Method of rendering a color image for an output medium from symbolic image data

• US 5,384,901 A
• Filed: 10/22/1992
• Issued: 01/24/1995
• Est. Priority Date: 10/22/1992
• Status: Expired due to Term

• Current Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Original Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Inventors: Glassner, Andrew S., Stone, Maureen C., Marimont, David H.
• Primary Examiner(s): Nguyen, Phu K.
• Application Number:
  US07/965,685
• Time in Patent Office: 824 Days
• Field of Search: 340/703, 340/747, 340/750, 395/129, 395/131, 345/153, 345/154, 345/155, 358/456, 358/443, 358/80, 358/75
• US Class Current: 345/591
• CPC Class Codes: G06T 11/001 Texturing; Colouring; Generation of texture or colour inpainting G06T5/77
• CPC Class Codes: H04N 1/6058 Reduction of colour to a range of reproducible colours, e.g. to ink- reproducible colour gamut

Abstract

A method of rendering a color image on a designated output medium is disclosed which maps colors to the gamut of the designated output medium while preserving the semantic consistency of the object color and illumination information in the image. The method performs gamut mapping earlier in the image synthesis process than current gamut method methods, at the point where information about object primitives and their spectral attributes in a scene description is available, but after the fixed scene geometry has been determined by the rendering system. The method makes use of the output of a symbolic rendering system which produces symbolic pixel expressions, having basis spectra variables which represent the interplay of light and object primitives in the scene description, and spectral data having color information about the light and object primitives in the scene, and which is indexed to the basis spectra variables. The method performs spectral change calculations using the symbolic pixel expressions, the spectral data, and spectral information about the gamut of the specific output medium to determine the modifications to the original spectral data that need to be made to the individual object primitives in the scene in order to produce image pixel colors which are in the gamut of the output medium. The resulting image colors are locally and globally consistent with the semantics of the image, make effective use of as much of the available gamut of the display device as is possible, and require no further post-rendering gamut mapping prior to display or reproduction on the designated medium.

Claims

1. A method of producing an output color image having colors included in a device gamut of a color reproduction device;

the method comprising;

providing symbolic color image data symbolically representing a plurality of original image colors from an original color image rendered from a scene description having a plurality of object primitives;

providing indexed color description data defining an original color for each of the object primitives in the scene description;

the symbolic color image data being indexed by the indexed color description data so that evaluating the symbolic color image data using the indexed color description data produces the plurality of original image colors of the original color image;

providing device gamut color data representing the device gamut of the color reproduction device;

performing spectral change calculations using the symbolic color image data, the indexed color description data, and the device gamut color data to produce modified indexed color description data;

the spectral change calculations modifying the indexed color description data until the output color image satisfies an image metric defining a desired relationship among the original color image, the device gamut and the output color image; and

producing the output color image having the colors included in the device gamut of the color reproduction device using the symbolic color image data and the modified indexed color description data.

2. The method of claim 1 wherein performing spectral change calculations includes

(a) determining color modification data to be used to modify the indexed color description data;

(b) evaluating the symbolic color image data with the indexed color description data and the color modification data to generate a current color image composed of a plurality of modified colors;

(c) generating a target color image from the current color image using the device gamut color data;

each of a plurality of target colors composing the target color image being one of the colors included in the device gamut of the color reproduction device and corresponding to one of the modified colors of the current color image;

(d) using the plurality of target colors composing the target color image to determine whether the current color image satisfies the image metric defining the desired relationship between the original color image, the device gamut and the output color image;

(e) if the current color image does not satisfy the image metric, repeating (a), (b), (c), and (d) until the current color image satisfies the image metric; and

(f) modifying the indexed color description data with the color modification data to produce the modified indexed color description data.

3. The method of claim 2 wherein

each modified color of the plurality of modified colors composing the current color image and each corresponding target color of the plurality of target colors composing the target color image is represented as a point in a color space mathematically representing colors;

and

the image metric defines the desired relationship between the original color image, the device gamut and the output color image according to a distance in the color space between one of the modified colors in the current color image and the corresponding one of the plurality of target colors in the target color image.

