Color Conversion Method, Color Conversion Table Created by the Color Conversion Method, Image Processing Apparatus, and Color Conversion Program

• US 20090310157A1
• Filed: 05/20/2009
• Published: 12/17/2009
• Est. Priority Date: 06/17/2008
• Status: Active Grant

• Current Assignee: Seiko Epson Corporation (Seiko Group)
• Original Assignee: Seiko Epson Corporation (Seiko Group)
• Inventors: WADA, Takeshi
• Granted Patent: US 8,077,353 B2
• Time in Patent Office: Days
• US Class Current: 358/1.900
• CPC Class Codes: H04N 1/6058 Reduction of colour to a range of reproducible colours, e.g. to ink- reproducible colour gamut

Abstract

There is provided a color conversion method. The method includes: a black correction step of converting the first black corresponding to the first color data having a tone value of 0 to the second black corresponding to the second color data having a tone value of 0 so that when the first black corresponding to the first color data having a tone value other than 0 is converted to a value smaller than a quantization threshold of tone values 0 and 1 of the second color data, a conversion destination becomes greater than or equal to the quantization threshold; a recording agent limit quantity calculating step of setting an upper limit of the sum of tone values for each pixel of the second color data with respect to a predetermined print medium; and a chromatic color correcting step of converting the second chromatic color so that the second color data after conversion in the black correction step does not exceed the upper limit.

Claims

1. A color conversion method for converting first color data in a first color space to second color data in a second color space, based on a first profile that defines the correspondence between the first color space that is expressed by a first chromatic color and a first black and a predetermined device-independent color space, a second profile that defines the correspondence between the second color space that is expressed by a second chromatic color and a second black and the device-independent color space, and a color conversion rule in the device-independent color space, the method comprising:

a black correction step of converting the first black corresponding to the first color data having a tone value of 0 to the second black corresponding to the second color data having a tone value of 0 so that when the first black corresponding to the first color data having a tone value other than 0 is converted to a value smaller than a quantization threshold of tone values 0 and 1 of the second color data, a conversion destination becomes greater than or equal to the quantization threshold;

a recording agent limit quantity calculating step of setting an upper limit of the sum of tone values for each pixel of the second color data with respect to a predetermined print medium; and

a chromatic color correcting step of converting the second chromatic color so that the second color data after conversion in the black correction step does not exceed the upper limit.

2. The color conversion method according to claim 1, wherein in the chromatic color correcting step, the second chromatic color of the conversion destination is calculated by an interpolation calculation on the basis of a combination that does not exceed the upper limit, among combinations of the second chromatic color and the second black defined in the second profile.

3. The color conversion method according to claim 1, wherein in the chromatic color correcting step, the chromatic colors are converted to have nearest hue values before and after the conversion.

4. The color conversion method according to claim 1, further comprising a color gamut conversion step of forming a second chromatic color gamut that represents a color reproducible range producible by the second chromatic color without changing a quantity of black on the basis of the upper limit so that a color point located outside the second chromatic color gamut is converted to a color point located inside the second chromatic color gamut.

5. The color conversion method according to claim 1, wherein when for a predetermined hue in the predetermined device-independent color space, there is an out-of-gamut region which is out of a color gamut of a second chromatic color gamut that represents a color reproducible range expressible by the second chromatic color without changing a quantity of the second black within the range of the upper limit and which is in a color gamut of a second color space formed by the first profile and the color conversion rule, the method further comprises:

a detecting step of detecting a lightness at a maximum chroma of the color gamut of the second chromatic color gamut in the predetermined hue;

a color gamut conversion step of compressing a color gamut shape of the second color space based on a color gamut shape of the second chromatic color gamut by performing a color gamut conversion process of mapping to be in the color gamut of the second chromatic color gamut while compressing both chromas and lightnesses on a high-lightness side of the lightness detected in the detecting step, while performing a color gamut conversion process of mapping to be in the color gamut of the second chromatic color gamut while maintaining approximately constant lightnesses on a low-lightness side of the lightness detected in the detecting step and compressing chromas, thereby defining the correspondence between the color gamut of the second color space and the color gamut of the second chromatic color gamut; and

a color conversion step of converting the first color data to the second color data on the basis of the correspondence.

