JP4872616B2 - Image processing apparatus, image processing method, and program - Google Patents

Description

  The present invention relates to an image processing apparatus that corrects the color of image data.

  In order to cope with various outputs such as printing with a printer and display on a display, techniques for variously converting colors of image data have been developed. As one of the color conversion techniques, there is a method of using a table (color lookup table: CLUT) in which values to be converted are associated with converted values. The color conversion technique using the CLUT can perform conversion with a higher degree of freedom than a method using calculation, but has a disadvantage that a relatively large memory capacity is required to store the table.

  This disadvantage becomes more prominent as the range of colors on the input side (color values to be converted) becomes wider. On the other hand, in general, in a color space such as L * a * b *, values that do not exist as colors, such as lightness 0 and saturation 100, are included. However, in the color conversion method using the CLUT, it is necessary to set conversion values relating to such non-existing values as a table, which is wasteful.

  Therefore, an effective color gamut is extracted and defined from a color space having a relatively wide color gamut, and the colors included in the image data are converted into colors within the effective color gamut before the colors in the CLUT. By performing the conversion process, it is possible to use a CLUT having an effective color gamut as an input value, and a technique for effectively using the CLUT has been considered.

However, when converting from a color space having a relatively wide color gamut to an effective color gamut, there is a problem with a method for adjusting a color outside the effective color gamut. As this adjustment method, for example, the method disclosed in Patent Document 1 can be adopted.
JP 2004-201184 A

  However, in the method disclosed in Patent Document 1, a color with a value exceeding the value of the color gamut is converted to a color within the color gamut by changing the lightness while maintaining the hue and saturation. In addition, a color having a value lower than the color gamut is converted to a color within the color gamut by changing the saturation while maintaining the hue and brightness. In this way, as a result of changing the brightness or saturation alone, an unintended color may appear.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an image processing apparatus capable of adjusting a color while considering the balance between lightness and saturation.

  The present invention for solving the problems of the conventional example is an image processing apparatus, wherein each of color values in image data to be processed is converted into a value in a predetermined color space; Means for storing a pre-defined value range with respect to a predetermined color space, and an offset for adjustment when any color value is converted to a value exceeding the upper limit of the stored value range by the conversion An offset calculation means for calculating a value, and the calculated offset value is added to each value after the conversion, and each value after the conversion is such that the maximum value of each value after the addition becomes the upper limit of the color space. And means for correcting.

  Here, the offset calculation means may calculate an offset value for converting a value after conversion exceeding the upper limit of the range into a value representing a color of target luminance determined by a predetermined method.

  Further, the target luminance is a color when the color represented by the converted value is a target color, while maintaining the chromaticity of the target color while changing the luminance to a color within the range, and It may be set between the brightness of the target color.

  Further, the target luminance is a color when the color represented by the converted value is a target color, while maintaining the chromaticity of the target color while changing the luminance to a color within the range, and It may be a value obtained by weighted averaging the luminance of the target color using a predetermined ratio.

  Furthermore, an image processing apparatus according to an aspect of the present invention includes means for converting a color value in image data to be processed into a value in a predetermined color space, and a value range defined in advance for the predetermined color space. When the color value is converted to a value outside the stored value range by the conversion, the chromaticity of the target color is set with the color represented by the converted value as the target color. A means for setting a target luminance between the luminance when the luminance is adjusted close to white, the luminance is adjusted to a color within the range, and the luminance of the target color, and is expressed by the converted value. And a means for correcting the color to be set to the color of the set target luminance.

  The target luminance is a luminance when the color represented by the converted value is the target color, the chromaticity of the target color is changed to be close to white, and the luminance is changed to be adjusted to a color within the range. And the luminance of the target color may be a weighted average value using a predetermined ratio.

  Here, the ratio may be determined based on at least one of the brightness or hue of the color represented by the converted value, and the ratio may be determined when the converted value exceeds the upper limit of the range. The value after conversion may be set to a different value when the value is below the lower limit of the range.

