On 08/10/2016 06:27 AM, Elle Stone wrote: > Hi Marti and all, > > When soft proofing with high bit depth GIMP, which uses LCMS, the gamut > check isn't reliable. I've confirmed some of the issues using tificc at > the command line, so it's not GIMP code per se. > > 1. When using the gamut check, when soft proofing to a destination color > space that supports unbounded ICC profile conversions, the "bounded" > color gamut of the source color space affects results. So colors that > are marked as out of gamut in one source color space will show as in > gamut in another source color space. In general, the closer the source > and destination color space gamuts are, the larger the proportion of > colors that are shown as in gamut with respect to the destination color > space. This is true even if the entire image is actually out of gamut > with respect to the selected destination color space. > > 2. The gamut check results vary depending on the TRC - this affects soft > proofing not just to profiles that support unbounded conversions, but > also to LUT printer profiles that don't support unbounded conversions: > > * The LCMS gamut check gives different results depending on > whether the source color space has a linear gamma TRC or else has a more > perceptually uniform TRC > > * When editing in a linear gamma RGB color space, the LCMS gamut > check can easily show most or all of the image as out of gamut, even > when the entire color gamut of the source image does fit in the > destination color space's color gamut. Dark and midtone colors are > marked as out of gamut. > > * For source color spaces with more or less perceptually uniform > TRCs it seems the exact TRC of the source color space makes a difference > in the results of soft proofing. > > Best regards, > Elle >
In case it might be helpful, here are links with screenshots illustrating some problems with LCMS gamut checks: * http://ninedegreesbelow.com/files/soft-proof/soft-proofing.html (this link has sample images if anyone wants to verify some of the issues with current LCMS soft proofing, and also screenshots comparing soft proofing results from several different image editors). * http://ninedegreesbelow.com/bug-reports/soft-proofing-problems.html (screenshots just from GIMP 2.9) Currently it seems that LCMS gamut checks only give correct results if several conditions are met: 1. The source color profile's "bounded/clipped/display range" color gamut must encompass all the image colors. This means means soft proofing while editing using unbounded/unclipped floating point processing only gives correct results if in fact no image colors are out of gamut with respect to the color space in which the image is being edited. 2. The source color profile must have a perceptually uniform TRC. This means anyone editing in a linear gamma color space sees incorrect gamut checks. Also the destination color space can't support "unbounded/unclipped" profile conversions, but this topic is already covered in other threads. Also currently LCMS gamut checks don't take into account whether or not "black point compensation" has been selected. I can see arguments either way, but apparently PhotoShop does produce different gamut checks depending on whether BPC is activated or not. Best regards, Elle -- http://ninedegreesbelow.com Color management and free/libre photography ------------------------------------------------------------------------------ Developer Access Program for Intel Xeon Phi Processors Access to Intel Xeon Phi processor-based developer platforms. With one year of Intel Parallel Studio XE. Training and support from Colfax. Order your platform today. http://sdm.link/xeonphi _______________________________________________ Lcms-user mailing list Lcms-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/lcms-user
