JP2005303953A - Image processing apparatus, image processing method and program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To more effectively remove a see-through component contained in image data. <P>SOLUTION: A control section 11 of an image processing apparatus 1 executes a program PRG to perform see-through removal processing. Concretely, it is discriminated whether a certain region of image data inputted by an image reading apparatus 2 is an area gradation region or a density gradation region, and for the region discriminated as the area gradation region, its colors of pixels within said region are replaced using representative colors stored in a color table TBL. Furthermore, backing removal processing is performed on the density gradation region. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a technique for reducing a show-through component included in image data.

When reading a document with images formed on both sides with a scanner or the like to obtain image data, the image formed on the back side of the scanned surface of the document is reflected on the scanned surface. It may be in a state that “the back side is seen through”. This phenomenon is generally called “show-through” and occurs particularly noticeably when, for example, thin paper is used for a document, or when a color material such as ink or toner forming an image has a high density.
An example of show-through is shown in FIG. As shown in the figure, the mirrored image of the image formed on the back surface is reflected in the low-density and dull edge state in the image where the show-through occurs. In the following, in the image data, a component indicating an image of the document surface (read surface) is expressed as “original image component”, and a component indicating a mirror image of the image on the back side of the document is expressed as “show-through component”.
Generally, the show-through component is an unnecessary component for the user, and it is desired to remove it from the image data. For this reason, various techniques for removing the show-through component have been proposed. As a representative example, there is a technique for performing a background removal process.

In the background removal process, a density area lower than a certain level in an image is regarded as a background area indicating the color of the document surface itself (usually white), and the color of the background area is matched with the color of the document surface. In this way, the show-through is removed by performing density conversion. This is a technique that utilizes the low density of the show-through component, and all the colors in the low density area of the image data are rounded to the color of the document surface.
However, when the background removal process is performed, the low-density area of the original image cannot be distinguished from the show-through component, and thus the low-density area of the original image is mistaken for show-through and is removed. For this reason, a part of the original image is lost, resulting in a problem that the quality of the image data is lowered.

As a technique for solving this problem, for example, there is a technique described in Patent Document 1. In Patent Document 1, in a document in which gradation expression is made by a change in the halftone dot area ratio (that is, a halftone document), first, after detecting a halftone portion, only a low density region other than the halftone portion is detected. It is shown that a background removal process is performed. In this way, it is possible to prevent the low-density halftone dot portion from being mistaken for show-through and being removed.
However, in the above-described processing, the background removal processing is not performed on the halftone dot portion and the high density region, so that the show-through component remains in such a region. In particular, in low density areas of halftone dots, high density uniform color areas or pattern areas, it is not possible to reduce show-through, and as a side effect of background removal, the gradation difference becomes large and show-through occurs. May become more prominent.

JP 2002-77607 A

  The present invention has been made in view of the above-described problems, and an object thereof is to provide a technique for more effectively removing show-through.

In order to solve the above-described problems, the present invention provides a storage unit that stores representative color information indicating a plurality of representative colors, an image acquisition unit that acquires image data, and image data acquired by the image acquisition unit. A color conversion unit that compares the color data of each pixel in the image data with each of the representative color information and converts the color data into a representative color that minimizes the color difference when gradation expression is made by an area gradation method; When the image data acquired by the image acquisition means is expressed by gradation using the density gradation method, density data below the determined density level is uniform among the density data of the image data. An image processing apparatus comprising density conversion means for converting density data is provided.
Alternatively, the present invention provides a storage unit that stores representative color information indicating a plurality of representative colors, an image acquisition unit that acquires image data obtained by reading a document, and a document image indicated by the acquired image data. Discriminating means for discriminating whether an image area is an area gradation area in which gradation expression is made by an area gradation method or a density gradation area in which gradation expression is made by a density gradation method; When the determination unit determines that the image area is an area gradation area, the color of each pixel included in the image area is compared with each of a plurality of representative colors indicated by the representative color information, and the color difference is minimized. Color conversion means for converting to a representative color, and when the discrimination means discriminates the image area as a density gradation area, the density of the area portion below the determined density level in the image area is uniform. Do the conversion To provide an image processing apparatus and a degree conversion means.
According to this image processing apparatus, it is possible to determine whether a certain area in the document image is an area gradation area or a density gradation area, and to remove the show-through by an appropriate method for each area. Become.

In a more preferred aspect of the image processing apparatus of the present invention, the image data converted by the color conversion unit or the density conversion unit is output to an image forming apparatus that forms an image on a recording material. Means.
In this way, the image forming apparatus can form the image data from which the show-through has been removed on the recording material.

