JP4274043B2 - Image processing apparatus, image forming apparatus, image processing method, and program - Google Patents

Description

  The present invention relates to so-called limited color processing, and more particularly to a technique for improving the image quality of image data subjected to limited color processing.

In order to reduce the amount of image data, so-called limited color processing is performed. In this case, the limited colorization processing 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 “color”). This is replaced with “representative color”).
As a method used in the limited color processing, for example, there is an exhaustive search method. In this method, each pixel is compared with all the representative colors, and converted into a representative color having the smallest color difference, that is, the distance between the pixel of interest and the representative color in a certain color space (see, for example, Non-Patent Document 1). .

By performing this limited color processing on the image data read by the scanner or the like, it is possible to remove or reduce the show-through component and noise component mixed in the image data, thereby suppressing deterioration in image quality. it can.
FIG. 9 is an enlarged view of a part of a document and image data obtained by reading this part, in order to explain suppression of image quality deterioration due to the limited color processing. In the figure, the original is uniformly white. At this time, the pixel P of the image data is recorded as white, but the pixel _PN is recorded as gray having a density of 10%, for example, because noise is mixed therein. For such image data, for example, white (W), cyan (C), magenta (M), yellow (Y), black (K), red (R), green (G), and blue (B). When limited color processing using eight colors as representative colors is performed, the pixel _PN is converted to a representative color that minimizes the color difference, that is, white, and as a result, the original color of the original is restored, and noise is restored. It is possible to remove components. Also, the show-through component can be similarly removed by the limited color processing.

In the case of performing the limited color processing for the purpose of preventing the deterioration of the image quality, it is desirable that as few representative colors as possible be used. This is because increasing the representative colors makes it difficult to restore the above-described noise to the original color of the original. For example, in the case of FIG. 9 described above, if it contains gray density of 15% of the representative color, a concentration of 10% of the pixel P _N is will be converted to gray levels of 15% rather than the original white noise Rather than removing it, the result is rather emphasized.
Therefore, when the limited color processing is performed for such a purpose, the representative colors used are several colors including the color of the original (paper color) and the color material (toner etc.) used. About ten or more colors are preferable, and the optimum combination of the representative colors can be obtained experimentally or empirically (hereinafter, such a plurality of representative colors are collectively referred to as a “color table”). .

However, as the number of representative colors constituting the color table decreases, it has become clear that there is a problem that the gray balance of the image data subjected to the limited color processing tends to be lost. As the cause, for example, there are the following two points.
First, if the representative colors constituting the color table are arranged in a certain color space, it is difficult to maintain symmetry with respect to the axis of the achromatic color when there are few representative colors. As an example, in the RGB color space, the axis of the achromatic color is represented by a straight line connecting coordinate points where R = G = B in the RGB color space. The possibility that the representative colors are evenly arranged with respect to this straight line is low. As a result, there is a high possibility that the image data subjected to the limited color processing using such a color table will lose the gray balance.

Secondly, a low-saturation pixel in the image data, that is, a pixel having a hue close to gray, can be easily converted to a chromatic representative color by the above-described limited color processing.
FIG. 10 is a schematic diagram of three representative colors in a certain color space, which is shown to explain this problem. In the figure, C _K , C _W , and C _Br indicate the representative colors of black, white, and brown, respectively, and P indicates a pixel to be subjected to the limited color processing. A line segment l is a straight line connecting the representative colors C _K and C _W , and the line segment l indicates an achromatic color. Since the pixel P is in the vicinity of the line segment l, the pixel P is a color that is perceived as “slightly brownish gray” by the human eye. However, if this pixel P is simply subjected to the limited colorization process based on only the distance from the representative color as described above, this pixel P will be converted to brown, Create a gap. In addition, since the pixels in the vicinity of the line segment l in the RGB color space, that is, the pixels that are visually close to the achromatic color are converted into chromatic colors, the gray balance is lost.

P. Heckbert, "Color Image Quantization for Frame Buffer Display", ComputerGraphics, vol.16, p.297-304, 1982

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a technique for performing limited color processing on image data while maintaining a good gray balance.

