JP3937714B2 - Color correction table generation method, image processing apparatus, image processing method, and recording medium - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to image processing in a case where an input color reproduction region and an output color reproduction region are different, and more specifically, to a gray data mapping method in monitor matching.
[0002]
[Prior art]
When the monitor image signal is hard-copied using a printer or the like, color conversion is necessary because each color system is different.
[0003]
The monitor image is a color image formed by an additive color method, and the RGB color coordinate system of the phosphor is used as the color system. On the other hand, for photographic paper, a color image is constructed by the subtractive color method, and its color system is L ^* a ^* b ^* The color system such as is used. In such a case, it is necessary to convert signal data (color correction) between these color systems.
[0004]
In addition, the color reproduction area of media such as photographic paper and the color reproduction area of monitors such as color CRT are the same L ^* a ^* b ^* In general, these color reproduction regions for the color system are different from each other. For example, if the monitor is an input color system and the media is an output color system, the color reproduction area of the output color system will be narrower than the input color reproduction area. When color image information exceeding the region is input, there is no output color system value corresponding to this.
[0005]
[Problems to be solved by the invention]
However, if color correction data related to color image information that exceeds the color reproduction area of the output color system is appropriately compressed within the color reproduction area, the hue, saturation, or brightness will be in a direction different from human visual characteristics. It is corrected and it is very unnatural. In particular, when the white background of the monitor is compared with the white background of the media, the monitor looks slightly blue. In comparison, the printed gray appears yellowish. Further, since the white background and gray of the media are different for each medium, the print result is also different for each medium. To perform monitor matching with natural colors, it is necessary to keep the gray color constant regardless of the media.
[0006]
The present invention has been made to solve the above-described problems, and provides a color correction table generation method, an image processing method, an image processing apparatus, and a recording medium that are capable of more faithful color reproduction regardless of media. Is an issue.
[0007]
[Means for Solving the Problems]
Furthermore, in view of the above problems, the invention described in claim 1 is a method for generating a color correction table used in an image output apparatus, and an image output apparatus gamut acquisition step of acquiring an image output apparatus gamut for a predetermined medium. A gray curve acquisition step of acquiring a gray curve passing through a predetermined gray color point based on the image output device gamut, and a color point in the image input device gamut based on the gray curve. And a color correction table generation step for generating a color correction table to be converted into points.
[0008]
According to the method for generating a color correction table used in the image output apparatus configured as described above, the image output apparatus gamut for a predetermined medium is acquired by the image output apparatus gamut acquisition step. Then, a gray curve passing through a predetermined gray color point is acquired based on the image output device gamut by the gray curve acquisition step, and a color point in the image input device gamut is acquired based on the gray curve by the color correction table generation step. A color correction table for converting the image data into color points in the image output device gamut is generated.
[0009]
The invention according to claim 2 corresponds to each of a plurality of media used in the image output apparatus. The hue of each color point converted by the color correction table is As the transition from the low concentration region to the middle concentration region, the overall approach is approached, and as the transition from the middle concentration region to the high concentration region is completed, the distance increases. Configured as Using a plurality of color correction tables, the color points in the image input device gamut are converted into the color points in the image output device gamut and output.
[0010]
Further, the invention described in claim 3 is the image processing device according to claim 2, and is configured to convert the gray axis of the image input device gamut into a gray curve of the image output device gamut.
[0011]
The invention according to claim 4 is the image processing apparatus according to claim 2 or 3, wherein the color point in the image input device gamut outside the image output device gamut is represented on the gray curve of the image output device gamut. Is configured to transform toward a predetermined point.
[0012]
The invention according to claim 5 is the image processing apparatus according to any one of claims 2 to 4, further comprising media selection means for selecting a medium to be used in the image output apparatus. Image processing is performed using a color correction table corresponding to the medium selected by the selection means.
[0013]
The invention according to claim 6 corresponds to each of a plurality of media used in the image output apparatus. The hue of each color point converted by the color correction table is As the transition from the low concentration region to the middle concentration region, the overall approach is approached, and as the transition from the middle concentration region to the high concentration region is completed, the distance increases. Configured as Using a plurality of color correction tables, the color points in the image input device gamut are converted into the color points in the image output device gamut and output.
