[Lcms-user] Soft proofing problems with gamut checks

[Elle S. <ell...@ni...>]

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
-- 
http://ninedegreesbelow.com
Color management and free/libre photography