In a more preferred aspect of the image processing apparatus according to the present invention, the determination unit includes a density change rate calculation unit that calculates a density change rate between adjacent pixels or neighboring pixels in the image area, and the density change. An edge pixel determination unit that determines that a pixel having a density change rate calculated by the rate calculation unit equal to or greater than a threshold is an edge pixel, and the image region includes a predetermined number or more of edge pixels. Determines that the image area is an area gradation area, and if the number of edge pixels is less than the determined number, the image area is determined to be a density gradation area.
Alternatively, the determination unit includes a density calculation unit that calculates a density of each pixel included in the image region, and a density pattern generation unit that generates a density pattern having regularity, and is calculated by the density calculation unit. When the density pattern of the image area and the generated density pattern are matched, and it is determined that both of these patterns match at a certain ratio or more, the image area is determined to be an area gradation area, When it is determined that these two patterns do not match at a certain ratio or more, the image area is determined to be a density gradation area.
Alternatively, the determination unit includes an analysis unit that analyzes a spatial frequency characteristic of the image area, and among the spatial frequency characteristics analyzed by the analysis unit, when it is determined that a high frequency component appears at a predetermined frequency or more The image area is determined to be an area gradation area, and when it is determined that a high-frequency component appears at a frequency lower than a predetermined frequency, the image area is determined to be a density gradation area.
In this way, it is possible to satisfactorily determine whether the image area is an area gradation area or a density gradation area.

  The determining means may perform the determination using the entire image area indicated by the input image data as one image area, or extract a plurality of image areas from the image area indicated by the input image data, It is also possible to perform the discrimination in each image area.

In a more preferred aspect of the image processing apparatus of the present invention, the density conversion unit may determine a density determined in the area when the determination unit determines that the area is a density gradation area. The density of the portion below the level is converted into the density of the portion where no image is formed on the surface to be read of the document.
In this way, the show-through can be removed satisfactorily.

Further, as a more preferable aspect, the image processing apparatus of the present invention further includes a first frequency conversion unit that emphasizes a high frequency region of image data, and the color conversion unit uses the first frequency conversion unit to convert the high frequency region. The conversion is performed on the emphasized image data.
In this way, it is possible to correct the MTF (Modulation Transfer Function) characteristic at the time of scanning of the input image in the area gradation region of the document image, and to faithfully restore the document image.

Further, as a more preferable aspect, the image processing apparatus of the present invention includes a second frequency conversion unit that emphasizes a low frequency region of the image data, and the density conversion unit uses the second frequency conversion unit to reduce the frequency. The conversion is performed on the image data in which the region is emphasized.
Alternatively, a smoothing unit that performs a smoothing process on the image data is provided, and the density conversion unit performs the conversion on the image data on which the smoothing process has been performed.
In this way, noise in the density gradation area of the original image can be reduced, and a clearer image can be obtained.

Further, as a more preferable aspect, the image processing apparatus of the present invention includes first compression means for performing lossless compression on the image data converted by the color conversion means.
In this way, it is possible to efficiently reduce the volume of image data without losing the gradation information of the area gradation area of the document image.

Moreover, the image processing apparatus of the present invention includes, as a more preferable aspect, a second compression unit that performs irreversible compression on the image data converted by the density conversion unit.
In this way, it is possible to efficiently reduce the volume of image data without visually losing the gradation information in the density gradation region of the original image.

In a more preferred aspect of the image processing apparatus of the present invention, the representative color indicated by the representative color information stored in the storage unit is the color of a color material used for forming an image on the document. Including.
More preferably, the representative color indicated by the representative color information stored in the storage unit is obtained by subtracting the color of the plurality of color materials used for forming an image on the original and the color of the plurality of color materials. Including mixed colors.
More preferably, the representative color indicated by the representative color information stored in the storage unit includes a color of a portion where no image is formed on the read surface of the document.
In this way, it is possible to restore the color of the area gradation area of the document image as a color more faithful to the document.

The present invention also provides an image acquisition step in which the image processing apparatus acquires image data obtained by reading a document, and a region in which the image data acquired in the image acquisition step has a gradation by area gradation. A determination step of determining whether the area gradation region is expressed or a density gradation region expressed by gradation, and the region is determined to be an area gradation region in the determination step; In this case, the color of each pixel in the area is compared with a plurality of predetermined representative colors and converted to a representative color having the smallest color difference, while the area is determined as a density gradation area in the determination step. In this case, it is also possible to provide an image processing method including a conversion step of converting the density of a portion of the area below a determined density level.
Alternatively, according to the present invention, a computer stores storage means for storing representative color information indicating a plurality of representative colors, image acquisition means for acquiring image data obtained by reading a document, and the acquired image data Discrimination to determine whether an image area in a document image to be displayed is an area gradation area expressed by gradation using the area gradation method or a density gradation area expressed by gradation according to the density gradation method And when the determining unit determines that the image region is an area gradation region, the color of each pixel included in the image region is compared with each of a plurality of representative colors indicated by the representative color information, Color conversion means for converting to a representative color that minimizes the color difference, and when the determination means determines that the image area is a density gradation area, the density of the area portion below the determined density level in the image area Uniform It is also possible to provide a program for functioning as a density conversion means for performing comprising conversion.

