time of the laser beam emitted from each of the optical writing devices 442M, 442C, 442Y, and 442Bk. Can be changed.
[0130]
Further, the head configuration of the ink jet recording apparatus is not limited to that of the above-described embodiment, but may be a thermal ink jet head using a heating resistor or a device equipped with an electrostatic ink jet head using a diaphragm and electrodes. In the above embodiment, the present invention is applied to an image forming apparatus. However, the present invention can be similarly applied to image processing and gradation reproduction when outputting image data to an image display apparatus.
[0131]
【The invention's effect】
As described above, according to the tone reproduction method according to the present invention, when a multi-tone image is thresholded at some densities by the dither matrix, the multi-tone image is formed in a line tone in a predetermined direction, and other than the tone. In the part, the configuration having a high-pass filter characteristic is employed, so that the number of lines can be suppressed from being reduced, the continuity of the line tone can be enhanced, and good image quality can be obtained even in low resolution / high speed recording. become.
[0132]
According to the threshold value matrix according to the present invention, at least a part of the density is formed in a line tone in a predetermined direction, and a portion other than the tone has a high-pass filter characteristic. As a result, the continuity of the line tone can be enhanced, and particularly, gradation reproduction that can obtain good image quality even at low resolution / high speed recording can be performed.
[0133]
According to the image processing method of the present invention, a multi-value image is decomposed into a plurality of color components, and an original image of at least one color component is used as an input image, and a binary or multi-valued image is formed using a dither matrix. Since the tone reproduction method according to the present invention is executed at the time of conversion processing into data, good image quality can be obtained.
[0134]
According to the image processing apparatus of the present invention, the image processing method of the present invention is executed when processing image data for outputting an image composed of a plurality of dots, so that good image quality can be obtained.
[0135]
According to the printer driver of the present invention, the image processing method of the present invention is executed when processing image data for an image forming apparatus for forming an image composed of a plurality of dots. Also, good image quality can be obtained.
[0136]
According to the image forming apparatus of the present invention, the image processing method of the present invention is executed when forming an image composed of a plurality of dots, so that good image quality can be obtained even in low-resolution / high-speed recording.
[Brief description of the drawings]
FIG. 1 is a schematic perspective explanatory view of a mechanism showing an example of an ink jet recording apparatus as an image forming apparatus.
FIG. 2 is an explanatory side view of the mechanism.
FIG. 3 is an exploded perspective view showing an example of a head of the recording apparatus.
FIG. 4 is an explanatory cross-sectional view of the head along the liquid chamber longitudinal direction.
FIG. 5 is an enlarged explanatory view of a main part of FIG. 4;
FIG. 6 is an explanatory cross-sectional view of the head taken along a lateral direction of the liquid chamber.
FIG. 7 is an explanatory plan view of a nozzle plate of the head.
FIG. 8 is a block diagram illustrating an outline of a control unit of the recording apparatus.
FIG. 9 is a block diagram of a part related to head drive control of the control unit.
FIG. 10 is a block diagram showing an example of the head drive circuit of FIG. 9;
FIG. 11 is an explanatory diagram for describing an operation of a portion related to the head drive control;
FIG. 12 is a block diagram illustrating an example of a configuration of a host which is an image processing apparatus according to the present invention equipped with a printer driver according to the present invention including a threshold matrix according to the present invention;
FIG. 13 is an explanatory diagram showing a dot pattern after Bayer-type dither processing and error diffusion processing for an input image;
FIG. 14 is an explanatory diagram showing image data after Bayer-type dither processing on an input image;
FIG. 15 is an explanatory diagram when the granularity of a 300 dpi Bayer-type dither pattern and an error diffusion pattern are measured every 10% density.
FIG. 16 is an explanatory diagram for explaining mechanical fluctuation factors in the inkjet recording apparatus.
FIG. 17 is an explanatory diagram for explaining interference between a Bayer-type dither pattern and mechanical fluctuation of a printing apparatus;
FIG. 18 is an explanatory diagram for explaining a dot arrangement pattern of an oblique tone in the threshold value matrix according to the present invention.
FIG. 19 is an explanatory diagram for describing a dot pattern of a parallel line dither pattern and a basic tone change of a gradation level of the electrophotographic recording apparatus.
FIG. 20 is an explanatory diagram for describing a dot pattern and a basic tone change of a gradation level when the method is applied to an ink jet recording apparatus.
FIG. 21 is an explanatory diagram for describing a base tone formed by tiling a dither mask;
FIG. 22 is an explanatory diagram for describing the tiling and the base tone when the mask has one dot per one gradation level;
FIG. 23 is an explanatory diagram serving to explain the tiling and basic tone when the mask has two dots per gradation level;
FIG. 24 is an explanatory diagram provided for describing tiling and basic tone when a mask has three dots per gradation level;
FIG. 25 is an explanatory diagram for describing division of a basic matrix into sub-matrices;
FIG. 26 is an explanatory diagram illustrating a different example of a matrix pattern having a line tone in a predetermined direction.
FIG. 27 is an explanatory diagram illustrating an example of a gradation pattern and a matrix pattern in which the number of lines is reduced in some densities.
FIG. 28 is an explanatory diagram illustrating human visual characteristics.
FIG. 29 is an explanatory diagram of a portion other than a base tone having a high-pass filter characteristic of a matrix pattern.
FIG. 30 is an explanatory diagram for describing a gradation gradation image using a matrix having a high-pass filter characteristic in a predetermined line tone and a portion other than the tone.
FIG. 31 is an explanatory diagram illustrating a matrix difference image between partial densities;
FIG. 32 is an explanatory diagram for describing a power spectrum distribution on a polar coordinate of a matrix according to the present invention;
FIG. 33 is an explanatory diagram for describing a power spectrum distribution on polar coordinates of a conventional Bayer dither matrix;
FIG. 34 is an explanatory diagram for describing a power spectrum distribution on a polar coordinate of a conventional centralized dither matrix;
FIG. 35 is a block diagram for explaining an image forming apparatus according to the present invention;
FIG. 36 is a schematic explanatory view of a thermal transfer recording apparatus.
FIG. 37 is a schematic configuration diagram of an image forming apparatus using an electrophotographic method.
FIG. 38 is a schematic configuration diagram of a process cartridge of the image forming apparatus.
FIG. 39 is an explanatory diagram for explaining a change in dot size;
FIG. 40 is an explanatory diagram for describing dots that are actually arranged when gradation expression is performed by binarization and reduction of values;
FIG. 41 is an explanatory view serving to explain a binarization process using a dither method;
FIG. 42 is an explanatory diagram serving to explain the correspondence between size-modulated dots and dither masks in the reduction processing;
FIG. 43 is an explanatory diagram illustrating an example of a dither mask of each dot size.
FIG. 44 is an explanatory diagram illustrating a procedure of a binary error diffusion process.
[Explanation of symbols]
13: carriage, 14: recording head, 87: drive waveform generation circuit, 88: head drive circuit, 91: main control unit, 101: printer driver, 105: threshold value matrix.

