On Sun, Feb 21, 2010 at 07:36:58AM -0500, Greg Troxel wrote: > > The word "gamma" gets used to mean at least three things, and they are > often blurred.
You're quite right, of course. > The problem, besides lack of round tuits, is as Martin points out: > there's something about shadow processing that is unpleasing when one is > pedantic about this, and that looks better when steps that are arguably > incorrect are used. Since this is all about making nice photographs, > that's where we still are. Actually we know pretty well what the problem is - colour management systems seem to generally optimise for the assumption that data will be spaced roughly linearly in perceptual space, rather than in absolute luminosity. I looked into this quite a while back. The same issue affects e.g. both lcms and Argyll, and my impression from Graeme Gill's comments on the subject was that it was likely to be a widespread assumption for CMS code. We can just work around this. It doesn't stop us being pedantic and handling gamma correctly. It just means that we need to get data into a roughly perceptually linear encoding before giving it to the CMS. The exact gamma function used to do this has negligible effect and does not need to be exposed as a user control. For the common case where the Adobe matrices are used, I think we should do the following: - Not display any gamma/linearity settings. - Generate a profile on the fly based on the RGB->XYZ matrices and an assumed fixed input gamma of 2.2. I posted patches to do this a long time back, and can bring them up to date if required. - Transform the linear data to a gamma-2.2 encoding after demosaicing, and before passing to the CMS. This can use the existing gamma/linearity application code. - Let the CMS do the rest. For the case where a pre-existing ICC profile is used, we can do essentially the same, but offer a checkbox option to say that the profile was designed to take data with some specific gamma function already applied. *Only* if this is ticked would the gamma/linearity options then appear; preferably as well as options for standard curves like the sRGB one. The gamma encoding applied to the linear data before passing to the CMS would then be adjusted accordingly. Martin ------------------------------------------------------------------------------ Download Intel® Parallel Studio Eval Try the new software tools for yourself. Speed compiling, find bugs proactively, and fine-tune applications for parallel performance. See why Intel Parallel Studio got high marks during beta. http://p.sf.net/sfu/intel-sw-dev _______________________________________________ ufraw-devel mailing list ufraw-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/ufraw-devel