[0014]
The invention according to claim 7 corresponds to each of a plurality of media used in the image output apparatus. The hue of each color point converted by the color correction table is As the transition from the low concentration region to the middle concentration region, the overall approach is approached, and as the transition from the middle concentration region to the high concentration region is completed, the distance increases. Configured as A computer-readable recording medium storing a program for causing a computer to execute image processing performed using a plurality of color correction tables, wherein colors in an image input device gamut are referred to the color correction table The computer is configured to be readable by a computer that records a program for causing a computer to execute processing for converting a point into a color point in an image output device gamut and outputting the color point.
[0015]
The invention according to claim 8 is a computer-readable recording medium that records a plurality of color correction tables for converting and outputting color points in the image input device gamut into color points in the image output device gamut, The plurality of color correction tables respectively correspond to a plurality of media used in the image output device, and the plurality of color correction tables Each color point transformed by As the hue of the color shifts from the low density area to the medium density area, it approaches as a whole, and as the hue shifts from the medium density area to the high density area, it moves away as a whole. Can be read by a computer that records multiple color correction tables configured Configured.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.
[0017]
FIG. 1 is a functional block diagram of an image processing apparatus according to an embodiment of the present invention, and FIG. 2 is a schematic block diagram showing a specific hardware configuration example of the image processing apparatus.
[0018]
In FIG. 1, the image processing device 20 performs desired image processing on the color image data of the color image generated by the application 12 d and outputs the color image data subjected to the image processing to the image output device 30. Here, the color image data represents the strength of each element color while color-separating the color image for each predetermined element color, and is represented by gray when it is a chromatic color and mixed at a predetermined ratio. It consists of a neutral color and black. On the other hand, regarding the display image data generated by the application 12d, the video driver 12b writes it in a predetermined image memory and displays it on the display 17a via the hardware circuit. In the present embodiment, a case where the image output apparatus 30 such as a printer performs print processing based on CMYK data will be described.
[0019]
In the display 17a, the color image is displayed in gradation in three primary colors (R, G, B) of red, green, and blue for each pixel arranged vertically and horizontally, but in the printer of this embodiment, cyan, magenta, yellow, black The four colors (C, M, Y, K) are printed without gradation display. Therefore, for color printing, color conversion work from the display of the three primary colors (R, G, B) of red, green and blue to the display of four colors (C, M, Y, K) of cyan, magenta, yellow, and black, It is necessary to perform gradation conversion from gradation display to display without gradation.
[0020]
The image processing apparatus 20 uses a first color conversion unit 20a for converting display RGB image data generated by the application 12d into image data of the Luv coordinate system, and a color correction table selected by the media selection unit 20i. A color correction unit 20b for reading from the correction LUT storage unit 20h and converting the display-use Luv coordinate system image data into printer-use Luv coordinate system image data with reference to the read color correction table; and a Luv coordinate system image A second color conversion unit 20c that converts data into CMYK (cyan, magenta, yellow, and black) data; and a binarization unit 20d that converts the CMYK data into binary data.
[0021]
The image processing apparatus 20 further includes a display gamut generation unit 20g that generates a display gamut based on the RGB image data generated by the application 12d, and a printer that generates a printer gamut based on the color measurement result of the colorimeter 37. A gamut generation unit 20e, a color correction LUT generation unit 20f that generates a color correction table (LUT) based on the generated printer gamut and display gamut, a color correction LUT storage unit 20h that stores the generated color correction LUT, A media selection unit 20i that selects a type of media such as printing paper. Details of the processing of these components will be described later.
[0022]
In the present embodiment, a computer system is employed as an example of hardware that realizes such an image processing apparatus. FIG. 2 is a block diagram showing the computer system. The computer system includes a scanner 11a, a digital still camera 11b, and a video camera 11c as image input devices, and is connected to a computer main body 12. Each input device can generate image data in which an image is expressed by a dot matrix pixel and output the image data to the computer main body 12. Here, the image data is displayed in 256 gradations for each of the three primary colors of RGB. About 16.7 million colors can be expressed.