In order to achieve the above object, the present invention is included in image acquisition means for acquiring image data representing an image, storage means for storing a plurality of representative colors, and image data acquired by the image acquisition means. Representative color specifying means for comparing the color of a pixel with each of a plurality of representative colors stored in the storage means and specifying a first representative color having the smallest color difference; and the image data in a certain color space First distance calculation means for calculating first distance information based on a distance between a coordinate point indicating the color of the pixel included in the color axis and a gray axis represented by a plurality of achromatic colors in the color space; a coordinate point indicating the color of a pixel included in the image data in the color space, the second distance information based on the distance between the coordinate points indicating the first representative color identified by the representative color specifying means Second to calculate A distance calculation means, wherein when the value of the first distance information is smaller than the value of the second distance information, the color of the pixel, the gray axis than on the gray axis to the first representative color There is provided an image processing apparatus comprising color conversion means for converting to a second representative color adjacent to the image processing apparatus.
According to such an image processing device, it is possible to prevent a pixel having a hue close to gray from being converted to a chromatic color when performing the limited color processing on the image data, and to maintain a good gray balance. It becomes possible.

In a more preferred aspect of the image processing apparatus of the present invention, the first distance calculation unit is configured to use an achromatic representative color when the plurality of representative colors include an achromatic representative color. Determine the gray axis.
As a result, the pixels having a hue close to gray are converted into achromatic representative colors, so that it is possible to maintain a good gray balance.

In a more preferred aspect of the image processing apparatus of the present invention, the plurality of representative colors include an achromatic representative color, and the first distance calculation means includes the achromatic color stored in the storage means. A gray axis is determined based on the representative color, and first distance information is calculated based on the distance between the coordinate point and the gray axis.
More preferably, the plurality of representative colors include white and black representative colors.
More preferably, the plurality of representative colors include white, black, and gray representative colors.
As a result, the pixels having a hue close to gray are converted into achromatic representative colors, so that it is possible to maintain a good gray balance.

In a more preferred aspect of the image processing apparatus of the present invention, the color conversion unit may be configured such that when the value of the first distance information is smaller than the value of the second distance information, the color of the pixel Of the representative colors on or near the gray axis, the representative color having the smallest color difference from the pixel is defined as the second representative color .
As a result, it is possible to perform a more accurate limited color process while maintaining the gray balance of the image data.

In the image processing apparatus of the present invention, the first distance calculation unit determines a gray axis based on the image data acquired by the image acquisition unit, and is based on a distance between the coordinate point and the gray axis. It is also possible to calculate the first distance information.
Alternatively, in the image processing apparatus of the present invention, the first distance calculating unit determines a gray axis using data indicating achromatic colors stored in advance, and based on a distance between the coordinate point and the gray axis. It is also possible to calculate the first distance information.

In addition, the present invention provides an image acquisition step in which the image processing apparatus acquires image data representing an image, and pixel colors included in the image data acquired in the image acquisition step. A representative color specifying step for specifying the first representative color having the smallest color difference, a coordinate point indicating the color of the pixel included in the image data in a certain color space, and a plurality of coordinate points in the color space. A first distance calculation step for calculating first distance information based on a distance from a gray axis expressed by connecting achromatic colors of the color coordinates; and a coordinate point indicating a color of a pixel included in the image data in the color space When the second distance calculating step of calculating a second distance information based on the distance between the coordinate points indicating the first representative color identified by the representative color specifying means, the first distance information If the value is less than the value of the second distance information, and converts the color of the pixel, the second representative color said than on the gray axis to the first representative color close to the gray axis It is also possible to provide an image processing method including a color conversion step.

Further, the present invention is included in the image data acquired by the image acquisition unit that acquires image data representing an image, the storage control unit that stores a plurality of representative colors in the storage unit, and the image acquisition unit. Representative color specifying means for comparing the color of a pixel with each of a plurality of representative colors stored in the storage means and specifying a first representative color having the smallest color difference; and the image data in a certain color space First distance calculation means for calculating first distance information based on a distance between a coordinate point indicating the color of the pixel included in the color axis and a gray axis represented by a plurality of achromatic colors in the color space; a coordinate point indicating the color of a pixel included in the image data in the color space, the second distance information based on the distance between the coordinate points indicating the first representative color identified by the representative color specifying means Calculation A second distance calculating means for, if the value of the first distance information is smaller than the value of the second distance information, the color of the pixel, from on the gray axis to the first representative color Can also be provided as a program for functioning as color conversion means for converting to the second representative color close to the gray axis .

