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Pel Color Encoding with Room for Overflow

IP.com Disclosure Number: IPCOM000108462D
Original Publication Date: 1992-Jun-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 2 page(s) / 86K

Publishing Venue

IBM

Related People

Deacon, JJ: AUTHOR [+2]

Abstract

Often three color values must be combined into a single computer word for use in a lookup table in image processing. In this article the three color values are first used as pointers into three lookup tables, and the numbers from these tables are summed to produce the combined result. The states of this sum map to a colorspace, with edge lengths that do not need to be powers of 2, can provide room for overflow and underflow of the original color values, and can allocate states among colors in a nonlinear fashion that better matches visual response. (Image Omitted)

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Pel Color Encoding with Room for Overflow

       Often three color values must be combined into a single
computer word for use in a lookup table in image processing. In this
article the three color values are first used as pointers into three
lookup tables, and the numbers from these tables are summed to
produce the combined result.  The states of this sum map to a
colorspace, with edge lengths that do not need to be powers of 2, can
provide room for overflow and underflow of the original color values,
and can allocate states among colors in a nonlinear fashion that
better matches visual response.

                            (Image Omitted)

      First the problem is stated.  There is a red, green, and blue
value which will be assembled into a single 16-bit color word.
Several things can be done with this one word that contains all the
colors; one of these things is to use it as a pointer into a lookup
table that tells the closest color available in some color palette.

      A common algorithm used in the past received the red, green,
and blue values, clipped their range to eliminate underflow and
overflow, normalized their range so red and blue each occupied 5 bits
and green 6 bits, then copied these 5+6+5 bits into contiguous places
in the 16-bit color word.  This process limited the number of states
of each color to be a power of 2.

      Next the characteristics of an ideal solution are stated.  To
match the human eye, red should have twice the number of states as
blue, and green four times as many. Also, to allow for temporary
overflow for use in error diffusion, states beyond pure white or
black should be represented. The color values begin as binary
numbers, with black as 0 and white a power of 2 minus 1, for example
15 or 31 or 63.  Let:
  blue  go from  0 > 15  total range of -2 > 17  for 20 total
states   red   go from  0 > 31  total range of -4...