In the present invention, show-through is removed by applying a so-called limited color technique. Here, the limited colorization means that the color of each pixel of image data formed with multiple colors (for example, about 16.77 million colors) is smaller in number (for example, 256 colors) than the original image data (hereinafter referred to as this). (Represented as “representative color”), thereby reducing the number of gradations of the image data and reducing the data amount of the image data.
The representative colors used for general limited colors are about several hundred colors, but in the present invention, the number of colors is much smaller than the number of typical representative colors, for example, representatives of about several to more than a dozen colors. It is desirable to use color. Further, it is desirable that the representative colors include the color of the color material used for forming an image on the original and the paper color. Hereinafter, the limited color processing in the present invention will be described together with the reason.

FIG. 12 is an enlarged view showing a part of a document P. In the figure, the document P is white and halftone D _C formed by the color material of cyan (C) onto the sheet (W), the halftone dots D _M is regularly formed by the color material of magenta (M) Are arranged in a plurality.
This document P is an area gradation document. That is, the document P is a document in which gradation is expressed by changing the area of a halftone dot or the like, and color shading is expressed by changing the halftone dot area ratio. That is, the gradation change in the document P is discontinuous when viewed microscopically.
As shown in the figure, the color existing on the original P is white of the paper color, cyan (cyan) of the toner, and blue (B) which is a subtractive color mixture of cyan and magenta. However, since these halftone dots are minute and have a size smaller than the human visual resolution, humans perceive them as “light blue” in which these colors are mixed (intermediate color mixture).

In printing, in general, an original is represented by four types of color materials, cyan (C), magenta (M), yellow (Y), and black (K). The color gamut that can be reproduced in such a manuscript is 3 points indicating C, M, and Y on the chromaticity diagram and 3 indicating red (R), green (G), and blue (B) which are subtractive color mixture thereof. It is an area indicated by a hexagon connecting points. However, since the colors actually present on the original are only the color of the color material, the subtractive color mixture thereof, and the paper color (W) as described above, if this original is viewed microscopically, C, M, Y, K, R, G, B, and W are formed with only eight colors.
That is, when the original is read by the image reading device, if the image reading device has a sufficient input resolution for reading halftone dots, the image data obtained by the image reading device is C, M, Y, K. , R, G, B, and W, the pixel is composed of only eight pixels.

The inventor pays attention to this point, and as described above, C, M, Y, K, R, and G are applied to the image data of the original formed by the four color materials of C, M, Y, and K. , B, and W were tried for the limited color processing using eight representative colors. Then, it became clear that the output image obtained in this way can reproduce the original image on the original very faithfully. The principle will be briefly described below.
For example, assume that the color of a certain part on the original is black (density 100%). When this document is read by an image reading device such as a scanner, the pixels corresponding to the read portion are not actually detected due to the performance of the image reading device itself (such as changes over time of the light source and light receiving element and dynamic range) and other noises. It shows a slightly different color, for example, gray with a density of 98%. However, when the above-described limited color processing is performed on this pixel, this pixel is converted into a color having the smallest color difference among the representative colors of C, M, Y, K, R, G, B, and W, that is, black. It is replaced and restored to the color information of the original document.
Based on such a principle, the limited color processing corrects the error of the image data satisfactorily unless the error of the read image data causes a color difference that can be replaced with another representative color. It is possible to restore the color information of the original image.

  What should be noted here is that the limited color processing enables removal of show-through. For example, when show-through occurs in a portion of image data that is originally white (density 0%), if the image formed on the back side is black, the show-through component is light gray (for example, density 3%). ). However, if a limited color process is applied to such a portion, it is replaced with white in the same manner as described above. In addition, since this is the same when show-through occurs in other color portions of the image data, the removal of the show-through by the limiting color processing forms not only the blank portion on the paper but also an image. It becomes effective even in the part which is.

As described above, it has been clarified that the limited color processing can effectively remove the show-through for the area gradation original. However, the present inventor has removed the show-through by the limited color processing. I found that there was one problem with doing. The problem is that the limited color processing does not function effectively for density gradation originals. Here, the density gradation original is an original in which the gradation change of the color material is continuous, such as a photographic image formed on photographic paper. For example, it is assumed that a certain part on the document continuously changes from white to black, and the image data read by the image reading apparatus is as shown in the pixel row of FIG. At this time, the pixel p ₀ is white having a density of 0%, and the pixels p ₁ , p ₂ , p ₃ , and p ₄ are grays having a density of 20%, 40%, 60%, and 80%, respectively. ₅ is black (concentration 100%). When the above-described limited color processing is performed on such a pixel column, the pixels p ₀ , p ₁ , and p ₂ are replaced with white, and p ₃ , p ₄ , and p ₅ are replaced with black, The gradation information contained in the image data is significantly impaired.