  Hereinafter, the configuration and operation of an image processing apparatus according to an embodiment of the present invention will be described. In the description of the present embodiment, the RGB color space is used as the color space. Here, the RGB color space is a color space represented by an orthogonal coordinate system with red (R), green (G), and blue (B) as axes. For example, coordinate values indicating “red”, “green”, “blue”, “black”, and “white” on the RGB color space are (1, 0, 0), (0, 1, 0), (0, 0, 1), (0, 0, 0), (1, 1, 1). In addition, these coordinate values are hereinafter referred to as “RGB values” as appropriate.

(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 various types of image processing to be described later on input image data. Execute the operation. At this time, the image data is temporarily stored in the RAM 113. The ROM 112 stores various values such as threshold values Th _W and Th _K described later. The HDD 12 is a storage device, and stores a program PRG for executing image processing and a color table TBL used in the limited colorization processing described later. The input IF 13 acquires image data from the image reading device 2, and the output IF 14 outputs the image data subjected to image processing 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 is cyan (C), magenta (M), yellow (Y), red (R), green (G), blue (B), light blue (Lb), orange. (O), yellowish green (Yg), brown (Br), dark green (Dg), indigo (Db), white (W), and black (K). The indicated RGB values are stored in the HDD 12. In the following, the RGB values indicating these representative colors are expressed as C ₀ , C ₁ , C ₂ ,..., C ₁₃ , respectively.

  The image reading apparatus 2 is a so-called scanner, and includes a platen, a light source, an optical system, a light receiving element, and a signal processing unit (not shown). The light source irradiates light on a document placed on the platen, and the reflected light is emitted. 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 shading correction on the image signal, and supplies the signal to the image processing apparatus 1 as image data. To do. 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, image processing performed by the image processing apparatus 1 under the above-described configuration will be described. This process is a process realized by the control unit 11 of the image processing apparatus 1 executing the program PRG.
FIG. 3 is a flowchart showing image processing executed by the image processing apparatus 1 of the present embodiment. As shown in the figure, first, the control unit 11 of the image processing apparatus 1 acquires the image data supplied from the image reading apparatus 2 via the input IF 13 (step S1). At this time, the image data supplied from the image reading apparatus 2 is subjected to various processes such as AD conversion, gamma conversion, or shading correction. And the control part 11 performs a limited colorization process with respect to this image data (step S2).
FIG. 4 is a flowchart showing this limited color processing. As shown in the figure, first, the control unit 11 refers to the color table TBL stored in the HDD 12, RGB value C ₀ of the representative color, C _1, C _2, ..., a C ₁₃ RAM 113 Is temporarily stored (step S21). Subsequently, the control unit 11 specifies a representative color having the maximum brightness from these representative colors, and sets this as max (C _n ) (step S22).

Next, the control unit 11 determines the whiteness of the representative color max (C _n ) (step S23). Specifically, the control unit 11 compares the RGB value of the representative color max (C _n ) with a point that is completely white, that is, (1, 1, 1) if indicated by coordinate values, and obtains the difference D _W. . Here, the difference D _W is given as the sum of the differences between the R, G, and B coordinates of the RGB value of white and the RGB value of the representative color max (C _n ). That is, assuming that the RGB value of max (C _n ) is (r ₁ , g ₁ , b ₁ ), D _W = (1−r ₁ ) + (1−g ₁ ) + (1−b ₁ ). .
Then, the control unit 11 compares this difference with a predetermined threshold value Th _W, and if Th _W > D _W , the representative color max (C _n ) is regarded as white and the value of the white flag FlagW is set to “1”. And On the other hand, if Th _W ≦ D _W , the representative color max (C _n ) is regarded as not white, and the value of the white flag FlagW is set to “0”. In the present embodiment, since max (C _n ) is white, the value of the white flag FlagW is “1”.

