JP2003069822A - Image processor, image processing method and recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To appropriately correct a main object in an image photographed in an inappropriate exposure state without causing overcorrection while maintaining a gradation property. SOLUTION: A gradation correction processing part 102 calculates gradation correction quantity on the basis of lightness information of an object area and prepares a gradation correction curve with respect to image data inputted from an image input device 100 such as a digital camera. Meanwhile, the gradation correction processing part 102 extracts a control area from a lightness area to which the object area does not belong to decide whether or not correction quantity in the control area is appropriate, and uses a control condition (such as the slope of a tangent line of the gradation correction curve with a lower limit) of the control area to control the gradation correction quantity when the correction quantity in the control area is not appropriate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus in an image output system, a printer system, an image display system or the like for automatically correcting the gradation of color image data, and is applied to a digital camera, a scanner or the like. Is.

[0002]

2. Description of the Related Art In recent years, due to the widespread use of personal computers, the Internet, household printers, and the increase in storage capacity of hard disks, there are increasing opportunities to handle photographic images with digital data. Along with this, digital cameras, film scanners, etc. are generally used as input means for digital photographic image data (hereinafter referred to as image data).

A digital camera is provided with an automatic exposure control device for always keeping the optimum exposure during photographing. There are various exposure control methods, but in general, the screen is divided into multiple appropriate areas for light intensity detection, weighted averages are taken for each area, and the aperture and shutter speed are adjusted. Target.

However, these exposure control methods vary from company to company, and there are cases where they do not operate properly depending on the shooting conditions, so there is no perfect method. In particular, when there is a light source directly behind the main subject and there is a large difference in brightness between the background and the main subject, and in the backlight condition, if the subject does not have an exposure, the background brightness will pull the exposure and correct the exposure to a negative value.
The subject appears dark.

Since nighttime stroboscopic photography is premised on that the subject is exposed to stroboscopic light, the aperture and shutter speed are fixed to predetermined values. In other words, if the distance between the strobe and the subject is too far, the light does not reach, and in this case the subject also appears dark.

[0006] Here, the exposure correction means adjusting an object having an inappropriate brightness for a scene to a brightness suitable for the scene. For example, it is to lighten a subject that is dark due to insufficient exposure, a subject that is dark due to backlight, or to darken an overexposed subject.
Exposure compensation in a camera is generally performed by changing an aperture and a shutter speed in order to adjust the amount of incident light entering a lens. In a printer or a display, it means a process for optimizing the brightness of an output signal with respect to the brightness of an input signal by using an input / output conversion function (gradation correction curve) or the like. It should be noted that since the purpose of correcting the exposure and adjusting the brightness of the image data is the same as that of the gradation correction processing of the present invention, it will be referred to as gradation correction processing hereinafter.

In order to deal with such an inappropriate exposure state at the time of photographing a digital image, many techniques for automatically correcting gradation of image data have been proposed.

For example, the entire original image is read to calculate the main image characteristic amount of the original image, the predetermined density range of the subject is calculated from the calculated main image characteristic amount, and only the pixels within the predetermined density range are calculated from the entire original image. There is an exposure amount determination method that appropriately corrects an image with a large density difference between the background and the subject, such as a backlight image, by extracting the original image feature amount and determining the exposure amount from the original image feature amount. (See Japanese Patent Laid-Open No. 7-301867).

There is also an image processing method in which backlight conversion is performed by creating a conversion curve that does not cause saturation in the entire image (see Japanese Patent Laid-Open No. 10-79885).

[0010]

What is most difficult in performing the gradation correction process is to appropriately correct the brightness of the object and maintain the gradation, while preventing overcorrection. In other words, if the correction amount is obtained only from the subject part as in the conventional case,
Gradation loss and hue shift occur in the image after the correction process.

The present invention has been made in view of the above problems, and an object of the present invention is to perform overcorrection while preserving gradation in a main subject of an image photographed in an inappropriate exposure state. An object of the present invention is to provide an image processing apparatus, an image processing method, and a recording medium that can be appropriately corrected with the maximum effect without causing them.