  Furthermore, an image processing method according to another aspect of the present invention provides a procedure for converting a color value in image data to be processed into a value in a predetermined color space, and the color value is converted into the predetermined value by the conversion. A step of calculating an offset value for adjustment when the value is converted to a value exceeding the upper limit of a predetermined range with respect to the color space, and adding the calculated offset value to the converted value, And a step of correcting so that the later maximum value becomes the upper limit of the color space.

  Furthermore, a program according to another aspect of the present invention provides a computer with a procedure for converting a color value in image data to be processed into a value in a predetermined color space, and the color value is converted into a value by the conversion. A procedure for calculating an offset value for adjustment when converted to a value exceeding the upper limit of a predetermined range for a predetermined color space, and adding the calculated offset value to the converted value, Correcting the maximum value after the addition to be the upper limit of the color space.

  Embodiments of the present invention will be described with reference to the drawings. The image processing apparatus according to the embodiment of the present invention is realized by, for example, a printer server, and includes a control unit 11, a storage unit 12, and a communication unit 13, as shown in FIG. The image processing apparatus 1 is connected to an output device 2 such as a printer and a client device 3.

  The control unit 11 is a CPU or the like, and operates according to a program stored in the storage unit 12. In the present embodiment, the control unit 11 performs the process of adjusting the color included in the image data with the image data received from the client device 3 as a processing target. Then, the adjusted image data is output to the output device 2.

  The storage unit 12 is a memory element, a disk device, or the like, and holds a program executed by the control unit 11. This program may be provided in a state of being stored in a storage medium such as a DVD-ROM or a CD-ROM, and may be copied and stored in the storage unit 12. The storage unit 12 also operates as a work memory for the control unit 11.

  The communication unit 13 is a communication unit such as a network interface, and transmits and receives data to and from the output device 2 and the client device 3. Here, the image data received from the client device 3 is output to the control unit 11. Further, the image data is transmitted to the output device 2 in accordance with an instruction input from the control unit 11.

  Here, the specific processing contents of the control unit 11 will be described. The control unit 11 receives image data to be printed from the client device 3 by the output device 2 (here, a printer) which is an output side device. This image data is, for example, bitmap image data.

  As shown in FIG. 2 functionally, the image processing apparatus 1 according to the present embodiment performs the software processing of the control unit 11 so that the color space conversion unit 21, the post-processing unit 22, the color conversion unit 23, It is comprised including. The color space conversion unit 21 converts the value (color value) of each pixel included in the bitmap image data to be processed into a value in a predetermined color space. Here, it is assumed that a value range (value range) representing a significant color is determined in advance in the predetermined color space.

  The post-processing unit 22 performs a process of adjusting the value to a value in the range when it is determined that the converted value is outside the above-described range based on the conversion result in the color space conversion unit 21. The contents of this process will be described later.

  The color conversion unit 23 uses the color value output from the post-processing unit 22 as an input value, refers to a predetermined CLUT, and outputs an output value (for example, a CMYK value) associated with the input value. Here, for example, it may be output to the output device 2. If the input value is not on the CLUT, at least one value close to the input value is found from the CLUT, and the output value is obtained by interpolation or extrapolation using the output value associated with the found value. Calculate. Here, the input value of the CLUT may be a value within a predetermined range. This is because the post-processing unit 22 always corrects the converted value to a value within a predetermined value range.

  Here, the operation of the post-processing unit 22 will be described. In the following example, it is assumed that the color space conversion unit 21 converts color values into RGB (color space consisting of Red, Green, and Blue) values, and each of R, G, and B components is “0” to “1”. The range in between is defined as a significant color gamut.

  The post-processing unit 22 first determines whether or not the converted value is below the range, and if it is below, performs a corresponding process, and then determines whether or not the converted value is above the range. However, if it exceeds, the corresponding processing is executed.

  That is, as shown in FIG. 3, the post-processing unit 22 acquires the minimum value minRGB among the component values of the converted RGB values (hereinafter, this value is referred to as a target value for distinction) ( S11). Then, it is checked whether or not the minimum value minRGB is less than “0” which is the minimum value in the range (that is, a negative value) (S12). That is, it is checked whether the value of any component of the target value is negative.