Subsequently, the control unit 11 specifies a representative color having the minimum brightness from the representative colors stored in the RAM 13, and sets this as min (C _n ) (step S24). Then, the control unit 11 determines the blackness of the representative color min (C _n ) in substantially the same manner as in step S23 described above (step S25). Specifically, the control unit 11 compares the RGB value of the representative color min (C _n ) with the complete black, that is, the point that becomes (0, 0, 0) in terms of coordinate values, and obtains the difference D _K. . Here, the difference D _K is given as the sum of the differences between the R, G, and B coordinates of the black RGB value and the RGB value of the representative color min (C _n ). That is, _if the RGB value of min (C _n ) is (r ₂ , g ₂ , b ₂ ), D _K = (0−r ₂ ) + (0−g ₂ ) + (0−b ₂ ). .
Then, the control unit 11 compares this difference with a predetermined threshold value Th _K, and if Th _K > D _K , the representative color min (C _n ) is regarded as black, and the value of the black flag FlagK is set to “1”. And On the other hand, if Th _K ≦ D _K , the representative color min (C _n ) is regarded as not black, and the value of the black flag FlagK is set to “0”. In the present embodiment, since min (C _n ) is black, the value of the black flag FlagK is “1”.

As described above, when the white flag FlagW and the black flag FlagK are obtained, the control unit 11 subsequently determines whether or not the values of the white flag FlagW and the black flag FlagK are both “1” (step S26). That is, here, the control unit 11 determines whether or not the color table TBL includes representative colors that can be regarded as white and black.
The control unit 11 performs different color conversion processing according to the determination result. In the present embodiment, the values of the white flag FlagW and the black flag FlagK are both “1”, and the above determination result is affirmative (step S26; YES). Therefore, here, the color conversion process A in step S27 will be described first.

FIG. 5 is a flowchart showing the color conversion process A in step S27. As shown in the figure, the control unit 11 first determines a straight line that passes through the representative colors max (C _n ) and min (C _n ) in the RGB color space (step S271). Since this straight line passes through the representative colors max (C _n ) and min (C _n ), that is, white and black, any point on this straight line indicates an achromatic color (white, black, or gray). Become. Hereinafter, this straight line is referred to as a “gray axis”.
Next, the control unit 11 calculates the distance D _G of this gray axis and a pixel P included in the image data (step S272). Then, the distance between all the representative color control unit 11 in the pixel P and a color table TBL, namely to calculate the color difference, the distance between the representative color C _n and the pixel P of the color difference is minimum and D _C (Step S273). Then, the control unit 11 compares the distance D _G with the distance D _C (Step S274). In comparing the distance D _G and the distance D _C , a predetermined coefficient a is multiplied to weight the distance D _G. The value of the coefficient a is a value obtained experimentally or empirically, but is set to “1” here.

The control unit 11 performs different processing according to the above comparison result. First, when the distance D _G between the pixel P and the gray axis is smaller than the distance D _C with respect to the representative color C _n that minimizes the color difference from the pixel P (step S274; YES), the control unit 11 performs this operation. The color of the pixel P is converted to the closest representative color on the gray axis (step S275). That is, in the present embodiment, the pixel P is converted to white or black.
On the other hand, when the distance D _G between the pixel P and the gray axis is not less than the distance D _C with respect to the representative color C _n that minimizes the color difference from the pixel P (step S274; NO), the control unit 11 The color of the pixel P is converted to the color of the representative color C _n that minimizes the color difference from the pixel P (step S276).

