Friday, August 26, 2011

Those TRCs

[Please note that this post is part of a series describing my first wobbly steps into the world of ICC color profiles. The aim is to build an ICC color profile editor from scratch. Seeing how I started this project with no knowledgde whatsoever on the subject, it's wise to realise that some of my findings described in these posts are incorrect. The incorrect conclusions are usually corrected in later posts. For the whole series in one go, click here.]

So, my assumption in previous color post turned out to be correct: The TRCs (Tone Reproduction Curve) work on the actual color translation.

It's a similar effect as the calibration curves have on the screen.

When I edit the TRCs, the colors of my proofing photo change, even when calibration is turned off.

Mind you, I'm still struggling with some problems. The TRCs are stored quite differently per profile. Sometimes 256 values, sometimes 512 values, sometimes as a parametric curve. And more annoyingly: they seem to work in the reverse direction. Pulling blue down increases blue in the color transformation, as opposed to the calibration curves.

I'm not fully sure if that's supposed to happen or if I am making programming mistakes here. Seeing how my changes (after saving) result in the expected (changed) curve when I reopen the profile, I'm inclined to believe this is what's supposed to happen...

I do have some ways of testing that, so that's one of the things to try out.

And it's not hard to understand why the hardware (the colorimeter) is necessary, because it's almost impossible to tweak these things on sight, especially since I'm now dealing with six curves (calibration and profiling), which all have an effect on the colors.

I don't intend to edit TRC curves at this point (although the application will be able to). Most of my profiles have 'gamma 2.2' stored in the TRC (as text) and that seems to be sufficient. I was a bit unsure about TRCs claiming gamma 2.2 without any additional curve data stored, but I guess now that the color translator (the CMM) knows how to deal with this.

A gamma curve isn't difficult to create when the value is known.

It will however be one of my next steps anyway: figure out what happens in profiles that don't have TRC data, but simply 'gamma #.#' stated. Does the gamma indeed adapt in the photo when I change it from 2.2 to say 1.8?

Once that's done, I think my profile editor needs a name and will see the daylight as a first version of a more elaborate application, because there's still more to edit.

Will take some time though, first I have to tame those TRCs in all their incarnations...

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