JP7230481B2 - Color conversion table correction unit and color conversion table correction program - Google Patents

Description

The present invention relates to a color conversion table correction unit and a color conversion table correction program for performing color conversion based on the relationship between the color distribution of an input image and the color gamut of an output device.

When an image is output to an output device such as a printer or display unit, the color distribution of the image is divided between the inside of the color gamut of the output device (dark colors and low saturation colors) and the colors outside the color gamut (high saturation colors). may be mixed. If such an image is output as it is, colors outside the color gamut will not be output correctly in the output device, resulting in a drop in color reproducibility.
In such an image, if colors outside the color gamut are mapped (clipped) to the surface of the color gamut, the difference in brightness and saturation between the colors inside and outside the color gamut will disappear, resulting in lower contrast. result in a distorted image. On the other hand, if mapping is performed to reduce the color space as a whole, the contrast is likely to be maintained, but the image becomes undesirable in terms of colorimetric values.
On the other hand, Japanese Patent Application Laid-Open No. 2002-200002 proposes a technique of switching the color gamut conversion method according to the color distribution of the image. Specifically, if the color distribution histogram is uniform, mapping (compression of the color gamut) is performed, and if the distribution is peaky, clipping (mapping to the color gamut surface) is performed.

Japanese Patent Application Laid-Open No. 2002-27263

However, in the prior art, there are problems such as unnecessary color gamut compression and failure to ensure color reproducibility and contrast.

The present invention has been made against the background of the above circumstances, and is capable of preventing unnecessary color gamut compression when outputting with an output device based on an input image, ensuring color reproducibility and ensuring contrast. It is an object of the present invention to provide a compatible color conversion table correction unit and color conversion table correction program.

Among the color conversion table correcting units of the present invention, the first form comprises an input image, an acquiring unit for acquiring the color reproduction range of an output device, and acquiring the color distribution of the input image using the color space based on the input image. and a control unit for
The control unit determines whether the color distribution is distributed inside or outside a color gamut that is a color reproduction gamut of the output device, and performs color conversion at a color gamut boundary according to the positional relationship between the color distribution and the color gamut. modify the color conversion table by changing the method,
Further, the control unit maps colors distributed outside the color gamut onto the surface of the color gamut, compresses colors inside the color gamut further inside the color gamut, When the color difference between the inner distribution position and the out-of-gamut color is larger than a predetermined value, the mapping level of the color inside the color gamut is decreased, and when the color difference is smaller than the predetermined value, It is characterized by increasing the mapping level of colors inside the color gamut .

Another aspect of the invention is a color conversion table correction unit according to the aspect of the invention, wherein the control unit converts a first color space of the input image into a device-independent second color space; The color distribution is obtained by a color space of .

According to another aspect of the invention, there is provided a color conversion table correction unit according to the aspect of the invention, wherein the control unit obtains a color distribution of one object of the input image as the color distribution.

According to another aspect of the invention of a color conversion table correction unit, in the above aspect of the invention, the control unit corrects the color according to whether the color distribution is inside the color gamut or outside the color gamut. It is characterized by changing the color conversion method of the range boundary.

In another aspect of the invention, there is provided a color conversion table correction unit according to the above aspect of the invention, wherein the color distribution has a certain contrast between the color distribution inside the color gamut and the distribution outside the color gamut. The method is characterized in that the color conversion method for the color gamut boundary portion is changed depending on whether.

According to another aspect of the invention of a color conversion table correction unit, in the invention of the above aspect, the control unit determines whether the color distribution is distributed inside the color gamut and outside the color gamut, The color conversion method for the color gamut boundary is changed depending on whether the hue distribution is within a certain range.

According to another aspect of the invention of a color conversion table correction unit, in the invention of the above aspect, the control unit determines whether the color distribution is distributed inside the color gamut and outside the color gamut, The color conversion method of the color gamut boundary is changed depending on whether the distribution occupies a certain area or more.

In another aspect of the invention, there is provided a color conversion table correction unit according to the above aspect of the invention, wherein the color distribution compresses colors inside the color gamut according to distribution positions inside the color gamut. It is characterized by changing the level.

