JP4105539B2 - Image processing apparatus, image forming apparatus including the same, image processing method, image processing program, and recording medium - Google Patents

Description

【００８７】
図１３は、領域分離処理を示すフローチャートである。ステップｓ２１では注目画素が網点領域に属する画素であるか否かを表す信号Screenが生成され、ステップｓ２２では注目画素が文字領域に属する画素であるか否かを表す信号Textが生成され、そしてステップｓ２３では注目画素が有彩であるか無彩であるかを表す信号Colorが生成される。それぞれ並列に処理され得られた３つの信号を基に、ステップｓ２４にて総合判定処理が行われる。ステップｓ２５では、総合判定処理の結果を示す領域分離信号が、黒生成／下色除去部１５および中間調生成部１６へ引き渡される。
【００８８】
図１４は、総合判定処理を示すフローチャートである。ステップｓ３１では信号Screenが「１」であるか否かを判断し、「１」であればステップｓ３４に進み、「０」であればステップｓ３２に進む。ステップｓ３４では、注目画素が網点領域に属すると判定し、領域分離信号として網点を示す「００」を出力する。ステップｓ３２では、信号Textが「１」であるか否かを判断する。信号Textが「１」であればステップｓ３３に進み、「０」であればステップｓ３７に進む。
【００８９】
ステップｓ３７では、注目画素が写真領域または下地領域に属すると判定し、領域分離信号として写真領域または下地領域を示す「１１」を出力する。ステップｓ３３では、信号Colorが「１」であるか否かを判断する。信号Colorが「１」であればステップｓ３５に進み、「０」であればステップｓ３６に進む。ステップｓ３５では、注目画素が黒文字領域に属すると判定し、領域分離信号として黒文字を示す「０１」を出力する。ステップｓ３６では、注目画素が色文字に属すると判定し、領域分離信号として色文字を示す「１０」を出力する。
【００９０】
以上のように、黒生成／下色除去部１５および中間調生成部１６は、領域分離信号を受け取り、それぞれの領域に応じた最適な処理を行うことができる。空間フィルタ部１０は、第１の実施形態と同様に、フィルタ係数設定部１３によりエッジ数を基に設定されたフィルタ係数を用いることで、領域分離結果によらず最適なフィルタ処理を行うことができる。
【００９１】
領域分離結果に基づいてフィルタ係数を切り替えた場合、たとえば、領域を誤判定した箇所については不適切なフィルタ処理が行われ周囲とのギャップが発生するので局所的に目立つ画質劣化を招くことになる。また、網点領域とも文字領域とも判断し難い箇所に対しては、はっきりとした領域分離結果を割り当てることができず、最適なフィルタ処理が行えないという問題もある。しかしながら、立上り／立下りエッジの数に応じてフィルタ係数を切り替えれば、領域を誤判定した箇所でも局所的なギャップが生じることはなく、また領域を確定できない箇所に対しても図１０（ｃ）に示したフィルタ係数C’のように最適なフィルタ係数を用いて処理を行うことができるため、出力画質の向上が実現できる。
【００９２】
図１５は、本発明の第３の実施形態である画像形成装置６１の構成を示すブロック図である。画像形成装置６１は、画像入力装置２、画像出力装置４および画像処理装置６３からなる。第２の実施形態の画像形成装置５１を構成する部位と同様の動作を行う部位については、同じ符号を付して説明を省略する。画像形成装置５１と異なるのは、領域分離処理部１７が立上り／立下りエッジ数に基づいて領域分離を行うことである。領域分離処理部１７は、有彩／無彩判定部１７１、立上り／立下りエッジカウント部１７２および総合判定部１７３から構成され、注目画素が黒文字、色文字、網点、その他（写真や下地など）のいずれの領域に属するものであるかが判定される。そしてその判定結果を示す領域分離信号は、黒生成／下色除去部１５および中間調生成部１６にそれぞれ引き渡される。
【００９３】
有彩／無彩判定部１７１は、入力処理部７からの出力信号に基づいて、注目画素が有彩であるか無彩であるかの判定を行う。立上り／立下りエッジカウント部１７２は、入力処理部７からの出力信号に基づいて、SumX_Up，SumX_Down，SumY_Up，SumY_Downを計数する。総合判定部１７３は、有彩／無彩判定部１７１の判定結果および立上り／立下りエッジカウント部１７２が計数したエッジ数に基づいて領域判定を行う。
【００９４】
フィルタ係数設定部１３は、立上り／立下りエッジカウント部１７２が計数したエッジ数に基づいてフィルタ係数を設定し、空間フィルタ部１０に出力する。
【００９５】
図１６は、領域分離処理を示すフローチャートである。ステップｓ４１では、立上り／立下りエッジカウント部１７２がSumX_Up，SumX_Down，SumY_Up，SumY_Downを計数して出力する。ステップＳ４２では、有彩／無彩判定部１７１が有彩か否かを判断して信号Colorを出力する。ステップｓ４３では、総合判定部１７３が注目画素の属する領域を判定して領域分離信号を出力する。ステップｓ４４では、領域分離信号が黒生成／下色除去部１５および中間調生成部１６へ引き渡される。
【００９６】
図１７は、総合判定処理を示すフローチャートである。ステップｓ５１では、SumX_Up > th_J1かつSumX_Down > th_J1かつSumY_Up > th_J1かつSumY_Down > th_J1（th_J1は閾値）の条件を満たすか否かを判断する。この条件式は、主走査方向における立上りエッジおよび立下りエッジの数がいずれも多く、かつ副走査方向においてもそれが成り立つような領域、つまり網点領域の特徴を表すものである。ステップＳ５１で条件を満たす場合は、ステップｓ５４に進み、注目画素が網点領域に属すると判定し、領域分離信号として網点を示す「００」を出力する。
【００９７】
ステップＳ５１で条件を満たさない場合はステップｓ５２に進み、th_J1≧SumX_Up＞th_J2かつth_J1≧SumX_Down＞th_J2の条件、またはth_J1≧SumY_Up＞th_J2かつth_J1≧SumY_Down＞th_J2の条件（th_J2は閾値）のいずれかを満たすか否かを判断する。この条件式は、主走査方向における立上りエッジおよび立下りエッジの数がいずれもある程度多い領域、もしくは副走査方向においてそれが成り立つ領域、つまり文字領域の特徴を表すものである。条件を満たす場合はステップｓ５３へ進み、条件を満たさない場合はステップｓ５７に進む。
【００９８】
ステップｓ５７では、注目画素が写真領域または下地領域に属すると判定し、領域分離信号として写真領域または下地領域を示す「１１」を出力する。
【００９９】
ステップｓ５３では、信号Colorが「１」であるか否かを判断する。信号Colorが「１」であればステップｓ５５に進み、「０」であればステップｓ５６に進む。ステップｓ５５では、注目画素が黒文字領域に属すると判定し、領域分離信号として黒文字を示す「０１」を出力する。ステップｓ５６では、注目画素が色文字に属すると判定し、領域分離信号として色文字を示す「１０」を出力する。
【０１００】
黒生成／下色除去部１５および中間調生成部１６は、領域分離信号を受け取り、それぞれの領域に応じた最適な処理を行うことができる。空間フィルタ部１０は、第１の実施形態と同様に、フィルタ係数設定部１３によりエッジ数を基に設定されたフィルタ係数を用いることで、領域分離結果によらず最適なフィルタ処理を行うことができる。さらに領域分離処理において、各種領域の判定には、立上り／立下りエッジカウント部１７２のエッジ数の計数結果を用いるので、処理データ量を削減して処理を高速化することができ、第２の実施形態と比較して回路規模を大幅に縮小できる。
【０１０１】
以上では、本発明の実施形態として画像形成装置を用いて説明したが、本発明はコンピュータに実行させるための画像処理プログラムおよび画像処理プログラムを記録したコンピュータ読み取り可能な記録媒体とすることもできる。
【０１０２】
この結果、適切なフィルタ処理や領域分離処理を備える画像処理方法をコンピュータに実行させる画像処理プログラムおよび画像処理プログラムを記録した記録媒体を提供することができる。なお、本実施形態では、この記録媒体としては、マイクロコンピュータで処理が行われるためにメモリ、たとえばＲＯＭのようなものそのものであっても良いし、また、外部記憶装置としてプログラム読み取り装置が設けられ、そこに記録媒体を挿入することで読み取り可能な記録媒体であっても良い。
【０１０３】
いずれの場合においても、格納されている画像処理プログラムは、マイクロプロセッサがアクセスして実行させる構成であっても良いし、あるいは、画像処理プログラムを読み出し、読み出した画像処理プログラムを、マイクロコンピュータのプログラム記憶エリアにダウンロードし、そのプログラムを実行する方式であってもよい。このダウンロード用のプログラムは予め本体装置に格納されているものとする。
【０１０４】
ここで、上記記録媒体としては、本体と分離可能に構成される記録媒体であり、磁気テープやカセットテープなどのテープ系、フレキシブルディスクやハードディスクなどの磁気ディスクやＣＤ−ＲＯＭ（Compact Disc-Read Only Memory）／ＭＯ（Magneto Optical）ディスク／ＭＤ（Mini Disk）／ＤＶＤ（Digital Versatile Disk）などの光ディスクのディスク系、ＩＣ（Integrated Circuit）カード（メモリカードを含む）／光カードなどのカード系、あるいはマスクＲＯＭ、ＥＰＲＯＭ（Erasable Programmable Read Only Memory）、ＥＥＰＲＯＭ（Electrically Erasable Programmable Read Only Memory）、フラッシュＲＯＭなどの半導体メモリを含めた固定的にプログラムを担持する媒体であっても良い。
【０１０５】
また、上記のような記録媒体に限らず、インターネットを含む通信ネットワークを接続可能なコンピュータシステムが通信ネットワークからプログラムをダウンロードするように流動的にプログラムを担持する媒体であっても良い。なお、このように通信ネットワークからプログラムをダウンロードする場合には、そのダウンロード用のプログラムは予めコンピュータシステムに格納しておくか、あるいは別な記録媒体からインストールされるものであっても良い。
【０１０６】
上記記録媒体は、デジタルカラー画像形成装置やコンピュータシステムに備えられるプログラム読み取り装置により読み取られることで上述した画像処理方法が実行される。特に、後者の場合、閾値の変更など画像処理方法をユーザの好みに応じて使用することが可能となる。
【０１０７】
図１８は、コンピュータシステム１０１の構成例を示す図である。コンピュータシステム１０１は、フラットベッドスキャナ、フィルムスキャナ、デジタルカメラなどの画像入力装置１０２、所定の画像処理プログラムがロードされることにより上記画像処理方法を含む様々な処理が行われるコンピュータ１０３、コンピュータの処理結果を表示するＣＲＴ（Cathode Ray Tube）ディスプレイ、液晶ディスプレイなどの画像表示装置１０４およびコンピュータの処理結果を紙などに出力するプリンタ１０５、画像処理の指示などをユーザが入力するためのキーボード１０６ａおよびマウス１０６ｂなどの入力装置１０６から構成されるさらには、電話線を介してサーバーなどに接続するための通信手段としてのモデムなどが備えられる。
【０１０８】
画像処理方法をこのコンピュータシステムで実行する場合、閾値の設定を任意に変更することが容易になり、また、画像表示装置に示される結果に応じて改めて設定し直すなどユーザの好みに応じた処理が可能となる。閾値の変更を行うには、キーボードやマウスを用いて直接数値を入力したり、閾値を表すシンボルをドラッグしたりすることにより設定される。
【０１０９】
【発明の効果】
以上のように本発明によれば、画素ブロック内のエッジ数に基づいて、複数のフィルタ係数から１つのフィルタ係数を選択するので、画素ブロックの特徴に応じた適切なフィルタ処理を行うことができ、出力画像の画質を向上させることができる。
【０１１０】
また本発明によれば、画素ブロック内のエッジ数に基づいて、複数のフィルタ係数から１つのフィルタ係数を選択するので、画素ブロックの特徴に応じた適切なフィルタ処理を行うことができ、出力画像の画質を向上させることができる。さらに、フィルタ処理は領域判定結果に依存しないので、領域判定の誤判定などによる画質劣化を抑制し、領域判定処理では明確に分離できない領域についても適切に処理を行うことができる。
【０１１１】
また本発明によれば、領域判定部は計数部が計数したエッジ数に基づいて領域判定を行うので、従来の領域判定処理に比べて必要な計算量を削減し、回路規模を縮小することができる。
【０１１２】
また本発明によれば、閾値を用いることで、容易にフィルタ係数を選択することができる。
【０１１３】
また本発明によれば、走査方向および立上り、立下りを区別してエッジ数を計数することで、より適したフィルタ係数を選択することができる。
【０１１４】
また本発明によれば、隣接する画素間の濃度差と、適切に設定した第３の閾値とを比較して立上りおよび立下りエッジを検出するので、写真領域や下地領域のエッジを検出せず、より正確にエッジ数を計数することができる。
【０１１５】
また本発明によれば、適切にフィルタ処理が施された画像データを出力することができるので、高画質な静止画像を形成することができる。
【０１１６】
また本発明によれば、上記の画像処理方法をコンピュータに実行させるための画像処理プログラムとして提供することができる。
【０１１７】
また本発明によれば、上記の画像処理方法をコンピュータに実行させるための画像処理プログラムを記録したコンピュータ読み取り可能な記録媒体として提供することができる。
【図面の簡単な説明】
【図１】本発明の第１の実施形態である画像形成装置１の構成を示すブロック図である。
【図２】立上り／立下りエッジカウント部１２の構成を示すブロック図である。
【図３】画素ブロック１００の一例を示す図である。
【図４】立上りエッジおよび立下りエッジを示す模式図である。
【図５】ライン内カウント部１２１の構成を示すブロック図である。
