JP4260774B2 - Image processing apparatus, image forming apparatus, image processing method, image processing program, and recording medium - Google Patents

Description

  The present invention relates to an image processing apparatus and an image processing method for identifying the type of input image data.

  2. Description of the Related Art Conventionally, in image forming apparatuses such as color copiers, various types of originals such as character originals and character printed photo originals in which characters and printed photographs are mixed are used. In order to perform optimum processing on these various originals, an original type selection mode called a character print photograph mode, a character mode, and a print photograph mode is provided. There is also known a color copying machine that automatically selects a document type selection mode by performing a pre-scan of a document.

  As a method of automatically selecting a document type, conventionally, a halftone dot, a character edge, and other areas are recognized from the input document, and the area of the input document is configured according to the size and ratio of each area. There is known a method of recognizing whether or not a document is selected according to the document type.

  For example, Patent Document 1 discloses a technique for detecting characters on halftone dots from an input document, and performing resolution-priority image processing and outputting when the document is recognized as a document containing many characters on halftone dots. ing.

In Patent Document 2, it is determined whether or not the document is chromatic, and whether or not the number of halftone pixels is a predetermined number. The document is chromatic and the number of halftone pixels is predetermined. A technique for performing color reproduction by determining that a halftone dot pattern is present when the number is greater than or equal to the number is disclosed.
JP-A-10-285394 (published October 23, 1998) JP 2002-247390 (released on August 30, 2002)

  Although the above-mentioned Patent Document 1 can detect a character on a halftone dot, the character on a halftone dot is a character on a halftone dot region, and is composed of halftone dots frequently used in maps, pamphlets, catalogs, and the like. It is not a character (hereinafter referred to as a halftone character). For this reason, it is difficult for the technique of Patent Document 1 to accurately recognize halftone characters. Also, even if it can be recognized, moire may occur if the color of halftone characters changes or halftone areas are included in the document because resolution priority processing is performed. There is.

  The halftone dot character area is similar to the halftone dot area because it is composed of halftone dots. Therefore, in the technique described in Patent Document 2, there is a high possibility that the halftone character region is erroneously detected as a halftone dot pattern. If it is determined that the halftone dot character is not a halftone dot pattern, the technique disclosed in Japanese Patent Laid-Open No. 2004-228867 makes a monochrome copy, so that the color halftone character is reproduced as a monochrome halftone character. Reproducibility is greatly impaired.

  In view of the above-described problems, the present invention is to realize an image processing apparatus capable of accurately detecting whether or not input image data includes halftone characters. Another object is to realize an image processing apparatus capable of improving the reproducibility of halftone characters.

  In order to solve the above problems, an image processing apparatus according to the present invention includes a halftone character edge region detection unit that detects a halftone character edge region that is a character edge composed of halftone dots from input image data, And a determination unit that determines whether or not the input image data includes a halftone character based on a detection result by the halftone character edge region detection unit.

  Also, the image processing method of the present invention includes a halftone character edge region detecting step for detecting a halftone character edge region which is an edge of a character composed of halftone dots from input image data, and the halftone character edge region detecting step. And a determination step of determining whether or not the input image data includes halftone characters based on the detection result in.

  According to the above configuration, a halftone character edge region that is an edge of a character composed of halftone dots is detected from the input image data, and whether or not the input image data includes halftone characters based on the detection result. judge. As a result, since there is no edge in the halftone dot area other than the halftone character, a simple halftone area is not detected as a halftone character edge area. Further, the edge of a character that is not composed of halftone dots is not erroneously detected as the edge of a halftone character. As a result, it is possible to accurately identify whether the input image data includes halftone characters.

  In addition to the above configuration, the image processing apparatus of the present invention further includes a halftone dot region detection unit that detects a halftone dot region from the input image data, and the halftone dot character edge region detection unit performs a smoothing process. Preferably, a character edge region is extracted from the input image data, and the halftone dot region detected by the halftone dot region detection unit is detected as the halftone dot character edge region.

  According to said structure, a character edge area | region is extracted from the input image data which performed the smoothing process. By performing the smoothing process, the features of the halftone dot region are removed from the input image data. Therefore, the character edge region extracted from the input image data subjected to the smoothing process includes an edge region of normal characters and an edge region of halftone characters. Therefore, the area that is the character edge area extracted from the input image data that has been subjected to the smoothing process and that is the halftone area that has been detected from the input image data that has not been subjected to the smoothing process is composed of halftone dots. This is the edge area of a halftone character, that is, a halftone dot character. Therefore, according to the above configuration, a dot character edge can be detected with high accuracy.

  Furthermore, in addition to the above-described configuration, the image processing apparatus of the present invention is configured to convert input image data into at least three areas of the halftone character edge area, the character edge area, and the halftone area based on the determination result of the determination unit. It is preferable to provide a region separation unit that separates into two.

  According to the above configuration, for example, when the determination result of the determination unit indicates that the input image data includes halftone characters, the halftone character edge region is preferentially separated from the input image data. As a result, the halftone dot character edge region is not erroneously separated as a halftone dot region. That is, the dot character edge region can be separated with high accuracy.

  Furthermore, in addition to the above configuration, the image processing apparatus of the present invention performs an enhancement filter process on the character edge region separated by the region separation unit, and applies to the halftone dot region separated by the region separation unit. It is preferable to include a spatial filter unit that performs smoothing filter processing and performs emphasis filter processing weaker than the character edge region on the halftone character edge region separated by the region separation unit.

  According to the above configuration, the character reproducibility is improved because the emphasis filter process is performed on the character edge region separated by the region separation unit. In addition, since the smoothing filter process is performed on the halftone dot regions separated by the region separation unit, the occurrence of moire can be suppressed. Further, since the emphasis filter processing weaker than that of the character edge region is performed on the halftone character edge region separated by the region separation unit, it is possible to suppress a change in the shade of the halftone character.

  Furthermore, in addition to the above-described configuration, the image processing apparatus of the present invention generates the same black as the halftone dot region separated by the region separation unit for the halftone character edge region separated by the region separation unit. It is preferable to provide a black generation unit that generates black at a rate.

