ColorSync behaviour on pure Gamma TRC
ColorSync behaviour on pure Gamma TRC
- Subject: ColorSync behaviour on pure Gamma TRC
- From: Rémi Achard via colorsync-users <email@hidden>
- Date: Wed, 12 Oct 2022 11:53:06 +0100
Hi,
I have been looking at some unexpected results coming from simple gamma
profiles, as the ones you can generate via DisplayCAL synthetic profile
tool. I found that the gamma curve is actually replaced by a piecewise
linear segment + gamma curve using a parametric curve tag. I was wondering
if there are any reasons for ColorSync to do that or if anything is wrong
in my assertion of the situation. I saw that an earlier topic mentioned a
similar issue in Aug 2019 (
https://lists.apple.com/archives/colorsync-users/2019/Aug/msg00005.html),
but I'm not sure I understand how to reply to it directly so spinning this
new thread.
To inspect ColorSync transformations, I re-purposed an example Objective-C
program converting image colour space from an older Apple Tech Note /
Documentation and added some debug print to get the transform details of
the ColorSync fragment.
Some truncated output from a sRGB to AdobeRGB conversion (for demonstration
purposes only), focusing on TRC handling.
sRGB EOTF decoding part:
"com.apple.cmm.ParamCurve3" = (
"2.4",
"0.9478673",
"0.0521327",
"0.07739938",
"0.04045",
0,
0
);
"com.apple.cmm.PureGammaOriginalTRC" = 0;
AdobeRGB EOTF encoding part:
"com.apple.cmm.ParamCurve3" = (
"0.4545455",
1,
0,
16,
"0.006200788",
0,
0
);
"com.apple.cmm.PureGammaOriginalTRC" = 1;
You can see the AdobeRGB 2.2 gamma is gone (notice the
PureGammaOriginalTRC=1) and replaced with a piecewise curve. According to
my tests, this happens for simple gamma curv tag as well as para tag with
type 1. If I use a para tag of type 3 emulating a pure gamma, I can get
ColorSync to respect the pure gamma. When I plug the numbers into a
homebrew parametric curve implementation, it does match what is happening
in ColorSync which lead me to believe this method of inspection is working.
Any idea what is happening?
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