[0023]
A floppy disk drive 13a, a hard disk 13b, and a CD-ROM drive 13c as external auxiliary storage devices are connected to the computer main body 12, and main programs related to the system are recorded on the hard disk 13b. Necessary programs and the like can be read from a CD-ROM or the like as appropriate. In addition, a modem 14a is connected as a communication device for connecting the computer main body 12 to an external network or the like, and it can be connected to an external network via the public communication line, and can be installed by downloading software and data. It has become. In this example, the modem 14a accesses the outside via a telephone line, but it is also possible to adopt a configuration where the network is accessed via a LAN adapter. In addition, a keyboard 15a and a mouse 15b are also connected for operating the computer main body 12.
[0024]
Furthermore, a display 17a and a color printer 17b are provided as image output devices. The display 17a has a display area of 800 pixels in the horizontal direction and 600 pixels in the vertical direction, and the above-described display of 16.7 million colors can be performed for each pixel. This resolution is merely an example, and can be appropriately changed such as 640 × 480 pixels or 1024 × 768 pixels.
[0025]
The color printer 17b is an ink jet printer, and can print an image by adding dots on printing paper as a medium using four color inks of CMYK. The image density is capable of high-density printing such as 360 × 360 dpi or 720 × 720 dpi, but the gradation expression is a two-gradation expression such as whether or not color ink is applied. On the other hand, in order to display or output to the image output device while inputting an image using such an image input device, a predetermined program is executed in the computer main body 12. Among them, an operating system (OS) 12a is operating as a basic program, and the operating system 12a performs a print output to a display driver (DSP DRV) 12b for displaying on the display 17a and a color printer 17b. A printer driver (PRT DRV) 12c is installed. These drivers 12b and 12c depend on the models of the display 17a and the color printer 17b, and can be additionally modified to the operating system 12a according to the respective models. In addition, depending on the model, additional functions beyond standard processing can be realized. That is, various additional processes within an allowable range can be realized while maintaining a common processing system on the standard system called the operating system 12a.
[0026]
As a premise for executing such a program, the computer main body 12 includes a CPU 12e, a RAM 12f, a ROM 12g, an I / O 12h, etc., and the CPU 12e that executes arithmetic processing uses the RAM 12f as a temporary work area or a setting storage area. While using it as a program area, a basic program written in the ROM 12g is executed as appropriate to control external devices and internal devices connected via the I / O 12h.
[0027]
Here, the application 12d is executed on the operating system 12a as a basic program. The processing contents of the application 12d are various. The operation of the keyboard 15a and the mouse 15b as operation devices is monitored, and when operated, various external devices are appropriately controlled to execute corresponding arithmetic processing, Furthermore, the processing result is displayed on the display 17a or output to the color printer 17b.
[0028]
In such a computer system, it is possible to acquire image data with the scanner 11a as an image input device, execute predetermined image processing by the application 12d, and then display and output the image data on a display 17a or a color printer 17b as an image output device. It is.
[0029]
The display driver 12b and the printer driver 12c are stored in the hard disk 13b, and are read and operated by the computer main body 12 at startup. At the time of introduction, it is recorded on a medium such as a CD-ROM or floppy disk and installed. Therefore, these media constitute a medium on which an image processing program is recorded. The image processing program itself is also included within the scope of the present invention. In this embodiment, the image processing apparatus is realized as a computer system. However, such a computer system is not necessarily required, and any system that requires image processing according to the present invention for similar image data may be used. Good. For example, a system in which an image processing apparatus that performs image processing according to the present invention is incorporated in a digital still camera and a color printer is used to print the image processed image data may be used. In a color printer that inputs and prints image data without going through a computer system, the image processing according to the present invention is automatically performed on image data inputted through a scanner, a digital still camera, a modem, or the like. It is also possible to configure to perform printing processing.
[0030]
In addition, the present invention can naturally be applied to various apparatuses that handle image data such as a color facsimile apparatus and a color copying apparatus.
[0031]
A color correction table generation processing program executed by the image processing apparatus 20 shown in FIG. 1 will be described below with reference to FIGS.
