[argyllcms] Re: Measuring a colorcheker on screen
- From: Graeme Gill <graeme@xxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Wed, 23 Jan 2008 11:43:31 +1100
Lars Tore Gustavsen wrote:
I found an old Seybold Report with the title "Measuring the quality of
ICC profiles"
http://www.heidelberg.com/wwwbinaries/bin/files/dotcom/en/prinect/prinect_profile_toolbox.pdf
. On monitor profiles they have three objective test for the quality.
The are, page 13, archived gamma, DE difference in whitepoint from
target D50 and the last one average DE for a colorcheker measured on
monitor.
Gamma is not as easy to verify as it may seem, due to the
multitude of ways that the non-zero black point can be
accounted for.
When I afterwards did a "profile -v -as Colorcheker" I get an avg DE
on 5.6. I get some real big mismatch on a few of the patches (set
profcheck output) There is DE of 60 on patch 16. I guess this yellow
patch is out of gamut. I find it hard to understand why they have
included out of gamut measurements.
Color checker is essentially a print based chart, and for subtractive
processes yellow is a primary, so naturally it is often more
saturated than the secondary yellow created by an additive device
like a monitor.
I also get a quit big error on patch 20( its L*81) with a DE of about
18. I don't understand why since I get pretty nice numbers when I did
the calibration with dispcal. I have attached a log for my
calibration/profiling today.
There are lots of traps to fall into with such an exercise. You
are essentially doing an absolute comparison, so the rendering
needs to be setup to be absolute as well. Often the white
points mismatch (reflective standard D50, emissive D65),
and there is even poor agreement on how ICC emissive profiles
should record the absolute behaviour of a display. So
you'd need to stick to Argyll based CMM to eliminate
the latter as a possible source of error.
Graeme Gill.
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