JP3865651B2 - Color image processing method, color image processing apparatus, program, and recording medium - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a color image processing method, a color image processing apparatus, a program for executing the color image processing method, and a computer-readable recording medium storing the program, and more particularly, show-through in digital color image processing. In particular, the present invention is suitable for application to color image forming apparatuses such as color copying machines, color printers, and color facsimiles.
[0002]
[Prior art]
With the widespread use of image input devices such as scanners, digital cameras, and digital copying machines, it has become common for document images on paper media such as books, magazines, catalogs, advertisements, and newspapers to be input in color.
When inputting using these image input devices, when the document is printed on both sides of a thin sheet with high transmittance, the image on the back side is seen through and mixed into the image on the front side. Will occur.
Furthermore, in the case of a color document, the background color is often not uniform, and there are often mixed patterns, patterns, and photographs.
For example, as shown in FIG. 7, a back surface image in which a black image is placed on a blue image shows through on an orange front image.
[0003]
For this reason, the problem of removing “show-through” from an input image by digital image processing has been considered difficult to handle, although it is important for improving image quality.
Most of the technologies for eliminating such show-through include capturing both sides of a document as digital data with a scanner, etc., and matching the front and back sides to detect the show-through component, and the brightness and color of the detected part. Show-through is reduced (for example, JP-A-5-22572, JP-A-6-62216, JP-A-8-265563, JP-A-9-233319, JP-A-9 No.-312770, JP-A-10-262147, JP-A-2000-22946, JP-A-2000-92324, JP-A-2000-188668, JP-A-2000-59569, JP-A-2000-134419. No. publication etc.).
In this technique, since front and back images are matched, there are a non-linear misalignment problem and a paper transmittance calculation problem.
[0004]
In addition, in the method of reducing the show-through component from the image on one side while keeping the reflectance of the pressure plate low (for example, see Japanese Patent Application Laid-Open No. 11-298694), a region other than the document portion appears dark when output. There is.
Further, in a method of separately providing a transmittance measuring device and adjusting exposure of a document according to the measurement result to reduce show-through from an image on one side (see, for example, Japanese Patent Laid-Open No. 2000-137355), the transmittance Since a measuring device is required and an image is corrected by adjusting exposure, it cannot be directly applied to digital data, and is not versatile.
The point that this technique cannot be applied directly to digital data is the same as the technique disclosed in Japanese Patent Application Laid-Open No. 11-298694, and it becomes a problem in today's network environment.
[0005]
In addition, methods for detecting and reducing a show-through component from an image on one side without a mechanical device such as a pressure plate have been proposed (for example, Japanese Patent Laid-Open Nos. 7-30757, 10-65921, and 10-257325, JP-A-11-187266, etc.). In any of these methods, the image density distribution is adaptively changed by the value of the evaluation function prepared in advance for the density histogram of the image.
[0006]
Also, only one image is used as an input image, and without using a histogram for the entire image, a method of reducing show-through not only in the halftone area but also in the case where there is a high-saturation area on the back side (for example, JP In Japanese Patent Laid-Open No. 2001-169080), the show-through area and the foreground image are mainly separated based on edge information extracted from the input image, and the show-through area is selectively corrected.
[0007]
[Problems to be solved by the invention]
As described above, in the method of correcting the show-through by reading the double-sided image with a scanner or the like, since the double-sided image is read with the scanner, the apparatus becomes large in size and requires special hardware. Necessary and low versatility.
[0008]
Also, in the method of correcting show-through from single-sided information, the threshold for noise removal is determined from the distribution of the luminance histogram of the original document, aiming to deal with not only normal images but also images with much dirt. However, in addition to requiring a histogram of the entire image in advance, it only changes the threshold value for removing noise adaptively. Either the removal or the show-through remains, and the reproduction of the halftone part of the foreground and the show-through correction are not compatible.
[0009]
In Japanese Patent Laid-Open No. 2001-169080, the size of the show-through that can be corrected tends to depend on the size of the reference range. Therefore, when processing show-through in a wide range, the process is performed multiple times on the entire image. It was necessary to repeat.
Such a method is less likely to be a problem when the entire image can be accessed randomly, but is not suitable for processing that has a strong one-time performance, such as sequential processing.
[0010]
In addition, the above-described conventional technique is premised on the iterative process for the entire image and the creation of a histogram for the entire image, so that it is difficult to implement by sequential processing.
[0011]
The present invention has been made in consideration of the above-mentioned circumstances, and is a color that can read an image of only one side of a document with a scanner or the like and can process a wide range of show-through by sequential processing from the image on one side. An object of the present invention is to provide an image processing method, a color image processing apparatus, a program for executing the color image processing method, and a computer-readable recording medium on which the program is recorded.