In another aspect of the invention of a color conversion table correction unit, in the invention of the above aspect, the control unit corrects the color gamut boundary for each object existing within a page of the input image or within a file within a plurality of pages. Change the color conversion method for some parts.

In another aspect of the invention of a color conversion table correction unit, in the invention of the above aspect, the control unit allows the user to select whether or not to change the color conversion method of the color gamut boundary according to the color distribution of each object. to

The invention of the color conversion table correction unit of another aspect is the invention of the above aspect, wherein the control unit can specify the color conversion table for each object or can specify the color conversion table for each object type in the user's selection. or automatically specify the color conversion method for the color gamut boundary.

A color conversion table correction program of the present invention is a program executed by a control unit for obtaining a color distribution of an input image in a color space based on the input image,
The program causes the control unit to determine whether the color distribution is distributed inside or outside the color gamut of the output device, and according to the positional relationship between the color distribution and the color gamut that is the color gamut of the output device, Modify the color conversion table by changing the color conversion method for the color gamut boundary ,
Colors distributed outside the color gamut are mapped onto the color gamut surface, colors inside the color gamut are compressed further inside the color gamut, and distribution positions inside the color gamut are mapped to the color gamut surface. If the color difference with the out-of-gamut color is greater than a predetermined value, the mapping level of the color inside the color gamut is reduced, and if the color difference is smaller than the predetermined value, the color inside the color gamut A step of increasing the mapping level of is executed .

According to the present invention, the color conversion method can be switched according to the color distribution of the image, unnecessary color gamut compression can be prevented, and both color reproducibility and contrast can be ensured. effective.

FIG. 4 is a diagram showing an input image and color distribution after color correction in the present embodiment; FIG. 10 is a diagram showing an input image and a color distribution after color correction in a conventional example; 1 is a schematic diagram showing an image output system including a conversion table corrector of this embodiment; FIG. It is a figure which shows the control block in this embodiment. It is a flow chart which shows a processing procedure of this embodiment. Similarly, it is a flow chart showing a procedure for mapping colors outside the color gamut onto the surface of the color gamut and mapping colors inside the color gamut inside. FIG. 10 is a diagram showing an input image and a color distribution after color correction according to the present embodiment together with processing contents; Similarly, it is a figure which shows the user interface in the setting of a process. Similarly, it is a figure explaining the conversion example of a source profile and a destination profile. Similarly, it is a diagram explaining an example of converting the input CMYK values into the target CMYK values through the LUT of the device link profile.

Prior to describing the embodiments of the present invention, first, the prior art and its problems will be described with reference to FIG.
FIG. 2A shows an example of an input image and color distribution. In the color distribution, the vertical axis represents lightness, and the horizontal axis represents saturation. Solid-line triangles represent the color gamut of the printer used. If colors outside the color gamut are clipped to the surface of the color gamut when printing with a printer, the image will become a low-contrast image (Fig. B).
That is, when colors inside and outside the color gamut are distributed among image objects in the page, contrast is reduced by simply clipping the outside of the color gamut.

Therefore, in the present embodiment, when such a color distribution exists, the decrease in contrast can be prevented by mapping the colors within the color gamut inside (Fig. 1C). Moreover, since processing is performed for each object, unnecessary color gamut compression processing can be omitted. For example, if a color inside the gamut is the same as an important color, such as a corporate color, the important color can be kept in colorimetric values without being mapped.

EMBODIMENT OF THE INVENTION Below, embodiment of this invention is concretely described based on an accompanying drawing.
FIG. 3 shows a schematic configuration of the image output system 100, which includes a printer 1 for outputting an image on a recording medium such as paper, a controller 2 for processing image data, and a color conversion table for creating and correcting a color conversion table. It has a creation (correction) device 3 and an output instruction terminal 4, which are connected by a network such as LAN and WAN. A color conversion table creation (correction) device 3 receives images and profile information from the controller 2, and performs color conversion mapping calculations for color gamut boundaries based on the color distribution of each image and the color gamut information of the printer. , modify the printer profile of the controller 2. Register as many printer profiles as the number of objects. The controller 2 performs color conversion processing based on the corrected printer profile. The color conversion table creating (correcting) device 3 has a configuration as a color conversion correcting section of the present invention.
A specific description will be given below.