[0032]
FIG. 3 shows a flowchart for explaining a color correction table generation processing program executed by the image processing apparatus 20 of the embodiment. As shown in FIG. 3, a medium to be printed is selected by the media selection unit 20i (step 40), and a 256 gradation pattern 36 of CMYK ink is printed on the selected predetermined medium (step 42). . The printed pattern is colorimetrically measured by the colorimeter 37, and the colorimetric data for each colorimetric pattern is supplied to the printer gamut generation unit 20e (step 44). The printer gamut generation unit 20e generates a printer gamut in the L * a * b * space based on the colorimetric data (step 46), and a white point A (achromatic color) on the selected predetermined medium. (Color point having the maximum luminance value) and black point B (color point having the minimum luminance value of the achromatic color) are obtained (step 48).
[0033]
FIG. 4 shows a gray axis curve for each medium projected on the L * a * plane, and FIG. 5 shows a gray axis curve for each medium projected on the L * b * plane. 4 and 5, Ai is a white point for each medium, and Bi is a black point for each medium (i = 1 to 5). As shown in FIGS. 4 and 5, the black point and the white point are different depending on the media, and gray has a different color for each media. Here, i = 1 is a case where the medium is plain paper, i = 2 is a case where the medium is a super fine dedicated paper, i = 3 is a case where the medium is photo paper 2, and i = 4 is a medium. Is the case of a dedicated glossy film, and i = 5 is the case of PM photo paper.
[0034]
Therefore, according to the present invention, the gray axis is divided into the white point Ai, the black point Bi, and the gray color point C in a predetermined medium density region (L * = 50, a * = − 1, b * = − 5). And converted to a gray curve passing through. Here, the point C is a fixed point that does not depend on the medium, and even if color compression is performed by configuring the gray curve so that it passes through a fixed point (point C) that does not depend on the medium, a predetermined gray is applied to the medium. A constant color can be achieved without stopping. For this reason, as shown in FIGS. 4 and 5, the plurality of gray curves corresponding to a plurality of media have hues approaching from the low density area to the medium density area and transition from the medium density area to the high density area. Accordingly, the hue is separated. Accordingly, the color correction table output generated using the gray curve also approaches the hue as it transitions from the low density region to the medium density region, and the hue separates as it transitions from the medium density region to the high density region.
[0035]
The printer gamut generation unit 20e obtains the curve ACB as follows (step 50). The curve ACB is obtained by joining a curve connecting point Ai and point C where the inclination is zero at point C and a curve connecting point Bi and point C where the inclination is infinite at point C. can get.
[0036]
A curve connecting point Ai and point C and a curve connecting point Bi and point C are:
y = P × x ² + Q × x + R (1)
The coefficients P, Q, and R need to be obtained by the curve AC and the curve BC, respectively. In addition, the expression obtained by differentiating the expression (1) is
y ′ = 2 × P × x + Q (2)
It is.
[0037]
The condition for obtaining the curve ACB is that for the curve AC, the curve represented by the equation (1) passes through the points Ai and C, and y ′ = 0 at the point C. For the curve BC, the curve represented by the equation (1) passes through the points Bi and C, and y ′ = 0 at the point C.
[0038]
Next, the printer gamut generating unit 20e performs normalization processing by converting the gray curve and the printer gamut from the L * a * b * space to the Luv space (step 52). The transformation from L * a * b * space to Luv space is
L = LA / (LAi−LBi) × (L * −LBi) (3)
u = aA / (aAi−aBi) × (a * −aBi) (4)
v = bA / (bAi−bBi) × (b * −bBi) (5)
Is done by. Here, L *, a *, and b * are data before conversion, and L, u, and v are data after conversion. Further, (LA, aA, bA) is a display white point, (LAi, aAi, bAi) is a media white point, and (LBi, aBi, bBi) is a media black point. Gray curves converted to the Luv space and normalized are shown in FIG. 6 and FIG. In this way, the printer gamut generation unit 20e generates a normalized printer gamut (output system gamut) (step 54) and supplies it to the color correction LUT generation unit 20f.
[0039]
On the other hand, the display gamut generation unit 20g generates a display gamut (input system gamut) based on the RGB data generated by the application 12d (step 56), and supplies it to the color correction LUT generation unit 20f.
