[Lcms-user] Soft proofing problems when the destination profile supports unbounded profile conversions

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

Hi Marti and all,

I am unable to get useable results when using GIMP (which uses LCMS) to 
soft proof from an RGB working space profile to an output profile.

Soft proofing only provides useable visual results if the destination 
profile doesn't support unbounded ICC profile conversions (this is with 
the gamut checks disabled, as the gamut checks introduce additional issues):

      * Soft proofing to a LUT printer profile that doesn't support 
unbounded ICC profile conversions works the way I expect soft proofing 
to work: Out of gamut colors are clipped during soft proofing and so 
they display as they would be displayed if the image were actually 
converted to the destination profile.

      * Soft proofing to a matrix RGB profile with a point curve (such 
as a V2 sRGB profile) also works as expected - again the out of gamut 
colors are clipped during the soft proofing conversion.

      * But soft proofing to a matrix RGB profile that has a parametric 
TRC (such as V4 sRGB with a parametric TRC) does *not* produce the 
expected results. Instead of being clipped to the color gamut of the 
destination color space, the soft proofed colors are simply converted 
using channel values that are less than zero and/or greater than 1.0f. 
This is the case even when the image is at integer precision, and so of 
course the image colors will be clipped as soon as the actual ICC 
profile conversion is done.

       So when soft proofing to profiles that support unbounded profile 
conversions the "soft proofed" colors look exactly like they would look 
if the user hadn't activated soft proofing. So the user has no visual 
indication that colors might actually be out of gamut, which entirely 
defeats the point of soft proofing.

       Would it be possible to put in a flag for soft proofing that says 
"after transforming to the destination profile clip the channel values 
to fit within the bounded color gamut of the destination color space, 
even if the profile supports unbounded conversons"?

Or even make it the default behavior that soft proofing results are 
clipped to the bounded color gamut of the destination color space? 
Personally I can't think of a single use case for soft proofing where 
results of soft proofing shouldn't reflect the actual *bounded* color 
gamut of the destination color space.

Speaking of flags, I've tried modifying the GIMP soft proofing transform 
code to use "cmsFLAGS_NONEGATIVES". But using or not using this flag 
doesn't seem to have any effect on the soft proofing transform (though 
it definitely affects normal ICC profile conversions). Is this by 
design? Or perhaps is there an issue with the GIMP code that sets up the 
soft proofing transform?

Best regards,
Elle
-- 
http://ninedegreesbelow.com
Color management and free/libre photography