4. A method of producing an output color image comprising:

(a) assigning one of a plurality of change variables to a respective one of a plurality of symbolic color image data items;

each change variable having a current value;

(b) evaluating the plurality of symbolic color image data items using a plurality of indexed color description data items each indexed to respective ones of the symbolic color image data items to produce a current color image composed of current colors;

each current value of one of the plurality of change variables being applied to the indexed color description data item indexed by the respective ones of the symbolic color image data items;

(c) generating, from the current colors of the current color image, a target color image using device gamut color data defining a gamut of device colors of a color reproduction device;

each of a plurality of target colors composing the target color image being one of the device colors in the gamut and corresponding to a respective one of the current colors of the current color image;

(d) calculating difference data between each of the current colors and the corresponding one of the target colors;

(e) determining an incremental change quantity for the current value of each of the plurality of change variables using the difference data;

(f) determining from the difference data and from the incremental change quantities whether the current color image satisfies image metric data defining a desired relationship between the current color image and the output color image;

(g) if the current color image does not satisfy the image metric data, updating the current value of each of the change variables with the incremental change quantity therefor, and repeating steps (b), (c), (d), (e), (f) and (g); and

(h) applying the current value of each change variable to the indexed color description data item indexed by the respective symbolic color image data item to which the change variable was assigned to produce modified indexed color description data items;

the modified indexed color description data items, when evaluated with the symbolic color image data items, producing the output color image.

5. The method of claim 4 wherein

each modified color of the plurality of modified colors composing the current color image and each corresponding target color of the plurality of target colors composing the target color image is represented as a point in a color space mathematically representing colors;

and

the image metric data defines the desired relationship between the current color image and the output color image according to a distance in the color space between one of the current colors in the current color image and the corresponding target color in the target color image.

6. A method of operating a machine to produce an output color image having in-gamut colors included in a device gamut of a color reproduction device;

the machine including a memory for storing data and a processor connected for accessing and storing the data in the memory;

the method comprising;

(a) operating the processor of the machine to assign a change variable to a symbolic spectral component included in each of a plurality of symbolic color image data items stored in the memory of the machine;

each of the symbolic color image data items defining one of a plurality of original image pixel color data items composing an original color image rendered from a scene description;

(b) operating the processor of the machine to determine a new value for the change variable to produce a modified symbolic spectral component;

(c) operating the processor of the machine to evaluate each symbolic color image data item including the modified symbolic spectral component using one of a plurality of indexed color description data items to produce image pixel color data items composing a current color image;

each indexed color description data item defining an original color of one of a plurality of object primitives in the scene description;

the symbolic spectral component included in each symbolic color image data item indexing a respective one of the plurality of indexed color description data items to the symbolic color image data item;

(d) operating the processor of the machine to produce a plurality of target image pixel color data items composing a target color image from the image pixel color data items composing the current color image using a plurality of device gamut color data items representing the device gamut of the color reproduction device;

each target image pixel color data item being a device color in the device gamut;

(e) operating the processor to determine if the current color image satisfies an image metric defining a desired relationship between the original color image and the output color image;

(f) if the current color image does not satisfy the image metric, operating the processor to repeat steps (b), (c), (d), and (e) until the current color image satisfies the image metric; and

(g) operating the processor to apply the new value of the change variable assigned to the symbolic spectral component to the respective indexed color description data item indexed thereto to produce a plurality of modified indexed color description data items;

the plurality of modified indexed color description data items, when evaluated with the symbolic color image data items, producing the output color image having the in-gamut colors.

7. The method of claim 6 wherein operating the processor to determine a new value for the change variable includes the steps of:

(b.1) operating the processor to determine an incremental change quantity from a current value of the change variable; and

(b.2) operating the processor to apply the incremental change quantity to the current value of the change variable to obtain the new value.

