JP4711542B2 - How to make a mask for halftone processing - Google Patents

Description

【００６６】
式（１）のＶは、人間の視覚特性を近似した関数（ＶＴＦ）であり、次式（２）で与えられる。
【００６７】
【数２】

【００６８】
なお、Ｌは、観察距離であり、ｄは、解像度である。また、ＶＴＦについては、様々な論文等で報告されているが、文献によって微妙に計数等が異なっているものもあり、ＶＴＦは、式（２）に限定されるものではなく、解像度と観察距離をパラメータとした広義のＶＴＦに対応するものである。また、ＶＴＦは、あくまでも人間の視覚特性を基にしていることが前提であるが、例えば、式（２）の係数違いや、ｆ（ｘ，ｙ）＜５．０の範囲を線形式に置き換えたものもＶＴＦに含まれるものとする。
【００６９】
そして、図５における最小ドット数ｎは、再現画素サイズＤにより決定する。通常、２〜３ドット分を集めることで、再現画素サイズＤを満たすことができる。なお、最小ドット数ｎは、必要以上にドットを集めると、相互作用によって帯電量が増加し、ドットゲインによって逆に画素の太りが発生してしまうため、あくまでも再現画素サイズＤを満たす最小ドット数とする。
【００７０】
切り換えを行う解像度は、上述のように、２〜３ドット分を集めることで再現画素サイズＤを満たすことができるため、ハイライト部では最高解像度に対して縦横２倍、すなわち、１／２の解像度に設定することが好ましい。すなわち、切り換えを行う解像度を、１／２倍、２倍という関係にすることで、ドット配置周期を同期させることができるためである。切り換え前後でドット配置周期が異なると、切り換え前に配置されたドットによって選択可能な位置が制限されたり、また、その逆に、必要以上に空きスペースが出現し、結果としてマスク品質を落とすこととなるからである。
【００７１】
次に、切り換えを行う階調レベルについては、図６に示すように、解像度を落とした画素の周囲にある程度残っている空間に設定することが好ましい。例えば、ハイライト側の画素を再現するために２×２の計４ドットを集中させる必要がある場合、この４ドット画素同士が結合しないまでも非常に近い位置に隣接すると、双方の電荷が影響しあって急激なドットゲインが生じてしまうおそれがある。この４ドット画素が互いに近接しないだけの距離を置くようにすると、例えば面積率２５％、すなわち階調レベル６４／２５５レベル付近に切り換えレベルを設定することが好ましい。
【００７２】
この切り換えレベル以降は１ドット画素に切り換えて配置が行われる。このとき、選択可能なドット位置の近傍には、低解像度時に配置した４ドット画素が存在するため、相互作用によって、ハイライト部では再現できなかった１ドット画素も適切に再現される。ただし、４ドット画素側にも僅かながらドットゲインが発生するため、濃度保存特性を考慮して、各階調レベル毎にＯＮ（オン）となるドット数の調整を行う。解像度の切り換え等を含めて考えると、ドット配置の自由度を確保するためには、最低でも３２×３２ドット以上の大きさのマスクとして設計を行う必要がある。
【００７３】
このように、本実施の形態によれば、画像データをハーフトーンイメージに変換する閾値マトリックスとしての周波数変調型のハーフトーン処理用マスクを作成するに際して、変換したハーフトーンイメージを出力する画像出力装置の画素再現特性に応じて、マスク最適時に用いる評価関数を制御してハーフトーン処理用マスクを作成している。
【００７４】
したがって、画素再現の不安定な階調領域を有する電子写真方式や熱転写方式等であっても好適なハーフトーン処理を行うことのできるハーフトーン処理用マスクを作成することができる。
【００７５】
また、評価関数として人間の視覚特性を近似した関数を用い、画像出力装置の画素再現特性に応じて、人間の視覚特性を近似した関数の解像度パラメータの切り換えを行っている。
【００７６】
したがって、ハーフトーン処理用マスクの品質を向上させるとともに、ＶＴＦが解像度パラメータを持つことを利用して、画像出力装置の画素径変動に合わせて解像度パラメータを調整することができ、画像出力装置に応じて最適化することのできるハーフトーン処理用マスクを作成することができる。
【００７７】
さらに、人間の視覚特性を近似した関数の解像度パラメータを、もとの解像度に対して、２倍または１／２倍の解像度に切り換えている。
【００７８】
したがって、お互いに最適化されるドット配置を同周期として、低解像度時の最適化パターンの隙間に高解像度時の最適化パターンを互いに干渉せずに形成することのできるハーフトーン処理用マスクを作成することができる。
【００７９】
また、画像データをハーフトーンイメージに変換する閾値マトリックスとしての周波数変調型のハーフトーン処理用マスクを作成するに際して、変換したハーフトーンイメージを画像出力装置で出力する際の画像サイズに応じて、マスク最適時に用いる評価関数を制御してハーフトーン処理用マスクを作成している。
【００８０】
したがって、画像サイズによって好適な観察距離が変わることを利用して、複写機等のある程度使用する用紙サイズが固定された画像出力装置において、用紙サイズをハーフトーン処理用マスクの最適化に取り入れて、用紙毎に最適なハーフトーン処理用マスクを作成することができる。
【００８１】