[0012]

According to the present invention, a subject area extracting means for extracting a main subject area, a correction amount calculating means for calculating a gradation correction degree from information on the subject area,
A control area extracting means for extracting a predetermined control area from a lightness area to which the main subject area does not belong; a determining means for determining whether or not the gradation correction degree is controlled using a predetermined control condition in the control area; By providing the control means for controlling the gradation correction degree according to the judgment result of the means, while correcting the gradation of the subject area, overcorrection of the highlight area or the shadow area is prevented, and the scene or the image output device is provided. The corresponding gradation correction is performed.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be specifically described below with reference to the drawings. (Embodiment 1) FIG. 1 shows the overall construction of Embodiment 1 of the present invention. In FIG. 1, an image input device 100 such as a scanner, a digital camera or a video camera outputs to the image processing device 101 image data of a photographed image in which a photograph or the like is represented as pixels in a matrix. The gradation correction processing unit 102 in the image processing apparatus 101 calculates an optimum evaluation value,
The gradation correction process is executed using the evaluation value. The image output device 103 such as a display or a printer outputs the image data that has been subjected to the gradation correction processing. In the case of the present embodiment, photographed data such as a photograph is suitable as the image data. In the present embodiment, the image data will be described below as color data.

FIG. 2 shows a gradation correction processing unit 10 according to the present invention.
2 shows the configuration. First, the following processing is executed on the image data 200 input by the image input device 100. The subject area extracting means 201 extracts the main subject area on the image data. Here, the extraction of the subject area may be performed by using a known technique that has been conventionally proposed.

Next, the control area extracting means 202 obtains a control area from the lightness area other than the lightness area to which the subject area belongs in the lightness distribution. In the following description, it is assumed that the input image to be corrected in this embodiment is backlit, and the brightness is used as the brightness information.

In the case of a backlit image, the subject is dark against the background and is underexposed. Therefore, the control area is a brighter area than the brightness area to which the subject area belongs. FIG. 3 shows the subject region of the backlight image extracted by the subject region extracting means. Further, FIG. 4 shows a histogram of the luminance area to which the subject area belongs and the control area.

Next, the process for obtaining the gradation correction curve f0 (x) will be described with reference to FIGS. Here, in order to simplify the explanation, the minimum luminance value sh of the input image data sh
Is sh = 0, and the maximum value hl is hl = 255.

The correction amount calculating means 206 calculates a statistic value 204 such as a median value (hereinafter, the median value A is used as a statistic value for explanation) from the extracted luminance distribution 203 of the main photographic subject region, and calculates the main object. Get a characteristic brightness.

Subsequently, the correction amount calculation means 206 determines whether or not the brightness of the subject is appropriate based on the median value A, and if not, obtains a temporary correction amount δ205. It should be noted that since it is assumed here that the exposure is insufficient due to the backlight condition,
> 0, for example δ as a function of the median brightness
Set as follows.

Next, the gradation correction curve f0 (x) is calculated. If δ = 0, then f0 (x) = x.

The case where δ ≠ 0 will be described. The gradation correction curve f0 (x) is represented by a combination of three linear functions, as shown in FIG.
1 (x), 12 (x), and 13 (x) are calculated as follows for each luminance region. y = f0 (x) l1 (x) = α1 · x (0 ≦ x <A) = l2 (x) = α2 · x + β2 (A ≦ x <C) l3 (x) = α3 · x + β3 (C ≦ x < 255) Expression (1) Note that α1 = {(A + δ) / A} α2 = 1, β2 = δ, (A ≦ x <C) α3 = {(255−C−δ) / (255−C)}, β3 = {(255 · δ) / (255−C)} Formula (2), and in the calculation 207 of f0 (x), the temporary functions are l1 (x), l2 (x), l3 according to the correction amount δ. It is calculated in the order of (x).

That is, from the shadow point to the median value A of the input luminance value, l1 (x) connecting the coordinates (0, 0), (A, B) (where B = A + δ), the luminance area of the main subject area Up to the maximum value C at 12 (x) that keeps the slope 1 of the curve, and the coordinates (C,
D3, (255, 255), (where D = C + δ) is connected to l3 (x) as a gradation correction curve. FIG. 6 shows a gradation correction curve.

