🚀 go-pugleaf

In article <1icliuINNfkj@cascade.cs.ubc.ca>, jwu@cs.ubc.ca (Joseph Wu) writes:
> salewski@tnt.uni-hannover.de (Carsten Salewski) writes:
> 
> :	R = V/cv + Y
> :	B = U/cu + Y
> :	G = (Y - 0.299 R - 0.114 B) / 0.587

This is obviously referring to the PAL television YUV colour space, it is a
simple linear transfrom of RGB so nothing is gained by using it.

> 
> I don't know what you are trying to convert here, but it is definitely
> not CMY to RGB. I don't know what problems Mr. Friedrich is having, but
> the way that he mentioned is the correct one i.e. inverting the components
> 
> 	[ R, G, B ] = [ 1, 1, 1 ] - [ C, M, Y ].
> 

A simple inverse would be good and dandy if we didn't have to consider the
range of colours reproducible using CMY(K) printing, and those on a colour
video monitor (Colour Gamut). The problem is that the CRT produces a greater
range of colours than CMYK can. Add to this the odd spectral reflectances found
from printing inks, there are real problems. As mentioned by one other poster,
Foley, van Dam et al, is a good starting point. The most definitive work on
colour theory and its application to computer graphics is by Hall (Can't find
the ref for it but it is in Foley et al (2nd ed.).

Hall talks of the methods used to convert from one colour model to another.

The method I used in a raytracer recently, was to convert from CMYK to
standard, CIE XYZ coordinates, this gives a device independent representation,
you then use the chromaticity values for your monitor to do the conversion.

The hard bit is the conversion from CMYK to XYZ, you really need reflectance
data for the printing inks AND the combinations.

Good luck, this is one of the more difficult problems!

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Alastair Thomson,                   | Phone +64-3-479-8347
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