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
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