  According to the above configuration, since the halftone dot area and the halftone dot edge area are generated with the same black generation rate, the gap between the halftone dot character edge area and the adjacent halftone dot area is black. Less.

  Furthermore, in addition to the above-described configuration, the image processing apparatus of the present invention has the same undercolor as the halftone dot region separated by the region separation unit with respect to the halftone character edge region separated by the region separation unit. It is preferable to provide a lower color removing unit that performs a lower color removing process at a removal rate.

  According to the above configuration, the halftone dot area and the halftone dot edge area are subjected to the undercolor removal process with the same undercolor removal rate, so that the halftone dot edge area and the adjacent halftone dot area are undercolored. The removal rate gap is reduced.

  In addition, as described above, the image forming apparatus of the present invention includes the above-described image processing apparatus and the image output unit that forms an image based on the image data processed by the image processing apparatus. Whether or not the image data includes halftone characters can be detected with high accuracy, and the reproducibility of the halftone characters can be improved.

  The image processing apparatus may be realized by a computer. In this case, an image processing program for causing the image processing apparatus to be realized by the computer by causing the computer to operate as each unit, and a computer that records the image processing program A readable recording medium falls within the scope of the present invention.

  The image processing apparatus according to the present invention includes a halftone character edge region detection unit that detects a halftone character edge region that is an edge of a character composed of halftone dots from input image data, and the halftone character edge region detection unit. And a determination unit that determines whether the input image data includes halftone characters based on the detection result. Therefore, it is possible to accurately detect whether or not the input image data includes halftone characters.

  An embodiment of the present invention will be described below with reference to FIGS.

<Overall Configuration of Image Forming Apparatus>
As shown in FIG. 2, the image forming apparatus according to the present embodiment includes an image processing apparatus 2, a color image input apparatus 1 and a color image output apparatus 3 connected to the image processing apparatus 2. In the present embodiment, a digital color copying machine will be described as an example of the image forming apparatus.

  The color image input device (reading device) 1 includes, for example, a scanner unit provided with a CCD (Charge Coupled Device). The reflected light image from the original image is converted into RGB (R: red, G: green, B: Blue) The analog signal is read by the CCD and output to the image processing apparatus 2.

  The color image output device 3 is a device that outputs a predetermined image processing result performed by the image processing device 2. The color image output device 3 is an electrophotographic printer or an inkjet printer.

  The image processing apparatus 2 includes an A / D (analog / digital) conversion unit 11, an input correction unit 12, a document type identification unit 14, a region separation unit 15, a color correction unit 13, a black generation / under color removal unit (black generation unit). , An under color removal unit) 16, a spatial filter processing unit 17, and a halftone generation unit 18.

  A color image signal (RGB analog signal) input by the CCD line sensor of the color image input device 1 is converted into an RGB digital signal by the A / D converter 11. The RGB digital signal is converted by the input correction unit 12 from various distortions generated in the illumination system, imaging system, and imaging system of the color image input device 1, and the reflectance signal is converted into a density signal suitable for image processing. And converted to a CMY (C: cyan, M: magenta, Y: yellow) signal which is a complementary color of the RGB signal.

  The document type identification unit 14 detects what area exists on the document, and based on the detection result, the document type determination process, for example, the input document is a character document, a character / print photo It is determined whether it is a manuscript, a character / photographic paper photographic manuscript, a printed photographic manuscript, or a photographic paper photographic manuscript. The document type identification unit 14 outputs a document type signal indicating the determination result to the region separation unit 15. Details of the document type identification unit 14 will be described later.

  The region separation unit 15 uses a document type signal indicating the determination result in the document type identification unit 14, and whether each pixel belongs to, for example, a halftone dot region, a character edge region, or a halftone character edge region. The region identification signal indicating the determination result is output to the black generation / undercolor removal unit 16, the spatial filter unit 17, and the halftone generation unit 18.

  The color correction unit 13 performs color correction processing for removing color turbidity based on spectral characteristics of CMY (C: cyan, M: magenta, Y: yellow) color materials including unnecessary absorption components in order to realize faithful color reproduction. Is to do.

  The black generation / under color removal unit 16 performs black generation processing for generating a black (K) signal from the CMY three-color signals after color correction, while corresponding to the K signal obtained by black generation from the original CMY signal. Undercolor removal processing for subtracting the UCR signal to generate a new CMY signal is performed. As a result of these processes (black generation process / under color removal process), the CMY three-color signal is converted into a CMYK four-color signal.

  The spatial filter processing unit 17 performs edge enhancement processing and smoothing processing on the CMYK signal.

  The halftone generation unit 18 performs gradation reproduction processing for finally dividing an image into pixels and reproducing each gradation.

  Based on the image data output from the halftone generator 18, an output image is formed by the color image output unit 3. The above processing is performed by a CPU (Central Processing Unit) (not shown).

<About the document type identification unit>
Next, details of the document type identification process in the document type identification unit 14 will be described. FIG. 1 is a block diagram illustrating a configuration of the document type identification unit 14.

  As shown in FIG. 1, the document type identification unit 14 includes a character edge region detection unit 21, a halftone dot region detection unit 22, a halftone dot character edge region detection unit 23, a pixel number counting unit 24, and a document type determination unit (determination). Part) 25. The document type identification unit 14 of the present embodiment identifies whether the input document belongs to a character document, a character / print photo document, a print photo document, or the other (photographic paper photo document).

  The character edge region detection unit 21 determines whether or not the pixel of interest is a character edge region, and outputs a character edge pixel signal indicating the determination result to the pixel count unit 24.

  The halftone dot region detection unit 22 determines whether or not the target pixel is a halftone dot region, and outputs a halftone pixel signal indicating the determination result to the pixel number counting unit 24 and the halftone character edge region detection unit 23. Is.

  The halftone dot edge region detection unit 23 refers to the halftone pixel signal from the halftone dot region detection unit 22 to determine whether or not the pixel of interest is an edge region of a halftone character, and indicates the determination result. The halftone character edge pixel signal is output to the pixel number counting unit 24.

  The pixel number counting unit 24 counts the number of pixels in the character edge region, the halftone dot region, and the halftone character edge region, and outputs each count value to the document type determination unit 25. The document type determination unit 25 determines the document type based on each count value, and outputs a document type signal indicating the determination result.