さらに、この場合にも、評価関数として人間の視覚特性を近似した関数を用い、出力する際の画像サイズに応じて、人間の視覚特性を近似した関数の観察距離パラメータの切り換えを行っている。
【００８２】
したがって、マスクの品質を向上させることができるとともに、ＶＴＦが観察距離パラメータを持つことを利用して、画像出力装置の用紙サイズに応じて最適化することのできるハーフトーン処理用マスクを作成することができる。
【００８３】
また、ハーフトーン処理用マスクとして、３２×３２以上の大きさのマスクを作成しているので、１階調レベル当たりのオン（ＯＮ）ドット数を増やすことができ、階調間でＯＮドットの数を調整することのできるハーフトーン処理用マスクを作成することができる。
【００８４】
以上の説明では、作像エンジンが主に電子写真方式の作像エンジンである場合について説明したが、作像エンジンとしては、電子写真方式に限るものではなく、例えば、熱転写方式の作像エンジンや近年急速に普及の進んだインクジェット方式の作像エンジンのマスクとしても、同様に適用することができる。
【００８５】
インクジェット方式の作像エンジンの画像出力装置、例えば、インクジェットプリンタの場合、図１に示したように、インク滴の容量・粘度・飛翔スピード等の関係から、飛翔中にインク滴が分裂して用紙表面に着弾することがあるが、インク滴の微細化が急速に行われており、最高解像度での出力が可能である。
【００８６】
ところが、ほとんどインクジェットプリンタは、シリアル走査型の噴射ヘッドで記録を行うため、インク滴の微細化はそのまま記録時間の増大につながる。そこで、従来から噴射ノズルの増加や噴射タイミングの短縮化も試みられているが、装置のサイズ・コスト・安定性といった点で限界があり、解像度が要求されない、または、判別のつき難い部分では大きなインク滴を噴射し、高画質が要求される部分では微細なインク滴を噴射することで記録時間の短縮を図る方法が用いられているのがほとんどである。
【００８７】
したがって、本実施の形態をインクジェットプリンタに適用する場合には、上記電子写真方式の場合とは逆になるが、ドットが目に付かないように微少なインク滴で記録が行われるハイライト側からミドルにかけては、ＶＴＦの解像度パラメータを最高解像度でＦＭマスクを最適化し、大きなインク滴が使用されるシャドー側にかけては、解像度を落として最適化を行う。
【００８８】
すなわち、インクの重ね撃ちが可能なインクジェット方式では、インクの重なった部分はより濃く表現されてしまうため、大きなインク滴によるドットの重なりが規則的に配置されていると非常に目立ってしまう。ところが、上述のように、大きなインク滴が使用されるシャドー側にかけては、解像度を落として最適化を行うと、大インク滴によるドットの重なりも大ドットのサイズに合わせた最適化が施されるため、ドットの重なりによる規則的な配置が目に付きにくい高品質な画像を再現することができる。
【００８９】
なお、上記説明では、主にＶＴＦの解像度に着目してＦＭマスクの最適化を行っているが、もう一つのパラメータである「観察距離」に着目して最適化を行うこともできる。
【００９０】
例えば、ハーフトーン処理用のマスクを搭載または専用のソフトウエアに組み込む形で利用する画像出力装置、例えば、複写機やファクシミリ装置、また、レーザ方式、熱転写方式、インクジェット方式等のプリンタでは、出力する用紙サイズは、ほぼ数種類に限定することができ、観察距離は、この用紙サイズによって好適な距離が変わるため、画像出力装置が搭載する用紙モードのそれぞれに対応した観察距離を設定してＦＭマスクの最適化を行うことで、用紙サイズに最適なＦＭマスクを作成することができる。
【００９１】
また、観察距離と同時に画素再現性を考慮することで、より高品質なＦＭマスクを作成することができる。
【００９２】
さらに、上述のようにして作成したＦＭマスクを、他のハーフトーン処理と組み合わせて用いることもできる。
【００９３】
例えば、誤差拡散法では、「ワーム」と呼ばれる独特のテクスチャーが発生しやすく、これを防ぐために、閾値にランダムノイズを付加する方法が一般的に知られている。この閾値に、上記ＦＭマスクを利用することで、本来、解像度や観察距離といった概念を持たない誤差拡散法に対しても、解像度や観察距離を考慮したハーフトーン処理を行うことができる。
【００９４】
また、上記説明では、ハイライト側の階調レベル１／２５５から設計が開始されているが、図７に示すように、途中のレベルから設計を開始してもよい。なお、図７では、図５と同一の処理ステップについては、同一のステップナンバを付して、その説明を省略する。
【００９５】
すなわち、再現画素サイズＤから最適な解像度ｄ１を算出すると（ステップＳ１０２）、スタートレベルがレベルｇよりハイライト側かチェックし（ステップＳ２０１）、ハイライト側であると、ＶＴＦの解像度を最適な解像度ｄ１に設定し、１画素を最小ドット数ｎでハイライト側に向けてＦＭマスクの最適化を行う（ステップＳ２０２）。
【００９６】
次に、階調レベル１／２５５に達したかチェックし（ステップＳ２０３）、階調レベル１／２５５に達していないときには、ステップＳ２０２に戻って、同様の処理を繰り返す。ステップＳ２０３で、階調レベル１／２５５に達すると、ＶＴＦの解像度を最適な解像度ｄ１に設定し、１画素を最小ドット数ｎでスタートレベルからシャドー側に向けてＦＭマスクの最適化を行い（ステップＳ２０４）、以下、上記図５の場合と同様に、切り換えレベルｇかチェックする（ステップＳ１０４）。ステップＳ１０４で、切り換えレベルｇになると、ＶＴＦの解像度を最高解像度に戻し、１画素１ドットで、シャドー側に向かってＦＭマスクの最適化を行う（ステップＳ１０５）。