If any of the components has a negative value, the luminance of the color represented by the target value is calculated as the initial luminance value (S13). As a specific example, the initial luminance value Yorg is
Yorg = 0.2126 × R + 0.7152 × G + 0.0722 × B
And calculate. When Yorg is less than “0”, Yorg = 0 is corrected.

Then, the post-processing unit 22 adds the absolute value of minRGB to each component so that the minimum value among the components of the target value is “0” (S14). Here minRGB is negative, so
R '= R-minRGB
G '= G-minRGB
B '= B-minRGB
And it is sufficient. The RGB value after this adjustment is obtained by changing the chromaticity of the target value close to the white direction and changing the luminance at the same time, and is a value when converted to a color in the range.

The post-processing unit 22 further calculates the luminance Ycuf of the RGB value after adjustment (S15).
Ycuf = 0.2126 x R '+ 0.7152 x G' + 0.0722 x B '

  Here, the post-processing unit 22 checks whether Ycuf = 0 (S16). If Ycuf = 0, the values of the R, G, and B components are set to “0” (S17), Is output (S18), and the process ends.

  If Ycuf is not 0 in step S16, the target luminance Ytgt is changed to the luminance Ycuf when the chromaticity of the target color is changed to be close to white, and the luminance is changed and adjusted to a color within the range. And the original luminance Yorg are set (S19).

Specifically, the coefficient β (0 ≦ β ≦ 1) is used,
Ytgt = β × Yorg + (1−β) × Ycuf
As a result, the target luminance is calculated.

Then, the post-processing unit 22 uses the RGB component value of the target value and the target luminance as the output RGB value,
Rout = R ′ × Ytgt / Ycuf
Gout = G ′ × Ytgt / Ycuf
Bout = B ′ × Ytgt / Ycuf
(S20), the process proceeds to step S18, and this value is output.

  In step S12, if none of the components of the target value is negative (that is, if any component is in the range and the target value is in the range), the target value is output as it is. (S21), the process ends.

  Next, as illustrated in FIG. 4, the post-processing unit 22 determines whether any component of the target value exceeds “1” that is the maximum value of the significant color gamut (whether it exceeds the upper limit of the range). Whether or not is checked (S1). This can be determined, for example, by selecting the maximum value maxRGB among the components of the target value and determining whether the maximum value exceeds “1”.

If the converted value exceeds the upper limit of the range, the post-processing unit 22 calculates the luminance of the color represented by the target value as the initial luminance value (S2). As a specific example, the initial luminance value Yorg is
Yorg = 0.2126 × R + 0.7152 × G + 0.0722 × B
And calculate. Then, each component of the target value is divided by the maximum component value maxRGB among the target values. That is, the target value is normalized by the maximum value of the component (S3). In other words,
R '= R / maxRGB
G '= G / maxRGB
B '= B / maxRGB
Is calculated.

  Further, the luminance value Ymin of the target value after normalization is calculated (S4).

  Ymin = 0.2126 x R '+ 0.7152 x G' + 0.0722 x B '

  This Ymin value corresponds to a luminance value when converted into a value within a predetermined range by changing the luminance while maintaining the ratio (chromaticity) between the components of the target value.

  Schematically in L * a * b * space, as shown in FIG. 5, with respect to the color gamut γ corresponding to the value range when each component of the target value is in the range of “0” to “1”. When the color represented by the RGB value (target value) after conversion by the color space conversion unit 21 is outside P of this color gamut γ, the luminance Yorg calculated in the process S2 is , The brightness of the original target value itself. Therefore, when the luminance is stored and corrected to a color within the color gamut γ, the color corresponds to the position Q where Y = Yorg and the outer edge of the color gamut γ intersect.

  When the luminance is changed while preserving chromaticity, the color corresponds to the position R where Y = Ymin and the outer edge of the color gamut γ intersect.

