Piotr Legiecki wrote:
Hello Guys,

For a long time we were waiting for GIMP to become a real pro tool for serious photographers. The scarcity of well designed tools for UNIX is obvious. What *have to be* changed as soon as possible:

1) Levels dialog - *linear* histogram as in histogram tool.
Grey point should use Gamma settings as follows:
User click on grey surface no matter what lightness it has and Gimp calculate Gamma using pure Gamma model:

R'Gamma = log(Grey)/log(R') ; G'Gamma = log(Grey)/log(G') ; B'Gamma = log(Grey)/log(B') ;

Grey = 0.299*R' + 0.587*G' + 0.114*B' ;

This is good stuff. I have been aware of these problems, and I am incorporating their solutions into the compositing core that will eventually become a part of gimp (gegl). What you are essentially describing is the Rec. 601 converstion from nonlinear RGB to nonlinear video luma, and then calculating the the correction factors on RGB by pulling out gamma correction on the non-linear RGB.

The actual coefficients there depend on the calibration of your monitor, and the colorspace in which the image was captured and in most cases what you want is sRGB, not Rec. 601 (which is fine for video, but not so great for computer monitors). I don't have the sRGB coordinates off hand, but I did take note of them. But what gimp will do in the future is request that the color management system convert the luma, which hopefully will have an ICC calibrated profile to back the transformation.

2) Grey point (and histogram) inside Curves Dialog using Shlick mapping method:

p = ( Grey*({R'G'B'}-2^bits) )/ ( {R'G'B'}*(1-2^bits) )

{R'G'B'} = p*{R'G'B'} / ( p*{R'G'B'}-{R'G'B'}+2^bits )

I am not familar with this algorithm. Could you please provide a source?

The problem with current tools (mentioned above,) is in their almost uselessness for photographers. Well, at least in a way to get perfect results in short time.

I was wondering if people noticed this. Yeah, without these corrections, the RGB->Gray converstion can get hosed as well.

Please consider a.s.a.p.

They are indeed being considered. Color is a hard topic, and I want to get it right.

Piotr Legiecki


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