【００９７】
また、ステップＳ２０１で、スタートレベルがレベルｇよりハイライト側でないときには、ＶＴＦを最高解像度に設定し、１画素を１ドットでハイライト側に向かって最適化し（ステップＳ２０５）、切り換えレベルｇかチェックする（ステップＳ２０６）。ステップＳ２０６で、切り換えレベルｇでないときには、ステップＳ２０５に戻って、上記同様の処理し、切り換えレベルｇになると、ＶＴＦの解像度を最適な解像度ｄ１に、最小ドット数をｎに設定して、ハイライト側に向かって最適化する（ステップＳ２０７）。そして、階調レベル１／２５５に達したかチェックし（ステップＳ２０８）、階調レベル１／２５５に達していないときには、ステップＳ２０７に戻って、同様の処理を繰り返す。ステップＳ２０８で、階調レベル１／２５５に達すると、ＶＴＦの解像度を最高解像度に戻し、１画素１ドットで、シャドー側に向かってＦＭマスクの最適化を行う（ステップＳ１０５）。
【００９８】
そして、上記ハーフトーン処理用マスク（ＦＭマスク）やこのハーフトーン処理用マスクを用いた誤差拡散処理方法のプログラムは、所定の記録媒体、例えば、専用のＲＯＭに書き込んで画像処理モジュールとして画像出力装置に搭載したり、画像出力装置の制御用のプログラムとして、ＦＤ（フロッピーディスク：登録商標）やＣＤ（Compact Disc）等の記録媒体に書き込んだり、また、ネットワークを介してコンピューター等にインストールして使用することができる。
【００９９】
そして、このようなハーフトーン処理用マスク作成方法で作成したハーフトーン処理用マスクや誤差拡散処理方法のプログラムを記録した記録媒体を画像出力装置に読み取らせて、画像処理モジュールとして搭載させることで、容易に適切なハーフトーン処理を行う画像出力装置を実現することができる。
【０１００】
この場合、図８に示すように、例えば、画像出力装置１として、コンピュータ１０にプリンタ２０が接続されたものに適用すると、コンピュータ１０のＯＳ（Operating System ）の描画関連の中間モジュール（ＧＤＩ）３０からの画像データに対して、コンピュータ１０に搭載されているプリンタドライバ１１で、上記画像処理のカラーマッチングまでの処理を行い、プリンタ２０に搭載した処理モジュールで中間処理を含む残りの処理を行う。なお、図８では、コンピュータ１０側とプリンタ２０側で画像処理を分けて行っているが、近年のコンピュータ１０は、高速化しており、全てソフトウェア化した方が処理が速い場合もあるため、全ての画像処理をコンピュータ１０側で行うようにしてもよい。
【０１０１】
以上、本発明者によってなされた発明を好適な実施の形態に基づき具体的に説明したが、本発明は上記のものに限定されるものではなく、その要旨を逸脱しない範囲で種々変更可能であることはいうまでもない。
【０１０２】
【発明の効果】
本発明によれば、ハーフトーン処理用マスクの品質を向上させるとともに、ＶＴＦ（人間の視覚特性を近似した関数）が解像度パラメータを持つことを利用して、画像出力装置の階調に応じた画素径変動に合わせて解像度パラメータを調整することができ、画像出力装置に応じて最適化することができるハーフトーン処理用マスクを作成することができる。
【図面の簡単な説明】
【図１】画像のハイライト部におけるドット再現性をハーフトーン処理方法及び作像エンジン別に示す図。
【図２】電子写真方式における帯電電荷の分布の説明図。
【図３】ハイライト部を１本のビームで画素形成を行っている状態（ａ）と複数のビームで画素形成を行っている状態（ｂ）を示す図。
【図４】分散系の画素配列（ａ）の個々の画素に対して画素径変動の要素を付加している状態（ｂ）を示す図。
【図５】本実施の形態によるＦＭマスク作成処理を示すフローチャート。
【図６】画素間の距離と再現される画素サイズとの関係を示す図。
【図７】階調レベルの途中からマスク設計を行う処理を含むＦＭマスク作成処理を示すフローチャート。
【図８】本実施の形態のハーフトーン処理用マスク作成方法で作成されたハーフトーン処理用マスク及びこのハーフトーン処理用マスクを用いた誤差拡散処理法を適用した画像出力装置の概略構成図。
【符号の説明】
１ 画像出力装置
１０ コンピュータ
１１ プリンタドライバ
２０ プリンタ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for making a mask for halftone processing. To the law In particular, a good threshold mask for individual image output devices such as an electrophotographic recording device in which pixel reproduction characteristics tend to vary and an inkjet recording device that balances image quality and output speed depending on the size of pixels How to create a halftone mask for creating To the law Related.
