I encountered a couple of stumbling blocks while working on Gimp color management, so I decided to give my brain a rest and work on some other projects. Sometimes enlightenment happens when you stop thinking about a coding problem. Regarding high bit depth color conversions, the stumbling block is still the background conversions between the sRGB tone response curve and and the linear gamma tone response curve - all those Babl conversions I've mentioned before. I was hoping that by removing all the deprecated functions the problem with high bit depth color conversion being subverted by the background TRC conversions would be resolved, but it wasn't. I am unsure how to procede, although I have pinpointed exactly where in the Gimp code the Babl functions are called (it's not in the lcms plugin code) and which particular Babl functions are being called. So I temporarily put the lcms.c plug-in high bit depth color conversion code aside and started working on the code that is used to convert from the image color space to the monitor color space. In the existing "image color space to monitor color space" code, the image RGB values are converted to 8 bits *before* the RGB values are sent to the code that converts from the image color space to the monitor profile. Which means the monitor display of linear gamma color space images has severely posterized deep shadow detail (the image itself isn't damaged, just the monitor display of the image). In images that don't have much deep shadow detail, the damage isn't visible. But in images that do have deep shadow detail, the damage is obvious. I can post pictures if anyone is interested. Shadows are not posterized, even in a linear gamma color space, if you convert from the linear gamma color space to the display color space at 16-bits or higher, and *then* convert from 16-bits to 8-bits to send to the monitor. But you do get posterization if you convert from linear gamma 16-bits to linear gamma 8-bits *before* converting to the monitor display space. In addition to the monitor display shadow posterization problem, newer high end monitors are using 10- and 12-bit color depth, and I think the Cairo developers are also working on the higher bit depth display code. But at this point Gimp is hard-coded to hand 8-bit values to Cairo. I've been tracing the "image color space to monitor color space" code execution path, looking for a place to intervene to give the color-space-to-monitor-space lcms profile conversion 16-bit integer or 32-bit floating point values instead of 8-bit values. It turns out that the relevant "image color space to monitor display space" code is spread out over quite a few gimp, babl, and gegl c-code files, and makes heavy use of Cairo-related functions. I apologize for the long delay in posting an update. Alas, the only enlightenment I have found during that long delay is that to make further progress I will need help from someone well-versed in Gimp/Babl/Gegl/Cairo code. Kindest regards, Elle Stone -- http://ninedegreesbelow.com Articles and tutorials on open source digital imaging and photography _______________________________________________ gimp-developer-list mailing list gimp-developer-list@xxxxxxxxx https://mail.gnome.org/mailman/listinfo/gimp-developer-list
