Re: Spot Color tables in Best ColorProof
Re: Spot Color tables in Best ColorProof
- Subject: Re: Spot Color tables in Best ColorProof
- From: Graeme Gill <email@hidden>
- Date: Wed, 04 Jun 2003 17:32:13 +1000
Roberto Michelena wrote:
>
The problem lies in the gamut mapping built into the profiles.
For spot color reproduction, it's generally best to use the
absolute colorimetric intent table. Most profiling software
should use a "map to nearest" gamut clipping algorithm
for out of gamut colors in the colorimetic table (ie.
they will strive to have any in-gamut color reproduced
exactly.) In practice, colors near the edge of the gamut may
have greater errors due to the finite clut table resolution (the
colors in the last row next to the edge of the gamut get
distorted because the out of gamut device values are
constrained to be between 0.0 and 1.0) Using the largest
possible clut resolution minimizes this error.
Of course it's up to the profiling software as to what
colorspace to "map to nearest". L*a*b* gives OK results,
but there are issues with it (blues mapping to purple
being just one), that seem to be a result of it
being created to evaluate small color differences,
rather than the large ones that appear in gamut mapping.
This is all an area of very active research, but there
seem to be no definitive answers yet.
I've found that CIECAM97s3, with the addition of
allowance for the Helmholtz-Kohlraush effect to be
slightly better than using L*a*b*, but then there
are lots of other candidates, such as CIECAM02, IPT etc.
Graeme Gill.
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