[0002]
[Prior art]
Conventionally, methods for creating a halftone image in digital image processing are roughly classified into a “dither method” and an “error diffusion method”, each having the following characteristics.
[0003]
First, the dither method is light in arithmetic processing, capable of high-speed processing, is suitable for low to medium image quality, and moiré countermeasures are essential. On the other hand, the error diffusion method has a heavy burden of arithmetic processing, a low processing speed, a high image quality, high speed, and low cost.
[0004]
As described above, since both the dither method and the error diffusion method have advantages and disadvantages, they are generally used in accordance with the configuration of the image output apparatus and the type of image.
[0005]
Then, in the late 1900s, a halftone processing method having the advantages of these two methods was proposed. Generally, it is a method called “FM mask method”, “FM screen method”, “blue noise mask method”, etc., and is classified into “dither method”, in particular, frequency modulation type (as with Bayer type). FM (Frequency Modulation) is a method of mask comparison. This halftone processing method is a mask comparison method by adding a frequency characteristic (Blue Noise characteristic: BN characteristic) composed only of a high-frequency component having no low-frequency component at the time of threshold determination. The periodicity found in molds and dot-concentrated dithering can be eliminated, the resistance to moire can be increased, and resolution characteristics close to error diffusion can be achieved, so various approaches from various fields including the printing field. Has been tried.
[0006]
As an early attempt, in “Method and apparatus for halftoning a grayscale image using a blue noise mask” described in Japanese Patent No. 2622429, a completely random (white noise) dot pattern is Fourier transformed. After conversion and filtering with a filter having a BN (blue noise) characteristic, an attempt is made to construct an ideal FM mask by performing an inverse Fourier transform.
[0007]
Japanese Patent Laid-Open No. 6-6586 has proposed a method of multiplying a function (MTF) that approximates visual characteristics after performing Fourier transform.
[0008]
[Problems to be solved by the invention]
However, such a conventional halftone processing method needs to be improved when performing appropriate processing in consideration of an image output unit, human visual characteristics, and the like.
[0009]
In other words, these conventional halftone processing methods perform appropriate halftone processing when, for example, a very ideal system in which the pixel diameter, shape, and resolution to be reproduced are balanced is targeted. However, in reality, it is difficult to perform complete digitization due to fluctuations in the output unit (including the input unit in some cases), human visual characteristics (MTF), etc. It is necessary to consider various factors.
[0010]
For example, in the technique described in Japanese Patent No. 2622429, etc., the characteristic is simply determined by the cut-off frequency calculated from the gradation level and the resolution, so it is not necessarily optimal for human visual characteristics. In the technique described in Japanese Patent No. 2622429, there is no consideration for fluctuations in the output system and the like, and there is a need for improvement in performing appropriate mask processing. It was.
[0011]
In the technique described in Japanese Patent Laid-Open No. 6-6586, a function (MTF) approximating visual characteristics is used as an evaluation function for optimizing a mask. A distance (observation distance) between an image and a person viewing it can be taken into consideration, and a mask more suitable for human eyes can be constructed. However, the details of this technique are not described in detail because it is not intended to use MTF as an evaluation function, and it is considered that resolution, observation distance, pixel diameter, etc. are substantially fixed, and image output is performed. There was a problem that it was not possible to respond appropriately to the fluctuation of the department.
[0012]
Therefore, The present invention improves the quality of a halftone processing mask and uses the fact that VTF (a function approximating human visual characteristics) has a resolution parameter to change the pixel diameter according to the gradation of an image output device. A halftone mask that can be adjusted according to the image output device and can be adjusted according to the image output device. For the purpose.
