🚀 go-pugleaf

In article <1993Jan4.134820.1522@tom.rz.uni-passau.de> friedric@diogenes.uni-passau.de (Carsten Friedrich) writes:
>Hi,
>
>I have a big problem converting colors from cmy to rgb and vice versa.
>The usual aproach, to just invert the components seems to be incorrect.
>The following facts make me think so:

>- if you print a bar of yellow on the screen with brightness ranging from 
>0% to 100% you'll see that it is slightly greenish

You are using (r,g,b) = (1,1,0) of the CRT ( using its own phosphors )
to "define" yellow. This has nothing much to do with any other definition of
yellow. You are working in device dependent space and you have to measure your
CRT to derive the device independent data in your favorite device-independent
color space.

>- if you look at an image that is scanned with a very good scanner 
>(like hell) you will see that e.g. red colors in real turn to purple colors 
>on the screen

Do you mean that colors you perceive to be red ( under what illumination )
on film/paper appear to have changed hue on the CRT? Even for device independent
descriptions, cross-media matches are very difficult and a field of ongoing
research. Means: don't expect a match.

>- if you look at color tables printed in 7 color-printing technique you'll see
>that the inverse color of cyan is not red, but a kind of orange. Also the inverse
>colors of magenta and yellow, are not the green and blue you use in rgb.
>if you trust the color tables it seems not to be just a linear offset, but sth.
>more complex. 

Your basic problem seems to be that you are operating in device dependent
color spaces ( rgb and cmy or even better cmyk are generally created with a
specific output medium/dye-set etc. in mind).

>Can anyone help me solve this problem ???

Even a rough color calibration to any device independent color space should
reduce the problem dramatically. You might try a device independent rgb ( like
Xerox rgb 8-) ) which does in general not span the available gamut, or a
luminance chrominance system like YCC,YES,YHS,Y??.

>thanx and greetings,

>carsten

Reiner