[0012]
[Means for Solving the Problems]
In order to solve the above-described problems, the color image processing method according to claim 1 of the present invention reduces show-through and cover-up on an input image obtained by digitally inputting a document color-printed on a paper surface. In the color image processing method, only the information within a specific reference range around the pixel of the input image is used, and the result of the show-through correction processing for this pixel is held as the primary correction result. By using it for the show-through correction process after this pixel, the show-through and the show-through are selectively reduced without repeating the entire image.
According to a second aspect of the present invention, in the color image processing method according to the first aspect, a show-through correction process is further performed on a specific reference range of the primary correction result, and the show-through correction process is performed. A high-order correction result is obtained by repeating locally, and a final show-through corrected image is obtained from these correction results.
According to a third aspect of the present invention, in the color image processing method according to the first or second aspect, only a region determined to be a halftone dot region based on the information obtained from the input image is used as a reference range. A correction process is performed.
According to a fourth aspect of the present invention, in the color image processing method according to the first or second aspect, the show-through correction processing is performed using only a halftone dot area where the halftone dot area ratio of the input image is within a certain range as a reference range. It is characterized by giving.
[0013]
The color image processing apparatus according to claim 5 of the present invention is the color image processing apparatus for reducing show-through and see-through with respect to an input image obtained by digitally inputting a document color-printed on paper. By using only the information within a specific reference range around the pixel of the input image, the show-through correction process is performed on the pixel and is retained as a primary correction result. The show-through correction processing is performed, and the show-through correction processing is locally repeated and held as a high-order correction result, and the show-through correction is finalized from these correction results and the input image. And a synthesis processing unit that synthesizes the output image, and selectively reduces the show-through and the show-through without repeating the entire image.
According to a sixth aspect of the present invention, there is provided a program for causing a computer to execute the color image processing method according to any one of the first to fourth aspects.
A recording medium according to a seventh aspect of the present invention is a computer-readable recording medium on which the color image processing program according to the sixth aspect is recorded.
[0014]
With the configuration as described above, it is possible to correct a show-through wider than the conventional one.
In addition, the show-through of a plurality of layers can be corrected accurately.
In addition, since one pixel in the higher-order correction result also includes surrounding information, the mounting efficiency is improved rather than simply expanding the reference range.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram showing a configuration of a digital color copying machine to which a color image processing apparatus according to an embodiment of the present invention is applied.
In FIG. 1, the color image processing apparatus includes at least an image input unit 10, a pre-processing unit 20, an image processing unit 30, a post-processing unit 40, and an image output unit 50.
[0016]
The image input unit 10 uses a CCD (Charged Coupled Device) element to read a document and perform A / D conversion, and then R (red), G (green), and B (blue) 3 for each read pixel. Image data is output as a color image signal. The image input unit 10 also performs processing such as shading correction and γ correction on the image signal obtained from each CCD element.
[0017]
The preprocessing unit 20 performs smoothing on the image data input by the image input unit 10 by performing an average value of pixel values within the smoothing reference range or appropriate thinning, and the Lab color from the RGB color space. Color space conversion is performed on the space or the like so that the color feature amount can be easily extracted in the subsequent processing. Depending on the type of the input image, this process may be omitted.
[0018]
The image processing unit 30 performs a show-through correction process on the preprocessed image data, and performs general image processing such as isolated point removal, color correction, and moire reduction on the resulting image data.
In the show-through correction process, the show-through detection and correction process is particularly limited as long as it is a method capable of determining whether a pixel to be processed is included in the show-through area and calculating a corrected pixel value for each pixel. Not what you want.
[0019]
The post-processing unit 40 performs color correction on the image data processed by the image processing unit 30, performs black generation and undercolor removal on the result, performs γ conversion in accordance with the characteristics of the output device, performs dither processing, The image data is processed so as to reproduce the halftone of the image by the error diffusion method.
The image output unit 50 is driven according to the image data output from the post-processing unit 40 and outputs the image to the color output device.
[0020]
Next, with reference to FIG. 2, an outline of the process for the show-through correction process of the image processing unit 30 according to the present invention will be described.
Hereinafter, for the sake of simplicity, the input image is referred to as I, the current target pixel to be processed as T, and the reference range set around the target pixel as W0 (range of input image I).
[0021]
(Procedure 1)
Based on the information obtained from the reference range W0, it is determined whether or not the target pixel T is included in the show-through area. If the target pixel T is included in the show-through area, a result of correcting the show-through is obtained (FIG. 2 ( A)). This correction is called “primary correction”, and the corrected result is called “primary correction result (R1)”.