[0025]
[Means for Solving the Problems]
The invention of claim 1 converts image data into a halftone image, One pixel can be formed by only one recording dot or a plurality of adjacent recording dots. Image output device Then, each pixel of the halftone image is switched between the formation with only one recording dot and the formation with the plurality of adjacent recording dots according to the gradation. A halftone processing mask creation method for creating a halftone processing mask for output, wherein the mask creation method uses a resolution-dependent function approximating human visual characteristics as an evaluation function. Including the process Form with adjacent multiple recording dots Gradation when pixels are placed close to each other and kept at a distance that does not cause a sharp dot gain. level Switch tone As a level Set , In the mask optimization process, the resolution set in the function is set lower on the highlight side than the shadow side on the switching gradation level in accordance with the number of the adjacent recording dots forming one pixel. For the halftone processing It is characterized by optimizing the mask.
[0051]
DETAILED DESCRIPTION OF THE INVENTION
DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described in detail based on the accompanying drawings. The embodiments described below are preferred embodiments of the present invention, and thus various technically preferable limitations are given. However, the scope of the present invention is particularly limited in the following description. As long as there is no description which limits, it is not restricted to these aspects.
[0052]
First, the premise of this invention is demonstrated based on FIGS. FIG. 1 shows digital data, an electrophotographic method, or a thermal transfer method, in which a halftone image is generated and processed by an error diffusion method, a Bayer method, an AM mask method, and an FM mask method for a solid image of a certain gradation level. 2 shows an image recorded and output by the ink jet method and an image recorded and output by the ink jet method.
[0053]
In the binarization processing generally used, image signals having an information amount of 8 bits or more are classified into “0” and “1” levels, and pixel outputs “OFF” and “ON” are classified. To generate an image. As the halftone processing methods, the four types shown in FIG. 1 are common, and methods other than the error diffusion method use a threshold mask for halftoning in advance.
[0054]
Normally, in digital image processing, “ON” is always calculated to be output with a certain level, but actually, as shown in FIG. The shape, size, position, and density of the reproduced pixel are greatly affected by the image engine characteristics.
[0055]
For example, in an electrophotographic image forming engine, an ideal pixel can be formed if the resolution is sufficiently low for engine performance. However, when the resolution is as high as the engine performance, as shown in FIG. 1, the pixels themselves are lost without being reproduced, or are reproduced as pixels far from the ideal. As shown in FIG. 2, the distribution of the electrostatic latent image formed on the photosensitive member is biased with respect to the beam spot diameter that determines the minimum recording size. This is because information deteriorates every time an image forming process such as transfer of image data is performed. Even in the image forming engine of the thermal transfer system, there are differences in the system of charge and heat, but the same problem as above can be seen.
[0056]
Many physical correction processes for this phenomenon have been devised in the past, but they tend to be greatly influenced by environmental conditions such as humidity and temperature, material types such as paper type, toner and image output device deterioration over time, In effect, the effect is not so high. In particular, as the resolution of the image output apparatus increases, this tendency becomes more prominent. Therefore, in the case of FIG. 3A, in the highlight portion that should be originally recorded at the highest resolution, FIG. As shown in (b), pixels are surely formed by concentrating several beams and reducing the resolution of the pixel diameter.
[0057]
In image processing, as shown in FIG. 3B, when pixels are formed with a plurality of beams, an image as shown in FIG. 4A is usually shown in FIG. 4B. A method of adding an element of variation in pixel diameter (AM: Amplitued Modulation) to each pixel after creating a dispersed pixel array is adopted.
[0058]
However, in this method, since there is no relationship between the initially set dispersion pixel array and the AM characteristic added later, the pixel diameter and the pixel size are added to add the BN characteristic like the FM mask. For masks designed with careful pixel array calculations, the quality is greatly reduced.
[0059]
Therefore, in the present embodiment, an evaluation function used for FM mask optimization, specifically, human's optimization, in accordance with the timing (gradation level) at which the pixel diameter is switched in accordance with such characteristics of the image forming engine. The mask is optimized while switching the function (VTF) parameter considering the visual characteristics. Hereinafter, the contents of the present embodiment will be described with reference to FIGS. In the following description, a case where an electrophotographic image forming engine is used as the image forming engine will be described.
[0060]
Masking while changing parameters of a function (VTF) approximating human visual characteristics as an evaluation function used for FM mask optimization in accordance with the timing (gradation level) of switching the pixel diameter according to the characteristics of the image forming engine The optimization process when performing the optimization is as shown in FIG.
[0061]
That is, first, based on the characteristics of the image output device, the minimum number of dots n required to reproduce one pixel, the pixel size D that is actually reproduced, and one pixel / one dot are switched. tone The level g is calculated (step S101). Next, the optimum resolution d1 is calculated from the reproduction pixel size D (step S102), the resolution of the function (VTF) approximating human visual characteristics is set to d1, and one pixel is set to the minimum number of dots n to highlight. The FM mask is optimized from the side (step S103).