(Overall process flow in the document type identification section)
FIG. 3 is a flowchart showing an outline of the flow of processing in the document type identification unit 14. First, from the CMY image data from the input correction unit 12, a halftone dot region detection process (S 1) for determining whether or not the target pixel in the halftone dot region detection unit 22 is a halftone dot region, and a character edge region detection unit 21 Character edge region detection processing (S2) for determining whether or not the pixel of interest is a character edge region, and a halftone dot for determining whether or not the pixel of interest in the halftone character edge region detector 23 is a halftone character edge region Character edge region detection processing (S3) is performed.

  Thereafter, the pixel count unit 24 counts the number of pixels belonging to each region (S4), and the document type determination unit 25 determines the document type based on the count value counted by the pixel number count unit 24 (S5).

<< Flow of halftone dot detection process >>
Next, each process will be described in detail. FIG. 4 is a flowchart showing the flow of halftone dot region detection processing in the halftone dot region detector 22. In the following description, a case where halftone dot region detection processing is performed for any one of C, M, and Y planes will be described.

The halftone dot region detection unit 22 is characterized in that the halftone dot region has a characteristic a: large density fluctuation in a small region,
Feature b: The density of the halftone dot is higher than the background,
It is determined whether or not each pixel is a halftone dot region by utilizing the characteristic of

  First, in S11, the halftone dot region detection unit 22 calculates a pixel value V (M, N) within a preset M × N pixel mask (for example, a 7 × 7 pixel mask) as shown in FIG. The maximum value VMax, the minimum value VMin, and the average density value Vave are calculated.

  Next, for each pixel in the pixel mask, it is determined whether the pixel value is larger than the average density value VAve (S12). When the pixel value is larger than the average density value Vave, the halftone dot region detection unit 22 sets the pixel value to 255 (S13). On the other hand, when the pixel value is smaller than the average density value Vave, the halftone dot region detection unit 22 sets the pixel value to 0 (S14).

  That is, the halftone dot area detection unit 22 performs binarization based on VAve so that the density fluctuations that are the characteristics of the halftone dots can be easily detected in S12 to S14.

  Next, in S15, the halftone dot region detection unit 22 calculates the number of inversions RMain in the main scanning direction and the number of inversions RSub in the sub scanning direction of the pixels in the pixel mask as shown in FIG.

For example, in the main scanning direction:
| V (m, n) -V (m, n-1) |> 0
When it is satisfied, it is determined that the pixel has been inverted, and RMain is incremented by 1, and this is performed for all main scanning directions in the pixel mask.

On the other hand, in the sub-scanning direction,
| V (m, n) -V (m-1, n) |> 0
When it is satisfied that the pixel has been inverted, RSub is incremented by 1, and this is performed for all sub-scanning directions in the pixel mask.

  Note that the arrows shown in FIG. 5 represent all combinations in which the above calculation is performed within the pixel mask. The arrows in the left graph in FIG. 5 indicate the main scanning direction, and the arrows in the right graph in FIG. 5 indicate the sub-scanning direction.

Next, the halftone dot area detection unit 22 determines whether or not the density difference between the maximum density value VMax and the minimum density value VMin and the average density value VAve is larger than preset threshold values ThMax and ThMin, that is, The following conditional expressions: VMax−VAve> ThMax and VAve−VMin> ThMin (1)
It is determined whether or not the condition is satisfied (S16). Thereby, it can be determined whether or not the target pixel has the feature a of the halftone dot region (the density variation within the small region is large).

  If the conditional expression (1) is satisfied, the process proceeds to S17, and otherwise, the process proceeds to S19.

In S17, the halftone dot region detection unit 22 determines whether or not the number of inversions RMain in the main scanning direction and the number of inversions RSsub in the sub-scanning direction calculated in S15 are larger than preset threshold values ThMain and ThSub, respectively, The following conditional expression: RMain> ThMain and RSub> ThSub (2)
It is determined whether or not the above is satisfied. Thereby, it can be determined whether or not the target pixel has the feature b of the halftone dot region (the density of the halftone dot is larger than that of the background).

  If the conditional expression (2) is satisfied, the process proceeds to S18, and if not, the process proceeds to S19.

  If both the conditional expressions (1) and (2) are satisfied, in S18, the halftone dot region detection unit 22 determines that the pixel of interest that is the central pixel of the pixel mask is a halftone dot region. That is, the pixel of interest is determined to be a halftone dot region, assuming that it has both the features a and b of the halftone dot region. On the other hand, when at least one of the conditional expressions (1) and (2) is not satisfied, in S19, the halftone dot region detection unit 22 determines the target pixel as a non-halftone dot region. Thereafter, the halftone dot region detection unit 22 outputs a halftone pixel signal indicating whether the target pixel belongs to a halftone dot region or a non-halftone dot region.

  Next, the halftone dot region detection unit 22 determines whether or not the determination as to whether or not all the pixels are halftone dot regions has ended (S20). If the determination for all the pixels has not been completed, the processes of S11 to S19 are repeated for the pixels that have not been determined. In this way, the halftone dot area detection unit 22 determines whether or not all pixels are halftone dot areas.

<< Character edge detection process flow >>
Next, the flow of character edge detection processing in the character edge region detection unit 21 will be described with reference to the flowchart of FIG. In the following description, a case where character edge detection processing is performed for any one of C, M, and Y planes will be described.

The character edge area detection unit 21 is characterized in that the character edge area has a characteristic c: a density difference (edge) in a certain direction,
It is determined whether or not each pixel is a character edge region by utilizing this characteristic.

  First, the character edge region detection unit 21 applies a pixel of interest f (x, y) that is a central pixel in a preset M × N pixel mask (for example, 3 × 3 pixel mask) as shown in FIG. On the other hand, i = −1 and j = −1 are set as initial variables (S21).

Subsequently, the character edge region detection unit 21 uses the following conditional expression based on a preset threshold ThEdge:
| F (x, y) -f (x + i, y + j) |> ThEdge (3)
Is determined (S22).