An input image is transmitted from the output instruction terminal 4 to the controller 2, and the controller 2 performs image processing such as RIP. The processed input image is sent by the controller 2 to the color conversion table creation (correction) unit 3, and the color conversion table is corrected. The controller 2 corrects the image based on the corrected color conversion table, sends the corrected image to the printer 1, and the printer 1 forms an image on a recording medium.

A color conversion table creation (correction) device 3 receives images and profile information from the controller 2, and performs color conversion mapping calculations for color gamut boundaries based on the color distribution of each image and the color gamut information of the printer. , modify the printer profile of the controller. Register as many printer profiles as the number of objects. The controller performs color conversion processing based on the corrected printer profile. That is, the color conversion table creation (correction) device 3 stores the control unit and program of the present invention.

Next, control blocks of the image output system 100 will be described with reference to FIG.
The output instruction terminal 4 and the printer 1 are connected to the controller 2 via the network 5 .
The output instruction terminal 4 is provided with an operation unit and the like, and a user can issue a print instruction by means of an operation unit instruction or the like.
The printer 1 has an image forming unit (not shown), receives processed image data sent from the controller 2, and prints on a recording medium such as paper.

The controller 2 has an input image from the output instruction terminal 4 and a raster image processor (RIP) 20 that sends the processed image to the printer 1 and instructs output. A raster image processor (RIP) 20 converts a vector image or the like into raster image data. The controller 2 also has a source (target) profile acquisition unit 21 . The source (target) profile obtains a profile indicating the color space of the rasterized image data and a target profile.
Further, the controller 2 has a printer (destination) profile acquisition section 22 . A printer (destination) profile acquisition unit 22 acquires a printer profile from a printer or the like.
Further, the controller 2 has a color conversion (profile application calculation) section 23 . A color conversion (profile application calculation) unit 23 applies a profile to perform color conversion.
The controller 2 is connected to the color conversion table creation (correction) device 3 via the network 5 as described above.

An image acquisition unit 30 connected to the controller 2 is connected to the color conversion table creation (correction) device 3 . The image acquisition unit 30 acquires the image rasterized by the controller 2 .
The color conversion table creation (correction) device 3 also has a color conversion section (CMYK→L ^* a ^* b) 31 . A color conversion unit (CMYK→L ^* a ^* b) 31 changes the color space of the CMYK image data to the L ^* a ^* b ^* color space. The color space of the CMYK image data corresponds to the first color space, and the L ^* a ^* b ^* color space corresponds to the second color space.

Further, the color conversion table creation (correction) device 3 has a color distribution acquisition section 32 . The color distribution acquisition unit 32 acquires color distribution from the color-converted image data.
Further, the color conversion table creation (correction) device 3 has a printer color gamut acquisition section 33 . The color gamut of the printer can be obtained from the printer (destination) profile acquired by the controller 2 .
Further, the color conversion table creation (correction) device 3 has a color gamut boundary color conversion mapping calculator 34 . The gamut boundary color conversion mapping calculator 34 performs calculations to map colors on the gamut boundary.

Further, the color conversion table creation (correction) device 3 has a profile correction section 35 . The profile correction unit 35 corrects the profile, that is, the table, based on the result calculated by the color gamut boundary color conversion mapping calculation unit 34 . The corrected profile is sent to the controller 2, and color conversion is performed by applying the corrected profile. The color-converted image data is sent from the controller 2 to the printer and printed.
In the color conversion table creation (correction) device 3, a color conversion section (CMYK→L ^* a ^* b ^* ) 31, a color distribution acquisition section 32, a color boundary color conversion mapping calculation section 34, and a profile correction section 35 are configured in accordance with this embodiment. Then, it functions as a control unit of the present invention. Also, the image acquisition unit 30 and the printer color gamut acquisition unit 33 function as an acquisition unit of the present invention in this embodiment. The programs that operate in these units include the program of the present invention.

Next, the procedure of each process for image data will be described with reference to the flowchart of FIG. First, a color conversion table is obtained (step s1), and colorimetric values are obtained from the input image (step s2).
In acquiring the color conversion table, both the source profile and the destination profile (see Appendix) are read. When the input image is a scanner, a scanner profile for converting the RGB values of the scanner into colorimetric values (L ^* a ^* b ^* and XYZ values) is required.