[0062]
Then, the switching level g is checked (step S104). When the switching level g is reached, the resolution of the VTF is returned to the maximum resolution, and the FM mask is optimized toward the shadow side by one dot per pixel (step S105). .
[0063]
As this optimization method, random noise (White Noise) or error diffusion processing is used to temporarily convert the binary bitmap into the power spectrum, the relative distance between dots on the bitmap, or their positions. The repulsive potential based on the relationship is calculated, and the dot position where the total value is minimized is searched for and replaced.
[0064]
For example, the following formula (1) is used as a formula for calculating an evaluation value when a power spectrum is used.
[0065]
[Expression 1]

[0066]
V in Expression (1) is a function (VTF) that approximates human visual characteristics, and is given by Expression (2) below.
[0067]
[Expression 2]

[0068]
Note that L is an observation distance and d is a resolution. Further, VTF has been reported in various papers and the like, but there are some that are slightly different in counting and the like depending on the literature, and VTF is not limited to Equation (2), but resolution and observation distance. This corresponds to the VTF in a broad sense using as a parameter. VTF is premised on the basis of human visual characteristics, but for example, the coefficient difference in equation (2) or the range of f (x, y) <5.0 is replaced with a linear format. Are also included in the VTF.
[0069]
The minimum number n of dots in FIG. Usually, the reproduction pixel size D can be satisfied by collecting 2 to 3 dots. Note that the minimum number of dots n is that if the dots are collected more than necessary, the charge amount increases due to the interaction, and the pixel gain increases due to the dot gain. And
[0070]
Since the resolution for switching can satisfy the reproduction pixel size D by collecting 2 to 3 dots as described above, the highlight portion has twice the vertical and horizontal directions, that is, 1/2 of the maximum resolution. It is preferable to set the resolution. That is, the dot arrangement period can be synchronized by setting the resolution for switching to 1/2 times or 2 times. If the dot placement cycle is different before and after switching, the positions that can be selected are limited by the dots placed before switching, and conversely, more free space appears than necessary, resulting in poor mask quality. Because it becomes.
[0071]
Next, as shown in FIG. 6, it is preferable that the gradation level to be switched is set in a space that remains to some extent around the pixel whose resolution has been reduced. For example, when it is necessary to concentrate a total of 4 dots of 2 × 2 in order to reproduce the highlight-side pixels, if both the 4-dot pixels are adjacent to each other even if they are not connected, both charges are affected. As a result, there is a risk of abrupt dot gain. If a distance is set such that the 4-dot pixels are not close to each other, it is preferable to set the switching level, for example, at an area ratio of 25%, that is, near the gradation level 64/255 level.
[0072]
After this switching level, the arrangement is performed by switching to one dot pixel. At this time, since there are 4 dot pixels arranged at the time of low resolution in the vicinity of the selectable dot positions, 1 dot pixels that could not be reproduced in the highlight portion are appropriately reproduced by the interaction. However, since a slight dot gain is also generated on the 4-dot pixel side, the number of dots that are turned on for each gradation level is adjusted in consideration of density preservation characteristics. Considering resolution switching and the like, it is necessary to design a mask with a size of 32 × 32 dots or more at least in order to ensure the freedom of dot arrangement.
[0073]
As described above, according to the present embodiment, when creating a frequency modulation type halftone processing mask as a threshold matrix for converting image data into a halftone image, an image output device that outputs the converted halftone image The halftone processing mask is created by controlling the evaluation function used at the time of mask optimization in accordance with the pixel reproduction characteristics.
[0074]
Therefore, it is possible to create a halftone processing mask capable of performing a suitable halftone process even with an electrophotographic system, a thermal transfer system, or the like having a gradation region where pixel reproduction is unstable.
[0075]
Further, a function approximating human visual characteristics is used as the evaluation function, and the resolution parameter of the function approximating human visual characteristics is switched according to the pixel reproduction characteristics of the image output apparatus.
[0076]
Therefore, the quality of the halftone processing mask is improved, and the resolution parameter can be adjusted according to the pixel diameter variation of the image output device by utilizing the fact that the VTF has the resolution parameter. A mask for halftone processing that can be optimized.
[0077]
Furthermore, the resolution parameter of the function that approximates human visual characteristics is switched to a resolution that is twice or half the original resolution.
[0078]
Therefore, creating a halftone processing mask that can form optimized patterns at high resolution without interfering with each other in the gap between optimized patterns at low resolution with the same dot arrangement optimized for each other. can do.
[0079]
Further, when creating a frequency modulation type halftone processing mask as a threshold matrix for converting image data into a halftone image, the mask is set in accordance with the image size when the converted halftone image is output by the image output device. The halftone processing mask is created by controlling the evaluation function used at the optimum time.