【図６】立上り／立下りエッジカウント処理を示すフローチャートである。
【図７】フィルタ係数設定処理を示すフローチャートである。
【図８】各領域の画素濃度変化を示す模式図である。
【図９】閾値による切り分けとそれに応じたフィルタ係数の選択の例を示す図である。
【図１０】フィルタ係数の例を示す図である。
【図１１】本発明の第２の実施形態である画像形成装置５１の構成を示すブロック図である。
【図１２】有彩／無彩判定部１４１の構成を示すブロック図である。
【図１３】領域分離処理を示すフローチャートである。
【図１４】総合判定処理を示すフローチャートである。
【図１５】本発明の第３の実施形態である画像形成装置６１の構成を示すブロック図である。
【図１６】領域分離処理を示すフローチャートである。
【図１７】総合判定処理を示すフローチャートである。
【図１８】コンピュータシステム１０１の構成例を示す図である。
【符号の説明】
１，５１，６１ 画像形成装置
２，１０２ 画像入力装置
３，５３，６３ 画像処理装置
４ 画像出力装置
５ Ａ／Ｄ（アナログ／デジタル）変換部
６ シェーディング補正部
７ 入力処理部
８ 色補正部
９，１５ 黒生成／下色除去部
１０ 空間フィルタ部
１１，１６ 中間調生成部
１２，１７２ 立上り／立下りエッジカウント部
１３ フィルタ係数設定部
１４，１７ 領域分離処理部
２０，３３ 差分器
２１ 絶対値算出部
２２，３４ 比較器
２３ ＡＮＤ回路
２４ ＳＵＭ回路
３１ 最大値算出部
３２ 最小値算出部
１００ 画素ブロック
１０１ コンピュータシステム
１０３ コンピュータ
１０４ 画像表示装置
１０５ プリンタ
１０６ 入力装置
１０６ａ キーボード
１０６ｂ マウス
１２１ ライン内カウント部
１２２ 総和算出部
１４１，１７１ 有彩／無彩判定部
１４２ 文字判定部
１４３ 網点判定部
１４４，１７３ 総合判定部[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image processing apparatus that performs filter processing for improving image quality on image data in which a plurality of types of areas such as character areas and halftone areas are mixed, an image forming apparatus including the same, an image processing method, and image processing The present invention relates to a program and a recording medium.
[0002]
[Prior art]
Currently, in image processing performed in an image forming apparatus such as a digital copying machine, processing called filter processing is performed on input document image data in order to improve the quality of an output image. The filter process is a process of changing the characteristics of the output image by converting the gradation value of a pixel in a predetermined region by a certain coefficient. A reading device that reads a document image is composed of a line sensor in which CCDs (Charge Coupled Devices) are arranged in a line, and converts reflected light from the document into an electrical signal and outputs image data. In such a reading apparatus, since the document is read at the same interval as the CCD, the image data output from the reading apparatus has periodicity. When the original image is a halftone image, the original image itself to be read is periodically arranged. Therefore, moire occurs in the read image data due to interference between these cycles. In addition, when processing using halftone dots is performed in the gradation reproduction process when outputting an image, the halftone dots of the output image are periodically arranged, so that the output is interfered with the period of the original image and the period at the time of reading. Moire occurs in the image. In order to suppress the occurrence of such moire, the filter processing is intended to improve image quality by smoothing image data.
[0003]
The image has a so-called edge portion such as a boundary portion between the character and the background. In order to emphasize this edge portion, in the filter processing, image quality is improved by sharpening the image data.
[0004]
There are two types of methods for performing the filter processing as described above on image data. The first is a method of performing similar filter processing such as smoothing and edge enhancement on the entire image data. The second is a method in which image data is separated into different types of areas such as a halftone dot area, a character area, and a photographic area by area separation processing, and optimum filtering is performed according to each area.
[0005]
When filtering is performed by the first method, if the entire image data is smoothed, the edge portion of the character is blurred, resulting in poor reproducibility. If the entire image data is sharpened, moire due to halftone dots is generated. Occurs and the image quality deteriorates. In addition, when performing the filtering process by the second method, improper filtering process is performed by erroneously separating the areas by erroneously determining the halftone dot area as a character area. If inappropriate filter processing is performed, significant image quality degradation will occur. For example, as a result of performing the filtering process, if the partial sharpening is mistakenly performed in the area where smoothing is performed and the smooth density change is performed, the density change is rapidly increased only at that point, Peripheral pixels and output characteristics look very different. As described above, in the image processing, the filter processing is the processing that most affects the image quality of the output image.
[0006]
As a specific conventional technique for performing region separation processing and filter processing, the image forming apparatus described in Patent Document 1 extracts image feature amounts from rising / falling edges in input image information, and identifies the feature amounts. Region separation is performed in comparison with the standard. Also, image quality correction is performed according to the separated area. Note that the feature quantity that is actually extracted is the distance between the rising edge and the falling edge. If the distance is short, it is a halftone area. If the distance is long, it is a character area. Domain separation is performed.
[0007]
[Patent Document 1]
JP-A-6-62230
[0008]
[Problems to be solved by the invention]
The image forming apparatus described in Patent Document 1 has the following problems. The first problem is that an accurate region determination cannot be performed because the zero cross point is used for detecting the rising / falling edge. For example, since many rising / falling edges are detected in the photograph area, etc., depending on the distance between the rising edge and the falling edge, it may be misjudged as a halftone dot and smoothed, or it may be misjudged as a character and edge enhanced. Resulting in. The second problem is that the range for performing region determination is not clear. That is, there is no clear description of the configuration such as how much range the rising / falling is detected and discriminated, and to which pixel the selected filter processing is applied. The third problem is that the filter processing is switched according to the dot period (number of lines). To achieve this, in addition to the process of identifying the dot area, the dot period is detected. It is necessary to provide a separate process for this.
