> 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.
I think that this is a specific issue with shadow area an sRGB. One the one hand, the sRGB curve is extremely steep and on the other hand the linear data from cameras sensors is not very accurate. The CMS standard was not made with digital cameras in mind, and this is one place where it shows. > 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. We already do our best to get raw linear data. > 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. You actually don't need to apply the gamma curve at all. If the input profile has just a matrix and the output profile is sRGB, lcms will apply a sRGB gamma curve for you. The problem is that you get "hazy" output. If we get rid of the gamma/linearity controls, there should be some replacement that controls the shadows and it should have a reasonable default. Eventually, my goal is to have an option to have "16-bit linear" output, with a CMS profile attached, so that the image would look correct in any CMS aware software. Any patch that would get us closer to this goal is welcome. Udi ------------------------------------------------------------------------------ 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