In order to solve this problem, the number of representative colors may be increased. As the number of representative colors increases, the gradation information of the density gradation original can be held. However, if the number of representative colors is increased, it will be difficult to restore the color information of the area gradation original. The reason is that a pixel portion that is recognized differently from the original image, such as the light gray show-through component described above, is different from the representative color (white) that should be replaced by adding the representative color. This is because there is a possibility of being replaced with a color (for example, light gray). In addition, increasing the representative color also causes an adverse effect of increasing the processing time required for the limited color processing.
In other words, it can be said that the limited color processing as performed for the area gradation original is not suitable for the density gradation original.

Therefore, the present inventor discriminates whether the input image data is an area gradation original or a density gradation original, and shows-through the image data by different methods according to the determination result. Was to be removed. The specific configuration and operation will be described in detail below.
In the following description, the RGB color space is used for the description in the color space. For example, colors indicating coordinates (0, 0, 0), (1, 0, 0), (0, 1, 0), (0, 0, 1), and (1, 1, 1) in this color space Are black, red, green, blue and white, respectively. Hereinafter, for the sake of convenience, these coordinate points are referred to as “RGB values”.

(1) Configuration FIG. 1 is a block diagram showing a configuration of an image processing apparatus 1 according to an embodiment of the present invention. As shown in FIG. 1, the image processing apparatus 1 according to the present embodiment is a computer including a control unit 11, an HDD (Hard Disk Drive) 12, an input IF (Interface) 13, and an output IF 14. is there. The image processing apparatus 1 is connected to an image reading apparatus 2 and an image forming apparatus 3.
The control unit 11 includes a CPU (Central Processing Unit) 111, a ROM (Read Only Memory) 112, a RAM (Random Access Memory) 113, and the like, and performs a show-through removal process described later on the input image data. The various operations are executed. At this time, the image data is temporarily stored in the RAM 113. The HDD 12 is a storage device, and stores a program PRG for executing the show-through removal process and a color table TBL used for the limited color process. The input IF 13 acquires image data from the image reading device 2, and the output IF 14 outputs the image data on which the show-through removal processing has been performed by the image processing device 1 to the image forming device 3.

Here, the color table TBL will be described.
FIG. 2 is a diagram schematically showing the color table TBL in the present embodiment. As shown in the figure, the color table TBL has eight representative colors of C, M, Y, K, R, G, B, and W, and values indicating these colors, that is, RGB In terms of values, (0, 1, 1), (1, 0, 1), (1, 1, 0), (0, 0, 0), (1, 0, 0), (0, 1, Data of 0), (0, 0, 1), (1, 1, 1) is stored in the HDD 12. In the following, the RGB values indicating these representative colors are expressed as C ₁ , C ₂ , C ₃ ,..., C ₈ , respectively.

  The image reading device 2 is a so-called scanner, which includes a platen, a light source, an optical system, a light receiving element, and a signal processing unit (not shown), irradiates light on a document placed on the platen with a light source, and reflects the reflected light. The light receiving element receives light through the optical system and outputs an image signal. The signal processing unit performs various signal processing such as AD conversion on the image signal, and supplies the image data to the image processing apparatus 1. . The image forming apparatus 3 is, for example, an electrophotographic printer, and forms an image by forming a toner image corresponding to the image data output from the image processing apparatus 1 and fixing the toner image on a sheet.

(2) Operation Next, the show-through removal process executed by the image processing apparatus 1 under the above-described configuration will be described. This process is realized by the control unit 11 of the image processing apparatus 1 executing the program PRG.
FIG. 3 is a flowchart showing the procedure of the show-through removal process according to the present embodiment. As shown in the figure, first, the control unit 11 of the image processing apparatus 1 acquires image data from the outside via the input IF 13 (step S1). Subsequently, the control unit 11 divides the image data into a plurality of regions, and performs image data determination processing for determining whether each region is an area gradation region or a density gradation region (step S2). ).