6. The color conversion method according to claim 5, wherein in the color gamut conversion step, on the high-lightness side of the lightness detected in the detecting step, when a color gamut shape of the second color space is fitted to a color gamut shape of the second chromatic color gamut, a color gamut conversion process of mapping to be in the color gamut of the second color space is performed by gradually decreasing a compression ratio of lightnesses as a lightness to be compressed gets closer to the lightness detected in the detecting step.

7. The color conversion method according to claim 5, wherein in the color gamut conversion step, when a color gamut shape of the second color space is fitted to a color gamut shape of the second chromatic color gamut, a color gamut conversion process is performed while performing the compression so that at the lightness detected in the detecting step, a compression ratio and a compression direction on the high-lightness side of the lightness detected in the detecting step coincide with a compression ratio and a compression direction on the low-lightness side of the lightness detected in the detecting step.

8. The color conversion method according to claim 5, wherein in the color gamut conversion step, when the color gamut shape of the second color space is fitted to the color gamut shape of the second chromatic color gamut on the high-lightness side of the lightness detected in the detecting step, a color gamut conversion process of compressing the color gamut shape toward the lightness detected in the detecting step on a lightness axis of the device-independent color space while mapping into the color gamut of the second color space with a compression ratio corresponding to a distance from an outer shell of the color gamut of the second chromatic color gamut is performed.

9. The color conversion method according to claim 5, wherein in the color gamut conversion step, when the color gamut shape of the second color space is fitted to the color gamut shape of the second chromatic color gamut on the high-lightness side of the lightness detected in the detecting step, a color gamut conversion process of limiting a compression destination of the out-of-gamut region to a predetermined range from the outer shell of the second chromatic color gamut and normalizing the out-of-gamut region and the predetermined range in the compression direction with respect to the predetermined range is performed.

10. An image processing apparatus for converting first color data in a first color space to second color data in a second color space, based on a first profile that defines the correspondence between the first color space that is expressed by a first chromatic color and a first black and a predetermined device-independent color space, a second profile that defines the correspondence between the second color space that is expressed by a second chromatic color and a second black and the device-independent color space, and a color conversion rule in the device-independent color space, the apparatus comprising:

a black correction unit capable of converting the first black corresponding to the first color data having a tone value of 0 to the second black corresponding to the second color data having a tone value of 0 and converting the first black corresponding to the first color data having a tone value other than 0 to the second black corresponding to the second color data having a tone value other than 0;

a recording agent limit quantity calculating unit capable of setting an upper limit of the sum of tone values for each pixel of the second color data with respect to a predetermined print medium as a recording agent duty limit; and

a chromatic color correcting unit capable of converting the second chromatic color so that the second color data after conversion by the black correction unit meets the recording agent duty limit.

11. A computer readable record medium encoded a color conversion program for converting first color data in a first color space to second color data in a second color space, based on a first profile that defines the correspondence between the first color space that is expressed by a first chromatic color and a first black and a predetermined device-independent color space, a second profile that defines the correspondence between the second color space that is expressed by a second chromatic color and a second black and the device-independent color space, and a color conversion rule in the device-independent color space, the color conversion program causing a computer to implement:

a black correction function of converting the first black corresponding to the first color data having a tone value of 0 to the second black corresponding to the second color data having a tone value of 0 and converting the first black corresponding to the first color data having a tone value other than 0 to the second black corresponding to the second color data having a tone value other than 0;

a recording agent limit quantity calculating function of setting an upper limit of the sum of tone values for each pixel of the second color data with respect to a predetermined print medium as a recording agent duty limit; and

a chromatic color correcting function of converting the second chromatic color so that the second color data after conversion by the black correction function meets the recording agent duty limit.