On Aug 4, 2004, at 2:49 AM, Peter Kleinheider wrote:
Andy
When you do a relativ Colormetric Conversion from AdobeRGB to ISO Web
Coated, a different way of gray-generation gets used as of perceptual.
Ahh OK. While this is not defined in the ICC spec, it is possible for
different profile vendors to have different gray balance behavior
between rendering intents, in particular perceptual. Relative
Colorimetric ignores paper white. Because a neutral gray, without any
respect for the paper, is produced with this intent, it's the most
reliable because the human visual system will chromatically adapt to
the paper white (and the corresponding tint in grays due to paper white
showing through).
Gretag Macbeth has an option in ProfileMaker that defaults to this
behavior for perceptual as well. But there is an option to force a
neutral gray WITH respect for paper white. If you were to measure a
gray ramp made in RGB, converted to CMYK using the perceptual intent
with such a profile, the measurement would indicate a neutral gray (a*
and b* = 0 or close to it). The problem with this is that the human
visual system still chromatically adapts to paper white. So a yellow
stock still looks white to the humans, and now the gray ramp will look
very cool in comparison, not neutral. I have found this method of
rendering, taking paper white into account, when it comes to gray
balance to be a waste of time. If anyone has had good results with it
in certain situations, I'd like to hear about it.
When it comes to taking paper L* into account however (which Relative
Colorimetric does not, it assumes paper white has an L* of 100, which
can leader to heavier separations with decreasing paper L*), that's a
good thing.
Chris Murphy
Color Remedies (TM)
www.colorremedies.com/realworldcolor
---------------------------------------------------------
Co-author "Real World Color Management"
Published by PeachPit Press (ISBN 0-201-77340-6)