[argyllcms] Re: The right way for device input profiles and artificial light sources?
- From: Stephen T <stwebvanuatu@xxxxxxxxxxxx>
- To: "argyllcms@xxxxxxxxxxxxx" <argyllcms@xxxxxxxxxxxxx>
- Date: Sat, 7 Dec 2013 02:26:46 -0800 (PST)
Hello,
Tungsten + D50 XYZ is not equal to Sunlight + D50 XYZ. The sunlight profile
performs very poorly for photos taken with warm lighting.
I was wondering which of the following is most nearly "correct":
1) Tungsten + D50 XYZ?
2) Tungsten + A XYZ?
3) Tungsten + adapted A XYZ?
I think most people profile with option 1) but this doesn't seem right because
the XYZ changes when the illuminant changes.
Stephen.
________________________________
From: Gerhard Fürnkranz <nospam456@xxxxxx>
To: argyllcms@xxxxxxxxxxxxx
Sent: Wednesday, 4 December 2013 4:31 AM
Subject: [argyllcms] Re: The right way for device input profiles and artificial
light sources?
Shooting the target under say tungsten light while using D50-based reference
data is basically supposed to result in a profile which reproduces the shot of
a tungsten-lit scene as if the scene were lit with D50 instead of tungsten
light (within the various limits of the camera profiling process, of course).
--
Best Regards,
Gerhard
Stephen T <stwebvanuatu@xxxxxxxxxxxx> schrieb:
Hello,
>
>I have been playing around with camera profiles for tungsten and fluorescent
>lighting.
>
>As I understand, there are 2 things that happen when the light source changes:
>1) Device RGB changes (what the camera sees).
>2) XYZ changes (what the standard observer sees).
>
>I have made simple matrix profiles. There are 3 different profiling strategies
>I have tried:
>1) Profile with D50 reference data (ignore changes in XYZ).
>2) Profile with XYZ computed for the CIE illuminant most closely matching the
>light source (non-standard profile white point).
>3) Take XYZ from step 2 and perform a chromatic adaptation transform to D50
>(standard PCS white point).
>Which of these is recommended?
>
>
>Profiling errors with the 3 different XYZ are similar (a good fit doesn't mean
>the results will be sensible in practice however). Options 2 and 3 produce
>exactly the same rXYZ, gXYZ and bXYZ, only the WTPT is different. Options 2
>and 3 produce identical images in my workflow and I guess the non-standard
>white for option 2 is being accounted for?
>
>For real photos in warm light, I have found all 3 tungsten profiles are
>similar. The D50 reference data produces slightly warmer reds.
>
>
>There are greater differences in photos with fluorescent lighting, where the
>D50 reference data produces a warmer, more pleasing image. Options 2 and 3
>look a bit greenish. Perhaps this is because I profiled with a light source
>approximating F5 daylight and the test photos were shot with inferior, lower
>colour temperature lights?
>
>
>
>Compared to daylight profiles, the artificial light profiles do produce more
>realistic colours. I haven't done any quantitative tests yet.
>
>It's strange that the D50 reference data seem to be useable, as the
>colorimetry of the target
does change and the rXYZ, gXYZ, bXYZ tags can be very different. Here are some
colour differences I calculated after CAT:
>Mean DE94 between D50 and illuminant A after CAT to D50 = 1.5.
>Mean DE94 between D50 and illuminant F5 after CAT to D50 = 2.4.
>
>Appreciate any tips, advice.
>
>Stephen.
>
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