The input image may be data such as PDF, or may be a scanner input image and PDF. In the former case, the PDF data is RIP-processed, and the CMYK values and the source profile are used to obtain colorimetric values. In the latter case, colorimetric values are obtained from the scanner image using the scanner profile. The reason why the PDF is necessary in the latter case is that the object can be easily identified.

Next, the color gamut information of the output printer is obtained (step s3), and the color distribution of the object is obtained (step s4).
Acquiring the color gamut information of the output printer can acquire the color gamut information of the printer by using the A2B information of the destination profile of the printer.
Although the L ^* a ^* b ^* color space, which is a uniform color space, is desirable for obtaining the color distribution of an object, various color systems such as a color appearance model may be used.

From the color distribution, it is determined whether there is a color distribution inside and outside the color gamut of the printer within a predetermined color range (step s5).
A certain degree of contrast may be used as a judgment condition when judging whether a color exists both inside and outside the color gamut within a predetermined color range. Here, the predetermined color range may be easily separated by hue, or may be separated by divisions in consideration of lightness. This is because, if the color gamut is not separated, even colors of different systems (for example, red outside the color gamut and green inside the color gamut) will be judged to sandwich the color gamut boundary. In the case of such colors, there is a contrast due to the hue, so processing to maintain the contrast is unnecessary. Further, whether or not the object occupies a certain area of the object may be used as a criterion for determining whether or not the object exists in both. In a minute area, there is no need to maintain contrast, and it is better not to generate errors due to unnecessary mapping.

If it is determined in the above determination that there is no color distribution inside the color gamut and outside the color gamut (step s5, N), the colors outside the color gamut are mapped onto the color gamut surface (step s7), and the process proceeds to step s8. .
Out-of-gamut colors are mapped to the surface of the printer gamut if they are not both in-gamut and out-of-gamut.
If a color distribution exists inside the gamut and outside the gamut (step s5, Y), the colors outside the gamut are mapped onto the gamut surface, and the colors inside the gamut are mapped inside (step s7). , to step s8.
That is, when they exist both outside and inside the gamut, the out-of-gamut colors are mapped to the surface of the printer gamut, and the in-gamut colors are mapped to the inside.

At step s8, the color conversion table is corrected based on the calculation result at step s6 or s7.
This color conversion table points to the printer's destination profile.
Next, it is determined whether distribution within a predetermined color range has been confirmed in all color gamuts (step s9). In this processing, processing is performed for each of the predetermined color ranges that are separated, and it is confirmed whether or not the profile correction has been completed in the previous color region of the color distribution.

If the distribution within the predetermined color range has not been confirmed in all color gamuts (step s9, N), the process returns to step s5 to determine whether the color distribution exists within the predetermined color range.
If the distribution within the predetermined color range has been confirmed in all color gamuts (step s9, Y), it is determined whether the color conversion tables of all objects have been corrected (step s10).
If the color conversion tables of all objects have not been corrected (step s10, N),
Returning to step s4, the color distribution of the new object is obtained.
If the color conversion tables of all objects have been corrected (step s10, Y), the repair ends.

In the above procedure, since processing is performed for each object, unnecessary color gamut compression processing can be omitted. For example, if a color inside the gamut is the same as an important color, such as a corporate color, the important color can be kept in colorimetric values without being mapped.

A subroutine for processing for mapping colors outside the gamut onto the surface of the gamut and mapping colors inside the gamut to the inside will be described below with reference to FIG.
First, a mapping target is determined (step s20).
In order to map in-gamut and out-of-gamut colors, it is necessary to determine a target mapping destination. The destination of mapping may be the center of the color gamut, or the target may be switched for each hue or lightness.
Next, it is determined whether the target color is out of the color gamut (step s21).
If outside the color gamut (step s21, Y), the intersection of the line segment connecting the target color and the mapping target and the color gamut surface is obtained.
Next, the intersection is adopted as the destination of mapping (step s23). The color gamut surface is calculated by the above processing. If the colors outside the gamut are not uniform, there is also a method of mapping the outermost color to the surface even if it is out of the gamut and mapping the other out-of-gamut colors slightly inside in order to provide gradation. .