[0009]
An object of the present invention is to provide an image processing apparatus capable of improving the image quality of an image by performing optimum filter processing, an image forming apparatus including the image processing apparatus, an image processing method, an image processing program, and a recording medium. That is.
[0010]
[Means for Solving the Problems]
  The present invention includes a counting unit that counts the number of edges in a pixel block composed of a target pixel and its peripheral pixels included in image data indicating an image composed of a plurality of pixels,
  A coefficient setting unit that selects one filter coefficient from a plurality of filter coefficients based on the counted number of edges;
  A spatial filter unit that performs a filtering process on the input image data using the filter coefficient selected by the coefficient setting unit.,
  The counting unit individually counts the number of rising and falling edges in the main scanning direction and the number of rising and falling edges in the sub-scanning direction in the pixel block, and sums the number of four edges. Count,
  The coefficient setting unit compares the first threshold value, the second threshold value smaller than the first threshold value, and the intermediate threshold value smaller than the first threshold value and larger than the second threshold value with the counted edge number, and the number of edges Is equal to or greater than the first threshold, a filter coefficient having a smoothing characteristic is selected, and when the number of edges is smaller than the first threshold and equal to or greater than the intermediate threshold, smoothing and low frequency components are reduced. When a filter coefficient having a characteristic that combines frequency component enhancement is selected and the number of edges is smaller than the intermediate threshold and equal to or greater than the second threshold, a filter coefficient having a sharpening characteristic is selected, and the number of edges is When it is less than the second threshold, a filter coefficient having a characteristic that the density value of the target pixel becomes the converted value as it is is selected.An image processing apparatus.
[0011]
The filtering process performed on the image data to which the spatial filter unit is input is a process of converting a pixel value for a predetermined pixel using a predetermined filter coefficient. The filter coefficient is set as a set of numerical values in which coefficients corresponding to each pixel in the pixel block are arranged in a matrix according to conversion characteristics such as smoothing and sharpening.
[0012]
The number of edges in the pixel block represents the feature of the pixel of interest. If the number of edges is large, the pixel of interest is a pixel constituting a halftone dot region, and if the number of edges is small, the pixel of interest is a pixel constituting a photographic region and a background region, and if the number of edges is medium, The target pixel is a pixel constituting the character area. The present invention pays attention to these correlations and selects the optimum filter coefficient for the target pixel.
[0013]
  According to the present invention, the counting unit counts the number of edges in the pixel block including the target pixel and its peripheral pixels.Specifically, the number of rising edges and falling edges in the main scanning direction and the number of rising edges and falling edges in the sub-scanning direction in the pixel block are individually counted, and the total number of four edges is counted. To do.The coefficient setting unit selects one filter coefficient from a plurality of filter coefficients based on the counted number of edges. Since the number of edges represents the feature of the target pixel, the coefficient setting unit can select an optimum filter coefficient according to the feature of the target pixel. For example, if the target pixel is a pixel constituting a halftone dot region, a filter coefficient having a smoothing characteristic is selected to suppress moire, and if the target pixel is a pixel constituting a character region, the edge is emphasized. In order to achieve this, a filter coefficient having a sharpening characteristic is selected.Specifically, the first threshold, the second threshold smaller than the first threshold, and the intermediate threshold smaller than the first threshold and larger than the second threshold are compared with the counted number of edges, When it is equal to or greater than the first threshold, a filter coefficient having smoothing characteristics is selected, and when the number of edges is smaller than the first threshold and equal to or greater than the intermediate threshold, smoothing of the high frequency component and low frequency component When the filter coefficient having the characteristic of combining the enhancement of the image is selected and the number of edges is smaller than the intermediate threshold and not less than the second threshold, the filter coefficient having the sharpening characteristic is selected and the number of edges is the second. If it is less than the threshold value, a filter coefficient having a characteristic that the density value of the pixel of interest becomes the converted value as it is is selected.
[0014]
The spatial filter unit performs a filtering process using the filter coefficient selected by the coefficient setting unit.
[0015]
Accordingly, it is possible to perform an appropriate filter process according to the feature of the target pixel based on the number of edges in the pixel block, and it is possible to improve the image quality of the output image.
[0016]
  In the present invention, image data indicating an image composed of a plurality of pixels is input.
  A spatial filter unit that performs a filtering process on the input image data;
  When the input image data is color image data composed of a plurality of colors, a black generation / under color removal unit that replaces the portion where each color has substantially the same pixel density with black and converts it to color image data further added with black When,
  A halftone generation unit that performs gradation reproduction processing according to the output destination on the input image data;
  An area determination unit that determines which of the plurality of areas including at least the character area and the halftone dot area the target pixel belongs to;
  In the image processing apparatus in which at least one of the black generation / undercolor removal unit and the halftone generation unit performs processing according to the determination result of the region determination unit,
  A counting unit for counting the number of edges in a pixel block composed of a pixel of interest and its surrounding pixels;
  A coefficient setting unit that selects one filter coefficient from a plurality of filter coefficients based on the counted number of edges;
  The counting unit individually counts the number of rising and falling edges in the main scanning direction and the number of rising and falling edges in the sub-scanning direction in the pixel block, and sums the number of four edges. Count,
  The coefficient setting unit compares the first threshold value, the second threshold value smaller than the first threshold value, and the intermediate threshold value smaller than the first threshold value and larger than the second threshold value with the counted edge number, and the number of edges Is equal to or greater than the first threshold, a filter coefficient having a smoothing characteristic is selected, and when the number of edges is smaller than the first threshold and equal to or greater than the intermediate threshold, smoothing and low frequency components are reduced. When a filter coefficient having a characteristic that combines frequency component enhancement is selected and the number of edges is smaller than the intermediate threshold and equal to or greater than the second threshold, a filter coefficient having a sharpening characteristic is selected, and the number of edges is When it is less than the second threshold, a filter coefficient having a characteristic that the density value of the target pixel is directly converted into a converted value is selected,
  The spatial filter unit is an image processing device that performs filter processing using a filter coefficient selected by the coefficient setting unit.
[0017]
  According to the present invention,The counting unit counts the number of edges in the pixel block including the target pixel and its surrounding pixels. Specifically, the number of rising edges and falling edges in the main scanning direction and the number of rising edges and falling edges in the sub-scanning direction in the pixel block are individually counted, and the total number of four edges is counted. To do.The coefficient setting unit selects one filter coefficient from a plurality of filter coefficients based on the counted number of edges. Since the number of edges represents the feature of the target pixel, the coefficient setting unit can select an optimum filter coefficient according to the feature of the target pixel. For example, if the target pixel is a pixel constituting a halftone dot region, a filter coefficient having a smoothing characteristic is selected to suppress moire, and if the target pixel is a pixel constituting a character region, the edge is emphasized. In order to achieve this, a filter coefficient having a sharpening characteristic is selected.Specifically, the first threshold, the second threshold smaller than the first threshold, and the intermediate threshold smaller than the first threshold and larger than the second threshold are compared with the counted number of edges, When it is equal to or greater than the first threshold, a filter coefficient having smoothing characteristics is selected, and when the number of edges is smaller than the first threshold and equal to or greater than the intermediate threshold, smoothing of the high frequency component and low frequency component When the filter coefficient having the characteristic of combining the enhancement of the image is selected and the number of edges is smaller than the intermediate threshold and not less than the second threshold, the filter coefficient having the sharpening characteristic is selected and the number of edges is the second. If it is less than the threshold value, a filter coefficient having a characteristic that the density value of the pixel of interest becomes the converted value as it is is selected.
[0018]
Also, the black generation / under color removal unit has almost all of cyan, magenta, and yellow when the input image data is color image data composed of three colors of CMY (C: cyan, M: magenta, Y: yellow). A portion having the same pixel density is replaced with black and converted to color image data composed of four colors of CMYK (K: black). The halftone generation unit performs tone reproduction processing according to the output destination, that is, the output characteristics of the image output device such as resolution.
[0019]
The region determination unit determines whether the target pixel belongs to at least one of a plurality of regions including a character region and a halftone dot region, and at least one of the black generation / undercolor removal unit and the halftone generation unit is a region determination unit The process according to the determination result is performed.
[0020]
Accordingly, it is possible to perform an appropriate filter process according to the feature of the target pixel based on the number of edges in the pixel block, and it is possible to improve the image quality of the output image. Furthermore, since the filter process does not depend on the area determination result, image quality deterioration when the area determination is erroneously determined is suppressed, and an area that cannot be clearly separated by the area determination process, for example, a character in a halftone area is appropriately processed. It can be performed.
[0021]
In the invention, it is preferable that the area determination unit determines which of the plurality of areas including at least the character area and the halftone dot area the pixel of interest belongs based on the counted number of edges.
[0022]
According to the present invention, the region determination unit determines, based on the counted number of edges, whether the pixel of interest belongs to at least a plurality of regions including a character region and a dot region, and generates black / under color The removal unit / halftone generation unit performs processing according to the determination result of the region determination unit.
[0023]
As a result, it is possible to reduce the amount of calculation required compared to the conventional area determination processing and reduce the circuit scale.
[0030]
In the invention, it is preferable that the counting unit has a density difference greater than a third threshold value between a predetermined pixel in the pixel block and a pixel adjacent to the predetermined pixel in the scanning direction, and the density increases in the scanning direction. Is detected as a rising edge, and the density difference between a predetermined pixel in the pixel block and a pixel adjacent to the predetermined pixel in the scanning direction is larger than the third threshold value, and the density decreases in the scanning direction. A part is detected as a falling edge.