  If the conditional expression (3) is satisfied, the process proceeds to S27. On the other hand, when the conditional expression (3) is not satisfied, i and j are sequentially added by 1 by the processing of S23 to S26, and the determination of S22 is performed. That is, (i, j) is changed to (0, -1), (1, -1), (-1,0), (0,0), (1,0), (-1,1), (0 , 1) and (1, 1) in this order. However, when the conditional expression (3) is satisfied with any of the settings, the process proceeds to S27 as it is without changing to the remaining settings.

  The character edge region has the above-described feature c (there is a density difference (edge) in a certain direction). Therefore, if any one of the pixels in the pixel mask satisfies the conditional expression (3), in S27, the character edge region detection unit 21 determines that the pixel of interest has the feature c, and the pixel of interest is the character edge region. It is determined that On the other hand, when the conditional expression (3) is not satisfied for all the pixels in the pixel mask, the character edge region detection unit 21 determines in S28 that the pixel of interest is a non-character edge region. Then, the character edge region detection unit 22 outputs a character edge pixel signal indicating whether the target pixel belongs to the character edge region or the non-character edge region.

  Thereafter, the character edge region detection unit 21 determines whether or not the determination of whether or not all the pixels are character edge regions has been completed (S29). If the determination for all the pixels has not been completed, the processes of S21 to S28 are repeated for the pixels that have not been determined. In this way, the character edge area detection unit 21 determines whether or not all pixels are character edge areas.

<< Half-dot character edge detection processing >>
Next, the flow of halftone character edge detection processing in the halftone character edge region detection unit 23 will be described with reference to the flowchart of FIG. In the following description, a case where halftone character edge detection processing is performed for any one of C, M, and Y planes will be described.

The area of the halftone dot edge is
Feature a: Large variation in density within a small region,
Feature b: The density of the halftone dot is higher than the background,
Feature c: existence of a density difference (edge) in a certain direction,
The character edge area feature and the halftone dot area feature are combined. For this reason, even if the character edge detection processing shown in FIG. 6 is performed on the input image data, the halftone character edge cannot be detected with high accuracy.

  Therefore, the halftone character edge region detection unit 23 of the present embodiment first performs smoothing processing on the input image data in order to remove the features of the halftone region (S31).

  The smoothing process uses, for example, a median filter of 5 × 5 size having a characteristic of reducing MTF (Modulation Transfer Function) in the entire frequency band as shown in FIG. 9A, or FIG. As shown in (b), the MTF is amplified in the low frequency band to which the halftone character region belongs, and has the smoothing and emphasis characteristics that have the characteristic of reducing the MTF in the high frequency band to which the halftone region belongs. A mixed filter may be used.

Thereafter, the character edge detection process shown in FIG. 6 is performed (S32). Here, the detected character edge includes not only a halftone dot character edge but also a normal character edge. Accordingly, the halftone character edge region detection unit 23 uses the result of the halftone dot region detection process shown in FIG. 4 to determine whether the pixel of interest is a halftone region and a character edge region. (S33).

  When the target pixel is a halftone dot area and a character edge area, the halftone dot character edge area detection unit 23 determines that the target pixel is a halftone character edge area (S34). Then, the halftone character edge region detection unit 23 outputs a halftone character edge pixel signal indicating whether the pixel of interest belongs to a halftone character edge region or a non-halftone character edge region.

  After that, the halftone character edge area detection unit 23 determines whether or not the determination as to whether or not all the pixels are halftone character edge areas has been completed (S35). When the determination for all the pixels has not been completed, the processes of S31 to S34 are repeated for the pixels that have not been determined. In this way, the halftone character edge region detection unit 23 determines whether or not all pixels are halftone character edge regions.

<< Pixel count processing >>
Next, the pixel number counting process in the pixel number counting unit 24 will be described with reference to the flowchart of FIG.

  First, in S41, the count value EdgeCount of the number of pixels in the character edge region, the count value ScreenCount of the number of pixels in the halftone dot region, and the count value ScreenEdgeCount of the number of pixels in the halftone character edge region are set to the initial value 0.

  Thereafter, in the target pixel, it is determined whether or not the character edge pixel signal indicates a character edge region in any one of C, M, and Y planes (S42). When the character edge pixel signal indicates the character edge region, the pixel number counting unit 24 adds 1 to the character edge region count value EdgeCount (S43).

  Similarly, the pixel number counting unit 24 determines whether or not the halftone dot pixel signal indicates that it is a halftone dot region in any one of the C, M, and Y planes in the target pixel (S44). If the dot pixel signal indicates a halftone dot region, the halftone dot region count value ScreenCount is incremented by 1 (S45). Further, the pixel number counting unit 24 determines whether or not the halftone dot character edge pixel signal indicates the halftone dot character edge region in any one of C, M, and Y in the target pixel (S46). When the halftone character edge pixel signal indicates a halftone character edge region, the count value ScreenEdgeCount of the halftone character edge region is incremented by 1 (S47).

  The processes of S42 to S47 are performed on the pixels of the entire image (S48). Then, the pixel number counting unit 24 outputs the count values EdgeCount, ScreenCount, and ScreenEdgeCount counted for the pixels of the entire image to the document type determination unit 25.

<< Document Type Determination Process >>
Next, the flow of the document type determination process in the document type determination unit 25 will be described with reference to the flowchart of FIG.

  First, in S51, a character edge region determination flag EdgeFlag, a halftone dot region determination flag ScreenFlag, and a halftone character edge region determination flag ScreenEdgeFlag, which are indexes for determining the type of document, are set to an initial value 0. Next, it is determined whether or not the count value EdgeCount of the pixels belonging to the character edge region calculated by the pixel number counting unit 24 is greater than or equal to a preset threshold ThEdgeCount (S52), and the count value EdgeCount is greater than or equal to the threshold ThEdgeCount. If there is, the character edge region determination flag EdgeFlag is set to 1 (S53).

  Similarly, when it is determined that the count value ScreenCount of the pixels belonging to the halftone dot area is greater than or equal to a preset threshold value ThScreenCount (S54), the document type determination unit 25 sets the halftone dot area determination flag ScreenFlag to 1. (S55). If it is determined that the count value ScreenEdgeCount of the pixels belonging to the halftone character edge region is equal to or larger than a preset threshold value ThScreenEdgeCount (S56), the halftone dot region determination flag ScreenEdgeFlag is set to 1 (S57).