FIG. 4 is a flowchart showing the procedure of the above-described image data discrimination process. 5 to 8 are schematic diagrams of a certain area gradation region a and density gradation region b, which are exemplified for explaining this process. In FIG. 5, it is assumed that the halftone dot color of the area gradation area a is black and the color of the area b ₁ of the density gradation area b is black. Further, it is assumed that the color of the portion where no image is formed is white. In the following description, it is assumed that processing is performed on the region a and the region b.
First, the control unit 11 divides the input image data into regions of a predetermined size (for example, 10 × 15 pixels) (step S21). At this time, the arrangement of the pixels in the region a and the region b is as shown in FIG. Next, the control unit 11 calculates the density change rate with respect to the 0 ° direction of the regions a to b, and extracts edge pixels in the region by determining whether this value is equal to or greater than a predetermined threshold value. (Step S22). That is, the edge pixel indicates a pixel of a part that forms a contour in the image. FIG. 7- (1) shows a state in which edge pixels in the 0 ° direction are extracted. In the figure, colored pixels indicate edge pixels.
Subsequently, the control unit 11 extracts edge pixels in the 45 °, 90 °, and 135 ° directions in the same manner (steps S23 to S25). This situation is shown in FIGS. 7- (2), (3), and (4), respectively.

After extracting the edge pixels, then the control unit 11 accumulates the number of edge pixels extracted in the aforementioned step S22 to S25, calculates the edge pixel number N _E of the area (step S26). Referring to FIG. 8, the numbers of edge pixels in the regions a and b are “74” and “15”, respectively.
Then, the control unit 11 determines whether or not the number of edge pixels N _{E in} this region is larger than a threshold Th _E (here, “50”) (step S27). Since the number of pixels in the region a is “74”, the above determination is affirmative, and because the number of pixels in the region b is “15”, it is negative.

  If the above determination is affirmative (step S27; YES), the control unit 11 determines that this region is an “area gradation region” and stores the determination result (step S28). If the above determination is negative (step S27; NO), the control unit 11 determines that this area is a “density gradation area” and stores the determination result (step S29). And the control part 11 judges whether such discrimination | determination was complete | finished in all the areas (step S20), and if this judgment is negative (step S20; NO), the process from step S22 will be repeated, If this determination is affirmative (step S20; YES), this process is terminated.

  As described above, the area gradation region and the density gradation region can be discriminated by obtaining the density change rate in a certain region. Since the gradation change is continuous in the density gradation area, the density change rate is small in such a portion, and the edge portion is mostly determined as an edge pixel. On the other hand, since the area gradation region is expressed by halftone dots, each halftone dot is determined to be an edge pixel. Therefore, when the appearance frequency of the edge pixel is compared in the area gradation region and the density gradation region of the same size, the appearance frequency of the edge pixel is remarkably increased in the area gradation region. That is, this indicates that the density gradation original has a high correlation between adjacent pixels, and conversely, the area gradation original has a low correlation between adjacent pixels.

Here, returning to the flowchart of FIG. 3, the description of the processing after step S2 is continued.
When the determination of each region of the image data is completed, the control unit 11 refers to the determination result stored in step S2 to determine whether or not each region is an area gradation region (step S3). Different processing is executed according to the determination result.
First, when it is determined that the target region is an area gradation region (step S3; YES), the control unit 11 performs a filtering process that emphasizes high-frequency components for this region (region a). (Step S4). In a region where the correlation between adjacent pixels is low, that is, a region where the gradation change is discontinuous, the frequency of the high-frequency component increases when this is converted into a spatial frequency. Therefore, if processing for emphasizing high-frequency components is performed on such an area, the MTF (Modulation Transfer Function) characteristic at the time of scanning of the input image can be corrected, and the original image can be restored faithfully. .

Next, the control unit 11 performs a limited color process on each pixel in the region a (step S5). FIG. 9 is a flowchart showing the procedure of the limited color processing. As shown in the figure, first, the control unit 11 calculates the difference between the representative color C _n (cyan RGB value) and the RGB value of the target pixel, with n = 1 as an initial value, and these color differences ΔE. _n is obtained (step S51). The control unit 11 calculates the color difference ΔE _n for all representative colors. That is, when the process of step S51 is completed, the value of n is incremented (step S53), and these processes are repeated until n reaches the maximum value of 8.
When the color differences ΔE _n of all the representative colors are calculated (step S52; NO), the control unit 11 compares these color differences ΔE _n to identify the representative color that minimizes the color difference ΔE _n (step S54). The color of the target pixel is replaced with the representative color specified in step S54 (step S55). For example, if the minimum color difference ΔE _n is ΔE ₇ , that is, black, the color of the target pixel is replaced with black at this time.
The control unit 11 determines whether or not the above processing has been performed for all the pixels in the region a (step S56), and repeats the processing from the above step S51 while this determination is negative. When the determination becomes affirmative, the process is terminated, and the process returns to the flowchart of FIG.
When the limited color processing is completed, the control unit 11 performs reversible compression on the region a (step S6). By performing reversible compression on the area gradation region, the capacity can be efficiently reduced without losing the gradation information of this region.