Next, the judging means 208 judges whether or not the correction amount δ in the control range is appropriate. In the gradation correction curve f0 (x) calculated from the correction amount δ, l3
If the gradient α3 of (x) is too small, gradation collapse may occur in the highlight after gradation correction. Therefore, a lower limit of the threshold value Th is set for α3, and when α3 <Th, α3 is set.
≡Th (209). Then, in the case of α3≡Th, f0 (x) calculated by the gradation correction curve calculation unit 207
Instead of, a new gradation correction curve f1 (x) is calculated (210).

That is, a new gradation correction curve f1
(X) is represented by a combination of three linear functions, and the respective temporary functions l1 (x), l2 (x), l3
(X) is calculated in the order of l3 (x), l2 (x), and l1 (x).

L3 (x) has an inclination Th and coordinates (255,
Since it is a function that passes through (255), α3 = Th, β3 = 255 (1-Th) Equation (3). At this time, the output C ′ of the input C (FIG. 8) can be calculated using the equation (3), and C ′ = Th · C + 255 (1−Th).

L2 (x) is a gradient 1 and coordinates (C, C ')
.Alpha.2 = 1 and .beta.2 = (255-C) (1-Th) equation (4) because it is a function that passes through. At this time, the output A ′ of the input A (FIG. 8) can be calculated using the equation (4), and A ′ = A + (255−C) (1-Th).

L1 (x) is the coordinates (A, A '), (0,
Since it is a function that passes through 0), α1 = {A + (255-C) (1-Th)} / A Formula (5) As a result, a new gradation correction curve f1 (x)
Was calculated.

Therefore, in the gradation correction processing means 211, when the result of the judgment of the judgment means 208 is α3 ≧ Th, f0 (x) calculated by the gradation correction curve calculation unit 207 is used,
When α3 <Th, the gradation correction processing is performed using the new gradation correction curve f1 (x).

F0 (x) is used as the gradation correction curve,
The gradation correction processing of RGB color image data will be described below.

Input luminance value Yin (j) of image data (j
= 1, 2 ,. ． ． N and N define the output luminance value Y1 (j) after correction by the gradation correction curve f0 (x) for the number of pixels of the input data). Here, x is the input luminance value of the image data. The input brightness value is calculated by the following formula. The luminance Yin (j) of the j-th pixel (j = 1, 2, ... N-1, N) is the color image signal (Rin (j),
It can be expressed by the following equation using Gin (j) and Bin (j). Yin (j) = 0.299 · Rin (j) + 0.587 · Gin (j) +0.1 14 · Bin (j) Formula (6) After the gradation correction processing for the input luminance value Yin (j) The output luminance value Y1 (j) is calculated, and the gradation correction coefficient C0 (j) is calculated by the following formula. C0 (j) = Y1 (j) / Yin (j) = f0 (Yin (j)) / Yin (j) Formula (7) Input color image signal (Rin (j), Gin (j), B
in (j)) is converted by the following equation to obtain gradation correction color signals (R1 (j), G1 (j), B1 (j)). (R1 (j), G1 (j), B1 (j)) = C0 (j). (Rin (j), Gin (j), Bin (j)) Equation (8) Further, the input image signal is gray. , The gray level Yi
With n (j) as an input value, the output after tone correction processing may be obtained as an output gray level Y1 (j) = f0 (Yin (j)).

In normal image processing, clipping is performed when the reproduction range of the output device (display in this embodiment) 0 to 255 is exceeded. That is, the value less than 0 is replaced with 0, and the value greater than 255 is forcibly replaced with 255.
The above gradation correction processing is performed by the gradation correction processing means 211,
The corrected output image data 212 is obtained.

The gradation correction curve has an exponential function f as shown in FIG.
The case represented by 0 (x) will be described. First, a statistical value such as a median value is calculated from the extracted luminance distribution of the main subject area to obtain the characteristic brightness of the main subject.

Next, it is determined whether or not the brightness of the subject is appropriate based on the median value A, and if not, a correction amount δ is obtained. Note that here, it is assumed that the exposure is insufficient due to the backlit state, and thus 1>δ> 0. Next, the gradation correction curve f
Get 0 (x). y = f0 (x) = {x ^ δ} · 255 Equation (9) Next, for example, the gradient of the tangent line of f0 (x) at x = C, or the first-order differential coefficient d (f0 (x)) / dx ] (X = C) = lim (δ → 0) {(f0 (C + δ) −f0 (C)) / δ} = δ · C ^ (δ−1) · 255 Equation (10) is calculated and its value is calculated. Is greater than or equal to the threshold Th, and if it is greater than or equal to the threshold Th, the right side of the equation (10) is set to Th and the final correction amount Δ is obtained. That is, Δ satisfying the following formula Δ · C ^ (Δ−1) · 255 = Th formula (11) is set as a new correction amount.

