Color display apparatus and method of coding a color image

DETAILED DESCRIPTION Another way of measuring the limitations of the HVS is to assess its ability to detect small targets against a background.
This is referred to as the "target model" and is especially favoured by those working on military applications.
Working from assumptions derived from the first body of work, we expected a correlation between the area of a target and the contrast needed for it to be detected.
Experiment showed that in fact the linear size of the target (rather than the area) was the controlling factor, and we have discovered a theoretical justification for this.
It seems that no worker in this field has compared the results described by the frequency and target models of the HVS.
If we do compare them, we find that they apparently conflict.
We can show how these differences can be resolved, with important implications for the display of color images, and we can show that fewer bits are required for the display of color images than might be expected.
Accordingly, the present invention provides display apparatus for producing a color image in which the color of each picture element is defined by the intensities of component red, blue wherein the color green primaries and is coded as a binary number having r bits defining 2.
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r intensity levels for red, g bits defining 2.
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g intensity levels for green, and b bits defining 2.
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b intensity levels for blue, where g<7, (g-r)=1 or 2, and (g-b)=2.
Heretofore it was generally assumed that for the display of high quality color images up to 8 bits would be needed for each of three Red, Green and Blue primaries, i.
e.
24 bits in all would be needed to define each picture element in the color image.
The invention is therefore based on the surprising discovery that, for the average HVS, a maximum of 7 bits are needed for Green, with one or two bits less for Red, and two bits less for Blue.
It will be appreciated that this provides substantial savings in storage in high quality image processing and graphics systems.
While 7 bits for Green is the maximum needed in any circumstances, we have found that in practice for the majority of applications using high quality color cathode ray tube (CRT) displays, which have a resolution of substantially 4 pels (picture elements) per mm or greater and are typically viewed at a distance of 400 mm, a maximum of only 4 bits are needed for Green, with correspondingly 2 bits less for Blue and 1 or 2 bits less for Red depending on the wavelength of the Red primary