Claims (12)

In a tone reproduction method of converting a multi-tone image into binary or multi-value image data for each dot using a dither matrix, when the multi-tone image is thresholded at some densities by the dither matrix, A tone reproduction method characterized by being formed in a line tone in a predetermined direction and having a high-pass filter characteristic in a part other than the tone. 2. The gradation reproduction method according to claim 1, wherein when both dither matrices between a part of the density of the dither matrix are binary images, a difference image between the two images has a high-pass filter characteristic. Tone reproduction method. 3. The tone reproduction method according to claim 1, wherein when the multi-tone image is thresholded at a part of the density by the dither matrix, a line tone in a predetermined direction on a polar coordinate based on the two-dimensional spatial frequency is obtained. A tone reproduction method characterized in that the distribution of the power spectrum other than the angle is uniformly approximated. 4. The gradation reproducing method according to claim 1, wherein said base tone is a line-based tone. In a threshold matrix for converting a multi-tone image into binary or multi-value image data for each dot, at least a part of the density is formed in a line tone in a predetermined direction, and a high-pass filter characteristic is formed in a portion other than the tone. A threshold matrix having: 6. The threshold matrix according to claim 5, wherein, when both matrices between the partial densities are binary images, a difference image between the two images has a high-pass filter characteristic. 7. The threshold matrix according to claim 5, wherein a distribution of a power spectrum other than an angle of a line tone in a predetermined direction on a polar coordinate based on a two-dimensional spatial frequency is uniformly approximated for a part of the density of the multi-tone image. A threshold matrix. An image processing method for decomposing a multi-color image into a plurality of color components and converting the original image of at least one color component into binary or multi-value image data for each dot using a dither matrix as an input image, An image processing method, comprising: executing the gradation reproduction method according to claim 1. An image processing apparatus for processing image data for outputting an image composed of a plurality of dots, comprising: means for executing the image processing method according to claim 8. 9. A printer driver for processing image data for an image forming apparatus for forming an image composed of a plurality of dots, wherein the printer driver includes means for executing the image processing method according to claim 8. 9. An image forming apparatus for forming an image composed of a plurality of dots, comprising: means for executing the image processing method according to claim 8. The image forming apparatus according to claim 11, wherein the image forming apparatus is an ink jet recording apparatus or a thermal transfer recording apparatus.

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