  The process performed on the area gradation region is as described above. On the other hand, when it is determined that the target region is the density gradation region (step S3; NO), the control unit 11 reduces the low-frequency component for this region (region b) in order to reduce noise. A filter process for emphasizing is performed (step S7). Subsequently, the control unit 11 performs a known background removal process on the region b (step S8), and then performs irreversible compression (step S9). By performing irreversible compression on a region having high correlation between pixels such as a density gradation region, the loss of gradation information is small and the capacity can be efficiently reduced.

  The control unit 11 determines whether or not these processes have been performed for all regions (step S0). If this determination is negative (step S0; NO), the process from step S3 described above is repeated. If this determination is affirmative (step S0; YES), this process is terminated.

As described above, by performing the show-through removal process using the image processing apparatus 1 according to the present embodiment, in addition to removing the show-through of the area tone area extremely well, the density tone area It has become possible to reduce the show-through of images.
Moreover, according to the show-through removal process of the present embodiment, not only show-through removal but also restoration of the original image and noise removal can be performed satisfactorily.
When there is no show-through in the density gradation area, the background removal which is a conventional method may not be performed on the density gradation area.

(3) Modifications The implementation of the present invention is not limited to the above-described embodiment, and various modifications are possible. An example is shown below.
First, in the above-described embodiment, the description is made using the RGB color space. However, the present invention is not limited to such an aspect, and various color spaces can be used. For example, an L * a * b * color space or an L * U * V * color space may be used. The selection of the color difference formula is also arbitrary, and an appropriate color difference formula may be used in each color space.

  Further, in the above-described embodiment, it has been described that the show-through removal process is realized by the image processing apparatus 1 executing the program PRG. However, the present invention is not limited to such an embodiment. For example, the image processing apparatus may include an electronic circuit such as a filter that can realize the function of each step of the show-through removal process. Good. Of course, it can also be realized by an ASIC (Application Specific Integrated Circuit) having the above-described functions.

  Further, in the above-described embodiment, it has been described that the present invention is implemented by the image processing apparatus 1. However, the present invention is not limited to such an embodiment. In addition to the functions of the image processing apparatus 1 described above, an image forming apparatus (so-called composite) having the functions of the image reading apparatus 2 and the image forming apparatus 3 described above. Can also be implemented.

  In the above-described embodiment, only the show-through removal processing has been described as processing performed by the image processing apparatus 1 in order to facilitate understanding of the present invention. However, the image processing apparatus of the present invention is not limited to such an embodiment, and for example, to output a read original as image data such as AD conversion, γ conversion, color space conversion, shading correction, and screen processing. Various processes used in the above may be executed together.

  Further, in the above-described embodiment, it has been described that the image data indicating the document is divided into a plurality of regions, and the above-described image data determination processing and limited color processing are performed for each of the divided regions. However, it is of course possible to regard the entire image data as one area and perform the above-described processing on the entire image data.

  In the above-described embodiment, the image processing apparatus 1 has been described as storing the color table TBL in the HDD 12. However, the present invention is not limited to such an embodiment. For example, input image data is analyzed, a representative color corresponding to the image data is specified, and data indicating the representative color is generated each time. It may be. In this way, there is no need to store a color table.

  Alternatively, the image processing apparatus may store a plurality of color tables having different representative colors, and select the optimum color table according to the input image data and perform the limited color processing. As a specific process, for example, when the RGB value of each pixel of the image data is calculated and the process of replacing these pixels with the representative color is performed on each color table, the sum of the color differences is the smallest. The color table is selected and the limited color processing is performed. In this way, it is possible to restore the original image with higher accuracy.

  At this time, the representative colors are not limited to the eight colors C, M, Y, K, R, G, B, and W as described above, and other colors can be used as the representative colors. Of course, the representative colors can be increased or decreased. For example, if it is known in advance that the input image data is a black and white image, the representative colors need only be two colors B and W, and similarly, a special color is used for the input image data. If it is known in advance, the spot color may be added to the representative color. In order to enable such an operation, the image processing apparatus is provided with an operation unit that accepts an operation by the user, and the user can select an optimal one from a plurality of color tables according to the type of input image data. May be selected. In this way, it is possible to reduce the processing time while restoring the original image with higher accuracy.

In addition, various modifications can be made to the show-through removal process in the above-described embodiment. The modification in each step is shown below.
The filtering process of step S4 or step S7 and the compression process of step S6 or step S9 are not necessarily necessary processes from the viewpoint of removing show-through, and therefore these processes can be omitted.
Further, in step S7, it has been described that filter processing for emphasizing low-frequency components is performed as processing for reducing noise. However, the present invention is not limited to such processing, and for example, smoothing processing is performed using a smoothing filter. Is also possible.

