Rich Franzen
Pseudogrey, a high resolution (1,786 levels) scale of near-grey RGB triples.
The author described the design in a 1999 posting on the comp.graphics.algorithms newsgroup
I've finally implemented a concept that has been on the back of my mind for some time. I call it pseudoGrey. It is a method to encode more than 8 bits of greyscale within a 24-bit color image. Using the technique, exactly 1,786 levels of grey can be encoded and decoded. The algorithm borrows from the concept of luma, in that the “plusses” map roughly to the luma weights of the three color components.
To use this technique, you begin with a 12-bit greyscale number. The base 8-bit value for each rgb element is the 12-bit value right-shifted by four. Then 1 is added to none, one, or two of the components by examining the low-order nibble of input. On:
2.. 4 -> blue + 1 5.. 6 -> red + 1 7.. 8 -> red + 1, blue + 1 9..10 -> green + 1 11..13 -> green + 1, blue + 1 14..15 -> green + 1, red + 1I don't think anyone can actually see the difference between 256 and 1786 levels of grey. However, without degrading the color image, you preserve analytical detail which would otherwise be lost. Of course, you need to have started with at least 512 levels of grey to get any benefit. Film and many scanners do provide data sources that might take advantage of this technique.
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