  Based on the flag obtained as described above and a table as shown in FIG. 12 showing the determination criteria stored in advance, the document type determination unit 25 determines that the document is “character printed photograph”, “character”, “printed photograph”, “ It is determined whether it is “other (printing paper photograph)” (S58). Then, the document type determination unit 25 outputs a document type signal indicating the determination result to the region separation unit 15.

  For example, as shown in the table of FIG. 12, when the character edge region determination flag EdgeFlag, the halftone dot region determination flag ScreenFlag, and the halftone character edge region determination flag ScreenEdgeFlag are all 1, the character region and the halftone dot region are included in the document. Since the halftone dot character area is mixed, the document type determination unit 25 determines that the document is “character printed photograph (A)”. If the character edge area determination flag EdgeFlag and the halftone dot area determination flag ScreenFlag are 1 and the halftone dot edge edge determination flag ScreenEdgeFlag is 0, the character area and the halftone area are mixed in the document. The document type determination unit 25 determines that the document is “character printed photograph (B)”. When only the character edge region determination flag EdgeFlag is 1, only normal characters exist in the document, and the document type determination unit 25 determines that the document is a “character” document. When all the flags are 0, it is considered that the document is composed of continuous tone gradations represented by silver halide photography, and is an “other” document (for example, a photographic paper photo document). To be judged. The character-printed photo original is divided into (A) to (E), which will be described later.

  By the processing of the above-described document type identification unit 25, what kind of document the input image is is identified. Thereby, based on the document type signal, the region separation unit 15 can accurately detect which region each pixel belongs to.

<Region separation unit>
Next, details of the region separation unit 15 will be described. FIG. 13 is a block diagram showing the configuration of the region separation unit 15. As shown in the figure, the region separation unit 15 includes a character edge region detection unit 31, a halftone dot region detection unit 32, a halftone dot edge region detection unit 33, a region determination unit 34, and a chromatic / achromatic determination unit 35. ing.

  The processing in the character edge region detection unit 31, the halftone dot region detection unit 32, and the halftone character edge region detection unit 33 includes the character edge region detection unit 21, the halftone region detection unit 22, the halftone character, respectively, of the document type identification unit 14. The same processing as that of the edge region detection unit 23 is performed. However, the character edge region detection unit 31, the halftone dot region detection unit 32, and the halftone dot edge detection unit 33 change the threshold value used in the detection process according to the document type signal from the document type identification unit 14. Shall. Thereby, it is possible to accurately detect which region the pixel of interest belongs to.

  The character edge region detection unit 31 outputs a character edge signal indicating that the target pixel is a character edge region or a non-character edge signal indicating that the target pixel is a non-character edge region to the region determination unit 34. To do. Also, the halftone dot region detection unit 32 sends a halftone dot region signal indicating that the target pixel is a halftone dot region to the region determination unit 34 and the halftone dot character edge region detection unit 33, and the target pixel is a non-halftone dot region. A non-halftone dot signal indicating this is output to the area determination unit 34. Further, the halftone dot edge region detection unit 33 generates a halftone dot edge signal indicating that the target pixel is a halftone dot edge region, or a non-halftone dot indicating that the target pixel is a non-halftone character edge region. The character edge signal is output to the area determination unit 34.

  Based on the signals from the character edge region detection unit 31, the halftone dot region detection unit 32, and the halftone character edge region detection unit 33, the region determination unit 34 determines that the target pixel is a halftone dot region, a character edge region, and a halftone character edge. It is determined which one of the areas it belongs to. Note that the area determination unit 34 determines the character edge area detection unit 31, the halftone dot area in accordance with the determination priority shown in the left column of the table of FIG. 12 based on the document type signal from the document type identification unit 14 when determining the region. Assume that the priority of the signals from the detection unit 32 and the halftone character edge region detection unit 33 is changed.

  When the target pixel belongs to the halftone dot region, the region determination unit 34 displays a halftone dot region signal indicating that, and when the target pixel belongs to the halftone character region, the region determination unit 34 displays a halftone character edge region signal indicating that fact. If the pixel of interest belongs to another area, an other area signal indicating that fact is output to the subsequent block. In addition, when the target pixel belongs to the character edge region, the region determination unit 34 outputs a character edge region signal to the chromatic / achromatic determination unit 35.

  The chromatic / achromatic determining unit 35 determines, for a pixel determined to be a character edge region, whether the region is a color character or a black character. Details of the processing in the chromatic / achromatic determination unit 35 will be described later.

(Overall process flow in the region separation unit)
FIG. 14 is a flowchart showing the flow of processing in the region separation unit 15. As shown in FIG. 14, first, a halftone dot region detection process (S61) by the halftone dot region detection unit 32, a character edge region detection process (S62) by the character edge region detection unit 31, and a halftone character edge region detection unit 33. Thus, the halftone character edge region detection process (S63) is performed in parallel. These S61, S62, and S63 are the same as the processes of S1, S2, and S3.

  However, as described above, the character edge region detection unit 31, the halftone dot region detection unit 32, and the halftone character edge region detection unit 33 change the threshold according to the document type signal from the document type identification unit 14. And The halftone dot region detection unit 32 stores default setting values in advance as threshold values ThMax, ThMin, ThMain, ThSub. In addition, the character edge region detection unit 31 and the halftone character edge region detection unit 33 store a default setting value in advance as the threshold ThEdge.

  For example, when the document type signal indicates a “character-printed photo (A)” document or a “character-printed photo (B)” document, a character edge region detection unit 31, a halftone dot region detection unit 32, and a halftone dot character edge region detection unit 33 performs a detection process using a default set value. On the other hand, when the document type signal indicates a “character printed photograph (D)” document, it is considered that only characters or many characters exist on the document. Therefore, in order to facilitate detection of a character edge or a halftone character edge, the character edge region detection unit 31 and the halftone character edge region detection unit 33 set the threshold ThEdge to a value smaller than the default setting value (for example, the default setting value). To 0.7).

  When the document type signal is a “character printed photograph (C)” document or a “character printed photograph (E)” document, only a halftone dot area and halftone character, or a halftone dot area and halftone character is displayed on the original. It is thought that there are many. Therefore, in order to facilitate detection of the halftone dot region, the halftone dot region detection unit 32 sets the threshold values ThMax, ThMin, ThMain, and ThSub to values smaller than the default setting values (for example, 0.7 times the default setting value). change.

