JP2006237657A - Image processing apparatus, image correction program, and recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To effectively enhance the attractiveness of an image by correcting the lightness and the color saturation of the image. <P>SOLUTION: Image data are acquired from an input device 1 and an RGB value of the image data is converted into an HSV value in a CPU 3. A histogram is created for each of the lightness and the color saturation of the image data and the width of the gradation having a number-of-pixels ratio not grater than a threshold value is compressed. Thus, by enlarging the entire gradation width of the histogram in which part of the gradation width is compressed, the lightness and the color saturation are corrected, respectively. Moreover, in the histogram before compression of the gradation width, when the minimum gradation value and the maximum gradation value whose number-of-pixels ratio is other than 0 are respectively other than 0 and a full scale value, in the histogram with an enlarged gradation width, the minimum gradation value and the maximum gradation value whose number-of-pixels ratio is other than 0 are set to values other than 0 and the full scale value. Thus, the lightness and the color saturation are corrected. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image correction process for improving the appearance of a color image.
[0002]
[Prior art]
In an apparatus such as a scanner or a digital camera, light from an original or a subject is converted into a voltage signal corresponding to the amount of light by an image sensor such as a CCD, and the intensity of the voltage signal is converted into digital data to obtain image data. However, with the digital data obtained by converting the intensity of the voltage signal, the contrast of brightness and saturation is small, and the image may be a plain image with little sharpness. In order to solve this problem and improve the appearance of the image, various image correction methods have been proposed. For example, a frequency distribution or a cumulative frequency distribution is created for the lightness gradation, and the lightness gradation is corrected based on the distribution (Patent Documents 1 and 2), or image data is converted to hue, saturation, and lightness. There is a technique that corrects the dynamic range of lightness using the frequency distribution of lightness and corrects the saturation by adding a value corresponding to the correction result of the lightness to the saturation (Patent Document 3).
[0003]
[Patent Document 1]
JP 2000-306088 A [Patent Document 2]
JP-A-5-80193 [Patent Document 3]
Japanese Patent Laid-Open No. 8-32827
[Problems to be solved by the invention]
However, even if these image correction methods are used, it is not always possible to effectively improve the appearance of the image. For example, in the method described in Patent Document 1 or 2, since only lightness gradation is corrected, saturation contrast cannot be improved. Therefore, it is not possible to correct the gorgeousness of the color that is greatly affected by the saturation, which is not effective for improving the appearance.
[0005]
The present invention provides an image processing apparatus and an image correction program that improve the appearance of an image effectively by improving the contrast of saturation and lightness.
[0006]
[Means for Solving the Problems]
An image processing apparatus according to a first aspect of the present invention includes an input means for inputting color image data, a conversion means for converting color image data into hue, saturation and brightness, and for each gradation with respect to the total number of pixels with respect to saturation and brightness. And a histogram creation means for creating a saturation histogram and a brightness histogram, respectively, and correcting the saturation and the brightness based on the saturation histogram and the brightness histogram, respectively. Contrast is improved.
The invention according to claim 2 is applied to the image processing apparatus according to claim 1, and changes the gradation width of the saturation histogram and the brightness histogram based on the ratio of the number of pixels in the gradation, thereby adjusting the saturation and brightness. Each is to be corrected.
The invention according to claim 3 is applied to the image processing apparatus according to claim 2, and sets a threshold value for each ratio of the number of pixels in each gradation of the saturation histogram and the brightness histogram, and when the threshold value is less than the threshold value, A histogram compression unit that compresses the width of the gradation, and a histogram expansion unit that expands the width of the entire gradation of the saturation histogram and the lightness histogram in which a part of the gradation width is compressed by the histogram compression unit. An image processing apparatus, further comprising: correcting the saturation and the brightness by a histogram compression unit and a histogram expansion unit, respectively. The invention according to claim 4 is applied to the image processing apparatus according to claim 3, wherein the histogram enlargement means has a ratio of the number of pixels of 0 in the saturation histogram and the brightness histogram before the gradation width is compressed by the histogram compression means. Based on the minimum and maximum gradation values that are not, the minimum and maximum gradation values in which the ratio of the number of pixels in the saturation histogram and the lightness histogram in which the width of the entire gradation is expanded by the histogram expansion means are not zero are determined, When the minimum gradation value in the saturation histogram or lightness histogram before compression of the tone width is not 0, the gradation value is determined to be a value other than 0, and the gradation width before compression is compressed. If the maximum gradation value in the saturation histogram or brightness histogram is not the full scale value, determine the gradation value to a value other than the full scale value, and On the basis of the minimum and maximum grayscale values for saturation histogram and brightness histogram compressed part of the width of the gradation by the histogram compression means, and expands the width of the entire gradations.
