[argyllcms] Re: Camera profiling?
- From: Klaus Karcher <lists@xxxxxxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Tue, 27 May 2008 20:33:12 +0200
Graeme Gill wrote:
[...] One of the issues is
that the color as seen by an RGB sensor in a camera is
the product of the spectral characteristics of the illuminant,
the spectral characteristics of the subjects, and the spectral
sensitivities of the camera sensor. So a test chart may not
be a good substitute for the subjects you want to photograph
if it has substantially different spectral reflectance characteristics
In my experience, the metamerism failures Graeme mentioned are the key
problem. The more the spectral characteristics of the camera/scanner
sensor differs from the Standard Observer and the more the spectral
characteristics of the test chart differs from the subject, the less you
can expect from a profile.
In practice, you have to face with a multiple of the errors than those
for the chart itself:
e.g. for a profile made from a photographic chart (dcam)
"profile" reports:
peak err = 5.445740, avg err = 1.118582
verification with a different chart (laser print):
peak err = 18.542958, avg err = 7.665851
profile made from several charts (inkjet- and laser prints, Colorchecker
DC, photographic chart, ...):
peak err = 13.438882, avg err = 1.892351
verification with laser print as above:
peak err = 11.950804, avg err = 3.576600
=> you can improve the "overall" result slightly by using several
(different) charts, but this will not solve the key problem (metamerism
failures)
=> causal solutions:
1) better sensor (spectral sensitivities must satisfy the Luther
condition) => practical limitations
2) chart "tailored" to the subject (same pigments / spectral
characteristics)
3) Multispectral approach (more than 3 sensors)
Klaus
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