I noticed a problem when trying to view regular sRGB and linear gamma sRGB versions of the same image when using Krita 2.4: The linear gamma image was noticeably darker in the shadows. So I did some tests, including converting the linear and regular sRGB images to my monitor profile.
The problem turned out to be in the conversion from the image ICC profile to the monitor ICC profile. When converting from the linear gamma image to the monitor profile, the darkest shadows were eye-droppering at about half the value that they should have had. So I created a very simple test image composed of ten blocks: (0,0,0), (1,1,1), (2,2,2), (4,4,4), (8,8,8) and so on, up to (128,128,128), (255,255,255). Then I converted the test image to 16-bits, the corresponding RGB values being 257 times the 8-bit values. Upon using Krita 2.4 to convert the 16-bit linear gamma image to the monitor profile, or to regular sRGB, the second darkest color block ended up with RGB values half of what they should have been. I tried the same test using Cinepaint, cctiff, tificc, showFoto, ImageMagick, GraphicsMagick, and Gimp (at 8 bits only), as well as Krita. I also tried using linear and gamma 1.8 versions of prophoto. cctiff and tificc (using -c 0, which I habitually use and did not think twice about as perhaps being relevant) produced the same values. Cinepaint produced nearly the same values as cctiff and tificc. ALL the other image editing programs cut the darkest shadow values in half. This "cutting in half" of the darkest shadow values is visible and obvious in any image with substantial areas of important shadow detail. I had Cinepaint set in the color management options to use "don't Precalculate" rather than one of the other Cinepaint options (Low Resolution, High Resolution, CMM default). I wish I had realized that particular setting might make a difference, because it would have saved a lot of time and tedious testing. I don't know of any image editing program besides Cinepaint that offers the user the choice to use Low Res, High Res, CMM default, or "Don't Precalculate". I would guess that most or all use something like "CMM default", because I just checked, and Cinepaint, when set to use "CMM default" and "use black point compensation" produces the same halving of the shadow values as all the other image editors. At any rate, at this point every image editor that I tested, other than Cinepaint and the latest Krita 2.6 alpha, produces visibly damaged shadow areas if there is a linear gamma profile involved in an ICC profile conversion. -- http://ninedegreesbelow.com Articles and tutorials on open source digital imaging and photography ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Lcms-user mailing list Lcms-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/lcms-user