These conversions will be described with reference to the drawings as follows.
FIGS. 6 and 7 are diagrams illustrating the representative color C _n that minimizes the color difference between the pixel P (or pixel P ′) and the gray axis, and the pixel P. FIG. First, as shown in FIG. 6, the distance between the pixel P and the gray axis is represented by D _G , and the distance between the pixel P and the representative color C _n is represented by D _C. At this time, since D _G > D _C , the pixel P is converted to the representative color C _n .
On the other hand, in the example of FIG. 7, since D _G <D _C , the pixel P ′ is converted to an achromatic representative color instead of the representative color C _n . Here, comparing the distance between the pixel P ′ and the representative colors C ₁₂ (ie, white) and C ₁₃ (ie, black), the pixel P ′ is closer to the representative color C ₁₂ , and in this case, the pixel P ′ is converted to white. The

  When the color of the pixel P is thus converted, the control unit 11 determines whether or not the above-described processing has been performed for all the pixels of the image data (step S277), and while the unprocessed pixels remain, (Step S277; NO) When the above process is repeated from Step S272 and the above process is completed for all pixels (Step S277; YES), this process is terminated and the process returns to the flowchart of FIG.

On the other hand, when the determination result in step S26 is negative (step S26; NO), the control unit 11 executes a color conversion process B different from the color conversion process A described above.
This color conversion process B is different from the color conversion process A described above in whether or not a gray axis is set. As described above, the color conversion process B is performed when a representative color that can be regarded as white or black is not included in the representative color. In such a case, the color of a certain pixel in the image data is not converted to white or black, so that it is not necessary to perform a process like the color conversion process A in the first place. Based on the above, the color conversion process B will be described below.

FIG. 8 is a flowchart showing the color conversion process B in step S28. As shown in the figure, the control unit 11 calculates the distance, that is, the color difference between a certain pixel P included in the image data and all the representative colors in the color table TBL, and the representative color C that minimizes this color difference. _n is specified (step S281). Subsequently, the control unit 11 converts the color of the pixel P to the representative color C _n identified in step S281 (step S282). When the color of the pixel P is thus converted, the control unit 11 determines whether or not the above-described processing has been performed on all the pixels of the image data (step S283), and while the unprocessed pixels remain. (Step S283; NO) The above process is repeated from Step S281, and when the above process is completed for all pixels (Step S283; YES), this process is terminated and the process returns to the flowchart of FIG.

Since the description of the limited color processing is thus completed, the description returns to the flowchart of FIG. 3 and the description of the image processing of the present embodiment is continued.
When the limited color processing in step S2 is completed, the control unit 11 subsequently performs color space conversion (step S3). In the present embodiment, conversion from the RGB color space used so far to the CMYK color space used in the image forming apparatus 2 is performed. Thereafter, the control unit 11 performs screen processing on the halftone area of the image data (step S4). When the screen process is completed, the control unit 11 outputs the image data to the image forming apparatus 3 via the output IF 14 (step S5).

The image processing executed by the image processing apparatus 1 according to this embodiment has been described above. In this way, the following effects can be obtained by performing the above-described image processing on the image data read by the image reading device 2.
First, the image processing apparatus 1 according to the present embodiment performs the above-described limited colorization process, thereby reducing or reducing the show-through component and noise component mixed in the image data while reducing the data amount of the image data. This makes it possible to suppress degradation of image quality.
In addition, according to the limited color processing of the present embodiment, when the representative colors include colors that are considered white and black, the gray axis is calculated and the distance between this and the target pixel is compared. Thus, it becomes possible to convert a low-saturation pixel that has been converted into a representative chromatic color into an achromatic color on the gray axis. As a result, even when the limited color processing is performed with a small number of colors, the gray balance is not lost as in the prior art, and it is possible to obtain higher quality image data.
In addition, according to the limited color processing of the present embodiment, when image data whose gray balance is lost due to chromatic aberration, MTF (Modulation Transfer Function) characteristics, or color conversion characteristics of an image reading apparatus such as a scanner is input. In this case, since low-saturation pixels near the gray axis can be converted to achromatic colors on the gray axis, even if such image data is input, the gray balance is restored to a certain level. Is possible.

(3) Modifications The application of the present invention is not limited to the above-described embodiment, and various modifications can be made. An example is shown below.
In the above-described embodiment, it has been described that image data is input to the image processing apparatus 1 from the image reading apparatus 2 and the processed image data is output to the image forming apparatus 3. However, the image processing apparatus of the present invention is not limited to such an aspect, and may be an aspect in which image data is input from, for example, a general computer, or an aspect in which the image data is output to a storage device or the like. May be. In such a case, among the steps of the flowchart shown in FIG. 3, the color space conversion (step S3) and the screen processing (step S4) can be omitted as appropriate.