The above embodiment is an example for preventing overcorrection of highlight when correcting a backlit image, but the control method is the same in the case of shadow. That is,
An upper limit is set for α1 to suppress the generation of noise caused by the gradation correction processing, and a lower limit is set for α1 to suppress the collapse.

In this embodiment, the threshold Th is changed according to the scene of the image. Here, a scene refers to a state when an image is captured by a camera, and includes, for example, a backlight state, night flash photography, and underexposure state. If the input image is a backlit image, the subject may be located on the low brightness side and the background may have overexposure.
The gradation collapse of the highlight that is the background is not noticeable. Also, in the case of night flash photography, the subject is located on the highlight side, and gradation reproducibility is important. Therefore, the control conditions for the backlight image and the night flash image are set to Th.
When b and Thf are set, Thb <Thf is determined.

Further, in this embodiment, the threshold Th is changed according to the color reproducibility of the output device. For example, if the output image device is a monitor, the gradation reproducibility is good, and if it is a printer, the gradation reproducibility of highlights and shadows is particularly bad. Therefore,
Monitor and printer control conditions are Thm and Thp, respectively.
Then, it is determined that Thm <Thp.

Although the above embodiment has been described by using the luminance, the L value or R of the L * a * b color system is used instead of the luminance.
The G value of GB may be used. Further, various methods have been proposed as a method of extracting a subject, and any of them may be used. Further, although the median value is used as the statistic for calculating δ, an average value or the like may be used, and the control condition may be appropriately determined according to the output condition.

(Embodiment 2) Embodiment 2 is an embodiment in which the present invention is applied to an image output system. FIG. 9 shows a configuration according to the second embodiment of the present invention. In FIG. 9, 300 is
It is a computer for controlling the image output control device. When the operator selects the printer (1) 304 and receives an output command, the computer 300 captures an image and captures image data and various DTs.
Image data such as both monochrome and color photographs created by the P software is transmitted to the image output control device (1) 305.

The image output control device (1) 305 analyzes the image data transmitted from the computer 300, and outputs a plurality of output color components C (Cyan), M (which are control signals of an image forming device such as a color copying machine. Mazenta), Y (Y
yellow) and K (black K) are subjected to color conversion processing described later, the result is transferred to the image forming apparatus, and the image forming apparatus prints the transmitted image.

The image output control device (1) 305 performs gradation correction processing on the input image data and converts it into a control signal for the image forming apparatus (1).
301 and drawing means (1) 30 for performing halftone processing, etc.
2 and an image storage unit (storage buffer) (1) 303 for temporarily storing the print image for one page drawn by the drawing unit (1) 302.

The above-mentioned image output control device is a color conversion means for obtaining a printer output faithful to a display image in accordance with the characteristics of the image display apparatus and the characteristics of the printer, and resolution conversion means for converting the resolution corresponding to each model. Etc. may be used as a component.

The image data input to the above image output control device is specifically RGB (green, blue, red) gradation data, and the number of gradations of each color component is 8 bits.
= 256 gradations are generally used, but other gradation numbers such as 64 or 512 may be used.

The operation of the image output control device (1) 305 will be described below. Image data is color conversion means (1) 301.
Then, the color conversion means (1) 301 performs gradation correction processing according to the color reproduction range of the image forming apparatus (printer) (1) 304. Next, color conversion is performed based on the characteristics of the image display apparatus and the characteristics of the image forming apparatus (printer) used by the operator, and a plurality of output color components C ( Cya
n), M (Mazenta), Y (Yellow), K
It is converted into (blacK) data and sent to the drawing means and the image storage means (storage buffer).