The invention according to claim 5 is applied to the image processing apparatus according to claim 3 or 4, and the histogram compression means is such that the ratio of the number of pixels that is less than the threshold value in the saturation histogram or the brightness histogram is a predetermined ratio of the entire histogram. Thus, a threshold value is set.
According to a sixth aspect of the present invention, there is provided a color image correction program comprising: an input step for inputting image data; a conversion step for converting image data into hue, saturation and brightness; and each gradation for saturation and brightness relative to the total number of pixels. Saturation and brightness contrast by calculating the pixel ratio for each and creating a saturation histogram and brightness histogram, and correcting saturation and brightness respectively based on the saturation histogram and brightness histogram And a correction step for improving the computer.
The invention according to claim 7 is applied in the image correction program according to claim 6, and in the correction step, the gradation width of the saturation histogram and the brightness histogram is changed based on the ratio of the number of pixels in the gradation. This corrects the brightness and brightness respectively.
The invention of claim 8 is applied to the image correction program of claim 7, and the correction step sets a threshold value for the ratio of the number of pixels in each gradation of the saturation histogram and the brightness histogram, and is less than the threshold value. A histogram compression step that compresses the width of the gradation, and a saturation histogram and a lightness histogram in which a part of the gradation width is compressed by the histogram compression step, the histogram that expands the width of the entire gradation It comprises an enlargement step, and the saturation and lightness are corrected by the histogram compression step and the histogram enlargement step, respectively.
The invention according to claim 9 is applied to the image correction program according to claim 8, and in the histogram expansion step, the pixel number ratio in the saturation histogram and the brightness histogram before the gradation width is compressed by the histogram compression step is not zero. Based on the minimum and maximum gradation values, the minimum and maximum gradation values in which the ratio of the number of pixels in the saturation histogram and the brightness histogram in which the width of the entire gradation is expanded by the histogram expansion step is not 0 are determined, If the minimum gradation value in the saturation histogram or lightness histogram before compression of the width is not 0, the gradation value is determined to be a value other than 0, and the gradation before compression of the gradation width is determined. If the maximum gradation value in the brightness histogram or lightness histogram is not a full scale value, the gradation value is not a full scale value. In accordance with the determined minimum and maximum gradation values, the entire width of the gradation is expanded with respect to the saturation histogram and the lightness histogram in which a part of the gradation width is compressed by the histogram compression step. Is.
The invention according to claim 10 is applied to the image correction program according to claim 8 or 9, and in the histogram compression step, the ratio of the number of pixels that is less than the threshold value in the saturation histogram or the brightness histogram is a predetermined ratio of the entire histogram. Thus, a threshold value is set.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of an image processing apparatus according to the present invention is shown in FIG. The image processing apparatus in FIG. 1 receives color image data such as a captured image and corrects both the saturation and lightness of the image data based on the respective histograms, thereby improving the appearance of the image. Device. The image processing apparatus includes an input device 1, an interface 2, a CPU 3, a storage device 4, and a monitor 5, which are connected to each other by a bus line 6.
[0008]
The input device 1 is a device that supplies image data to be input to the image processing device. As the input device 1, for example, a scanner, a digital camera, a drive device of a storage device such as a CD-ROM or a memory card, or the like is used. Image data supplied from the input device 1 is input to the CPU 3 via the interface 2.
[0009]
The CPU 3 is a processor that executes an image correction processing program stored in the storage device 4 and corrects input image data. The storage device 4 stores an image correction processing program and also stores image data input to the CPU 3 from the input device 1 and image data corrected by the CPU 3. For example, an HDD or the like is used. Images based on these image data are displayed on the monitor 5.
[0010]
A flowchart of the image correction processing program executed in the CPU 3 is shown in FIG. When image data is input to the CPU 3, RGB / HSV conversion in step S1 is performed. In RGB / HSV conversion, R (red), G (green), and B (blue) values of input image data are converted to H (hue), S (saturation), and V (lightness) values. . A known method is used as the conversion method at this time.
