Method and apparatus for mapping a digital color image from a first color space to a second color space

• US 5,233,684 A
• Filed: 06/26/1990
• Issued: 08/03/1993
• Est. Priority Date: 06/26/1990
• Status: Expired due to Term

• Current Assignee: Hewlett-Packard Development Company, L.P. (HP Inc.)
• Original Assignee: Digital Equipment Corporation (HP Inc.)
• Inventors: Ulichney, Robert A.
• Primary Examiner(s): Zimmerman, Mark K.
• Application Number:
  US07/545,384
• Time in Patent Office: 1,134 Days
• Field of Search: 395/131, 340/703, 358/80
• US Class Current: 345/604
• 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 and apparatus including two look-up operations in two series coupled look-up tables coupled to an output of a dynamic quantization device to provide on-line representation mapping of digital color data from a first color space to a second color space.

Claims

1. An apparatus for mapping a digital color image comprising a plurality of pixel data elements, from a first color space to a second color space, which apparatus comprises:

a dynamic quantization device adapted to quantize each of the plurality of pixel data elements, represented in first color space components, to one of a preselected set of quantized color data values, represented in the first color space components;

a first look-up table coupled to said dynamic quantization device, said first look-up table receiving as input the quantized color data values corresponding to the plurality of pixel data elements;

said first look-up table containing a color index for each one of the quantized color values and being adapted to output a respective color index accessed by each one of the quantized color data values input to said first look-up table;

a second look-up table coupled to said first look-up table, said second look-up table receiving as input each color index accessed by the quantized color data values input to said first look-up table;

said second look-up table containing a unique n-bit data element for each color index and being adapted to output a respective n-bit data element in response to input of each color index accessed by the quantized color data values input to the first look-up table;

wherein each n-bit data element corresponds to at least one of the preselected set of quantized color data values and represents a respective corresponding at least one quantized color data value in second color space components;

wherein there are a fixed number of color indices, and wherein a number of quantized color data values in the preselected set of quantized color data values equals;

the number of color indices divided by a second color space-to-first color space volume ratio;

a look-up table generator coupled to each of the first and second look-up tables for generating and storing the color indices and the n-bit data elements in the first and second look-up tables, respectively;

wherein said look-up table generator is coupled to said dynamic quantization device and generates, as a function of the fixed number of color indices, and transmits to the dynamic quantization device, the number of quantized color data values in the preselected set of quantized color data values.

2. An apparatus for mapping a digital color image comprising a plurality of pixel data elements, from a first color space to a second color space, which apparatus comprises:

a dynamic quantization device adapted to quantize each of the plurality of pixel data elements, represented in first color space components, to one of a preselected set of quantized color data values, represented in the first color space components;

a first look-up table coupled to said dynamic quantization device, said first look-up table receiving as input the quantized color data values corresponding to the plurality of pixel data elements;

said first look-up table containing a color index for each one of the quantized color values and being adapted to output a respective color index accessed by each one of the quantized color data values input to said first look-up table;

a second look-up table coupled to said first look-up table, said second look-up table receiving as input each color index accessed by the quantized color data values input to said first look-up table;

said second look-up table containing a unique n-bit data element for each color index and being adapted to output a respective n-bit data element in response to input of each color index accessed by the quantized color data values input to the first look-up table;

wherein each n-bit data element corresponds to at least one of the preselected set of quantized color data values and represents a respective corresponding at least one quantized color data value in second color space components;

an address generator intercoupling said dynamic quantization device and said first look-up table, said address generator generates an address for each quantized data value output by said dynamic quantization device as a function of each respective quantized data value for access to said first look-up table;

wherein said first color space comprises a YUV color space and said second color space comprises an RGB color space, and wherein said address generator concantonates digital Y, U and V values of each one of the quantized color data values input to said address generator to generate the respective address.

3. An apparatus for mapping a digital color image comprising a plurality of pixel data elements, from a first color space to a second color space, which apparatus comprises:

a dynamic quantization device adapted to quantize each of the plurality of pixel data elements, represented in first color space components, to one of a preselected set of quantized color data values, represented in the first color space components, the dynamic quantization device including a color dithering system;

a first look-up table coupled to said dynamic quantization device, said first look-up table receiving as input the quantized color data values corresponding to the plurality of pixel data elements;

said first look-up table containing a color index for each one of the quantized color values and being adapted to output a respective color index accessed by each one of the quantized color data values input to said first look-up table; and

a second look-up table coupled to said first look-up table, said second look-up table receiving as input each color index accessed by the quantized color data values input to said first look-up table;

said second look-up table containing a unique n-bit data element for each color index and being adapted to output a respective n-bit data element in response to input of each color index accessed by the quantized color data values input to the first look-up table;

wherein each n-bit data element corresponds to at least one of the preselected set of quantized color data values and represents a respective corresponding at least one quantized color data value in second color space components.

4. A method for mapping a digital color image comprising a plurality of pixel data elements, from a first color space to a second color space, comprising the steps of:

(a) quantizing each one of the plurality of pixel data elements represented in first color space components to one of a preselected set of quantized color data values represented in the first color space components;

(b) calculating color indices for storage in a first look-up table by;

i) calculating a second color space value for each one of the preselected quantized color data values;

ii) ascertaining a feasibility of each second color space value calculated in step i);

iii) identifying each quantized color data value corresponding to a feasible second color space value as ascertained in step ii);

iv) mapping each quantized color data value corresponding to a non-feasible second color space value, as ascertained in step ii), to a representative feasible second color space value;

v) assigning a unique color index to each feasible second color space value as ascertained in step ii) and to each representative feasible second color space value as mapped in step iv); and

vi) pairing each quantized color data value with a respective one color index that corresponds to the feasible second color space value corresponding to the respective quantized color data value;

(c) utilizing each quantized color data value to look-up the corresponding color index stored in the first look-up table; and

(d) utilizing each color index to look-up a corresponding n-bit data element stored in a second look-up table, wherein each corresponding n-bit data element further corresponds to the quantized color data value utilized to look-up the corresponding color index, and represents the corresponding quantized color data value in second color space components.

5. The method of claim 4 comprising the further step of:

calculating the corresponding n-bit data elements by calculating an n-bit data element for each feasible second color space value and pairing each calculated n-bit data element with a respective one color index that corresponds to the feasible second color space value corresponding to the respective calculated n-bit data element.

6. The method of claim 4 wherein the step of mapping is carried out by:

i) projecting each quantized color data element corresponding to a non-feasible second color space value to a feasible second color space value;

ii) listing each feasible second color space value determined in the projecting step i) in a predetermined order;

iii) selecting certain ones of the feasible second color space values listed in the listing step ii), as a function of the predetermined order, to be the representative feasible second color space values; and

iv) pairing each representative feasible second color space value with at least one quantized color data value corresponding to a non-feasible second color space value.

7. The method of claim 6 wherein the predetermined order comprises a shortest distance to longest distance order wherein distance represents geometric distance from each quantized color data value to its projected-to feasible second color space value.

8. The method of claim 4 wherein the step of calculating color indices includes the further steps of:

determining whether any of the preselected set of quantized color data values corresponding to feasible second color space values correspond to a black or white second color space value;

assigning a color index to a black second color space value when none of the quantized color data values corresponds to a black second color space value; and

assigning a color index to a white second color space value when none of the quantized color data values corresponds to a white second color space value.