Conversion of color images to gray value images

• US 20030058475A1
• Filed: 08/23/2002
• Published: 03/27/2003
• Est. Priority Date: 08/23/2001
• Status: Active Grant

• Current Assignee: Océ-Technologies B.V. (Canon Inc.)
• Original Assignee: Océ-Technologies B.V. (Canon Inc.)
• Inventors: Hofman, Paul Michiel
• Granted Patent: US 7,440,138 B2
• Time in Patent Office: Days
• US Class Current: 358/3.01
• CPC Class Codes: H04N 1/40012 Conversion of colour to monochrome

Abstract

A method of converting a digital color image to a digital gray value image and in so doing distinguishably reproducing colors present in the color image is provided. The digital color image is built up of pixels each having a color value, and the digital gray value image is built up of pixels each having a gray value. The color values of the pixels are converted to gray values by the use of a dynamically adapted conversion function which is optimized on the basis of a comparison of color differences in the digital color image and gray value differences corresponding to the color differences in the digital gray value image. In this way, the colors which in conventional conversion methods are imaged on identical or substantially identical gray values can effectively be distinguishably reproduced.

Claims

1. A method of converting a digital color image to a digital gray value image and in so doing distinguishably reproducing colors present in the color image, the digital color image being built up of pixels each having a color value and the digital gray value image being built up of pixels each having a gray value, the method comprising:

converting the color values of the pixels to gray values by using a dynamically adapted conversion function which is optimized on the basis of a comparison of color differences in the digital color image and gray value differences corresponding to said color differences in the digital gray value image.

2. A method according to claim 1, wherein said optimization is carried out automatically in accordance with predetermined criteria.

3. A method according to claim 1, wherein said optimization is carried out automatically in accordance with predetermined criteria, for each color image for conversion or for each group of color images for conversion separately.

4. A method according to claim 1, wherein said conversion function is also optimized on the basis of a comparison of gray values in the digital gray value image after conversion with said conversion function and gray values valid for the same pixels in the digital gray value image after conversion with a predefined naï

ve conversion function.

5. A method according to any one of claim 1, wherein said optimization is based on color differences between a limited number of main color values, which are defined by a cluster analysis of the colors of the color image.

6. A method according to claim 5, wherein the conversion function is a combination of a pre-defined naï

ve gray value conversion and a localized correction function, wherein the correction function has a value which decreases with the distance in a color space between a color for conversion and a main color.

7. A method of converting a digital color image, built up of pixels, to a gray value image and in so doing distinguishably reproducing colors present in the color image, the method comprising:

(a) determining main color values of the digital color image by using a cluster analysis of the colors of the pixels in a color space;

(b) converting said main color values to gray values by using a predefined naï

ve conversion function;

(c) calculating a distance in a color space of main color values mutually to produce a color distance;

(d) calculating a distance on a gray value scale of gray values corresponding to the main color values mutually to produce a gray distance; and

(e) compiling an adapted conversion function for optimization of the agreement between corresponding gray distances and color distances.

8. A method according to claim 7, wherein the step (e) comprises calculating a localized correction function by using the optimization of the agreement between the corresponding gray distances and color distances for said main colors, and combining the nï

ve conversion function and the localized correction function.

9. An apparatus for converting a digital color image to a gray value image, the apparatus being provided with a memory for storing a digital color image and a processing unit for processing digital data, the apparatus comprising:

a cluster module for determining main color values of the digital color image by using a cluster analysis of the colors of the pixels in a color space;

a conversion module for converting color values to gray values by using a conversion function;

a color distance module for calculating the color distance of color values in a color space;

a gray distance module for calculating the gray distance of gray values on a gray value scale; and

an adaptation module for compiling an adaptive conversion function for optimization of the agreement between corresponding gray distances and color distances.

10. An apparatus according to claim 9, wherein the adaptation module is adapted to calculate a localized correction function by using the optimization of the agreement between the corresponding gray distances and color distances for said main colors, and combining the naï

ve conversion function and the localized correction function.

11. A computer program product embodied on a computer-readable medium, for converting a digital color image to a gray value image, the computer program product comprising computer-executable instructions for:

(a) determining main color values of the digital color image by using a cluster analysis of the colors of the pixels in a color space;

(b) converting said main color values to gray values by using a predefined naï

ve conversion function;

(c) calculating a distance in a color space of main color values mutually to produce a color distance;

(d) calculating a distance on a gray value scale of gray values corresponding to the main color values mutually to produce a gray distance; and

(e) compiling an adapted conversion function for optimization of the agreement between corresponding gray distances and color distances.

12. A computer program product embodied on a computer-readable medium, for converting a digital color image to a digital gray value image and in so doing distinguishably reproducing colors present in the color image, the digital color image being built up of pixels each having a color value and the digital gray value image being built up of pixels each having a gray value, the computer program product comprising computer-executable instructions for:

converting the color values of the pixels to gray values by using a dynamically adapted conversion function which is optimized on the basis of a comparison of color differences in the digital color image and gray value differences corresponding to said color differences in the digital gray value image.

13. A computer program product according to claim 12, wherein said optimization is carried out automatically in accordance with predetermined criteria.

14. A computer program product according to claim 12, wherein said conversion function is also optimized on the basis of a comparison of gray values in the digital gray value image after conversion with said conversion function and gray values valid for the same pixels in the digital gray value image after conversion with a predefined naï

ve conversion function.

15. A computer program product according to claim 12, wherein said optimization is based on color differences between a limited number of main color values, which are defined by a cluster analysis of the colors of the color image.

16. A digital copier provided with a scanner, a processing unit for processing digital image data and a printing unit, wherein the processing unit includes a conversion apparatus comprising:

a cluster module for determining main color values of a digital color image by using a cluster analysis of the colors of the pixels in a color space;

a conversion module for converting color values to gray values by using a conversion function;

a color distance module for calculating the color distance of color values in a color space;

a gray distance module for calculating the gray distance of gray values on a gray value scale; and

an adaptation module for compiling an adaptive conversion function for optimization of the agreement between corresponding gray distances and color distances.

17. A digital copier according to claim 16, further comprising:

a user interface provided with means for switching said conversion apparatus so that the color-to-gray value conversion is performed optionally by a pre-defined naï

ve conversion function or by an automatically adapted conversion function.

18. A printer provided with a network connection unit for receiving externally generated print orders, a processing unit for processing digital image data and a printing unit, wherein the processing unit is provided with a conversion apparatus for converting a digital color image to a gray value image, the conversion apparatus comprising:

a cluster module for determining main color values of the digital color image by using a cluster analysis of the colors of the pixels in a color space;

a conversion module for converting color values to gray values by using a conversion function;

a color distance module for calculating the color distance of color values in a color space;

a gray distance module for calculating the gray distance of gray values on a gray value scale; and

an adaptation module for compiling an adaptive conversion function for optimization of the agreement between corresponding gray distances and color distances.

19. A printer according to claim 18, wherein said conversion apparatus is programmable so that a color-to-gray value conversion is performed optionally by using a predefined naï

ve conversion function or by using an automatically adapted conversion function.