[0011]
In the brightness histogram creation in step S2, a histogram is created for the brightness of the image data converted by the RGB / HSV conversion in step S1. Here, for example, it is assumed that the histogram shown in FIG. 4 is created. In this lightness histogram, the horizontal axis represents the lightness gradation (i) from 0 to 255, and the vertical axis represents the pixel number ratio (fi) in the lightness gradation i. The pixel number ratio is the ratio of the number of pixels of the lightness gradation to the total number of pixels of the image.
[0012]
In the lightness histogram compression in step S3, for the lightness histogram created by creating the lightness histogram in step S2, it is determined whether or not the pixel number ratio Fi is greater than or equal to the threshold value for each lightness gradation i, and less than the threshold value. The width of the lightness gradation of the part which is is compressed. This is performed as follows by the subroutine processing shown in the flowchart of FIG.
[0013]
In step S31 of FIG. 3, the temporary memory variable TEMP is reset to zero, and the process proceeds to step S32. In step S32, the lightness gradations i and j are set to i = 0 and j = 0. Here, the lightness gradation i indicates the lightness gradation of the original histogram created before the lightness gradation width is compressed, that is, in the lightness histogram creation in step S2 of FIG. The lightness gradation j indicates the lightness gradation of the lightness histogram after the lightness gradation width is compressed, that is, after the lightness gradation width is compressed by the processing of the flowchart of FIG. In step S33, the flag FLAG is set to the initial value FALSE.
[0014]
Thereafter, in step S34, the pixel number ratio Fi is compared with the calculated threshold value S. This threshold value S is calculated based on a threshold designation input (step S320) performed in advance by the user. At this time, for example, it can be specified whether the threshold value is a constant or a threshold value corresponding to the lightness gradation. If it is a constant, the value input by the user is set as the threshold value S. On the other hand, when setting the threshold value according to the lightness gradation, the user designates the threshold value S as a function S (i) of the lightness gradation i. By this function S (i), the threshold value is calculated according to the lightness gradation. Furthermore, the user may be automatically set according to the image without inputting or specifying such a threshold value. For example, the threshold value S is set so that the total of the pixel number ratio Fi that is less than the threshold value becomes a predetermined ratio such as 10% of the entire histogram.
[0015]
If the pixel number ratio Fi is less than the threshold value S in step S34, the flag FLAG is set to TRUE in step S35. Further, in step S36, the pixel number ratio of the current lightness gradation i to the variable TEMP in the temporary memory. Add Fi. Thereafter, in step S37, the variable TEMP is compared with the threshold value S. When step S37 is executed for the first time, the determination result is always NO.
[0016]
If a negative determination is made in step S37, the variable TEMP is stored in the variable Gj in step S37A, and the process proceeds to step S38. In step S38, 1 is added to the lightness gradation i. In step S39, it is determined whether the lightness gradation i is 255 or more. If step S39 is negative, the process returns to step S34. At this time, if a negative determination is made in step S34, steps S35, S36, S37, S38, and S39 are repeated.
[0017]
If step S37 is positive, in step S310, the value of variable TEMP is stored in variable Gj. That is, when the pixel number ratios Fi of a plurality of continuous lightness gradations are both less than the threshold value S, those pixel number ratios Fi are integrated in step S36. When the integral value is equal to or greater than the threshold value S, the integral value is set as the pixel number ratio Gj in the lightness gradation j of the corrected histogram in step S310.
[0018]
When the process proceeds from step S310 to step S311, the variable TEMP is reset to zero, and 1 is added to the lightness gradation j in step S312. Next, in step S319, the flag FLAG is set to FALSE. Thereafter, the process returns to step S34 via steps S38 and S39. When step S34 is negative, the operation is performed as described above.
[0019]
On the other hand, if step S34 is affirmed, that is, if the pixel number ratio Fi for the current lightness gradation i is equal to or greater than the threshold value S, it is determined in step S313 whether the flag FLAG is TRUE. This step S313 changes the subsequent processing procedure depending on whether or not the pixel number ratio Fi at the previous processing cycle, that is, the lightness gradation 1 smaller than the current lightness gradation i is equal to or greater than the threshold value S. Is provided to do. If the pixel number ratio Fi in the previous processing cycle is greater than or equal to the threshold value S, a negative determination is made in step S313, and in step S314, the pixel number ratio Fi of the current lightness gradation i is stored in the variable Gj, and step S312 is performed. 1 is added to the lightness gradation j. Thereafter, the process returns to step S34 via steps S319, S38, and S39. When a negative determination is made in step S313, the operation is performed as described above.