[0080]
Therefore, by utilizing the fact that the preferred observation distance varies depending on the image size, in the image output device in which the paper size to be used to some extent is fixed, such as a copying machine, the paper size is taken into the optimization of the halftone processing mask, An optimum halftone processing mask can be created for each sheet.
[0081]
In this case as well, a function approximating human visual characteristics is used as an evaluation function, and the observation distance parameter of the function approximating human visual characteristics is switched according to the image size at the time of output.
[0082]
Therefore, it is possible to improve the quality of the mask and to create a halftone processing mask that can be optimized according to the paper size of the image output apparatus by using the fact that the VTF has an observation distance parameter. Can do.
[0083]
In addition, since a mask of 32 × 32 or more is created as a halftone processing mask, the number of ON (ON) dots per gradation level can be increased, and the ON dot between gradations can be increased. It is possible to create a halftone processing mask whose number can be adjusted.
[0084]
In the above description, the case where the image forming engine is mainly an electrophotographic image forming engine has been described. However, the image forming engine is not limited to the electrophotographic method, for example, a thermal transfer image forming engine, The present invention can also be applied in the same manner as a mask of an inkjet image forming engine that has been rapidly spread in recent years.
[0085]
In the case of an image output device of an ink jet image forming engine, for example, an ink jet printer, as shown in FIG. 1, due to the relationship between the ink droplet capacity, viscosity, flight speed, etc., the ink droplets break up during the flight and the paper Although it may land on the surface, the ink droplets are rapidly miniaturized, and output at the highest resolution is possible.
[0086]
However, since most ink jet printers perform recording with a serial scanning type ejection head, miniaturization of ink droplets directly leads to an increase in recording time. Thus, attempts have been made to increase the number of injection nozzles and shorten the injection timing. In most cases, a method for reducing the recording time by ejecting ink droplets and ejecting fine ink droplets in a portion where high image quality is required is used.
[0087]
Therefore, when this embodiment is applied to an ink jet printer, it is the reverse of the case of the electrophotographic method, but from the highlight side where recording is performed with minute ink droplets so that dots are not visible. In the middle, the FM mask is optimized with the resolution parameter of the VTF being the highest resolution, and the resolution is reduced and optimized on the shadow side where large ink droplets are used.
[0088]
In other words, in an ink jet system capable of overprinting ink, a portion where ink is overlapped is expressed darker, and therefore, when dots overlap by large ink droplets are regularly arranged, it becomes very conspicuous. However, as described above, on the shadow side where large ink droplets are used, when optimization is performed at a reduced resolution, the overlap of dots due to large ink droplets is also optimized in accordance with the size of the large dots. Therefore, it is possible to reproduce a high-quality image in which regular arrangement due to overlapping dots is not easily noticeable.
[0089]
In the above description, the FM mask is optimized mainly focusing on the resolution of the VTF. However, optimization can also be performed focusing on the “observation distance” which is another parameter.
[0090]
For example, an image output device that uses a mask for halftone processing or that is incorporated into dedicated software, such as a copier or a facsimile device, or a printer using a laser method, a thermal transfer method, an ink jet method, etc., outputs the image. The paper size can be limited to almost several types, and the observation distance varies depending on the paper size. Therefore, the observation distance corresponding to each of the paper modes installed in the image output apparatus is set to set the FM mask. By performing the optimization, an FM mask that is optimal for the paper size can be created.
[0091]
Further, by considering the pixel reproducibility simultaneously with the observation distance, a higher quality FM mask can be created.
[0092]
Furthermore, the FM mask created as described above can be used in combination with other halftone processing.
[0093]
For example, in the error diffusion method, a unique texture called “worm” is likely to be generated, and a method of adding random noise to the threshold is generally known to prevent this. By using the FM mask as the threshold value, halftone processing that considers resolution and observation distance can be performed even for an error diffusion method that originally has no concept of resolution and observation distance.
[0094]
In the above description, the design is started from the gradation level 1/255 on the highlight side. However, as shown in FIG. 7, the design may be started from an intermediate level. In FIG. 7, the same processing steps as those in FIG. 5 are denoted by the same step numbers, and the description thereof is omitted.
[0095]
That is, when the optimum resolution d1 is calculated from the reproduction pixel size D (step S102), it is checked whether the start level is the highlight side from the level g (step S201). If the start level is the highlight side, the VTF resolution is set to the optimum resolution. d1 is set, and the FM mask is optimized with one pixel facing the highlight side with the minimum number of dots n (step S202).