[0040]
By the normalization process, the white point and the black point of the medium are not close to the medium, and coincide with the white point and the black point of the display gamut generated by the display gamut generation unit 20g.
[0041]
Next, color correction LUT generation processing for a predetermined medium in step 58 will be described with reference to FIGS.
[0042]
FIG. 8 is a flowchart for explaining gray data mapping processing and color compression. The color correction LUT generation unit 20f obtains a correspondence relationship between the normalized printer gamut generated by the printer gamut 20e and the display gamut generated by the display gamut generation unit 20g in the Luv space (step 64). 9 and 10 are diagrams for explaining the correspondence between the normalized printer gamut and the display gamut. FIG. 9 is a diagram for explaining gray mapping, and FIG. 10 is a diagram for explaining color compression.
[0043]
Next, the color correction LUT generation unit 20f determines whether or not a predetermined color point in the display gamut exists in the printer gamut, that is, in the printer gamut (step 65). If a predetermined color point in the display gamut exists in the printer gamut (step 65, Yes), gray data mapping is performed (step 66). That is, as shown in FIG. 9, the points P1 to P4 on the gray axis of the display gamut are color-converted to points Q1 to Q4 on the gray curve obtained in step 50, respectively. Here, the luminance value of the point Pi on the gray axis of the display gamut is made equal to the luminance value of the point Qi on the gray curve to be color-converted. In this way, the gray color of a given media can be matched to the gray color of the display. Similarly, color points existing in the printer gamut that are not on the gray axis of the display gamut move in the direction of movement of the gray axis. The amount of movement becomes maximum in the medium density region, similar to the amount of movement on the gray axis.
[0044]
On the other hand, if the predetermined color point in the display gamut does not exist in the printer gamut (step 65, No), color compression processing is performed (step 67). That is, as shown in FIG. 10, the points P1 to P4 outside the printer gamut are color-converted to points Q1 to Q4 on the gray curve obtained in step 50, respectively. Here, an area in which the distance from the gray axis of the display gamut is within 85% of the distance from the gray axis of the display gamut to the printer gamut boundary surface is defined as an uncompressed area of the printer gamut, and from the gray axis of the display gamut. An area in which the distance is 85% to 100% of the distance from the gray axis of the display gamut to the printer gamut boundary surface is defined as a compressed area of the printer gamut. The point Pi outside the printer gamut is subjected to color conversion while preserving the gradation at a point Qi in the printer gamut compression region on the straight line connecting the point La and the point Pi on the gray curve.
[0045]
Next, the color correction LUT generation unit 20f generates a color correction table for performing color conversion in Steps 66 and 67 (Steps 58 and 68).
[0046]
The color correction LUT generated by the color correction LUT generation unit 20f in this way is stored in the color correction LUT storage unit 20h together with the identifier of the medium selected by the media selection unit 20i, for example, the name of the medium (step 60). ).
[0047]
Then, the process from step 40 to 60 is performed on all the media (step 62, Yes), and the process ends.
[0048]
Next, the operation of the printer driver 12c will be described with reference to FIG.
[0049]
As shown in FIG. 11, when the user instructs to start printing (step 70), it is determined whether or not a medium for printing is selected by the media selection unit 20i (step 72). When a predetermined medium is selected (step 70, Yes), the color correction table for the predetermined medium is read from the color correction LUT 20h and read into the RAM (step 74). Then, the color correction table is incorporated into the color correction unit 20b (step 76), and printing processing is performed (step 78).
[0050]
As described above, according to the embodiment, a color correction table capable of performing gray mapping for each medium is generated, and color correction is performed using a color correction table suitable for the medium to be used. Appropriate gray can be reproduced every time.
[Brief description of the drawings]
FIG. 1 is a functional block diagram of an image processing apparatus according to an embodiment of the present invention.
FIG. 2 is a schematic block diagram illustrating a specific hardware configuration example of an image processing apparatus according to an embodiment of the present invention.
FIG. 3 is a flowchart for explaining color correction table generation processing by the image processing apparatus 20 according to the present invention;
FIG. 4 is a diagram illustrating a gray axis curve for each medium projected on an L * a * plane.
