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LUMINANCE DEPENDENT SCALING OF CHROMINANCE DATA

IP.com Disclosure Number: IPCOM000027154D
Original Publication Date: 1995-Jun-30
Included in the Prior Art Database: 2004-Apr-07
Document File: 8 page(s) / 335K

Publishing Venue

Xerox Disclosure Journal

Abstract

As described in the Xerox Color Encoding Standard (XNSS 288811, March 1989), Xerox has proposed a device independent method for specifying colors. This color description specifies a color in terms of luminance, or Y value, and two chrominance components, designated E and S. This color space is a simple linear transformation of a color space which uses the red, green, and blue components of a color to describe the color. This red-green-blue color space will be called RGB space. Many devices such as scanners, printers and displays require that colors be specified in their own device dependent RGB color space coordinates. It is therefore necessary to transform between YES space and RGB space.

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XEROX DISCLOSURE JOURNAL

LUMINANCE DEPENDENT Proposed Classification SCALING OF CHROMINANCE U.S. C1.358/000 DATA Int. C1. H04n
Robert J. Rolleston

As described in the Xerox Color Encoding Standard (XNSS 288811, March
1989), Xerox has proposed a device independent method for specifying colors. This color description specifies a color in terms of luminance, or Y value, and two chrominance components, designated E and S. This color space is a simple linear transformation of a color space which uses the red, green, and blue components of a color to describe the color. This red-green-blue color space will be called RGB space. Many devices such as scanners, printers and displays require that colors be specified in their own device dependent RGB color space coordinates. It is therefore necessary to transform between YES space and RGB space.

It is common today to specify the RGB color space coordinates of a color by three 8-bit numbers, each being an integer between 0 and 255 inclusive. If the YES color space coordinates of a color are also to be represented by three 8-bit numbers, it will be necessary to map the actual E and S coordinates to the range of 0 to 255 inclusive for storage as unsigned integers. Proposed is a method of mapping E and S values which involves a shifting and scaling dependent upon the corresponding Y value of the color.

Current Luminance Independent Scaling

If each color space is quantized to a given number of levels, then there may be errors in transforming a color from one space to another and back again. A typical example of such an operation may be to scan in an image with colors specified in RGB space, convert the color descriptions to YES space for transmission over a network, and finally convert back to RGB space for display at a CRT.

Ifthe RGB color space is quantized to 8-bits, with values ranging from 0 to 255 for each of the components, the Y component will range from 0 to 255, and both the E and S components will range from -127.5 to + 127.5. If YES space is now linearly quantized to 8-bits for each component, and these values are transformed back to RGB space, some errors in color are introduced.

XEROX DISCLOSURE JOURNAL - Vol. 20, No. 3 MayiJune 1995 275

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LUMINANCE DEPENDENT SCALING OF CHROMINANCE DATA

(Cont'd)

In this example, all the E and S components were quantized by first adding a bias value of 127, then rounding to the nearest integer between 0 and 255 inclusive. These operations are expressed mathematically as

and

~

E' = (int)( E + 127. + .5)

S' = (int)(S + 127. + .5)

where the addition of .5 insures that the truncation performed by the int operation is equivalent to rounding to the nearest integer. The inverse mapping is

(3)

(1)

(2)

E =E' - 127.

and

S = S' - 127. (4)

The errors which result from this type of mapping are shown in a tabulated form in Table 1.

 Error Red Green Difference( LAB)

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

4

5

6

7...