For this determination and calculation of the correction result, for example, a method according to Japanese Patent Laid-Open No. 2001-169080 may be used. However, the method is not particularly limited as long as the method can calculate the above-described determination and the corrected pixel value for each pixel. Not.
[0022]
(Procedure 2)
The primary correction result is held in the buffer for the reference range W1 given in advance (FIG. 2B).
The reference range W1 may be set to the same size as W0, for example. However, in the scanning method generally called raster scan, the target pixel T is calculated using information on the reference range W0. The reference range W1 of the previous part is not defined.
[0023]
(Procedure 3)
Using the information obtained from the primary correction result held in the buffer in the reference range W1, the show-through correction result is calculated again for the target pixel T (FIG. 2C). The result calculated at this time is referred to as “secondary correction result (R2)”.
The show-through correction processing method for obtaining the secondary correction result R2 and the parameters necessary for the processing may be the same as the method and parameters in the procedure 1, or set to methods and values suitable for each. It is also possible to keep it.
[0024]
By repeatedly applying (Procedure 1) to (Procedure 3) below, it is possible to obtain higher-order correction results such as “third-order correction result (R3)”, “fourth-order correction result (R4)”. .
[0025]
Next, an optimum output for replacing the target pixel T is selected from the several correction results calculated above. For example, there are the following selection methods.
(1) From among all the correction results calculated for the target pixel T, the correction result that is brightest is selected. This is the simplest implementation.
(2) The secondary correction result is selected only when the values of the primary correction result and the secondary correction result are smaller than a predetermined threshold value.
(3) Within the reference range for calculating each correction result, an index representing variation such as dispersion is calculated, and the correction result calculated from the reference range having the smallest variation is selected.
[0026]
Since only the primary correction result is used in the conventional method, it is difficult to completely correct the show-through that greatly exceeds the reference range. However, as in the present invention, the higher-order correction result after the secondary correction result is difficult. By using, there are the following effects.
(1) It is possible to correct a show-through wider than that in the past.
(2) It is possible to accurately correct show-through of a plurality of layers.
(3) Since one pixel in the higher-order correction result also includes surrounding information, mounting efficiency is improved rather than simply expanding the reference range.
[0027]
Hereinafter, in order to simplify the description of the present invention, up to the third-order correction result is used, but it may be configured using a fourth-order or higher-order correction result.
FIG. 3 is a flowchart for explaining the processing procedure of the show-through correction processing of the present invention.
[0028]
It is read as digital data by an image input device such as a scanner, and is subjected to processing such as scanner γ correction. The input image data is smoothed by performing an average value of pixel values within the smoothing reference range or appropriate thinning, etc., and color space conversion is performed from the RGB color space to the Lab color space, etc. The process makes it easy to extract the color feature amount (step S1).
From this input image, an image that can be clearly determined to be a foreground image by “contents printed on the front side of the document” such as characters and photographs is extracted and protected from subsequent correction processing (step S2). This foreground extraction / protection will be described later with reference to FIG.
[0029]
Next, the entire input image (however, the area protected as the foreground is excluded) is set as the reference range W0, and the show-through correction (primary correction) is performed on the reference range (step S3).
In the following description, the reference range is described as the entire input image. However, by using the information obtained from the input image, the region determined to be a halftone dot region is limited to the reference range and the show-through correction process is performed. Good. Alternatively, the show-through correction process may be performed by limiting only the halftone dot area where the halftone dot area ratio of the input image is a certain range to the reference range.
[0030]
Examples of the show-through correction method include the following correction methods.
(1) Even with a method of extracting the brightest color (hereinafter referred to as the brightest color) in the reference range (for example, W0) and replacing the pixel to be processed (target) with the brightest color, a certain degree of show-through correction performance Is obtained.
(2) It is also effective to classify the pixels within the reference range into several classes and replace the target pixel with a color representing this class (hereinafter referred to as a representative color). When such a statistical approach is used, it is determined in advance whether or not the show-through correction is performed using an index (for example, variance) indicating pixel value variation within the reference range. Also good.
Further, when the foreground pixels of a certain ratio or more are included in the reference range, foreground protection can be more thoroughly performed by not correcting the pixel (i, j). Regarding the show-through correction processing, a processing procedure based on the simple method (1) will be described later with reference to FIG.
[0031]
Next, since the show-through correction obtained in step S3 may be erroneously corrected, the correction content is verified and corrected immediately after the show-through correction (step S4).