[0020]
If the pixel number ratio Fi in the previous processing cycle is less than the threshold value S, an affirmative determination is made in step S313, and in step S315, the value of the variable TEMP is stored in the variable Gj. In step S316, the pixel number ratio Fi is stored in the variable Gj + 1. Further, in step S317, the variable TEMP is reset to zero, and 2 is added to the lightness gradation j in step S318. Thereafter, the flag FLAG is set to FALSE in step S319, and the process returns to step S34 via steps S38 and S39. When step S313 is affirmed, the operation is performed as described above.
[0021]
In this way, the process is performed for all the lightness gradations i. Finally, an affirmative determination is made in step S39, and the process flow shown in FIG. 3 ends. In the processing described above, the pixel number ratio Fi that does not exceed the threshold value S is integrated in step S36, and stored in the variable Gj in step S37A, S310, or S315. By creating a histogram of the pixel number ratio Gj by the variable Gj for the lightness gradation j, the pixel number ratio Fi is less than the threshold in the histogram of the pixel number ratio Fi for the lightness gradation i before processing. The brightness gradation width of the portion is compressed.
[0022]
FIG. 5 shows a result obtained by compressing the gradation width of the brightness histogram of FIG. 4 by the processing of the flowchart of FIG. 3 described above. In the histogram of FIG. 5, the horizontal axis represents the lightness gradation j, and the vertical axis represents the pixel number ratio Gj by the variable Gj. Note that the lightness gradations indicated by α and β in FIG. 5 are the minimum value and the maximum value of the lightness gradation j whose pixel number ratio Gj is not 0, and are used in the process of step S4 described below.
[0023]
In the brightness histogram expansion in step S4 in FIG. 2, the gradation width of the brightness histogram compressed in the brightness histogram compression in step S3 is expanded. This is done as follows. First, in the lightness histogram of FIG. 4, that is, the lightness histogram before gradation width compression, the minimum value and the maximum value of the lightness gradation i whose pixel number ratio Fi is not 0 are obtained. Here, for example, it is assumed that the minimum value A ₀ and the maximum value B ₀ shown in FIG. 4 are obtained.
[0024]
Next, based on the obtained minimum value A ₀ and maximum value B ₀ , the lower limit value A and the upper limit value B of the lightness gradation i for expanding the compressed gradation width are determined. The lower limit A and the upper limit B are used when the minimum value A ₀ of the lightness gradation i is not 0 in the original histogram or when the maximum value B ₀ is not a full scale value, that is, there is no black or white in the original image. In this case, it is determined by a relationship obtained experimentally or statistically so that black and white do not exist even after the image correction process and the appearance of the image is improved. The relationship can be represented by a curve 70 as shown in FIG. Based on this relationship, based on the minimum value A ₀ and the maximum value B ₀ of the lightness gradation in the original histogram shown on the horizontal axis, the lower limit value A and the upper limit value of the lightness gradation when the gradation width is expanded shown on the vertical axis. B is determined. In this way, when black or white does not exist in the original image, black or white is prevented from existing in the corrected image. Thus, by expanding the brightness gradation width of the frequency distribution from zero to a full scale value in the method described in Patent Document 1, black and white are generated even in an image that originally does not have black or white, and the appearance of the image Can be prevented from changing greatly.
[0025]
Based on the lower limit value A and the upper limit value B determined in this way, the lightness gradation width of the histogram compressed in step S3 in FIG. 2 is expanded using the following conversion equation (1). At this time, the value of the lightness gradation j is converted by the equation (1), and the value of the pixel number ratio Gj in the lightness gradation j is left as it is, so that the width of the lightness gradation can be expanded. Note that α and β in Expression (1) are the minimum value α and the maximum value β of the lightness gradation j where the pixel number ratio Gj in FIG.
[Expression 1]
C (j) = (j−α) · {(B−A) / (β−α)} + A (1)
Where C (j): lightness gradation after conversion
As described above, the gradation width of the compressed brightness histogram is expanded. FIG. 6 shows an enlargement of the gradation width of the histogram of FIG. 5 by this processing. By enlarging the gradation width in this way, it is possible to enhance the contrast of the image and obtain a good-looking image.