  As described above, the character edge region detection unit 31, the halftone dot region detection unit 32, and the halftone character edge region detection unit 33 adjust the threshold according to the document type signal from the document type identification unit 14.

  Then, the character edge region detection unit 31 outputs a character edge signal indicating that the target pixel is a character edge region or a non-character edge signal indicating that the target pixel is a non-character edge region to the region determination unit 34 To do. Further, the halftone dot area detection unit 32 outputs a halftone dot signal indicating that the target pixel is a halftone dot area or a non-halftone dot signal indicating that the target pixel is a non-halftone dot area to the area determination unit 34. To do. Further, the halftone dot edge region detection unit 33 generates a halftone dot edge signal indicating that the target pixel is a halftone dot edge region, or a non-halftone dot indicating that the target pixel is a non-halftone character edge region. The character edge signal is output to the area determination unit 34.

  Next, in S64, the region determination unit 34 determines the region to which the target pixel belongs based on the character edge signal, the halftone dot signal, and the halftone character edge signal as the character edge region, the halftone dot region, and the halftone character edge region. Which one is determined (area determination processing). At this time, the region determination unit 34 determines the priority of the character edge signal, the halftone dot signal, and the halftone character edge signal based on the document type signal, and determines the region of the target pixel according to the priority. The detailed process flow of the area determination process in the area determination unit 34 will be described later.

  Next, when it is determined that the target pixel is the character edge region (Yes in S65), the region determination unit sends a character edge signal indicating that the target pixel is the character edge region to the chromatic / achromatic determination unit 35. Output.

  Thereafter, the chromatic / achromatic determination unit 35 performs a chromatic / achromatic determination process to determine whether the pixel of interest is a chromatic color or an achromatic color (S66). Then, the process ends. Details of the chromatic / achromatic determination processing in the chromatic / achromatic determination unit 35 will be described later.

  On the other hand, when the target pixel is other than the character edge region (that is, a halftone dot region, a halftone character edge region, or other region) (No in S65), the region determination unit 34 determines that the target pixel is a halftone dot region. , A dot character edge region signal indicating that the pixel of interest is a dot character edge region, or another region signal indicating that the pixel of interest is another region is output to the subsequent stage. Then, the process ends.

<< Area determination processing >>
Next, details of the area determination process in the area determination unit 34 in S64 will be described. FIG. 15 is a flowchart showing the flow of region determination processing when the document type signal indicates a “character-printed photo (A)” document.

  First, the area determination unit 34 prioritizes the halftone signal, the halftone character edge signal, and the character edge signal corresponding to the document type signal “character printed photograph (A)” based on the table of FIG. 12 stored in advance. Determine the degree. Here, the dot character edge signal is given the highest priority, followed by the dot signal and the character edge signal in this order.

  In step S71, the area determination unit 34 indicates that the signal obtained from the halftone dot edge area detection unit 33 is a halftone dot edge area in any one of C, M, and Y planes. It is determined whether it is an edge signal. When any one of C, M, and Y receives a halftone dot signal, the region determination unit 34 determines that the pixel of interest is a halftone character edge region (S72).

  On the other hand, if the C, M, and Y planes are non-halftone character edge signals, the region determination unit 34 determines that the signal obtained from the halftone dot region detection unit 32 is C, M, Y in S73. It is determined whether any of the planes is a halftone dot signal indicating a halftone dot region. When any one of C, M, and Y receives a halftone dot signal, the region determination unit 34 determines that the pixel of interest is a halftone dot region (S74).

  On the other hand, if the C, M, and Y planes are non-halftone signals, the region determination unit 34 determines that the signal obtained from the character edge region detection unit 31 is C, M, or Y in S75. It is determined whether or not the character edge signal indicates the character edge region in the plane. If any one of C, M, and Y receives a character edge signal, the region determination unit 34 determines that the pixel of interest is a character edge region (S76).

  On the other hand, if the C, M, and Y planes are non-character edge signals, the region determination unit 34 determines in S77 that the pixel of interest is another region (S77).

  In the above description, the case where the document type signal is “character printed photo (A)” has been described as an example, but the document type signal is another document (“character printed photos (B) to (E)”, “ Even in the case of “character”), the priority order of each signal is determined as shown in FIG. 12, and the region of the target pixel is determined based on the determined priority order. When the document type signal indicates “printed photograph”, the area determination unit 34 outputs a dot area signal for all pixels. When the document type signal indicates “other”, the area determination unit 34 outputs other area signals for all pixels.

<< Chromatic / Achromatic Determination Process >>
Next, the chromatic / achromatic determination process in the chromatic / achromatic determination unit 35 will be described. FIG. 16 is a flowchart showing a chromatic / achromatic determination process.

First, the chromatic / achromatic determination unit 35 that has received the character edge signal obtains the maximum (MaxColor) and minimum (MinColor) among the three values of C, M, and Y of the pixel of interest (S81). Next, the obtained maximum (MaxColor) and minimum (MinColor) and a predetermined threshold ThColor are the following conditional expressions:
MaxColor-MinColor> ThColor
It is determined whether or not the condition is satisfied (S82).

  When the above conditional expression is satisfied, the chromatic / achromatic determination unit 35 determines that the pixel of interest is a color character edge region (S83). On the other hand, when the conditional expression is not satisfied, the chromatic / achromatic determination unit 35 determines that the pixel of interest is a black character edge region (S84).

  As a result, the character edge region is classified into a color character edge region and a black character edge region.

<Processing according to region separation signal>
As described above, the region determination unit 34 and the chromatic / achromatic determination unit 35 determine which region each pixel belongs to, and a region separation signal (color character edge region, black character edge region, A signal indicating one of a halftone dot region, a halftone character edge region, and other regions) is output to the black generation / undercolor removal unit 16, the spatial filter unit 17, and the halftone generation unit 18. In each block, appropriate processing according to the region separation signal as shown in FIG. 16 is performed. Hereinafter, specific examples of processing in the black generation / undercolor removal unit 16, the spatial filter unit 17, and the halftone generation unit 18 will be described in order.