From the gradation data of RGB (green, blue, red),
Output color components C (Cyan), M (Mazenta), Y
The conversion into (Yellow) and K (blacK) data is color conversion into CMY by memory map interpolation. As shown in FIG. 10, when the RGB space is used as the input color space, the memory map interpolation is that the RGB space is divided into three-dimensional figures (here, cubes) of the same type, and the output value P at the input coordinates (RGB) is divided. In order to obtain, the cube including the coordinates of the input is selected, and the output values on the preset vertices of the eight points of the selected cube and the position of the input in the cube (distance from each vertex) Based on, linear interpolation is performed.
Here, in the case of the present embodiment, the output value P corresponds to the C, M, and Y values, respectively, and the actual input / output (L * a) is assigned to the coordinates (RGB) on the input space used for the interpolation calculation. * B * -CM
The values of C, M, and Y for RGB (L * a * b *) calculated by the least-squares method by measuring the relationship of Y) and using this data are preset. Further, the CMY signal is converted into a CMYK signal by an operation such as the following equation. K = α × min (C, M, Y) C ′ = C−β × K M ′ = M−β × K Y ′ = Y−β × K Note that the color conversion method is not limited to the above-described method, and may be an image. For example, the computer may record the characteristics of the display device in the header information of the image data and transmit the information. When the image output control device is mounted inside the computer, for example, an ICC (Inter Color Cons) may be used.
Ortium) standardized device profile may be read and used.

The image correction processing means 417 in the color conversion means 301 will be described with reference to FIG. The image correction processing means 417 is basically the same as that described in the first embodiment, and therefore its description is omitted. However, in this embodiment, the threshold information table 411 is provided. That is, since the reproduction range of each printer is different, the threshold value is also different for each printer. Similarly, the threshold value varies depending on the ink characteristics of the printer and the halftone processing performed thereafter. Therefore, in the color conversion means, the table 411 is provided with threshold information regarding the printers to be output.

Using the gradation correction curve calculated in the same manner as in the first embodiment, the grid point data of the color conversion table stored in the color conversion table storage unit 414 is rewritten (413). CMYK conversion coefficients are stored at each grid point in the RGB space. It is possible to speed up the process by rewriting the grid point data instead of performing the gradation correction on the RGB values of each pixel of the input image data.

Next, for the input image data, the rewritten color conversion table is selected from the color conversion table storage unit and sent to the interpolation calculation unit 415. In the interpolation calculation unit 415,
By referring to the color conversion table, for example, the memory map interpolation as described above is performed, and the image data is sent to the drawing means 416.

The image output control device may be mounted inside the color printer for color conversion, or may be mounted inside the computer. When implemented in a computer, the color conversion table described above includes information on all printers that can be output from the computer. Alternatively, the image data may be sent to the printer after being mounted in a printer control device provided independently of the color printer to perform color conversion.

Further, this embodiment can be executed by software, and in this case, it can be executed by a printer driver existing as a program in the computer.

FIG. 12 shows a system configuration example when the present invention is implemented by software. In this image output system, a workstation and a printer are connected.
The workstation realizes the function of the gradation correction processing described above, and includes a display, a keyboard, a program reading device, an arithmetic processing device, and the like.
The arithmetic processing unit is a CPU capable of executing various commands.
Further, ROM and RAM are connected by a bus. Further, a DISK, which is a mass storage device, and an NIC for communicating with devices on a network are connected to the bus.

The program reading device is a storage medium storing various program codes, that is, a floppy (registered trademark) disk, a hard disk, an optical disk (CD).
-ROM, CD-R, CD-R / W, DVD-ROM,
A device for reading a program code stored in a DVD-RAM, etc., a magneto-optical disk, a memory card, etc., and is, for example, a floppy disk drive, an optical disk drive, a magneto-optical disk drive, etc.

The program code stored in the storage medium is read by the program reading device and stored in the DISK or the like. By executing the program code stored in the DISK or the like by the CPU, the above-described image processing method or the like is performed. To be realized. Further, by executing the program code read by the computer, an OS (operating system) or device driver operating on the computer performs a part or all of the actual processing based on the instruction of the program code. ,
The case where the above-mentioned functions are achieved by the processing is also included.