[0027]
In this manner, a lightness histogram with lightness corrected in the lightness histogram creation in step S2, lightness histogram compression in step S3, and lightness histogram expansion in step S4 is created. The same processing is also performed for the saturation in the creation of the saturation histogram in step S5, the saturation histogram compression in step S6, and the expansion of the saturation histogram in step S7, thereby creating a saturation histogram with the corrected saturation. . In this way, by correcting the lightness and the saturation with the respective histograms, by adding the value determined by the lightness correction result in the method described in Patent Document 3 to the saturation, it is possible to reduce the originally low saturation portion. Can prevent over-saturation. In the HSV / RGB conversion in step S8, the corrected image data based on the brightness and saturation histograms are converted from HSV values to RGB values. The converted image data is output from the CPU 3 and stored in the storage device 4 or the image is displayed on the monitor 5.
[0028]
The image processing apparatus described above has the following operational effects.
(1) By correcting the saturation and the brightness based on the saturation histogram and the brightness histogram, respectively, it is possible to improve the contrast of the saturation and the brightness and effectively improve the appearance of the image.
(2) The saturation and brightness are corrected by changing the gradation width of the saturation histogram and the brightness histogram based on the ratio of the number of pixels in the gradation. At this time, if the ratio of the number of pixels in each gradation of the histogram is less than the set threshold value, the gradation width is compressed, thereby expanding the entire gradation width of the histogram in which a part of the gradation width is compressed. To correct the saturation and brightness respectively. Since it did in this way, since saturation and lightness can be correct | amended according to the pixel number ratio in each gradation of the histogram, the contrast of saturation and lightness can be improved appropriately according to an image.
(3) Minimum and maximum gradation values in which the pixel number ratio is not 0 in the histogram when the gradation width is expanded based on the minimum and maximum gradation values in which the pixel number ratio is not 0 in the histogram before gradation width compression If the former gradation value is not 0 or a full scale value, the latter gradation value is set to 0 or a value other than the full scale value. Since it did in this way, when black and white do not exist in an original image, black and white do not exist after an image correction process, and the appearance of an image can be improved.
(4) Since the threshold value is set so that the ratio of the number of pixels that is less than the threshold value in the histogram becomes a predetermined ratio of the entire histogram, the image correction process can be easily performed without a troublesome operation by the user. Can be executed.
[0029]
In the embodiment described above, the image correction processing program executed to realize the present invention is recorded on the recording medium of the storage device 4, and an example in which the CPU 3 executes this image correction processing program will be described. However, the present invention is not limited to this content. For example, the image correction processing program as described above may be stored in the input device 1 and executed. Further, this image correction processing program may be taken into a personal computer or an input device using a recording medium. Alternatively, the image correction processing program may be taken into a personal computer or an input device through an electric communication line such as the Internet.
[0030]
In the embodiment described above, the input unit is realized by the input device 1, and the conversion unit, the histogram creation unit, the histogram compression unit, and the histogram enlargement unit are realized by the CPU 3. However, these are merely examples, and each component is not limited to the above-described embodiment as long as the characteristics of the present invention are not impaired.
[0031]
【The invention's effect】
According to the present invention, since it is configured as described above, it is possible to effectively improve the appearance of a color image.
[Brief description of the drawings]
FIG. 1 is a diagram showing an embodiment of an image processing apparatus according to the present invention. FIG. 2 is a diagram showing a flowchart of an image correction processing program. FIG. 3 is a diagram showing a subroutine processing flowchart in lightness histogram compression. FIG. 5 is a diagram showing an example of a brightness histogram with a compressed gradation width. FIG. 6 is a diagram showing an example of a brightness histogram with an expanded compressed gradation width. The figure which shows the example of the relationship between the minimum value and the maximum value of the lightness gradation when the pixel number ratio in the lightness histogram is 0, and the minimum value and the maximum value of the lightness gradation when expanding the compressed gradation width Explanation of]
1: Input device 2: Interface 3: CPU
4: Storage device 5: Monitor 6: Bus line

Claims (10)

Input means for inputting color image data;
Conversion means for converting the color image data into hue, saturation and brightness;
Histogram creation means for calculating a ratio of the number of pixels for each gradation with respect to the total number of pixels for the saturation and brightness, and creating a saturation histogram and a brightness histogram,
An image processing apparatus, wherein the saturation and brightness contrast are improved by correcting the saturation and brightness based on the saturation histogram and the brightness histogram, respectively. The image processing apparatus according to claim 1.