FIG. 5 is a diagram showing a gray axis curve for each medium projected on an L * b * plane.
FIG. 6 is a diagram showing a gray curve that has been projected onto an L * u * plane and converted into a Luv space and normalized.
FIG. 7 is a diagram showing a gray curve that is projected onto an L * v * plane and converted into a Luv space and normalized.
FIG. 8 is a flowchart for explaining gray data mapping processing and color compression;
FIG. 9 is a diagram for explaining gray mapping, showing a correspondence relationship between a normalized printer gamut and a display gamut.
FIG. 10 is a diagram for explaining color compression by showing a correspondence relationship between a normalized printer gamut and a display gamut.
FIG. 11 is a flowchart for explaining the operation of the printer driver 12c.
[Explanation of symbols]
10 Image input device
11a scanner
11a2 scanner
11b Digital still camera
11b1 Digital still camera
11b2 Digital still camera
11c video camera
12 Computer body
12a operating system
12b Display driver
12c Printer driver
12d application
13a Floppy disk drive
13b hard disk
13c CD-ROM drive
14a modem
14a2 modem
15a keyboard
15b mouse
17a display
17b color printer
17b1 color printer
17b2 color printer
18a color facsimile machine
18b color copying machine
20 Image processing device
20a First color conversion unit
20b Color correction unit
20c Second color conversion unit
20d binarization part
20e Printer gamut generator
20f Color correction LUT generator
20g display gamut generator
20h Color correction LUT storage
20i media selector
30 Image output device
37 Colorimeter

Claims (8)

A method of generating a color correction table used in an image output device,
An image output device gamut acquisition step of acquiring an image output device gamut for each of a plurality of types of predetermined media;
Based on the image output device gamut, a gray curve acquisition step of acquiring a gray curve for each of a plurality of types of predetermined media;
A color correction table generating step for generating a color correction table for converting a color point in the image input device gamut into a color point in the image output device gamut based on the gray curve;
With
Each of the gray curves for each of a plurality of types of predetermined media passes through a common gray color point in the medium density region.
Method. The hue of each color point converted by the color correction table corresponding to each of the multiple media used in the image output device approaches as a whole from the low density area to the medium density area, and from the medium density area to the high density An image processing device that converts a color point in an image input device gamut into a color point in an image output device gamut using a plurality of color correction tables configured to be separated as a whole as the region moves. The image processing apparatus according to claim 2,
An image processing apparatus for converting a gray axis of an image input device gamut into a gray curve of an image output device gamut. The image processing apparatus according to claim 2 or 3,
An image processing apparatus that converts a color point in an image input device gamut outside a image output device gamut toward a predetermined point on a gray curve of the image output device gamut. An image processing apparatus according to any one of claims 2 to 4, wherein
Media selection means for selecting media used for the image output device;
An image processing apparatus in which image processing is performed using a color correction table corresponding to a medium selected by the media selection means. The hue of each color point converted by the color correction table corresponding to each of the multiple media used in the image output device approaches as a whole from the low density area to the medium density area, and from the medium density area to the high density An image processing method for converting a color point in an image input device gamut into a color point in an image output device gamut using a plurality of color correction tables configured so as to be separated as a whole as the region moves. The hue of each color point converted by the color correction table corresponding to each of the multiple media used in the image output device approaches as a whole from the low density area to the medium density area, and from the medium density area to the high density A computer-readable recording medium storing a program for causing a computer to execute image processing performed using a plurality of color correction tables configured to be separated as a whole as the region moves,
Referring to the color correction table, a computer-readable recording having recorded thereon a program for causing a computer to execute a process of converting a color point in the image input device gamut into a color point in the image output device gamut and outputting it. Medium. A computer-readable recording medium that records a plurality of color correction tables for converting color points in the image input device gamut into color points in the image output device gamut and outputting the converted color points.
The plurality of color correction tables respectively correspond to a plurality of media used in the image output device;
The hues of the respective color points converted by the plurality of color correction tables generally approach as the transition from the low density region to the medium density region, and generally separate as the transition from the medium density region to the high density region. A computer-readable recording medium on which a plurality of color correction tables recorded are recorded.

Images