At step s21, if the target color is not out of gamut (step s21, N), the distance between the inner color distribution and the outer distribution is measured (step s24). By measuring the distance, the contrast in the color distribution can be determined.
Next, the mapping amount of the inner color is calculated according to the distance (step s25).
Further, the target color is mapped in the mapping target direction by the above-described mapping amount, and is adopted as the mapping destination (step s26). It is desirable to change the amount of color mapping within the color gamut depending on the color distribution of the two. If the contrast due to the color distribution of the two is sufficiently large, the inner mapping amount may be small, but if the contrast is halved due to mapping outside the color gamut, a large mapping is required.

In the example of FIG. 7, there are only two points, but if the colors are distributed over a wide range to some extent, the mapping amount may be changed depending on the outer color and the inner color even within the color gamut. For example, move the inner color a lot and the outer color a little.
Although the above subroutine has been explained, as a modification, if it is difficult to ensure the contrast by mapping with the above method, there is also a method of maintaining the contrast by mapping in the hue direction. Especially in the case of graphic objects that are not continuous tone, it is often okay to map the hue.
Also, although the mapping level of the inner color is determined based on the contrast in the color distribution, it may be customized by the user. In that case, it would be even better if a preview display could be provided to support input.

In addition, execution/non-execution of the above processing may be switched according to the user's designation.
An example of a user interface 300 is shown in FIG.
The function of this embodiment is performed only when the setting of "optimization for each image" is made. If it is not valid, color conversion processing is performed in conformity with the conventional ICC profile. Also, it may be set for each object type (graphic, text, image).
In the illustrated example, graphic objects are calculated by conventional ICC profile saturation rendering, but for text and image objects, the color distribution is checked for each object and the profile is corrected as in the present embodiment.

In the user interface 300 of FIG. 8, settings can be made for each of object types such as graphic, text, and image. It has an image profile column 303 that displays the profile of the image. A rendering intent field is provided for each object, and a graphics field 310 defines "saturation", a text field 311 defines "optimize for each image", and a field 312 defines "optimize for each image" for images. ing. When the OK button 320 is pressed, the setting is confirmed, and when the cancel button 321 is pressed, the setting is canceled.

<Appendix>
Color conversion table correction The color conversion table is also called a profile. Among the profiles, the ICC profile (http://www.color.org/) is widely used not only in the printing industry but also in the IT industry. , has become the de facto standard.
Color management by ICC profile can be broadly divided into the following:
- A method using a device profile (hereinafter referred to as DP);
- There are two methods: a method using a device link profile (hereinafter referred to as DLP).

・In the case of DP Since device values (CMYK values and RGB values) are model-dependent values, DP is associated with model-independent values (XYZ values and L*a*b* values). The DP consists of an LUT from device values to model-independent values and an LUT from model-independent values to device values. , L*a*b* to CMYK LUT (referred to as B2A). This model-independent value is called a profile connection space because it becomes a color space that connects profiles.

Note that the rendering intent can be set according to the difference in color reproduction policy. This can be selected from perceptual, colorimetric, and saturation, and has three types of LUTs.
Color conversion requires the DP of the target device, which is called a source profile (also called a target profile). For example, a standard profile such as an offset press profile or Japan Color is selected. The DP of the output device is called a destination profile (sometimes called a printer profile). This means that the profile of the printer actually outputting is selected.
Then, the input CMYK values become model-independent values through A2B of the source profile, and are converted into CMYK values through B2A of the destination profile. (See Figure 9). Through this conversion, the printer (destination) can produce CMYK values that correspond to the CMYK values of the target device (source).

In the case of DLP DLP is the above conversion process, device value -> model independent value -> device value conversion, which is made into one LUT. In other words, it is an LUT that associates an input device value with an output device value. Conversion is only required once, and in the case of CMYK, there is no need to convert to three-dimensional L*a*b*, etc., so information loss (black plate information, etc., desired to remain) is reduced. However, it is not possible to acquire the model-independent value with only this information.
DLP has no concept of rendering intent and has only one LUT. Therefore, the input CMYK values can output the target CMYK values through the LUT of the device link profile (see FIG. 10).