An image processing apparatus, wherein the saturation and brightness are respectively corrected by changing a gradation width of the saturation histogram and the brightness histogram based on a pixel number ratio in the gradation. The image processing apparatus according to claim 2.
Histogram compression means for setting a threshold value for the ratio of the number of pixels in each gradation of the saturation histogram and the lightness histogram, and compressing the gradation width when it is less than the threshold value;
A histogram expansion unit that expands a width of the entire gradation with respect to the saturation histogram and the lightness histogram in which a part of the gradation width is compressed by the histogram compression unit, and
An image processing apparatus, wherein the saturation and lightness are corrected by the histogram compression means and histogram enlargement means, respectively. The image processing apparatus according to claim 3.
The histogram enlargement means is based on the minimum and maximum gradation values in which the pixel number ratio is not 0 in the saturation histogram and the brightness histogram before the gradation width is compressed by the histogram compression means. Determining the minimum and maximum gradation values in which the ratio of the number of pixels in the saturation histogram and the lightness histogram in which the width of the entire gradation is expanded is not 0,
When the minimum gradation value in the saturation histogram or lightness histogram before compression of the gradation width is not 0, the gradation value is determined to a value other than 0,
When the maximum gradation value in the saturation histogram or lightness histogram before compression of the gradation width is not a full scale value, the gradation value is determined to a value other than the full scale value,
Based on the determined minimum and maximum gradation values, with respect to the saturation histogram and lightness histogram in which a part of the gradation width is compressed by the histogram compression means, the width of the entire gradation is expanded. A featured image processing apparatus. The image processing apparatus according to claim 3 or 4,
The histogram compression means sets the threshold value such that a ratio of the number of pixels that is less than the threshold value in the saturation histogram or brightness histogram is a predetermined ratio of the entire histogram. apparatus. An input step for inputting image data;
A conversion step of converting the image data into hue, saturation and brightness;
A histogram creation step of calculating a ratio of the number of pixels for each gradation with respect to the total number of pixels for the saturation and brightness, and creating a saturation histogram and a brightness histogram;
An image correction program for a color image for executing, by a computer, a correction step for improving the saturation and brightness contrast by correcting the saturation and brightness based on the saturation histogram and brightness histogram, respectively. In the image correction program of claim 6,
In the correction step, the saturation and the brightness are corrected by changing the gradation width of the saturation histogram and the brightness histogram based on the pixel number ratio in the gradation, respectively. Image correction program. In the image correction program of claim 7,
The correction step sets a threshold value for the ratio of the number of pixels in each gradation of the saturation histogram and the lightness histogram, and compresses the width of the gradation when it is less than the threshold value; ,
A saturation histogram and a lightness histogram in which a part of the gradation width is compressed by the histogram compression step, and a histogram expansion step for expanding the entire gradation width,
An image correction program for a color image, wherein the saturation and brightness are corrected by the histogram compression step and the histogram expansion step, respectively. The image correction program according to claim 8, wherein
The histogram expansion step is based on the minimum and maximum gradation values in which the pixel number ratio is not 0 in the saturation histogram and the brightness histogram before the gradation width is compressed by the histogram compression step. Determining the minimum and maximum gradation values in which the ratio of the number of pixels in the saturation histogram and the lightness histogram in which the width of the entire gradation is expanded is not 0,
When the minimum gradation value in the saturation histogram or lightness histogram before compression of the gradation width is not 0, the gradation value is determined to a value other than 0,
When the maximum gradation value in the saturation histogram or lightness histogram before compression of the gradation width is not a full scale value, the gradation value is determined to a value other than the full scale value,
Based on the determined minimum and maximum gradation values, for the saturation histogram and lightness histogram in which a part of the gradation width is compressed by the histogram compression step, the width of the entire gradation is expanded. An image correction program for featured color images. The image correction program according to claim 8 or 9,
The histogram compression step sets the threshold value such that a ratio of the number of pixels that is less than the threshold value in the saturation histogram or the brightness histogram is a predetermined ratio of the entire histogram. Image correction program.

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