The verification and correction of the correction content may be performed immediately after the correction value is calculated in the show-through correction process.
In this verification, for example, the original pixel value (target) to be processed is compared with the pixel value after the show-through correction obtained in step S3, and if the difference is equal to or greater than a predetermined threshold value, it is determined as an erroneous correction. The predetermined threshold is a parameter that determines the strength of the show-through correction, and is preferably statistically calculated in advance, but may be set based on the user's desired processing intensity.
After this show-through correction is completed, the corrected image (primary correction result) is stored in a buffer.
[0032]
Next, a reference range W1 (see FIG. 2) is set for the correction result stored in this buffer, and the brightest color is extracted in the same manner as the show-through correction in step S3, and the target pixel is set to the brightest color. (Step S5), the erroneous correction is verified, and the secondary correction result is stored in the buffer (step S6).
Further, using the secondary correction result stored in the buffer, the tertiary correction result is similarly stored in the buffer (steps S7 and S8).
[0033]
Lastly, the primary correction result, the secondary correction result, the tertiary correction result, and the “color space conversion, smoothing, etc., which are the basis of these, are calculated and stored in the buffer. The final output image is synthesized using the “input image” (step S9).
In order to synthesize this output image, a value selected by any of the following is set for each pixel.
[0034]
(1) The brightest color is selected from the primary correction result, the secondary correction result, the tertiary correction result, and the “input image that has been subjected to color space conversion and smoothing”. This is the simplest implementation.
(2) The secondary correction result is selected only when the values of the primary correction result and the secondary correction result are smaller than a predetermined threshold value.
(3) Primary correction result, secondary correction result, tertiary correction result, and reference ranges set for each of these “input images subjected to color space conversion, smoothing, etc.” Within (W0, W1, W2,...), An index indicating variation such as dispersion is calculated, and a correction result calculated from a reference range with the least variation is selected. This method is a more accurate method.
[0035]
FIG. 4 is a flowchart for explaining the processing procedure for extracting the foreground from the input image.
The first pixel in the scan order is extracted from the input image and is defined as pixel (i, j) (step S11).
Based on information such as the characteristics of the scanner, the luminance value (lum) at pixel (i, j) is calculated (step S12).
In order to calculate the edge, the pixel value around the pixel (i, j) is held as the primary neighborhood (step S13).
The edge amount (edge) of the pixel (i, j) is calculated (step S14).
[0036]
From these luminance values (lum) and edge amounts (edge), it is determined whether the pixel (i, j) is in the foreground or non-foreground (step S15).
As a feature of the foreground image, for example, characters and the like exhibit strong edge strength, and therefore, when the edge amount is larger than a predetermined threshold, it is determined that the pixel is a foreground pixel.
Further, by determining together with the pixel value and the luminance value based on information such as the characteristics of the scanner, it is possible to more accurately determine that the pixel is a foreground pixel.
[0037]
If all the pixels have been processed, the process ends (Yes in step S16).
If there is still a pixel to be processed (No in step S16), the next pixel in the scan order is set as the pixel (i, j), and the process is repeated from step S12 (step S17).
[0038]
FIG. 5 is a flowchart for explaining the processing procedure of the show-through correction processing within the reference range.
The first pixel in the scan order is extracted from the input image and is set as pixel (i, j) (step S21).
The pixel value in the reference range around the pixel (i, j) is temporarily held (step S22). The brightest color within this reference range is extracted (step S23). The pixel value of the brightest color is assumed to be Mlum.
[0039]
If the pixel (i, j) is not in the foreground region, the pixel value of the pixel (i, j) is replaced with the pixel value Mlum of the brightest color (steps S24 and S25).
Otherwise, the pixel (i, j) is not corrected.
[0040]
If all the pixels have been processed, the process ends (Yes in step S26).
If there is still a pixel to be processed (No in step S26), the next pixel in the scan order is set as the pixel (i, j), and the process is repeated from step S22 (step S27).
[0041]
For each pixel, steps S12 to S15 in FIG. 4 may be executed before step S22 in FIG. 5, and the above-described foreground determination and correction processing may be executed simultaneously.
[0042]
FIG. 6 is a hardware configuration diagram for realizing the color image processing apparatus of the present invention by a computer. That is, the computer according to the present embodiment connects the image input device 1, the display device 2, the input device 3, the image printing device 4, the CPU (Central Processing Unit) 5, the RAM 6, the ROM 7 and the medium driving device 8 through the data bus 9. ing.
[0043]
First, each function of the above-described embodiment is programmed and stored in the ROM 7. After that, the CPU 5 reads out and executes the program stored in the ROM 7 to execute each function.