[0053]

As described above, according to the present invention, the following effects can be obtained. (1) By controlling the correction amount for gradation correction using the information of the lightness range to which the subject does not belong, it is possible to perform correction without causing gradation collapse. (2) By setting the lightness range brighter than the lightness range to which the subject area belongs to as the control area, it is possible to prevent the gradation gradation loss and the shadow gradation loss caused by overcorrection. (3) In the gradation correction processing using the gradation correction curve,
By setting a lower limit on the inclination of the tangent line of the gradation correction curve, it is possible to prevent the gradation from being overshadowed or the gradation being overshadowed due to overcorrection. (4) By providing an appropriate control condition for a scene, it is possible to perform gradation correction according to the scene. (5) By providing appropriate control conditions for the image output device, it is possible to perform gradation correction according to the image output device. (6) By providing an appropriate control condition for the characteristics of the image to be corrected, it is possible to perform gradation correction according to the image output device. (7) By obtaining the correction amount from the component representing the brightness belonging to the subject area, it is possible to obtain the tone correction amount suitable for the subject.

[Brief description of drawings]

FIG. 1 shows an overall configuration of a first embodiment of the present invention.

FIG. 2 shows a configuration of a gradation correction processing unit according to the present invention.

FIG. 3 shows a subject region of a backlight image extracted by a subject region extracting means.

FIG. 4 shows histograms of a brightness area and a control area to which a subject area belongs.

FIG. 5 shows the correction amount δ as a function of the median luminance value.

FIG. 6 shows a first example of a calculated gradation correction curve.

FIG. 7 shows a gradation correction curve represented by an exponential function.

FIG. 8 shows first and second examples of calculated gradation correction curves.

FIG. 9 shows a configuration according to a second exemplary embodiment of the present invention.

FIG. 10 is a diagram illustrating memory map interpolation.

FIG. 11 shows a configuration of image correction processing means in the color conversion means.

FIG. 12 shows a system configuration example when the present invention is implemented by software.

[Explanation of symbols]

100 image input device 101 image processing apparatus 102 gradation correction processing unit 103 image output device

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Claims (12)

[Claims] 1. An image processing apparatus for correcting the gradation of color image data, comprising object area extracting means for extracting an area of a main object from the color image data, and gradation based on information of the main object area. A correction amount calculation unit that calculates a correction amount, a control region extraction unit that extracts a predetermined control region from a lightness region to which the main subject region does not belong, and a gradation correction amount in the control region using a predetermined control condition. An image processing apparatus comprising: a determination unit that determines whether to control and a control unit that controls a gradation correction amount according to a determination result of the determination unit. 2. The image processing apparatus according to claim 1, wherein the control area is set to a lightness area brighter than a lightness area to which the main subject area belongs. 3. The image processing apparatus according to claim 1, wherein the control area is set to a lightness area darker than a lightness area to which the area of the main subject belongs. 4. The image processing apparatus according to claim 1, wherein as the control condition, an upper limit or a lower limit is set for the slope of the tangent line of the gradation correction curve. 5. The image processing apparatus according to claim 4, wherein the control condition for the gradation correction curve differs depending on the scene of the correction target image. 6. The image processing apparatus according to claim 5, wherein the lower limit of the inclination of the tangent line, which is the control condition, is set to be lower in a backlight scene than in a nighttime flash shooting scene. 7. The image processing apparatus according to claim 4, wherein the control condition for the gradation correction curve differs depending on the image output device. 8. The image processing apparatus according to claim 7, wherein the lower limit of the gradient of the tangent line, which is the control condition, is set to be lower for an image output device having better gradation reproducibility. 9. The image processing apparatus according to claim 4, wherein the control condition for the gradation correction curve differs depending on the characteristics of the correction target image. 10. The image processing apparatus according to claim 1, wherein the gradation correction amount is set using a statistical value obtained from a component representing brightness belonging to the main subject area. 11. A step of extracting an area of a main subject from color image data, a step of calculating a gradation correction amount based on lightness information of the main subject area, and a step of gradation in accordance with the calculated gradation correction amount. A step of calculating a correction curve, a step of extracting a control area from a lightness area to which the main subject area does not belong, a step of determining whether or not the gradation correction amount in the control area is appropriate, When it is determined that a new gradation correction curve is calculated using a predetermined control condition, gradation correction is performed on the color image data using the gradation correction curve or the new gradation correction curve. An image processing method comprising the step of performing processing. 12. A computer-readable recording medium recording a program for causing a computer to implement each step of the image processing method according to claim 12.