[0031]
According to the present invention, the counting unit has, as the rising edge, the density difference between the predetermined pixel in the pixel block and the pixel adjacent to the predetermined pixel in the scanning direction is larger than the third threshold, and the density is in the scanning direction. The part that is rising is detected. Further, as a falling edge, a portion where the density difference between a predetermined pixel in the pixel block and a pixel adjacent to the predetermined pixel in the scanning direction is larger than the third threshold and the density is decreasing in the scanning direction. To detect. Note that the third threshold value is set to be sufficiently larger than the density change between adjacent pixels in the photographic region and the background region.
[0032]
As a result, an area having a small density change in the local area, such as a photographic area or a background area, is not detected as an edge, so that the number of edges can be counted more accurately.
[0033]
The present invention also provides the above image processing apparatus,
An image forming apparatus comprising: an image output device that outputs image data processed by the image processing device.
[0034]
According to the present invention, the image data processed by the image processing apparatus is output from the image output apparatus.
[0035]
As a result, image data appropriately filtered can be output, and a high-quality still image can be formed.
[0036]
  Further, the present invention is a counting step for counting the number of edges in a pixel block composed of a target pixel and its peripheral pixels included in image data indicating an image composed of a plurality of pixels,
  A coefficient setting step of selecting one filter coefficient from a plurality of filter coefficients based on the counted number of edges;
  A spatial filter step of performing a filtering process on the input image data using the filter coefficient selected in the coefficient setting step.,
  In the counting step, the number of rising edges and falling edges in the main scanning direction and the number of rising edges and falling edges in the sub-scanning direction in the pixel block are individually counted, and the total number of four edges is calculated. Count,
  In the coefficient setting step, the first threshold value, the second threshold value smaller than the first threshold value, and the intermediate threshold value smaller than the first threshold value and larger than the second threshold value are compared with the counted number of edges, and the number of edges Is equal to or greater than the first threshold, a filter coefficient having a smoothing characteristic is selected, and when the number of edges is smaller than the first threshold and equal to or greater than the intermediate threshold, smoothing and low frequency components are reduced. When a filter coefficient having a characteristic that combines frequency component enhancement is selected and the number of edges is smaller than the intermediate threshold and equal to or greater than the second threshold, a filter coefficient having a sharpening characteristic is selected, and the number of edges is When it is less than the second threshold, a filter coefficient having a characteristic that the density value of the pixel of interest becomes the converted value as it is is selected.An image processing method characterized by this.
[0037]
  According to the present invention, the number of edges in the pixel block composed of the target pixel and its surrounding pixels is counted in the counting step.Specifically, the number of rising edges and falling edges in the main scanning direction and the number of rising edges and falling edges in the sub-scanning direction in the pixel block are individually counted, and the total number of four edges is counted. To do.In the coefficient setting step, one filter coefficient is selected from a plurality of filter coefficients based on the counted number of edges. Since the number of edges represents the feature of the target pixel, in the coefficient setting step, an optimum filter coefficient can be selected according to the feature of the target pixel. For example, if the target pixel is a pixel constituting a halftone dot region, a filter coefficient having a smoothing characteristic is selected to suppress moire, and if the target pixel is a pixel constituting a character region, the edge is emphasized. In order to achieve this, a filter coefficient having a sharpening characteristic is selected.Specifically, the first threshold, the second threshold smaller than the first threshold, and the intermediate threshold smaller than the first threshold and larger than the second threshold are compared with the counted number of edges, When it is equal to or greater than the first threshold, a filter coefficient having smoothing characteristics is selected, and when the number of edges is smaller than the first threshold and equal to or greater than the intermediate threshold, smoothing of the high frequency component and low frequency component When the filter coefficient having the characteristic of combining the enhancement of the image is selected and the number of edges is smaller than the intermediate threshold and not less than the second threshold, the filter coefficient having the sharpening characteristic is selected and the number of edges is the second. If it is less than the threshold value, a filter coefficient having a characteristic that the density value of the pixel of interest becomes the converted value as it is is selected.
[0038]
In the spatial filter process, a filter process is performed using the filter coefficient selected in the coefficient setting process.
[0039]
Accordingly, it is possible to perform an appropriate filter process according to the feature of the target pixel based on the number of edges in the pixel block, and it is possible to improve the image quality of the output image.
[0040]
  In the present invention, image data indicating an image composed of a plurality of pixels is input.
  A spatial filter process for performing filtering on input image data;
  An area determination step for determining whether the target pixel belongs to at least one of a plurality of areas including a character area and a halftone dot area;
  When the input image data is color image data composed of a plurality of colors, a black generation / under color removal step of replacing the portion where each color has substantially the same pixel density with black and converting it to color image data further adding black When,
  A halftone generation step for performing gradation reproduction processing according to the output destination for the input image data,
  In the image processing method for performing processing according to the determination result of the region determination step in at least one of the black generation / under color removal step and the halftone generation step,
  A counting step of counting the number of edges in a pixel block composed of a pixel of interest and its surrounding pixels;
  A coefficient setting step of selecting one filter coefficient from a plurality of filter coefficients based on the counted number of edges,
  In the counting step, the number of rising edges and falling edges in the main scanning direction and the number of rising edges and falling edges in the sub-scanning direction in the pixel block are individually counted, and the total number of four edges is calculated. Count,
  In the coefficient setting step, the first threshold value, the second threshold value smaller than the first threshold value, and the intermediate threshold value smaller than the first threshold value and larger than the second threshold value are compared with the counted number of edges, and the number of edges Is equal to or greater than the first threshold, a filter coefficient having a smoothing characteristic is selected, and when the number of edges is smaller than the first threshold and equal to or greater than the intermediate threshold, smoothing and low frequency components are reduced. When a filter coefficient having a characteristic that combines frequency component enhancement is selected and the number of edges is smaller than the intermediate threshold and equal to or greater than the second threshold, a filter coefficient having a sharpening characteristic is selected, and the number of edges is When it is less than the second threshold, a filter coefficient having a characteristic that the density value of the target pixel is directly converted into a converted value is selected,
  In the spatial filter process, the filter process is performed using the filter coefficient selected in the coefficient setting process.
[0041]
  According to the present invention,In the counting step, the number of edges in the pixel block including the target pixel and its surrounding pixels is counted. Specifically, the number of rising edges and falling edges in the main scanning direction and the number of rising edges and falling edges in the sub-scanning direction in the pixel block are individually counted, and the total number of four edges is counted. To do.In the coefficient setting step, one filter coefficient is selected from a plurality of filter coefficients based on the counted number of edges. Since the number of edges represents the feature of the target pixel, in the coefficient setting step, an optimum filter coefficient can be selected according to the feature of the target pixel. For example, if the target pixel is a pixel constituting a halftone dot region, a filter coefficient having a smoothing characteristic is selected to suppress moire, and if the target pixel is a pixel constituting a character region, the edge is emphasized. In order to achieve this, a filter coefficient having a sharpening characteristic is selected.Specifically, the first threshold, the second threshold smaller than the first threshold, and the intermediate threshold smaller than the first threshold and larger than the second threshold are compared with the counted number of edges, When it is equal to or greater than the first threshold, a filter coefficient having smoothing characteristics is selected, and when the number of edges is smaller than the first threshold and equal to or greater than the intermediate threshold, smoothing of the high frequency component and low frequency component When the filter coefficient having the characteristic of combining the enhancement of the image is selected and the number of edges is smaller than the intermediate threshold and not less than the second threshold, the filter coefficient having the sharpening characteristic is selected and the number of edges is the second. If it is less than the threshold value, a filter coefficient having a characteristic that the density value of the pixel of interest becomes the converted value as it is is selected.
[0042]
In the region determination step, it is determined whether the target pixel belongs to at least one of a plurality of regions including a character region and a halftone dot region, and at least one of the black generation / undercolor removal step and the halftone generation step is a region determination step The process according to the determination result is performed.
[0043]
Further, in the black generation / under color removal step, when the input image data is color image data composed of three colors of CMY, a portion where cyan, magenta, and yellow have substantially the same pixel density is replaced with black, and CMYK Convert to color image data consisting of four colors. In the halftone generation step, gradation reproduction processing according to the output characteristics of the image output device such as resolution is performed.
[0044]
Accordingly, it is possible to perform an appropriate filter process according to the feature of the target pixel based on the number of edges in the pixel block, and it is possible to improve the image quality of the output image. Furthermore, since the filter process does not depend on the area determination result, image quality deterioration when the area determination is erroneously determined is suppressed, and an area that cannot be clearly separated by the area determination process, for example, a character in a halftone area is appropriately processed. It can be performed.
[0045]
In the invention, it is preferable that the region determination step determines whether the pixel of interest belongs to at least a plurality of regions including a character region and a halftone region based on the counted number of edges.
[0046]
According to the present invention, the region determination step determines whether the pixel of interest belongs to at least a plurality of regions including a character region and a halftone region based on the counted number of edges, and generates black / under color The removal unit / halftone generation unit performs processing according to the determination result of the region determination step.
[0047]
Thereby, it is possible to reduce the amount of calculation required for the area determination processing and reduce the circuit scale.
[0048]
The present invention is also an image processing program for causing a computer to execute the above image processing method.
[0049]
According to the present invention, an image processing program for causing a computer to execute the above-described image processing method can be provided.
[0050]
A computer-readable recording medium recording an image processing program for causing a computer to execute the image processing method described above.
[0051]
According to the present invention, it can be provided as a computer-readable recording medium on which an image processing program for causing a computer to execute the above-described image processing method is recorded.
[0052]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail with reference to the drawings.
[0053]
FIG. 1 is a block diagram showing a configuration of an image forming apparatus 1 according to the first embodiment of the present invention. The image forming apparatus 1 is realized by a so-called digital color copying machine, digital color facsimile, or a complex machine of these, and includes an image input device 2, an image processing device 3, and an image output device 4.
[0054]
The image input device 2 is composed of, for example, a scanner having a CCD line sensor. The image input device 2 irradiates light on a document to be read and separates the light reflected by the document into RGB (R: red, G: green, B: blue). Converted into an electrical signal (reflectance signal).