The post-processing unit 22 maintains the target luminance Ytgt between the luminance Ymin when the luminance is changed and adjusted to a color within the range while maintaining the chromaticity, and the luminance Yorg that is the luminance of the converted color itself. Is set (S5). Specifically, the coefficient α (0 ≦ α ≦ 1) is used,
Ytgt = α × Yorg + (1−α) × Ymin
As a result, the target luminance is calculated.

  Thus, in FIG. 5, the target luminance Ytgt is set at a position represented by a ratio of α: 1−α between Yorg and Ymin.

  The post-processing unit 22 checks whether or not the target luminance Ytgt calculated here exceeds “1” (or “1” or more) (S6), and if it exceeds (or more), R, G, B Each output value is set to “1” (S7), and this RGB value is output (S8).

  On the other hand, if the target luminance Ytgt does not exceed “1” (or less than “1”) in the process S7, an adjustment offset value (here, each of the target values) related to each component of the color space after conversion. The offset value ofsRGB common to the components is calculated (S9).

  ofsRGB = (maxRGB × Ytgt−Yorg) / (1−Ytgt)

Then, the calculated offset value is added to each component of the target value, normalized by the maximum value of each component value after addition, and corrected so that the maximum value after addition falls within the range of the color space (S10). . As a specific example, the post-processing unit 22
Rout = (R + ofsRGB) / (maxRGB + ofsRGB)
Gout = (G + ofsRGB) / (maxRGB + ofsRGB)
Bout = (B + ofsRGB) / (maxRGB + ofsRGB)
And the process proceeds to step S8 to output RGB values having Rout, Gout, and Bout as components.

  According to this example, in FIG. 5, along the outer edge of the color gamut γ, the point S moved from the point R corresponding to Ymin to the point Q corresponding to Yorg by the offset value in the direction of white (L * = 1). It is corrected to the value of.

  If any component of the target values does not exceed “1”, which is the maximum value of the significant color gamut (does not exceed the upper limit of the range) in process S1, the process proceeds to process S21. The target value is output and the process ends.

  When the input image data is bitmap data, the processing shown in FIGS. 3 and 4 is performed for each pixel value, for example.

  In the present embodiment, after the color space conversion is performed in this way, the original luminance Yorg of the color and the chromaticity of the color for the color that has a value outside the range defined in the color space after the conversion. The target brightness Ytgt is determined between them, for example, based on the brightness Ymod (Ymin or Ycuf) when the brightness is changed while maintaining the brightness and corrected to a color within the range. Then, the converted color (the color represented by the target value) is corrected so that the target luminance value is obtained.

  Here, as described above, the target luminance Ytgt may be determined at a point divided by a predetermined ratio (α or β described above) between Yorg and Ymod. These ratios may be determined based on at least one of the lightness or hue of the color represented by the target value. That is, it can be determined empirically as a function of the lightness or hue of the color represented by the target value.

  The ratio α when the target value exceeds the upper limit of the range (overflow) and the ratio β when the target value falls below the lower limit of the range (underflow) may be set to different values. Good.

  As described above, according to the present embodiment, it is possible to perform color adjustment while considering the balance between lightness and saturation.

It is a block diagram showing the structure of the image processing apparatus which concerns on embodiment of this invention, and its connection example. It is a functional block diagram showing the example of the image processing apparatus which concerns on embodiment of this invention. It is a flowchart figure showing the process example of the image processing apparatus which concerns on embodiment of this invention. It is a flowchart figure showing the process example of the image processing apparatus which concerns on embodiment of this invention. It is the schematic showing the operation example of the image processing apparatus which concerns on embodiment of this invention.

Explanation of symbols

  1 image processing device, 2 output device, 3 client device, 11 control unit, 12 storage unit, 13 communication unit, 21 color space conversion unit, 22 post-processing unit, 23 color conversion unit.