Various modifications can be considered for the image data discrimination processing in step S2. An example is shown below.
(A) Method by halftone dot extraction In this method, a plurality of types of data representing halftone dot patterns are stored in the image processing apparatus, the halftone dot pattern generated based on this data is compared with image data, and Whether or not it is an area gradation region is determined based on the degree of coincidence. For example, a halftone dot pattern having a screen line number of 175 lpi (line per inch) is generated, the density pattern of the image data is collated with this halftone dot pattern, and an area where the image data exists is a predetermined threshold (for example, 50%) If the dot pattern matches the dot pattern, the region is discriminated as an area gradation region. At this time, as the halftone dot pattern, typical data such as 175 lpi, 150 lpi, 100 lpi, or 80 lpi may be stored.

(B) Method by Spatial Frequency Analysis This method utilizes the fact that the spatial frequency characteristic of the area gradation region includes a higher frequency component than that of the density gradation region. Specifically, spatial frequency analysis is performed on a certain region of image data, and if a component of a region having a predetermined frequency or higher appears at a certain frequency or higher, the region is determined to be an area gradation region. It is.

(C) Method by Edge Pixel Extraction This method is the same as the image data discrimination processing described in the above embodiment in that edge pixels are extracted, but whether or not it is a subsequent area gradation region. The determination method is different.
FIG. 10 is a diagram for explaining this determination method. In the following description, it is assumed that areas a and b are the same as areas a and b in FIG. As shown in the figure, in this method, the region is further divided into several blocks, the number N _E ′ of edge pixels is calculated for each block, and the edge pixels are equal to or greater than a predetermined threshold Th _E1. It is determined whether or not. A block for which this determination is positive is referred to as an “area gradation block”. Then, this determination is performed for each block, and when the number of area gradation blocks is equal to or greater than a predetermined threshold Th _E2 , this area is determined to be an area gradation area.

Here, FIG. 10 will be described as an example. Here, it is assumed that the thresholds Th _E1 and Th _E2 are “20” and “2”, respectively. As shown in the figure, the numbers N _E ′ of the edge pixels of the blocks 1, 2, 3, and 4 in the area a are “28”, “16”, “24”, and “14”, respectively. Therefore, block 1 and block 3 are area gradation blocks. At this time, since there are “2” area gradation blocks, this region a is determined to be an area gradation region. Further, the numbers N _E ′ of the edge pixels of the blocks 1, 2, 3, and 4 in the region b are “4”, “8”, “5”, and “0”, respectively, and are determined as area gradation blocks. Since there are “0” blocks, this region b is determined to be a density gradation region.

1 is a block diagram illustrating a configuration of an image processing apparatus according to an embodiment of the present invention. It is the figure which showed typically the color table in the same embodiment. 6 is a flowchart illustrating a procedure of show-through removal processing according to the embodiment. It is the flowchart which showed the procedure of the image data discrimination | determination process concerning the embodiment. It is the schematic diagram which showed the area gradation area | region and density | concentration gradation area | region in the embodiment. It is the model which showed arrangement | positioning of the pixel of an area gradation area | region and a density gradation area | region in the embodiment. It is the schematic diagram which showed a mode that an edge pixel was extracted in the same embodiment. It is a figure for demonstrating the number of edge pixels in the embodiment. It is the flowchart which showed the procedure of the limited colorization process which concerns on the same embodiment. It is a figure for demonstrating the discrimination method of an area gradation area | region. It is the figure which showed an example of show-through. It is the figure which expanded and showed a part of a certain original document. FIG. 6 is a diagram illustrating a certain area on a document that is continuously changing from white to black.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Image processing apparatus, 2 ... Image reading apparatus, 3 ... Image forming apparatus, 11 ... Control part, 111 ... CPU, 112 ... ROM, 113 ... RAM, 12 ... HDD, 13 ... Input IF, 14 ... Output IF.

Claims (19)