Although the present invention has been described above based on the above embodiment, appropriate modifications to the present embodiment are possible without departing from the scope of the present invention.

1 Printer 2 Controller 3 Color Conversion Table Creation (Correction) Device 4 Output Instruction Terminal 5 Network 21 Source (Target) Profile Acquisition Unit 22 Printer (Destination) Profile Acquisition Unit 23 Color Conversion (Profile Application Calculation) Unit 30 Image Acquisition Unit 32 Color distribution acquisition unit 33 Printer gamut acquisition unit 34 Gamut boundary color conversion mapping calculation unit 35 Profile correction unit 100 Image output system 300 User interface

Claims (12)

an input image and an acquisition unit that acquires the color gamut of the output device;
a control unit that acquires the color distribution of the input image in a color space based on the input image;
The control unit determines whether the color distribution is distributed inside or outside a color gamut that is a color reproduction gamut of the output device, and performs color conversion at a color gamut boundary according to the positional relationship between the color distribution and the color gamut. modify the color conversion table by changing the method,
Further, the control unit maps colors distributed outside the color gamut onto the surface of the color gamut, compresses colors inside the color gamut further inside the color gamut, When the color difference between the inner distribution position and the out-of-gamut color is larger than a predetermined value, the mapping level of the color inside the color gamut is decreased, and when the color difference is smaller than the predetermined value, A color conversion table correction unit that increases the mapping level of colors inside the color gamut . 2. The control unit according to claim 1, wherein the first color space of the input image is converted into a device-independent second color space, and the color distribution is obtained by the second color space. color conversion table correction part. 3. The color conversion table correction unit according to claim 1, wherein the control unit acquires the color distribution of one object of the input image as the color distribution. 1. The control unit changes the color conversion method for the color gamut boundary depending on whether the color distribution is inside the color gamut or outside the color gamut. 4. The color conversion table correction unit according to any one of 3. The control unit changes the color conversion method of the color gamut boundary part depending on whether the color distribution has a constant contrast between the color distribution inside the color gamut and the color distribution outside the color gamut. The color conversion table correction unit according to any one of claims 1 to 4. The control unit determines whether the color distribution is distributed inside the color gamut and outside the color gamut, depending on whether the hue distribution is within a certain range. 6. The color conversion table correction unit according to claim 1, wherein the color conversion method is changed. The control unit determines whether the color distribution is distributed inside the color gamut or outside the color gamut, depending on whether the color distribution occupies a certain area or more, and performs color conversion of the color gamut boundary. 7. The color conversion table correction unit according to claim 1, wherein the method is changed. 8. The control unit changes the compression level of the color inside the color gamut according to the distribution position inside the color gamut of the color distribution. Color conversion table correction unit described in . 9. The method according to any one of claims 1 to 8 , wherein the control unit changes the color conversion method of the color gamut boundary for each object existing within a page of the input image or within a file within a plurality of pages. Color conversion table correction part described. 10. The color conversion table correction according to any one of claims 1 to 9 , wherein the control unit enables a user to select whether or not to change the color conversion method for the color gamut boundary according to the color distribution of each object. Department. The control unit can select whether the color conversion table can be specified for each object, whether the color conversion table can be specified for each object type, or whether the color conversion method for the color gamut boundary can be specified automatically, in the selection by the user. 11. The color conversion table corrector according to claim 10 , wherein A program executed by a control unit for obtaining a color distribution of an input image in a color space based on the input image,
The program causes the control unit to determine whether the color distribution is distributed inside or outside the color gamut of the output device, and according to the positional relationship between the color distribution and the color gamut that is the color gamut of the output device, Modify the color conversion table by changing the color conversion method for the color gamut boundary ,
Colors distributed outside the color gamut are mapped onto the color gamut surface, colors inside the color gamut are compressed further inside the color gamut, and distribution positions inside the color gamut are mapped to the color gamut surface. If the color difference with the out-of-gamut color is greater than a predetermined value, the mapping level of the color inside the color gamut is reduced, and if the color difference is smaller than the predetermined value, the color inside the color gamut A color conversion table correction program for executing a step of increasing the mapping level of .

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