Re: CMM "interfering" with PS color picker?
Re: CMM "interfering" with PS color picker?
- Subject: Re: CMM "interfering" with PS color picker?
- From: Rolf Gierling <email@hidden>
- Date: Thu, 01 Feb 2001 13:41:12 +0100
There are a lot of things happening when converting RGB to CMYK:
First step: The CMM converts the RGB values to XYZ using the monitor
or workspace profile.
1. Taking a closer look on the profile reveals that adding the XYZ-values
of R,G and B dont match the whitepoint (nealy in Adobe 98 RGB, almost never
in a monitor profile)
Second step: The CMM converts the XYZ values to Lab.
1. Rounding errors happening here.
Third step: The CMM converts the Lab values to CMYK
1. Is the given Lab value in the Lookup table? If not,
interploate. errors arising here.
2. If the given Lab value is in the Lookup table, is it
really zero?
Interesting answer: take a look at the Euroscale Coated profile
that ships with Photoshop 6. If you input L=100, a=0, b=0 for example
in the ColorSync Profile inspector, the CMYK output is 14, 14, 14, 0.
(ranging from 0-255, 14 is about 5,49%).
3. rounding error in the information palette: 5,49% yields in 5% in
the palette.
Even more interesting: The Adobe CMM returns values of 0, 0, 0, 0.
The only conclusion I have what Adobe is doing is that they really control
if the given input values should be white and override the values with the
whitepoint values, as Chris Murphy already suggested.
Rolf Gierling