[0055]
The image processing apparatus 3 includes an A / D (analog / digital) conversion unit 5, a shading correction unit 6, an input processing unit 7, a color correction unit 8, a black generation / under color removal unit 9, a spatial filter unit 10, and a halftone generation. Part 11, rising / falling edge counting part 12, and filter coefficient setting part 13.
[0056]
A color image signal (RGB analog signal) input by the image input device 2 is converted into a digital signal (image data) by the A / D conversion unit 5, and an illumination system of the image input device 2 is converted by the shading correction unit 6. Various distortions generated in the imaging system and imaging system are removed.
[0057]
Thereafter, the reflectance signal is converted into a density signal suitable for image processing in the input processing unit 7, and a CMY (C: cyan, M: magenta, Y: yellow) signal that is a complementary color signal of the RGB signal is generated. Next, a process for improving color reproducibility is performed in the color correction unit 8, and converted into a CMYK (K: black) 4-color signal in the black generation / under color removal unit 9.
[0058]
Further, the spatial filter unit 10 performs edge enhancement processing and smoothing processing on the CMYK signal, and the halftone generation unit 11 performs gradation reproduction processing according to output characteristics such as the resolution of the image output device 4. Then, an output image is formed by the image output device 4 realized by an electrophotographic or inkjet printer. The image output device 4 may be an image display device such as a liquid crystal display. In this case, the color correction unit 8 performs a process of converting the RGB signal into an R′G′B ′ signal suitable for color reproduction of the image display device, and the process in the black generation / under color removal unit 9 is unnecessary. The R′G′B ′ signal output from the spatial filter unit 10 is output after the gradation is adjusted in the halftone generation unit 11 according to the characteristics of the image display device.
On the other hand, the output signal from the input processing unit 7 is transferred to the rising / falling edge counting unit 12 which is a counting unit, and the number of rising edges and falling edges in a pixel block including a plurality of pixels including the target pixel is determined. Counted. Thereafter, an optimum filter coefficient is set by the filter coefficient setting unit 13 based on the number of edges, and the coefficient is delivered to the spatial filter unit 10. A plurality of types of filter coefficients may be stored in advance according to the number of edges. The above processing is performed by, for example, CPU (Central Processing
Unit).
[0059]
The rising / falling edge counting unit 12 will be described in detail. FIG. 2 is a block diagram showing a configuration of the rising / falling edge counting unit 12. The rising / falling edge counting unit 12 includes an in-line counting unit 121 and a sum calculation unit 122. FIG. 3 is a diagram illustrating an example of the pixel block 100. The pixel block 100 is composed of M × N pixels centered on the target pixel, and a line in the main scanning direction is a line X and a line in the sub-scanning direction is a line Y. Since there are M × N pixels, the main scanning direction line is the line X₁~ Line X_NThe sub-scanning direction line is line Y₁~ Line Y_MAnd FIG. 4 is a schematic diagram showing a rising edge and a falling edge. In the figure, a case where a line having 9 pixels is scanned from left to right is shown. The rising edge, like the pixel A (density value fa) and the right adjacent pixel B (density value fb), has a large density difference between adjacent pixels, is equal to or greater than a predetermined value, and the density is increased ( This refers to the part fb> fa). The falling edge has a large density difference between adjacent pixels such as the pixel A ′ (density value fa ′) and the right adjacent pixel B ′ (density value fb ′), and is equal to or greater than a predetermined value. This means the part that is descending (fb ′ <fa ′). Therefore, the line shown in the figure includes one rising edge and one falling edge.
[0060]
The in-line count unit 121 generates a line X in the main scanning direction.₁~ Line X_NAnd line Y of the sub-scanning direction line₁~ Line Y_MIn-line count circuit 121X corresponding to each₁~ 121X_NAnd the in-line count unit 121Y₁~ 121Y_MConsists of. The in-line counting unit 121 counts rising edges and falling edges for each line, outputs NumUp_X as the number of rising edges in one line in the main scanning direction, and sets the number of falling edges in one line in the main scanning direction. NumDown_X is output, NumUp_Y is output as the number of rising edges in one line in the sub-scanning direction, and NumDown_Y is output as the number of falling edges in one line in the sub-scanning direction. For example, line X in the main scanning direction₁The counting result of NumUp_X₁And NumDown_X₁Is output as
[0061]
The sum total calculation unit 122 includes SUM circuits 122a to 122d corresponding to the number of rising edges in the main scanning direction, the number of falling edges in the main scanning direction, the number of rising edges in the sub scanning direction, and the number of falling edges in the sub scanning direction. The SUM circuits 122a to 122d calculate the sum of the count values output from the in-line count unit 121. The SUM circuit 122a calculates the sum SumX_Up of the number of rising edges in the main scanning direction, the SUM circuit 122b calculates the sum SumX_Down of the number of falling edges in the main scanning direction, and the SUM circuit 122c calculates the sum of the number of rising edges in the sub scanning direction. SumY_Up is calculated, and the SUM circuit 122 d calculates the sum SumY_Down of the number of falling edges in the sub-scanning direction, and outputs it to the filter coefficient setting unit 13.
[0062]
FIG. 5 shows an in-line count circuit 121X.₁It is a block diagram which shows the structure of these. In-line count circuit 121X₂~ 121X_NAnd in-line count circuit 121Y₁~ 121Y_MIn-line count circuit 121X₁In this case, the in-line count circuit 121X is the same as the configuration in FIG.₁The configuration of will be described. The in-line count unit 121 includes a difference unit 20, an absolute value calculation unit 21, comparators 22a and 22b, AND circuits 23a and 23b, SUM circuits 24a and 24b, and an inverter circuit 25. When the differentiator 20 calculates the difference (fa−fb) between the density values fa and fb of two adjacent pixels a and b in the line, the absolute value calculator 21 calculates the absolute value of the calculated difference value. . The pixel b is located in front of the pixel a in the scanning direction. The comparator 22a compares the difference absolute value with the threshold TH that is the third threshold, and outputs “1” if the difference absolute value is equal to or greater than the threshold TH and outputs “0” otherwise. Further, the comparator 22b determines whether or not the difference value is 0 or more, that is, a positive value or a negative value. If the difference value is 0 or more, “1” is output, otherwise “0” is output. To do.
[0063]
The output signals of the comparators 22a and 22b are transferred to the AND circuits 23a and 23b, and it is determined whether or not they are rising / falling edges. The output signal from the comparator 22b to the AND circuit 23b is inverted by the inverter circuit 25. The AND circuit 23a has a falling edge when the output from the comparator 22a is "1" and the output from the comparator 22b is "1", that is, when the difference value is greater than or equal to a threshold value and is positive. Therefore, “1” is output as the output signal Down, and “0” is output otherwise. On the other hand, the AND circuit 23b rises when the output from the comparator 22a is “1” and the output from the comparator 22b is “0”, that is, when the difference value is equal to or greater than the threshold value and negative. Since it is an edge, “1” is output as the output signal Up, and “0” is output otherwise.
[0064]
The above calculation process is repeated while shifting the target pixel in the line by one pixel in the scanning direction. The SUM circuit 24a counts the number of outputs of “Down = 1” during the repetition of the calculation process, and calculates the total in-line NumDown_X₁Is output. The SUM circuit 24b counts the number of output “Up = 1” and counts the sum NumUp_X in the line.₁Is output.
[0065]
The method for detecting the rising / falling edge is not limited to this, and for example, a method may be used in which a zero cross point is obtained and the density change in the vicinity thereof is detected.
[0066]
FIG. 6 is a flowchart showing rising / falling edge count processing. First, in step s1, density values of M × N pixels including the target pixel are held for each color plane. After that, in step s2, the line X in the main scanning direction with respect to the C plane₁~ X_NSums SumX_Up and SumX_Down of the rising / falling edges in FIG. In step s3, the line Y in the sub-scanning direction₁~ Y_MSums SumY_Up and SumY_Down of the rising / falling edges in FIG. At this time, the same processing is performed in parallel for the M plane and the Y plane. In step s4, the maximum values MAX {SumX_Up, SumX_Down, SumY_Up, SumY_Down} in each plane are obtained. To be handed over.
[0067]
FIG. 7 is a flowchart showing the filter coefficient setting process. First, in step s11, as described above, the rising / falling edge counting unit 12 causes the number of rising edges SumX_Up and the number of falling edges SumX_Down in the main scanning direction and the rising edges in the sub-scanning direction in the pixel block. The number SumY_Up and the number of falling edges SumY_Down are counted. FIG. 8 is a schematic diagram showing a change in pixel density in each region. The upward arrow indicates a rising edge, and the downward arrow indicates a falling edge. Since the halftone dot area includes substantially circular halftone dots, as shown in FIG. 8A, the number of edges included in one line is large, and there are many edges in the main scanning direction and sub-scanning direction. There are many downstream edges. Therefore, the sum of the number of edges becomes larger than in other areas.
[0068]
The character area includes many linear portions, and the distance between the rising edge and the falling edge is larger than the halftone dot area as shown in FIG. 8B. Furthermore, there is an edge biased in either the main scanning direction or the sub-scanning direction. Therefore, the total number of edges is smaller than the halftone dot region and larger than the photographic region and the background region.
[0069]
The photographic area includes high density pixels and low density pixels as in the other areas. However, as shown in FIG. 8C, the density change is gradual and the density difference between adjacent pixels is small. There is almost no falling edge. Further, as shown in FIG. 8D, the base region is composed of only low density pixels, and therefore there is no rising edge or falling edge. Accordingly, the total number of edges is very small.
[0070]
A filter coefficient is set based on the correspondence between each region and the total number of edges as described above. The filter coefficient is composed of numerical values arranged in a matrix, and a new gradation value is calculated by performing a convolution operation on the gradation values of the pixels corresponding to the matrix and these numerical values.