In the above-described embodiment, it has been described that the image processing of the present invention is realized by the image processing apparatus 1 executing the program PRG. However, the present invention is not limited to such an aspect. For example, the image processing apparatus may be configured to include an electronic circuit that can realize the functions of the respective steps of the image processing described above. Of course, it can also be realized by an ASIC (Application Specific Integrated Circuit) having the above-described functions.
In the above-described embodiment, the present invention has been described as being implemented by the image processing apparatus 1. However, the present invention is, for example, the image processing apparatus 1, the image reading apparatus 2, and the image forming apparatus 3 in the above-described embodiment. Needless to say, the present invention can be implemented by an image forming apparatus such as a so-called copying machine having both functions.

  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.

  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 specific processing, for example, when the RGB value of each pixel of the image data is calculated and the processing for converting these pixels into 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 cyan (C), magenta (M), yellow (Y), red (R), green (G), blue (B), light blue (Lb), orange (O) as described above. ), Yellow green (Yg), brown (Br), dark green (Dg), indigo (Db), white (W), black (K), and other colors are representative colors. It is also possible to increase or decrease 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 the above-described embodiment, the gray axis is described as a straight line passing through white and black. However, as described in the whiteness determination in step S23 and the blackness determination in step S25 in the flowchart of FIG. 4, the representative colors of white and black determined here are not necessarily completely white ( It is not necessary that the RGB value is (1, 1, 1) white) and the complete black (RGB value is (0, 0, 0) black). In the present invention, even if the above-described white or black is not strictly achromatic, the same effect as that obtained when these are achromatic can be obtained.

In the above-described embodiment, the representative colors existing on the gray axis are only two colors, white and black, but other representative colors may be on the gray axis. For example, if the middle color between white and black, that is, gray having a density of 50% is added to the representative color, the chromatic color in the middle density area (25 to 75%) near the gray axis can be converted to gray. In addition, it is possible to perform the limited color processing with higher accuracy than the conversion to black.
In addition, although it has been described that these representative colors exist on the gray axis, it is not always necessary to exist on the gray axis, and even if the representative color exists in the vicinity of the gray axis, There is an effect.

Further, in the above-described embodiment, at step S274 of the flowchart of FIG. 5, it has been described multiplying a predetermined coefficient a in order to perform the weighted distance D _G. However, it is of course possible to weight not the distance D _G but the distance D _C. Further, in this step S274, it is described that the process of step S276 is performed when the distance D _G (aD _G ) and the distance D _C have the same value. It is good also as what performs the process of S275.

  In the above-described embodiment, the gray axis is determined based on the representative color. However, the present invention is not limited to such an aspect. For example, the input image data is analyzed, and the pixel having the maximum brightness in this image data is regarded as white and the pixel having the minimum brightness is regarded as black, and the gray axis is determined based on these. Good. Alternatively, the distribution (histogram) of the input image data in a certain color space is obtained, the mode value in the high brightness area is regarded as white, and the mode value in the low brightness area is regarded as black, and the gray axis is determined based on these. You may do. The gray axis may be determined based on data indicating an achromatic color stored in advance in the HDD 12 or the like.

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. 5 is a flowchart illustrating image processing executed by the image processing apparatus according to the embodiment. It is the flowchart which showed the limited color processing in the same image processing. It is the flowchart which showed the color conversion process A in the same image processing. FIG. 4 is a diagram illustrating a representative color that minimizes a color difference between a pixel and a gray axis and the pixel in the embodiment. FIG. 4 is a diagram illustrating a representative color that minimizes a color difference between a pixel and a gray axis and the pixel in the embodiment. It is the flowchart which showed the color conversion process B in the same image processing. FIG. 3 is an enlarged view of a part of a certain original and image data obtained by reading this part, for explaining suppression of deterioration in image quality due to limited color processing. It is a schematic diagram of three representative colors in a certain color space.