(Black generation / under color removal processing)
The black generation / under color removal unit 16 uses a black generation curve stored in advance based on one of the average, minimum, and maximum values of the CMY values of each pixel output from the color correction unit. A (black) value is generated, a UCR value is obtained using the K value and a UCR curve stored in advance, and the obtained UCR value is subtracted from the CMY value. At this time, the black generation / under color removal unit 16 switches the black generation curve and the UCR curve as follows according to the region separation signal.
In the black character edge area, the black generation rate and UCR rate are increased to improve character reproducibility.
In the color character edge area, halftone dot area, and other areas, the black generation rate and UCR rate are lowered in order to prevent saturation reduction.
Since the halftone dot character edge region is adjacent to the halftone dot region, the same black generation rate and UCR rate as the halftone dot region are used in order to reduce the gap between the black generation rate and the UCR rate.

  From the above viewpoint, the black generation / under color removal unit 16 uses an upwardly convex black generation curve as shown by a solid line in FIG. 18 in the black character edge region, for example, a color character edge region, a halftone character edge region, In the halftone dot region and other regions, black generation is performed using a downwardly convex black generation curve as indicated by a broken line in FIG.

  Further, the black generation / undercolor removal unit 16 uses, for example, a substantially linear UCR curve as shown by a solid line in FIG. 19 in the black character edge region, so that the color character edge region, the halftone character edge region, the halftone dot region, etc. For example, UCR processing is performed using a downwardly convex UCR curve as indicated by a broken line in FIG.

(Spatial filter processing)
The spatial filter unit 17 performs enhancement or smoothing processing. At this time, the spatial filter unit 17 switches the spatial filter coefficient based on the region separation signal as follows.
-Emphasis filter processing is performed to improve character reproducibility in the black character edge region and the color character edge region.
-In the dot area, smooth filter processing is performed to suppress moire.
In the halftone character edge region, in order to suppress a change in the shade of the halftone character due to the filter processing, emphasis filter processing that is weaker than that used in the black character edge region and the color character edge region is performed.
In other areas, the same smoothing filter process as in the halftone dot area or the weak emphasis filter process similar to that in the halftone character area is performed in order to suppress the blur in the continuous tone portion represented by silver halide photography.

  From the above viewpoint, in the black character edge region and the color character edge region, for example, an emphasis filter having a characteristic of amplifying the MTF in the entire frequency band as shown by the broken line in FIG. In the halftone dot edge region, for example, weak emphasis having a characteristic of amplifying MTF weaker than the emphasis filter for the black character edge region and the color character edge region in the entire frequency band as shown by the solid line in FIG. Use a filter. In the halftone dot region, for example, a smoothing filter having such a characteristic that the MTF is reduced in the entire frequency band as shown by the solid line in FIG. In other regions, a weak emphasis filter having a spatial frequency characteristic as shown by a solid line in FIG. 20A or a smoothing filter having a spatial frequency characteristic as shown by a solid line in FIG. 20B is used.

(Halftone generation process)
The halftone generation unit 18 performs processing for reproducing the resolution of the character edge region, the gradation in the halftone dot region, and the like. For example, in the case of dither processing, which is one of the processes performed as halftone generation processing, the black character edge region has a small dither matrix size (for example, 1 × 1 pixel) from the viewpoint of emphasizing resolution, and the color character edge region, In the halftone dot edge region, halftone dot region, and other regions, those having a large dither matrix size (5 × 5 pixels) are used from the viewpoint of emphasizing gradation.

<Modification>
In the above description, the threshold for performing the region separation process is changed based on the document type signal to increase the region separation accuracy, and the details of the black generation / under color removal processing, spatial filter processing, and halftone generation processing for each pixel are described. It was supposed to switch.

  However, when the document type signal is “printed photograph” or “others”, the region separation unit 15 outputs the same signal to all the pixels. That is, the region separation unit 15 outputs a halftone dot region signal for all pixels when the document type signal is “printed photograph”, and outputs another region signal for all pixels when the document type signal is “other”. Therefore, when the document type signal is “printed photograph” or “others”, the document type identification unit 14 outputs the document type signal to the black generation / under color removal unit 16, the spatial filter unit 17, and the halftone generation unit 18, Each block may perform processing according to the document type signal as shown in FIG.

  That is, when the document type signal is “printed photo”, since there are many halftone dot areas in the document, the black generation / undercolor processing unit 16 prevents the black generation rate, Reduce the UCR rate. Further, the spatial filter unit 17 performs smoothing filter processing to suppress moire, and the halftone generation unit 18 uses a large dither matrix size with an emphasis on gradation.

  When the document type signal indicates “other”, since there are many photographic paper regions in the document, the black generation / under color processing unit 16 prevents the black generation rate, UCR, etc. Reduce the rate. In addition, the spatial filter unit 17 performs weak enhancement filter processing in order to suppress smoothing filter processing or blur of the photographic paper region. The halftone generator 18 uses a large dither matrix size with an emphasis on gradation.

  When the document type signal indicates “character printed photograph” or “character”, the processing as shown in FIG. 21 may be performed on each region after the region separation processing described above.

  According to the present invention, a recording medium on which a program for processing is recorded is recorded on a computer-readable recording medium on which a program to be executed is recorded.

  In the present embodiment, the recording medium may be a program medium such as a memory such as a ROM, or a program reader may be provided as an external storage device. It may be a program medium that can be read by being inserted.

  In any case, the stored program may be configured to be accessed and executed by the microprocessor, or in any case, the program is read and the read program is not illustrated in the microcomputer. The program may be downloaded to a non-program storage area and executed. It is assumed that this download program is stored in the main device in advance.

  Here, the program 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 floppy (registered trademark) disk or a hard disk, or a CD-ROM / MO /. Disk system for optical disks such as MD / DVD, card system such as IC card (including memory card) / optical card, mask ROM, EPROM (Erasable Programmable Read Only Memory), EEPROM (Electrically Erasable Programmable Read Only Memory), flash It may be a medium that carries a fixed program including a semiconductor memory such as a ROM.

  The recording medium is executed by being read by a program reading device provided in a digital color image forming apparatus or a computer system.