[0071]
In step s12, the sum Total of the number of edges SumX_Up, SumX_Down, SumY_Up, and SumY_Down is calculated. In step s13, it is determined whether or not the total Total is equal to or greater than a threshold thA that is a first threshold. If it is equal to or greater than the threshold thA, the process proceeds to step s15, and if it is smaller than the threshold thA, the process proceeds to step s14. In step s15, a filter coefficient suitable for the halftone dot region is selected from a plurality of types of filter coefficients stored in advance, and delivered to the spatial filter unit 10. As a filter coefficient suitable for a halftone dot region, a filter coefficient that can be smoothed is preferable.
[0072]
In step s14, it is determined whether or not the total Total is equal to or greater than a second threshold value threshold value thB (<threshold value thA). If it is equal to or greater than the threshold thB, the process proceeds to step s16. In step s16, a filter coefficient suitable for the character region is selected from a plurality of types of filter coefficients stored in advance, and delivered to the spatial filter unit 10. As a filter coefficient suitable for the character region, a filter coefficient capable of edge enhancement is preferable.
[0073]
In step s17, filter coefficients suitable for the photographic region and the background region are selected and delivered to the spatial filter unit 10.
[0074]
In the above example, the sum of the number of edges is divided into three types using threshold values in two stages and selected from the filter coefficients corresponding to each. However, more threshold values can be provided and set in more detail. It is. FIG. 9 shows an example of separation by a threshold value and selection of a filter coefficient corresponding to the separation. FIG. 9A shows an example in which the threshold value is set in three stages as in the case described with reference to the flowchart of FIG. FIG. 9B shows an example when the threshold value is set in four stages.
[0075]
Since the value of the sum Total is the sum of the number of edges in the pixel block, the finer the halftone dot, the greater the number of halftone dots present in the pixel block, and the greater the sum of the number of edges. Therefore, it is possible to determine the fineness (number of lines) of the halftone dot based on the value of the total Total even within the same halftone dot region. Therefore, it is not necessary to provide a separate process for detecting the number of lines of halftone dots. For example, when the total Total is equal to or greater than the threshold thA ', a filter coefficient A' suitable for a high-line number halftone dot region is selected, and when the total Total is smaller than the threshold thA 'and equal to or greater than the threshold thB', the low-line count is selected. A filter coefficient B ′ suitable for the halftone dot region is selected.
[0076]
In addition, for an area having characteristics that cannot be determined as a halftone dot area or a character area, it is possible to set a filter coefficient (such as a mixed filter) that can perform preferable processing in any case. When the total Total is smaller than the threshold thB 'and is equal to or larger than the threshold thC', a filter coefficient C 'suitable for the halftone area and the character area is selected. For the text area, photo area, and background area, the total Total is smaller than the threshold thC ', and if it is greater than or equal to the threshold thD', a filter coefficient D 'suitable for the text area is selected, and if the total Total is smaller than the threshold thD' A filter coefficient E ′ suitable for the photographic area and the background area is selected.
[0077]
FIG. 10 is a diagram illustrating an example of the filter coefficient illustrated in FIG. Since the filter coefficient A ′ (FIG. 10A) has characteristics with an emphasis on smoothing high-frequency components, it is suitable for a high-line-number halftone dot region containing many high-frequency components. Since the filter coefficient B '(FIG. 10B) has characteristics with an emphasis on smoothing low frequency components, it is suitable for a low-line-number halftone dot region containing many low frequency components. The filter coefficient C ′ (FIG. 10C) has characteristics that combine smoothing of high-frequency components and emphasis of low-frequency components, and is therefore suitable for regions that are difficult to determine as a halftone region or a character region. The filter coefficient D ′ (FIG. 10D) is optimal for the character region because it has characteristics focused on emphasizing all frequency components. The filter coefficient E ′ (FIG. 10E) is applied when the density value of the pixel of interest has a conversion value as it is, and it is not necessary to perform a filter process such as a photographic area or a background area. Note that the value and size of the filter coefficient are not limited to this.
[0078]
As described above, by selecting the filter coefficient based on the number of edges in the pixel block, it is possible to perform an appropriate filter process according to the feature of the target pixel.
[0079]
FIG. 11 is a block diagram showing a configuration of an image forming apparatus 51 according to the second embodiment of the present invention. The image forming apparatus 51 includes an image input apparatus 2, an image output apparatus 4, and an image processing apparatus 53. Parts that perform the same operations as the parts constituting the image forming apparatus 1 according to the first embodiment are denoted by the same reference numerals and description thereof is omitted. The difference from the image forming apparatus 1 is that the image processing device 53 further includes a region separation processing unit 14, and the black generation / undercolor removal unit 15 and the halftone generation unit 16 are the region separation results obtained by the region separation processing unit 14. Is to change the process based on
[0080]
The output signal from the input processing unit 7 is delivered to the rising / falling edge counting unit 12 and is also delivered to the region separation processing unit 14. The region separation processing unit 14 includes a chromatic / achromatic determination unit 141, a character determination unit 142, a halftone dot determination unit 143, and an overall determination unit 144. The pixel of interest is black characters, color characters, halftone dots, and others (photographs It is determined in which region of the background or the like it belongs. Then, the region separation signal indicating the determination result is delivered to the black generation / undercolor removal unit 15 and the halftone generation unit 16, respectively.
[0081]
FIG. 12 is a block diagram illustrating a configuration of the chromatic / achromatic determination unit 141. The chromatic / achromatic determination unit 141 includes a maximum value calculation unit 31, a minimum value calculation unit 32, a difference unit 33, and a comparator 34, and determines whether the target pixel is a chromatic color or an achromatic color. The result is output as a signal Color.
[0082]
First, the maximum value calculation unit 31 and the minimum value calculation unit 32 detect the maximum and minimum CMY density values of the target pixel. Next, a difference value between the maximum value and the minimum value is obtained by the subtractor 33. Then, the comparator 34 compares the difference value with the threshold value THcol. When the difference value is equal to or smaller than the threshold THcol, that is, when the difference in CMY density values of the target pixel is small, it is determined that the target pixel is an achromatic color, and “1” is output as the signal Color. When the difference value is larger than the threshold value THcol, that is, when the difference in CMY density values in the target pixel is large, it is determined that the target pixel is a chromatic color, and “0” is output as the signal Color.
[0083]
In the character determination unit 142, the character region is identified by utilizing the feature that the pixels belonging to the character region have a large density difference from the surrounding pixels (the edge is high). For example, a 3 × 3 pixel block centered on the target pixel is set, and the density difference between the target pixel and eight neighboring pixels adjacent thereto is obtained. If any one of the density differences is larger than the threshold THtext, the target pixel is determined to be a character pixel, and otherwise, it is determined to be a non-character pixel. This process is performed for each color of CMY individually, and if one of the pixels is determined to be a character pixel, “1” is output as the signal Text, otherwise “0” is output. .
[0084]
In the halftone dot determination unit 143, the halftone dot region is identified using the characteristics that “the density variation in the small area is large” and “the halftone dot density is higher than the background”. The following processing is performed within a block of P × Q (P and Q are natural numbers) pixels centered on the target pixel, and it is determined whether or not the target pixel is a pixel belonging to a halftone dot region. This process is performed individually for each color of CMY, and if one of the pixels is determined to be a “halftone pixel”, “1” is output as the signal Screen, otherwise “0” is output. Is output.
[0085]
1. For example, the average density value D for the central nine pixels in the pixel block_aveAnd binarize each pixel in the block to “0” or “1” using the average value. The maximum value D_max, Minimum value D_minAlso ask for.
2. Obtain the number of change points from 0 to 1 in the main scanning and sub-scan directions for the binarized data, respectively, and obtain the number of change points from 1 to 0._R, K_VAnd
3. Make the following decision: (B₁, B₂, T_R, T_V: Threshold)
[0086]
[Expression 1]

[0087]
FIG. 13 is a flowchart showing the region separation process. In step s21, a signal Screen is generated indicating whether or not the pixel of interest is a pixel belonging to the halftone dot region, and in step s22, a signal Text is generated indicating whether or not the pixel of interest is a pixel belonging to the character region. In step s23, a signal Color indicating whether the target pixel is chromatic or achromatic is generated. Based on the three signals obtained in parallel with each other, a comprehensive determination process is performed in step s24. In step s 25, a region separation signal indicating the result of the comprehensive determination process is delivered to the black generation / undercolor removal unit 15 and the halftone generation unit 16.
[0088]
FIG. 14 is a flowchart showing the comprehensive determination process. In step s31, it is determined whether or not the signal Screen is “1”. If “1”, the process proceeds to step s34, and if “0”, the process proceeds to step s32. In step s34, it is determined that the target pixel belongs to the halftone dot region, and "00" indicating the halftone dot is output as the region separation signal. In step s32, it is determined whether or not the signal Text is “1”. If the signal Text is “1”, the process proceeds to step s33, and if it is “0”, the process proceeds to step s37.
[0089]
In step s37, it is determined that the target pixel belongs to the photographic area or the background area, and “11” indicating the photographic area or the background area is output as an area separation signal. In step s33, it is determined whether or not the signal Color is “1”. If the signal Color is “1”, the process proceeds to step s35, and if it is “0”, the process proceeds to step s36. In step s35, it is determined that the target pixel belongs to the black character region, and “01” indicating the black character is output as the region separation signal. In step s36, it is determined that the target pixel belongs to the color character, and “10” indicating the color character is output as the region separation signal.
[0090]
As described above, the black generation / undercolor removal unit 15 and the halftone generation unit 16 can receive the region separation signal and perform optimal processing according to each region. As in the first embodiment, the spatial filter unit 10 uses the filter coefficient set based on the number of edges by the filter coefficient setting unit 13 to perform optimum filter processing regardless of the region separation result. it can.