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 (11)

Image acquisition means for acquiring image data representing an image;
Storage means for storing a plurality of representative colors;
The pixel color included in the image data acquired by the image acquisition unit is compared with each of the plurality of representative colors stored in the storage unit, and the first representative color that minimizes the color difference is specified. Representative color specifying means;
First distance information is calculated based on a distance between a coordinate point indicating the color of a pixel included in the image data in a certain color space and a gray axis represented by connecting a plurality of achromatic colors in the color space. 1 distance calculation means;
A coordinate point indicating the color of a pixel included in the image data in the color space, the second distance information based on the distance between the coordinate points indicating the first representative color identified by the representative color specifying means A second distance calculating means for calculating;
When the value of the first distance information is smaller than the value of the second distance information, the color of the pixel is set to the second on the gray axis or closer to the gray axis than the first representative color. An image processing apparatus comprising: color conversion means for converting to a representative color. The first distance calculating means includes
The image processing apparatus according to claim 1, wherein a gray axis is determined based on the representative color of the achromatic color when the representative color of the achromatic color is included in the plurality of representative colors. The plurality of representative colors include achromatic representative colors,
The first distance calculation means determines a gray axis based on the achromatic representative color stored in the storage means, and calculates first distance information based on the distance between the coordinate point and the gray axis. The image processing apparatus according to claim 1. The image processing apparatus according to claim 2, wherein the plurality of representative colors include white and black representative colors. The image processing apparatus according to claim 2, wherein the plurality of representative colors include representative colors of white, black, and gray. The color conversion means includes
When the value of the first distance information is smaller than the value of the second distance information, the color difference between the pixel and the representative color on or around the gray axis is the color difference of the pixel. 6. The image processing apparatus according to claim 4 , wherein the second representative color is a minimum representative color. The first distance calculating means includes
The image processing apparatus according to claim 1, wherein a gray axis is determined based on image data acquired by the image acquisition unit, and first distance information is calculated based on a distance between the coordinate point and the gray axis. The first distance calculating means includes
The image processing apparatus according to claim 1, wherein a gray axis is determined using data indicating an achromatic color stored in advance, and first distance information is calculated based on a distance between the coordinate point and the gray axis. An image forming apparatus comprising the image processing apparatus according to claim 1. The image processing device
An image acquisition step of acquiring image data representing the image;
A representative color specifying step of comparing the color of the pixel included in the image data acquired in the image acquisition step with each of a plurality of representative colors stored in advance and specifying a first representative color having the smallest color difference When,
First distance information is calculated based on a distance between a coordinate point indicating the color of a pixel included in the image data in a certain color space and a gray axis represented by connecting a plurality of achromatic colors in the color space. 1 distance calculation step;
A coordinate point indicating the color of a pixel included in the image data in the color space, the second distance information based on the distance between the coordinate points indicating the first representative color identified by the representative color specifying means A second distance calculating step for calculating;
When the value of the first distance information is smaller than the value of the second distance information, the color of the pixel is set to the second on the gray axis or closer to the gray axis than the first representative color. An image processing method comprising: a color conversion step for converting to a representative color. Computer
Image acquisition means for acquiring image data representing an image;
Storage control means for storing a plurality of representative colors in the storage means;
The pixel color included in the image data acquired by the image acquisition unit is compared with each of the plurality of representative colors stored in the storage unit, and the first representative color that minimizes the color difference is specified. Representative color specifying means;
First distance information is calculated based on a distance between a coordinate point indicating the color of a pixel included in the image data in a certain color space and a gray axis represented by connecting a plurality of achromatic colors in the color space. 1 distance calculation means;
A coordinate point indicating the color of a pixel included in the image data in the color space, the second distance information based on the distance between the coordinate points indicating the first representative color identified by the representative color specifying means A second distance calculating means for calculating;
When the value of the first distance information is smaller than the value of the second distance information, the color of the pixel is set to the second on the gray axis or closer to the gray axis than the first representative color. A program for functioning as a color conversion means for converting to a representative color.

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