[0096]
Next, it is checked whether the gradation level 1/255 has been reached (step S203). If the gradation level 1/255 has not been reached, the process returns to step S202 and the same processing is repeated. In step S203, when the gradation level reaches 1/255, the resolution of VTF is set to the optimum resolution d1, and the FM mask is optimized from the start level to the shadow side with the minimum number of dots n (1 pixel). In step S204), the switching level g is checked (step S104) as in the case of FIG. When the switching level is g in step S104, the resolution of the VTF is returned to the maximum resolution, and the FM mask is optimized toward the shadow side with one dot per pixel (step S105).
[0097]
In step S201, when the start level is not the highlight side from level g, VTF is set to the highest resolution, one pixel is optimized by one dot toward the highlight side (step S205), and the switching level g is checked. (Step S206). If it is not the switching level g in step S206, the process returns to step S205 and the same processing as described above is performed. When the switching level g is reached, the VTF resolution is set to the optimum resolution d1 and the minimum number of dots is set to n. Optimization is performed toward the side (step S207). Then, it is checked whether the gradation level 1/255 has been reached (step S208). If the gradation level 1/255 has not been reached, the process returns to step S207 and the same processing is repeated. When the gradation level reaches 1/255 in step S208, the VTF resolution is returned to the maximum resolution, and the FM mask is optimized toward the shadow side with one pixel per dot (step S105).
[0098]
The halftone processing mask (FM mask) and the program of the error diffusion processing method using the halftone processing mask are written in a predetermined recording medium, for example, a dedicated ROM, and the image output module is used as an image processing module. Installed on a PC, or written on a recording medium such as an FD (floppy disk: registered trademark) or CD (Compact Disc) as a program for controlling an image output device, or installed on a computer or the like via a network can do.
[0099]
Then, by causing the image output device to read the recording medium on which the program for the halftone processing mask and the error diffusion processing method created by such a halftone processing mask creation method is recorded, and mounted as an image processing module, An image output apparatus that easily performs appropriate halftone processing can be realized.
[0100]
In this case, as shown in FIG. 8, for example, when the image output apparatus 1 is applied to a computer 10 connected to a printer 20, an OS (Operating System) drawing-related intermediate module (GDI) 30 of the computer 10. Is processed by the printer driver 11 mounted on the computer 10 up to the color matching of the image processing, and the remaining processing including intermediate processing is performed by the processing module mounted on the printer 20. In FIG. 8, the image processing is separately performed on the computer 10 side and the printer 20 side. However, since the recent computer 10 has been speeded up, and if it is all software, the processing may be faster. The image processing may be performed on the computer 10 side.
[0101]
The invention made by the present inventor has been specifically described based on the preferred embodiments. However, the present invention is not limited to the above, and various modifications can be made without departing from the scope of the invention. Needless to say.
[0102]
【The invention's effect】
The present invention According to In addition to improving the quality of the halftone processing mask and using the fact that VTF (function approximating human visual characteristics) has a resolution parameter, the resolution is adjusted to the pixel diameter variation according to the gradation of the image output device. Parameters can be adjusted and optimized according to the image output device A halftone processing mask can be created.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating dot reproducibility in a highlight portion of an image for each halftone processing method and image forming engine.
FIG. 2 is an explanatory diagram of a distribution of charged charges in an electrophotographic system.
FIG. 3 is a diagram showing a state (a) in which pixel formation is performed with a single beam in a highlight portion and a state (b) in which pixel formation is performed with a plurality of beams.
FIG. 4 is a diagram illustrating a state (b) in which an element of pixel diameter variation is added to each pixel of a distributed pixel array (a).
FIG. 5 is a flowchart showing FM mask creation processing according to the present embodiment.
FIG. 6 is a diagram showing a relationship between a distance between pixels and a reproduced pixel size.
FIG. 7 is a flowchart showing FM mask creation processing including mask design processing in the middle of a gradation level.
FIG. 8 is a schematic configuration diagram of a halftone processing mask created by the halftone processing mask creation method of the present embodiment and an image output apparatus to which an error diffusion processing method using the halftone processing mask is applied.
[Explanation of symbols]
1 Image output device
10 Computer
11 Printer driver
20 Printer

Claims (1)

Convert image data to halftone image,
In the image output apparatus capable of forming one pixel with only one recording dot or a plurality of adjacent recording dots , each pixel of the halftone image is formed with only the one recording dot according to the gradation and A halftone processing mask creating method for creating a mask for halftone processing when switching to and forming with adjacent multiple recording dots ,
The mask generation method includes a mask optimization process using a resolution-dependent function approximating human visual characteristics as an evaluation function,
The gradation level when the pixels formed by the adjacent plural recording dots are arranged so as to maintain a distance close to each other so as not to cause an abrupt dot gain is set as a switching gradation level,
In the mask optimization process, the resolution set in the function is set lower on the highlight side than the shadow side on the switching gradation level in accordance with the number of the adjacent recording dots forming one pixel. To optimize the halftone processing mask,
A method for creating a mask for halftone processing.

Images