8. The method of claim 7 wherein each symbolic color image data item includes a symbolic pixel function of the change variable and the symbolic spectral component;

and

wherein the step of operating the processor to determine an incremental change quantity includes;

(i) operating the processor to differentiate each symbolic pixel function with respect to the change variable therein to produce a differentiated symbolic pixel function;

(ii) operating the processor to construct a jacobian matrix in the memory of the machine from the differentiated symbolic pixel function;

(iii) operating the processor to compute and store in the memory of the machine a pseudo-inverse jacobian matrix from the jacobian matrix; and

(iv) evaluating the pseudo-inverse jacobian matrix using the current value of each change variable to find the incremental change quantity for each change variable.

9. The method of claim 6 further including, after step (a) and before (step (b),

(a.1) operating the processor of the machine to initialize the change variable with an initial value having no modification effect on the symbolic spectral component;

(a.2) operating the processor of the machine to evaluate each symbolic color image data item using the respective one of the indexed color description data items indexed thereto to produce the plurality of original image pixel color data items composing the original color image;

(a.3) operating the processor of the machine to store the original image pixel color data items in the memory of the machine; and

wherein each original image pixel color data item composing the original color image and each target color pixel color data item composing the target color image is represented as a point in a color space mathematically representing colors; and

wherein the desired relationship defined by the image metric is a distance quantity in the color space between each of the original image pixel color data items composing the original color image and a corresponding one of the target image pixel color data items; and

wherein the current color image satisfies the desired relationship defined by the image metric when the distance quantity is minimized.

10. A machine comprising:

input circuitry for obtaining symbolic color image data items and indexed color description data items;

each indexed color description data item defining an original color of one of a plurality of object primitives in a scene description;

each symbolic color image data item defining one of a first plurality of image pixel color data items composing an original color image rendered from the scene description;

the symbolic color image data items having symbolic spectral components indexing the indexed color description data items to the symbolic color image data items;

memory for storing the symbolic color image data items and the indexed color description data items;

output circuitry for connecting the machine to an output medium and for transferring selected data from the memory to the output medium; and

a processor connected for accessing the input circuitry to receive the symbolic color image data items and the indexed color description data items and to store the symbolic color image data items and the indexed color description data items in the memory;

the processor further being connected for accessing the output circuitry to transfer the selected data from the memory to the output circuitry;

the processor further being connected for accessing the data stored in the memory;

the data stored in the memory further including;

device gamut color data items representing a gamut of device colors capable of reproduction by a color reproduction device; and

instruction data indicating instructions the processor executes;

the processor, in executing the instructions, performing spectral change calculations using the symbolic color image data items, the indexed color description data items, and the device gamut color data items to produce modified indexed color description data items;

the spectral change calculations modifying the indexed color description data items so that the modified indexed color description data items, when evaluated with the symbolic color image data items, produce a second plurality of image pixel color data items composing an in-gamut color image;

the in-gamut color image satisfying an image metric defining a relationship between the original color image, the gamut of device colors and the in-gamut color impact;

the processor further, in executing the instructions, storing the modified indexed color description data items in the memory.

11. The machine of claim 10 wherein the processor further, in executing the instructions, transfers the modified indexed color description data items and the symbolic color image data items from the memory to the output circuitry;

the output circuitry transferring the modified indexed color description data items and the symbolic color image data items to the connected output medium.

12. The machine of claim 11 wherein the connected output medium is a data communications circuit.

13. The machine of claim 10 wherein the processor further, in executing the instructions, evaluates the symbolic color image data items with the modified indexed color description data items to produce the second plurality of image pixel color data items composing the in-gamut color image;

and wherein the processor further, in executing the instructions, transfers the second plurality of image pixel color data items to the output circuitry for transferring to the output medium.

14. The machine of claim 13 wherein the output medium is the color reproduction device having the gamut of device colors represented by the device gamut color data items stored in the memory of the machine.

15. The machine of claim 13 wherein the output medium is a data communications circuit.