[0091]
When the filter coefficient is switched based on the region separation result, for example, inadequate filter processing is performed on a portion where the region is erroneously determined, and a gap with the surroundings is generated, leading to locally noticeable image quality degradation. . Further, there is a problem that a clear region separation result cannot be assigned to a portion that is difficult to determine as a halftone dot region or a character region, and optimal filter processing cannot be performed. However, if the filter coefficient is switched according to the number of rising / falling edges, a local gap does not occur even at a location where the region is erroneously determined, and the region where the region cannot be determined is also shown in FIG. Since it is possible to perform processing using an optimum filter coefficient such as the filter coefficient C ′ shown in the above, it is possible to improve the output image quality.
[0092]
FIG. 15 is a block diagram showing a configuration of an image forming apparatus 61 according to the third embodiment of the present invention. The image forming apparatus 61 includes an image input apparatus 2, an image output apparatus 4, and an image processing apparatus 63. Parts that perform the same operations as those constituting the image forming apparatus 51 of the second embodiment are denoted by the same reference numerals and description thereof is omitted. The difference from the image forming apparatus 51 is that the region separation processing unit 17 performs region separation based on the number of rising / falling edges. The region separation processing unit 17 includes a chromatic / achromatic determination unit 171, a rising / falling edge count unit 172, and a comprehensive determination unit 173, and the pixel of interest is black characters, color characters, halftone dots, and others (photograph, background, etc.) ) To which area it belongs. Then, the region separation signal indicating the determination result is delivered to the black generation / undercolor removal unit 15 and the halftone generation unit 16, respectively.
[0093]
The chromatic / achromatic determination unit 171 determines whether the target pixel is chromatic or achromatic based on the output signal from the input processing unit 7. The rising / falling edge counting unit 172 counts SumX_Up, SumX_Down, SumY_Up, and SumY_Down based on the output signal from the input processing unit 7. The overall determination unit 173 performs region determination based on the determination result of the chromatic / achromatic determination unit 171 and the number of edges counted by the rising / falling edge counting unit 172.
[0094]
The filter coefficient setting unit 13 sets a filter coefficient based on the number of edges counted by the rising / falling edge counting unit 172 and outputs the filter coefficient to the spatial filter unit 10.
[0095]
FIG. 16 is a flowchart showing the region separation process. In step s41, the rising / falling edge counting unit 172 counts and outputs SumX_Up, SumX_Down, SumY_Up, and SumY_Down. In step S42, the chromatic / achromatic determining unit 171 determines whether the color is chromatic or not, and outputs a signal Color. In step s43, the comprehensive determination unit 173 determines a region to which the target pixel belongs and outputs a region separation signal. In step s44, the area separation signal is delivered to the black generation / undercolor removal unit 15 and the halftone generation unit 16.
[0096]
FIG. 17 is a flowchart showing the comprehensive determination process. In step s51, it is determined whether or not the conditions of SumX_Up> th_J1, SumX_Down> th_J1, SumY_Up> th_J1, and SumY_Down> th_J1 (th_J1 is a threshold value) are satisfied. This conditional expression represents the characteristics of an area where the number of rising edges and falling edges in the main scanning direction are both large and in the sub scanning direction, that is, a halftone dot area. If the condition is satisfied in step S51, the process proceeds to step s54, where it is determined that the pixel of interest belongs to the halftone dot region, and “00” indicating the halftone dot is output as the region separation signal.
[0097]
If the condition is not satisfied in step S51, the process proceeds to step s52, and either th_J1 ≧ SumX_Up> th_J2 and th_J1 ≧ SumX_Down> th_J2 or th_J1 ≧ SumY_Up> th_J2 and th_J1 ≧ SumY_Down> th_J2 (th_J2 is a threshold value) It is determined whether or not the above is satisfied. This conditional expression represents the characteristics of an area where the number of rising edges and falling edges in the main scanning direction are both large to some extent, or an area where it is established in the sub-scanning direction, that is, a character area. If the condition is satisfied, the process proceeds to step s53. If the condition is not satisfied, the process proceeds to step s57.
[0098]
In step s57, it is determined that the target pixel belongs to the photographic area or the background area, and “11” indicating the photographic area or the background area is output as an area separation signal.
[0099]
In step s53, it is determined whether or not the signal Color is “1”. If the signal Color is “1”, the process proceeds to step s55, and if it is “0”, the process proceeds to step s56. In step s55, it is determined that the target pixel belongs to the black character region, and “01” indicating the black character is output as the region separation signal. In step s56, it is determined that the target pixel belongs to the color character, and “10” indicating the color character is output as the region separation signal.
[0100]
The black generation / undercolor removal unit 15 and the halftone generation unit 16 can receive the region separation signal and perform optimum processing according to each region. As in the first embodiment, the spatial filter unit 10 uses the filter coefficient set based on the number of edges by the filter coefficient setting unit 13 to perform optimum filter processing regardless of the region separation result. it can. Further, in the region separation processing, the count result of the number of edges of the rising / falling edge counting unit 172 is used for the determination of various regions, so that the processing data amount can be reduced and the processing speed can be increased. The circuit scale can be greatly reduced as compared with the embodiment.
[0101]
The image forming apparatus has been described above as an embodiment of the present invention. However, the present invention can also be an image processing program to be executed by a computer and a computer-readable recording medium on which the image processing program is recorded.
[0102]
As a result, it is possible to provide an image processing program for causing a computer to execute an image processing method including appropriate filter processing and region separation processing, and a recording medium on which the image processing program is recorded. In the present embodiment, the recording medium may be a memory such as a ROM itself because processing is performed by a microcomputer, and a program reading device is provided as an external storage device. The recording medium may be a recording medium that can be read by inserting the recording medium therein.
[0103]
In any case, the stored image processing program may be configured to be accessed and executed by the microprocessor, or the image processing program is read out and the read image processing program is stored in the microcomputer program. A method of downloading to a storage area and executing the program may be used. It is assumed that this download program is stored in the main device in advance.
[0104]
Here, the recording medium is a recording medium configured to be separable from the main body, such as a tape system such as a magnetic tape or a cassette tape, a magnetic disk such as a flexible disk or a hard disk, or a CD-ROM (Compact Disc-Read Only). Memory) / MO (Magneto Optical) disk / MD (Mini Disk) / DVD (Digital Versatile Disk) optical disk system, IC (Integrated Circuit) card (including memory card) / optical card system, etc. A medium carrying a fixed program including a semiconductor memory such as a mask ROM, an EPROM (Erasable Programmable Read Only Memory), an EEPROM (Electrically Erasable Programmable Read Only Memory), or a flash ROM may be used.
[0105]
Further, the present invention is not limited to the recording medium as described above, and may be a medium that fluidly carries a program so that a computer system that can connect to a communication network including the Internet downloads the program from the communication network. When the program is downloaded from the communication network in this way, the download program may be stored in advance in a computer system or may be installed from another recording medium.
[0106]
The recording medium is read by a program reading device provided in a digital color image forming apparatus or a computer system, whereby the above-described image processing method is executed. In particular, in the latter case, it is possible to use an image processing method such as changing a threshold according to the user's preference.
[0107]
FIG. 18 is a diagram illustrating a configuration example of the computer system 101. The computer system 101 includes an image input device 102 such as a flatbed scanner, a film scanner, and a digital camera, a computer 103 in which various processes including the image processing method are performed by loading a predetermined image processing program, and computer processing An image display device 104 such as a CRT (Cathode Ray Tube) display and a liquid crystal display for displaying the results, a printer 105 for outputting the processing results of the computer to paper, a keyboard 106a and a mouse for the user to input image processing instructions and the like Furthermore, a modem configured as a communication unit for connecting to a server or the like via a telephone line is provided.
[0108]
When the image processing method is executed by this computer system, it becomes easy to arbitrarily change the threshold setting, and the processing according to the user's preference such as re-setting according to the result shown in the image display device. Is possible. In order to change the threshold value, it is set by directly inputting a numerical value using a keyboard or a mouse or dragging a symbol representing the threshold value.
[0109]
【The invention's effect】
As described above, according to the present invention, since one filter coefficient is selected from a plurality of filter coefficients based on the number of edges in the pixel block, it is possible to perform appropriate filter processing according to the characteristics of the pixel block. The image quality of the output image can be improved.
[0110]
Further, according to the present invention, since one filter coefficient is selected from a plurality of filter coefficients based on the number of edges in the pixel block, it is possible to perform appropriate filter processing according to the characteristics of the pixel block, and output image Image quality can be improved. Furthermore, since the filter process does not depend on the area determination result, image quality deterioration due to an area determination error determination or the like can be suppressed, and an area that cannot be clearly separated by the area determination process can be appropriately processed.
[0111]
Further, according to the present invention, since the region determination unit performs region determination based on the number of edges counted by the counting unit, it is possible to reduce the required calculation amount and reduce the circuit scale as compared with the conventional region determination processing. it can.
[0112]
Further, according to the present invention, the filter coefficient can be easily selected by using the threshold value.
[0113]
Further, according to the present invention, it is possible to select a more suitable filter coefficient by counting the number of edges while distinguishing between the scanning direction, rising edge, and falling edge.
[0114]
Further, according to the present invention, since the rising edge and the falling edge are detected by comparing the density difference between adjacent pixels with the appropriately set third threshold value, the edges of the photographic area and the background area are not detected. The number of edges can be counted more accurately.
[0115]
In addition, according to the present invention, it is possible to output image data that has been appropriately filtered, so that a high-quality still image can be formed.
[0116]
Further, according to the present invention, it is possible to provide an image processing program for causing a computer to execute the above image processing method.