[0044]
When each function is executed, a color image input from the image input device 1 such as a scanner or a digital camera is accumulated in the RAM 6, read out from the RAM 6, and an image corrected for show-through as a result of a series of processing. Is generated and output to an image printing apparatus 4 such as a printer or a color digital copying machine.
In addition, the progress and result of the process are presented to the user through the display device 2 such as a display, and when necessary, the user inputs parameters necessary for the process from the input device 3 such as a keyboard or a touch panel. . Intermediate data created during the execution of the process is also stored in the RAM 6 and is read out by the CPU 5 and corrected / written as necessary.
[0045]
In the above description, the program recording medium is the ROM 7, but other recording media such as a semiconductor medium (for example, a non-volatile memory card), an optical medium (for example, a DVD-ROM, MO, MD, CD-R). Etc.) and magnetic media (for example, magnetic tape, flexible disk, etc.).
When the program is recorded on the recording medium as described above, the recording medium is loaded in the medium driving device 8, the stored program is read, and the installed program is executed by the CPU 5.
[0046]
Thus, the program itself read from the recording medium storing the program realizes the functions of the above-described embodiments, and the program and the recording medium storing the program also constitute the present invention.
[0047]
In addition, when the above-described program is stored in a storage device such as a magnetic disk of a server computer and downloaded by a user's computer connected via a network, or distributed in the form of distribution from the server computer, the server computer The storage device is also included in the recording medium of the present invention.
[0048]
【The invention's effect】
As described above, according to the present invention, it is possible to correct a show-through wider than the conventional one.
In addition, the show-through of a plurality of layers can be corrected accurately.
In addition, since one pixel in the higher-order correction result also includes surrounding information, the mounting efficiency is improved rather than simply expanding the reference range.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a digital color copying machine to which a color image processing apparatus according to an embodiment of the present invention is applied.
FIG. 2 is a diagram for explaining a relationship between a reference range and a correction result.
FIG. 3 is a flowchart for explaining a processing procedure of show-through correction processing according to the present invention;
FIG. 4 is a flowchart for explaining a processing procedure of foreground extraction and protection processing;
FIG. 5 is a flowchart for explaining a processing procedure of a show-through correction process within a reference range;
FIG. 6 is a hardware configuration diagram for realizing the color image processing apparatus of the present invention by a computer.
FIG. 7 is a diagram illustrating an example in which a show-through of a back image of a plurality of layers occurs in a front image.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Image input device, 2 ... Display device, 3 ... Input device, 4 ... Image printing device, 5 ... CPU, 6 ... RAM, 7 ... ROM, 8 ... Medium drive device, 9 ... Data bus, 10 ... Image input part 20 ... Pre-processing unit, 30 ... Image processing unit, 40 ... Post-processing unit, 50 ... Image output unit.

Claims (7)

In a color image processing method for reducing show-through and underlay on an input image obtained by digitally inputting a color-printed document on paper, only information within a specific reference range around pixels of the input image is obtained. The result of the show-through correction processing for this pixel is held as a primary correction result, and this primary correction result is used for the show-through correction processing for this pixel and thereafter, so that the entire image is not repeatedly processed. A color image processing method characterized by selectively reducing the show-through and underlay. 2. The color image processing method according to claim 1, further comprising performing a show-through correction process on a specific reference range of the primary correction result, and locally repeating the show-through correction process to obtain a higher-order correction result. And obtaining a final show-through corrected image from these correction results. 3. The color image processing method according to claim 1, wherein a show-through correction process is performed using only an area determined as a halftone dot area as a reference range based on information obtained from the input image. Image processing method. 3. The color image processing method according to claim 1, wherein a show-through correction process is performed using only a halftone dot area where a halftone dot area ratio of the input image is in a certain range as a reference range. Method. A color image processing apparatus that reduces show-through and cover-through for an input image obtained by digitally inputting a document color-printed on paper. In a color image processing apparatus, only information within a specific reference range around pixels of the input image is stored. And performing a show-through correction process on the pixel and holding it as a primary correction result, applying a show-through correction process to a specific reference range of the primary correction result, and performing the show-through correction process locally. A show-through correction output unit that repeatedly holds as a high-order correction result, and a synthesis processing unit that synthesizes the final show-through corrected output image from these correction results and the input image, and covers the entire image. A color image processing apparatus that selectively reduces show-through and cover-up without repeating processing. A program for causing a computer to execute the color image processing method according to any one of claims 1 to 4. A computer-readable recording medium on which the color image processing program according to claim 6 is recorded.

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