[argyllcms] Re: camera profiling with newest version of Argyll

  • From: Ben Goren <ben@xxxxxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Thu, 27 Aug 2015 19:51:06 -0700

On Aug 27, 2015, at 7:19 PM, Chad Johnson <chad.chadjohnson@xxxxxxxxx> wrote:

I do supply the spectral data. See
http://www.freelists.org/post/argyllcms/camera-profiling-with-newest-version-of-Argyll,9

I think just found (at least part of) the problem. To wit, I had previously
missed:

the cie file is from running spec2cie on the ti3 file and the spectrum file

That works for creating a general-purpose profile to be used under any
illuminant. If you develop your RAW file with a fixed white balance for D50 and
use such a profile, the resulting file will be a match for the actual
conditions of the scene. The camera essentially functions as a colorimeter and
records the absolute color values that hit the sensor.

But that's *not* what you want to do when you want to, as you suggested
earlier, profile the light for copy work. In that case, you want to use D50 for
the XYZ reference values. The profile will be valid only for the lighting in
which the chart is photographed; however, it will make any such photographs
appear as if they had been photographed with D50 -- the goal of copy work. Done
right, with a good printer profile and all other typical caveats, you can place
the original and the print side-by-side and they'll look the same -- even for
the most part under various lighting conditions, to the extent that the
pigmentation of the original isn't especially exotic.

So, a couple examples.

Imagine you're on a theatrical stage with all sorts of funky gelled lights.
We'll pick one corner of the stage that we'll work with and keep the light
constant -- but assume the light is weird and colorfully non-standard.

Put your chart there, photograph it, get your spectro measurement of the light,
build your profile with that used for the XYZ reference -- just as you've done.
(Or you could just pass it to colprof with -i scene.sp.) Take a picture with
the profile. Look at it on a properly calibrated laptop right there on stage;
the colors match. Now, point the camera to some other part of the set with a
completely different crazy lighting. Use the exact same profile and workflow,
and the colors on the laptop still match.

Now, use D50 for the reference values for the profile. The laptop image no
longer matches. Take the laptop outside along with whatever you took pictures
of; now they match. Make a print, go back in the theatre and put the print next
to the original, and they match. But you need a different such profile for each
light environment.

Does that help?

b&

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