[0117]
The present invention can also be provided as a computer-readable recording medium that records an image processing program for causing a computer to execute the above-described image processing method.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of an image forming apparatus 1 according to a first embodiment of the present invention.
FIG. 2 is a block diagram showing a configuration of a rising / falling edge counting unit 12;
3 is a diagram illustrating an example of a pixel block 100. FIG.
FIG. 4 is a schematic diagram showing a rising edge and a falling edge.
5 is a block diagram showing a configuration of an in-line count unit 121. FIG.
FIG. 6 is a flowchart showing rising / falling edge count processing;
FIG. 7 is a flowchart showing a filter coefficient setting process.
FIG. 8 is a schematic diagram showing a change in pixel density in each region.
FIG. 9 is a diagram illustrating an example of separation by a threshold and selection of a filter coefficient according to the separation.
FIG. 10 is a diagram illustrating an example of filter coefficients.
FIG. 11 is a block diagram illustrating a configuration of an image forming apparatus 51 according to a second embodiment of the present invention.
12 is a block diagram illustrating a configuration of a chromatic / achromatic determination unit 141. FIG.
FIG. 13 is a flowchart showing region separation processing.
FIG. 14 is a flowchart showing a comprehensive determination process.
FIG. 15 is a block diagram illustrating a configuration of an image forming apparatus 61 according to a third embodiment of the present invention.
FIG. 16 is a flowchart showing region separation processing;
FIG. 17 is a flowchart showing a comprehensive determination process.
FIG. 18 is a diagram illustrating a configuration example of a computer system 101.
[Explanation of symbols]
1, 51, 61 Image forming apparatus
2,102 image input device
3, 53, 63 Image processing device
4 Image output device
5 A / D (analog / digital) converter
6 Shading correction part
7 Input processing section
8 color correction part
9,15 Black generation / under color removal part
10 Spatial filter section
11, 16 Halftone generator
12,172 Rising / falling edge count section
13 Filter coefficient setting part
14, 17 Region separation processing unit
20, 33 Differencer
21 Absolute value calculator
22, 34 Comparator
23 AND circuit
24 SUM circuit
31 Maximum value calculator
32 Minimum value calculator
100 pixel block
101 Computer system
103 computer
104 Image display device
105 Printer
106 Input device
106a keyboard
106b mouse
121 In-line count section
122 Total calculation unit
141,171 Chromatic / Achromatic Judgment Unit
142 Character determination part
143 Halftone dot determination unit
144,173 Total judgment part

Claims (10)

A counting unit that counts the number of edges in a pixel block composed of a target pixel and its peripheral pixels included in image data indicating an image composed of a plurality of pixels;
A coefficient setting unit that selects one filter coefficient from a plurality of filter coefficients based on the counted number of edges;
A spatial filter unit that performs filter processing on input image data using the filter coefficient selected by the coefficient setting unit ;
The counting unit individually counts the number of rising and falling edges in the main scanning direction and the number of rising and falling edges in the sub-scanning direction in the pixel block, and sums the number of four edges. Count,
The coefficient setting unit compares the first threshold value, the second threshold value smaller than the first threshold value, and the intermediate threshold value smaller than the first threshold value and larger than the second threshold value with the counted edge number, and the number of edges Is equal to or greater than the first threshold, a filter coefficient having a smoothing characteristic is selected, and when the number of edges is smaller than the first threshold and equal to or greater than the intermediate threshold, smoothing and low frequency components are reduced. When a filter coefficient having a characteristic that combines frequency component enhancement is selected and the number of edges is smaller than the intermediate threshold and equal to or greater than the second threshold, a filter coefficient having a sharpening characteristic is selected, and the number of edges is when less than a second threshold value, the image processing apparatus according to claim you to select a filter coefficient having a characteristic density value of the pixel of interest is directly converted value. Image data indicating an image composed of a plurality of pixels is input,
A spatial filter unit that performs a filtering process on the input image data;
When the input image data is color image data composed of a plurality of colors, a black generation / under color removal unit that replaces the portion where each color has substantially the same pixel density with black and converts it to color image data further added with black When,
A halftone generation unit that performs gradation reproduction processing according to the output destination on the input image data;
An area determination unit that determines which of the plurality of areas including at least the character area and the halftone dot area the target pixel belongs to;
In the image processing apparatus in which at least one of the black generation / undercolor removal unit and the halftone generation unit performs processing according to the determination result of the region determination unit,
A counting unit for counting the number of edges in a pixel block composed of a pixel of interest and its surrounding pixels;
A coefficient setting unit that selects one filter coefficient from a plurality of filter coefficients based on the counted number of edges;
The counting unit individually counts the number of rising and falling edges in the main scanning direction and the number of rising and falling edges in the sub-scanning direction in the pixel block, and sums the number of four edges. Count,
The coefficient setting unit compares the first threshold value, the second threshold value smaller than the first threshold value, and the intermediate threshold value smaller than the first threshold value and larger than the second threshold value with the counted edge number, and the number of edges Is equal to or greater than the first threshold, a filter coefficient having a smoothing characteristic is selected, and when the number of edges is smaller than the first threshold and equal to or greater than the intermediate threshold, smoothing and low frequency components are reduced. When a filter coefficient having a characteristic that combines frequency component enhancement is selected and the number of edges is smaller than the intermediate threshold and equal to or greater than the second threshold, a filter coefficient having a sharpening characteristic is selected, and the number of edges is When it is less than the second threshold, a filter coefficient having a characteristic that the density value of the target pixel is directly converted into a converted value is selected,
The image processing apparatus, wherein the spatial filter unit performs a filter process using a filter coefficient selected by the coefficient setting unit.   The image according to claim 2, wherein the region determination unit determines, based on the counted number of edges, whether the target pixel belongs to at least one of a plurality of regions including a character region and a halftone dot region. Processing equipment. The counting unit has a rising edge where a density difference between a predetermined pixel in a pixel block and a pixel adjacent to the predetermined pixel in the scanning direction is larger than a third threshold and the density increases in the scanning direction. detected, the concentration difference between the predetermined pixel and an adjacent pixel in the scanning direction forward predetermined pixel in the pixel block is larger than the third threshold value, and a portion where the concentration is lowered in the scanning direction fall as The image processing apparatus according to claim 1, wherein the image processing apparatus is detected as an edge . An image processing apparatus according to any one of claims 1 to 4,
An image forming apparatus comprising: an image output device that outputs image data processed by the image processing device. A counting step of counting the number of edges in a pixel block composed of a target pixel and its peripheral pixels included in image data indicating an image composed of a plurality of pixels;
A coefficient setting step of selecting one filter coefficient from a plurality of filter coefficients based on the counted number of edges;
A spatial filter step of performing a filtering process on the input image data using the filter coefficient selected in the coefficient setting step.
In the counting step, the number of rising edges and falling edges in the main scanning direction and the number of rising edges and falling edges in the sub-scanning direction in the pixel block are individually counted, and the total number of four edges is calculated. Count,
In the coefficient setting step, the first threshold value, the second threshold value smaller than the first threshold value, and the intermediate threshold value smaller than the first threshold value and larger than the second threshold value are compared with the counted number of edges, and the number of edges Is equal to or greater than the first threshold, a filter coefficient having a smoothing characteristic is selected, and when the number of edges is smaller than the first threshold and equal to or greater than the intermediate threshold, smoothing and low frequency components are reduced. When a filter coefficient having a characteristic that combines frequency component enhancement is selected and the number of edges is smaller than the intermediate threshold and equal to or greater than the second threshold, a filter coefficient having a sharpening characteristic is selected, and the number of edges is when less than a second threshold value, images processing how to and selecting a filter coefficient having a characteristic density value of the pixel of interest is directly converted value. Image data indicating an image composed of a plurality of pixels is input,
A spatial filter process for performing filtering on input image data;
An area determination step for determining whether the target pixel belongs to at least one of a plurality of areas including a character area and a halftone dot area;
When the input image data is color image data composed of a plurality of colors, a black generation / under color removal step of replacing the portion where each color has substantially the same pixel density with black and converting it to color image data further adding black When,
A halftone generation step for performing gradation reproduction processing according to the output destination for the input image data,
In the image processing method for performing processing according to the determination result of the region determination step in at least one of the black generation / under color removal step and the halftone generation step,
A counting step of counting the number of edges in a pixel block composed of a pixel of interest and its surrounding pixels;
A coefficient setting step of selecting one filter coefficient from a plurality of filter coefficients based on the counted number of edges,
In the counting step, the number of rising edges and falling edges in the main scanning direction and the number of rising edges and falling edges in the sub-scanning direction in the pixel block are individually counted, and the total number of four edges is calculated. Count,
In the coefficient setting step, the first threshold value, the second threshold value smaller than the first threshold value, and the intermediate threshold value smaller than the first threshold value and larger than the second threshold value are compared with the counted number of edges, and the number of edges Is equal to or greater than the first threshold, a filter coefficient having a smoothing characteristic is selected, and when the number of edges is smaller than the first threshold and equal to or greater than the intermediate threshold, smoothing and low frequency components are reduced. When a filter coefficient having a characteristic that combines frequency component enhancement is selected and the number of edges is smaller than the intermediate threshold and equal to or greater than the second threshold, a filter coefficient having a sharpening characteristic is selected, and the number of edges is When it is less than the second threshold, a filter coefficient having a characteristic that the density value of the target pixel is directly converted into a converted value is selected,
Wherein the spatial filter process, image processing method characterized by performing the filtering process using the filter coefficient selected by the coefficient setting process. The image according to claim 7 , wherein the region determination step determines whether the target pixel belongs to at least one of a plurality of regions including a character region and a halftone region based on the counted number of edges. Processing method. An image processing program for causing a computer to execute the image processing method according to claim 7 or 8 . A computer-readable recording medium on which an image processing program for causing a computer to execute the image processing method according to claim 7 or 8 is recorded .

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