Storage means for storing representative color information indicating a plurality of representative colors;
Image acquisition means for acquiring image data;
When the image data acquired by the image acquisition means is expressed in gradation by the area gradation method, the color data of each pixel in the image data is compared with each of the representative color information, and the color difference is minimized. Color conversion means for converting to a representative color,
When the image data acquired by the image acquisition unit is expressed by gradation using a density gradation method, density data below a determined density level is uniform among density data of the image data. An image processing apparatus comprising: density conversion means for converting density data. Storage means for storing representative color information indicating a plurality of representative colors;
Image acquisition means for acquiring image data obtained by reading a document;
An area of the original image represented by the image data acquired by the image acquisition unit is an area gradation area in which gradation expression is made by an area gradation method and a density gradation in which gradation expression is made by a density gradation method. A discriminating means for discriminating between the region and
When the determination unit determines that the image area is an area gradation area, the color of each pixel included in the image area is compared with each of a plurality of representative colors indicated by the representative color information, and the color difference is minimized. Color conversion means for converting to a representative color,
Image processing comprising: density conversion means for performing conversion in which the density of an area portion lower than a determined density level in the image area becomes uniform when the determination means determines the image area as a density gradation area apparatus. The image processing apparatus according to claim 1, further comprising an output unit that outputs the image data converted by the color conversion unit or the density conversion unit to an image forming apparatus that forms an image on a recording material. The discrimination means includes
A density change rate calculating means for calculating a density change rate between adjacent pixels or neighboring pixels in the image region;
Edge pixel determination means for determining that a pixel having a density change rate calculated by the density change rate calculation means equal to or greater than a threshold is an edge pixel;
When the image region includes a predetermined number or more of edge pixels, the image region is determined to be an area gradation region, and when the number of edge pixels less than the determined number is included, The image processing apparatus according to claim 2, wherein the image area is determined to be a density gradation area. The discrimination means includes
Density calculating means for calculating the density of each pixel included in the image area;
A density pattern generating means for generating a density pattern having regularity,
If the density pattern of the image area calculated by the density calculation means matches the generated density pattern, and it is determined that both of these patterns match at a certain ratio or more, the image area is determined as an area scale. The image processing according to claim 2, wherein it is determined that the image area is a tone area, and if it is determined that the two patterns do not match at a certain ratio or more, the image area is determined as a density gradation area. apparatus. The discrimination means includes
Analyzing means for analyzing the spatial frequency characteristics of the image region;
Of the spatial frequency characteristics analyzed by the analyzing means, when it is determined that the high frequency component appears at a frequency higher than the determined frequency, the image region is determined to be an area gradation region, and the high frequency component is determined. The image processing apparatus according to claim 2, wherein when it is determined that the image appears with less frequency, the image area is determined to be a density gradation area. The image processing apparatus according to claim 2, wherein the determination unit performs the determination using the entire image area indicated by the input image data as one image area. The image processing apparatus according to claim 2, wherein the determination unit extracts a plurality of image regions from an image region indicated by the input image data, and performs the determination in each image region. The density conversion means, when the discrimination means discriminates that the area is a density gradation area, the density of the portion below the determined density level in the area on the surface to be read of the document. The image processing apparatus according to claim 2, wherein the image processing apparatus converts the density into a density of a portion where no is formed. First frequency conversion means for emphasizing a high-frequency region of image data;
The image processing apparatus according to claim 1, wherein the color conversion unit performs the conversion on the image data in which a high frequency region is emphasized by the first frequency conversion unit. A second frequency converting means for emphasizing a low frequency region of the image data;
The image processing apparatus according to claim 1, wherein the density conversion unit performs the conversion on the image data in which a low frequency region is emphasized by the second frequency conversion unit. Comprising a smoothing means for smoothing image data;
The image processing apparatus according to claim 1, wherein the density conversion unit performs the conversion on the image data on which the smoothing process has been performed. The image processing apparatus according to claim 1, further comprising a first compression unit that performs reversible compression on the image data converted by the color conversion unit. The image processing apparatus according to claim 1, further comprising: a second compression unit that performs irreversible compression on the image data converted by the density conversion unit. The image processing apparatus according to claim 1, wherein the representative color indicated by the representative color information stored in the storage unit includes a color of a color material used for forming an image on the document. The representative color indicated by the representative color information stored in the storage unit is represented by a color mixture of a plurality of color materials used for forming an image on the original and a subtractive color mixture of the colors of the plurality of color materials. The image processing apparatus according to claim 1, further comprising: The image processing apparatus according to claim 1, wherein the representative color indicated by the representative color information stored in the storage unit includes a color of a portion where an image is not formed on the surface to be read of the document. The image processing device
An image acquisition step of acquiring image data obtained by reading a document;
Whether an area of the image data acquired in the image acquisition step is an area gradation area expressed by gradation by area gradation or a density gradation area expressed by gradation by density gradation A determination step to determine;
When the region is determined to be an area gradation region in the determination step, the color of each pixel in the region is compared with a plurality of predetermined representative colors and converted to a representative color having the smallest color difference. An image processing method comprising: a conversion step of converting a density of a portion below a determined density level in the area when the area is determined to be a density gradation area in the determination step. Computer
Storage means for storing representative color information indicating a plurality of representative colors;
Image acquisition means for acquiring image data obtained by reading a document;
An image area in the original image indicated by the acquired image data is either an area gradation area in which gradation expression is made by the area gradation method or a density gradation area in which gradation expression is made by the density gradation method. Determining means for determining whether or not
When the determination unit determines that the image area is an area gradation area, the color of each pixel included in the image area is compared with each of a plurality of representative colors indicated by the representative color information, and the color difference is minimized. Color conversion means for converting to a representative color,
When the determination unit determines that the image area is a density gradation area, the image conversion unit is configured to function as a density conversion unit that performs conversion in which the density of a region portion below the determined density level in the image area is uniform. program.

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