  The computer system includes an image input device such as a flatbed scanner, a film scanner, and a digital camera, a computer that performs various processes such as the above image processing method by loading a predetermined program, and a CRT display that displays the processing results of the computer. An image display device such as a liquid crystal display and a printer that outputs the processing results of the computer to paper or the like.

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope shown in the claims. That is, embodiments obtained by combining technical means appropriately modified within the scope of the claims are also included in the technical scope of the present invention.

  According to the present invention, a document including halftone characters can be identified, and image processing suitable for a document including halftone characters can be performed. Accordingly, the present invention can be applied to an image forming apparatus (such as a printer) that captures an image from a document and outputs the captured image on paper or the like.

FIG. 3 is a block diagram illustrating a configuration of a document type identification unit included in the image processing apparatus according to the embodiment of the present invention. 1 is a block diagram illustrating a configuration of an image forming apparatus according to an embodiment of the present invention. 6 is a flowchart showing the overall flow of document type identification processing in a document type identification unit. It is a flowchart which shows the flow of a halftone dot area detection process. It is a figure which shows an example of the pixel mask used for the detection process of a halftone dot area | region. It is a flowchart which shows the flow of a character edge area | region detection process. It is a figure which shows an example of the pixel mask used for the detection process of a character edge area | region. It is a flowchart which shows the flow of a halftone character edge area | region detection process. (A) and (b) are figures which show the example of a smoothing filter. It is a flowchart which shows the flow of a pixel number count process. 6 is a flowchart illustrating a flow of document type determination processing. It is a figure which shows an example of the table of the determination criteria used for original document type determination processing. It is a block diagram which shows the structure of the area | region separation part with which the image processing apparatus of this embodiment is provided. It is a flowchart which shows the flow of the whole area | region separation process in an area | region separation part. It is a flowchart which shows the flow of an area | region determination process. It is a figure which shows an example of the process according to an area | region separation signal. It is a flowchart which shows the flow of chromatic / achromatic determination processing. It is a figure which shows an example of the black production | generation curve used for a black production | generation process. It is a figure which shows an example of the UCR curve used for under color removal processing. The filter used in a spatial filter part is shown, (a) is an example of an emphasis filter, (b) is a figure which shows an example of a smoothing filter. It is a figure which shows an example of the process according to a document classification signal.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Color image input device 2 Image processing device 3 Color image output device (image output part)
15 Region separation unit 16 Black generation / under color removal unit (black generation unit, under color removal unit)
17 Spatial filter unit 22 Halftone dot region detection unit 23 Halftone dot edge region detection unit 25 Document type determination unit (determination unit)

Claims (7)

A first condition in which a first pixel mask including a pixel of interest in the center and including a plurality of pixels is set for input image data, and a feature amount indicating density variation in the first pixel mask is greater than a first threshold; A pixel of interest that satisfies both the second condition that the number of inversions of the density of the pixels in the first pixel mask with the reference density value as a reference in a specific direction is greater than a preset second threshold is detected as a halftone dot area. A halftone dot detection unit;
For the input image data subjected to the smoothing process, a second pixel mask including a pixel of interest in the center and including a plurality of pixels is set, and a density difference between the pixel of interest and any pixel in the second pixel mask is determined in advance. A pixel of interest that satisfies a third condition that is greater than a set third threshold is extracted as a character edge region, and is an extracted character edge region and a region that is a halftone dot region detected by the halftone dot region detection unit. a halftone text edge area detecting unit that detects a halftone text edge area is an edge of a character made up of halftone dots,
A determination unit that determines whether or not the input image data includes a halftone character, based on a detection result by the halftone character edge region detection unit ;
A region separating unit that separates input image data into at least three regions of the halftone character edge region, the character edge region, and the halftone region based on the determination result of the determination unit;
Dot emphasis filter processing is performed on the character edge region separated by the region separation unit, smoothing filter processing is performed on the halftone dot region separated by the region separation unit, and halftone dots separated by the region separation unit An image processing apparatus comprising: a spatial filter unit that performs an emphasis filter process that is weaker than the character edge region on the character edge region . A black generation unit that generates black at the same black generation rate as the halftone dot region separated by the region separation unit with respect to the halftone character edge region separated by the region separation unit. Item 8. The image processing apparatus according to Item 1 . A lower color removal unit that performs a lower color removal process on the halftone character edge region separated by the region separation unit at the same lower color removal rate as the halftone dot region separated by the region separation unit; The image processing apparatus according to claim 1 , wherein: The image processing apparatus according to any one of claims 1 to 3 ,
An image forming apparatus comprising: an image output unit configured to form an image based on image data processed by the image processing apparatus.   A first condition in which a first pixel mask including a pixel of interest in the center and including a plurality of pixels is set for input image data, and a feature amount indicating density variation in the first pixel mask is greater than a first threshold; A pixel of interest that satisfies both the second condition that the number of inversions of the density of the pixels in the first pixel mask with the reference density value as a reference in a specific direction is greater than a preset second threshold is detected as a halftone dot area. Halftone dot detection step;
  A second pixel mask composed of a plurality of pixels including the target pixel in the center is set for the smoothed input image data, and the density difference between the target pixel and any pixel in the second pixel mask is determined in advance. A target pixel that satisfies the third condition that is greater than the set third threshold is extracted as a character edge region, and the extracted character edge region and a region that is a halftone dot region detected by the halftone dot region detection step A halftone dot edge region detection step for detecting a halftone dot edge region that is an edge of a character composed of halftone dots;
  A determination step of determining whether or not the input image data includes a halftone character, based on a detection result in the halftone character edge region detection step;
  An area separation step of separating the input image data into at least three areas of the halftone character edge area, the character edge area, and the halftone area based on the determination result of the determination step;
  The emphasis filter processing is performed on the character edge region separated by the region separation step, the smoothing filter processing is performed on the halftone region separated by the region separation step, and the halftone dot separated by the region separation step. And a spatial filter step for performing an emphasis filter process weaker than the character edge region on the character edge region. An image processing program for operating an image processing apparatus according to any one of claims 1 to 3, an image processing program for causing a computer to function as each section described above. A computer-readable recording medium on which the image processing program according to claim 6 is recorded.

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