Claims (10)

Means for converting each color value in the image data to be processed into a value in a predetermined color space;
Means for storing a pre-defined value range for the predetermined color space;
Offset calculation means for calculating an offset value for adjustment based on a predetermined target luminance when any color value is converted to a value exceeding the upper limit of the stored value range by the conversion. When,
The calculated offset value is added to each value after the conversion, and the maximum value of each value after the addition becomes the upper limit of the color space, and the luminance of each value after the conversion becomes the target luminance. Means for correcting each of the converted values;
Only including,
The target brightness is the brightness when the color represented by the converted value is the target color and the brightness is adjusted to a color within the value range while maintaining the chromaticity of the target color, and the target color An image processing apparatus characterized in that the brightness is set between the two . The image processing apparatus according to claim 1 .
The target brightness is the brightness when the color represented by the converted value is the target color and the brightness is adjusted to a color within the value range while maintaining the chromaticity of the target color, and the target color An image processing apparatus characterized in that the luminance is a value obtained by weighted averaging using a predetermined ratio. The offset calculating means is a component value maxRGB which is the maximum value in the converted color value.
Is multiplied by the target luminance value Ytgt, and the value obtained by subtracting the converted luminance Yorg is divided by the value obtained by subtracting the target luminance value Ytgt from the upper limit value of luminance to obtain the offset value. the image processing apparatus according to claim 1 or 2, characterized. The means for correcting each value after the conversion adds the offset value to the value of each component after the conversion, and corrects by normalizing with the maximum value of the value of each component after the addition. The image processing apparatus according to any one of claims 1 to 3 . Means for converting a color value in the image data to be processed into a value in a predetermined color space;
Means for storing a pre-defined value range for the predetermined color space;
When the color value is converted to a value outside the stored value range by the conversion, the color represented by the converted value is set as the target color, and the chromaticity of the target color is changed close to white. A means for setting a target luminance between the luminance when the luminance is adjusted to a color within the range and the luminance of the target color;
Means for correcting the color represented by the converted value to the color of the set target luminance;
An image processing apparatus comprising: The image processing apparatus according to claim 5 .
The target luminance is the color when the color represented by the converted value is the target color, the chromaticity of the target color is changed to be close to white, the luminance is changed to the color within the range, and the luminance is changed. An image processing apparatus, wherein the luminance of the target color is a value obtained by weighted averaging using a predetermined ratio. The image processing apparatus according to claim 6 .
The image processing apparatus according to claim 1, wherein the ratio is determined on the basis of at least one of color brightness and hue represented by the converted value. The image processing apparatus according to claim 7 .
The ratio is set to a different value when the converted value exceeds the upper limit of the range and when the converted value is below the lower limit of the range. . Converting each color value in the image data to be processed into a value in a predetermined color space;
Storing a predefined range of values for the predetermined color space;
An offset calculation step of calculating an offset value for adjustment based on a predetermined target brightness when any one of the color values is converted to a value exceeding the upper limit of the stored value range by the conversion. When,
The calculated offset value is added to each value after the conversion, and the maximum value of each value after the addition becomes the upper limit of the color space, and the luminance of each value after the conversion becomes the target luminance. , Correcting each value after the conversion;
Only including,
The target brightness is the brightness when the color represented by the converted value is the target color and the brightness is adjusted to a color within the value range while maintaining the chromaticity of the target color, and the target color An image processing method characterized in that the image processing method is set in between . Computer
Means for converting each color value in the image data to be processed into a value in a predetermined color space;
Means for storing a pre-defined value range for the predetermined color space;
Offset calculation means for calculating an offset value for adjustment based on a predetermined target luminance when any color value is converted to a value exceeding the upper limit of the stored value range by the conversion. When,
The calculated offset value is added to each value after the conversion, and the maximum value of each value after the addition becomes the upper limit of the color space, and the luminance of each value after the conversion becomes the target luminance. Means for correcting each of the converted values;
To function as,
The target brightness is the brightness when the color represented by the converted value is the target color and the brightness is adjusted to a color within the value range while maintaining the chromaticity of the target color, and the target color A program characterized by being set in between .

Images