Re: [Gimp-user] color management -- basic question

[Elle Stone <ellestone ninedegreesbelow com>]

On 01/11/2017 02:00 PM, Casey Connor wrote:
> Ok, thanks -- I was just confused because you said "LCMS soft proofing
> will report that all the colors are in gamut" but when I soft proofed to
> that profile it showed lots of colors that are out of gamut... this link
> <https://sourceforge.net/p/lcms/mailman/message/35271294/> makes me
> think it's just more complicated than that, so I won't worry about it.

No, my apologies, you are right, you will see out of gamut colors 
indicated in the test situation you describe. I was trying too hard to 
not cover a lot of cases in detail. To try to summarize the relevant 
details:

1. If you are editing at integer precision (8-bit integer, 16-bit 
integer, etc)

2. and the TRC of the source image ICC profile is reasonably close to 
being perceptually uniform

3a. and the image color gamut completely encompasses the soft proofing 
profile color gamut, or else 3b. if the soft proofing profile doesn't 
support unbounded ICC profile conversions,

* then the gamut check will be reasonably accurate.

So for example let's say you assign "Rec2020-elle-V4-labl.trc" to one of 
those very nice "all colors" images,  and you've chosen 
"sRGB-elle-V4-srgbtrc.icc" as the soft proofing profile. The gamut 
checks will be accurate.

Now assign Rec2020-elle-V4-srgbtrc.icc to the source image. The gamut 
checks will still be very, very close to accurate.

Now assign Rec2020-elle-V4-g18.icc to the source image. The gamut checks 
won't be quite as accurate. Gamma=1.8 is the standard TRC for 
ProPhotoRGB color space.

Now assign Rec2020-elle-V4-g10.icc to the source image. The gamut checks 
will be completely inaccurate.

The appearance of the image will keep changing as you assign the 
different profiles - in fact getting lighter and lighter as the TRC gets 
closer to gamma=1.0. But the thing to pay attention to is the gamut 
checks. It helps to have the gamut check color set to magenta so the 
gamut check areas stand out from the image colors.

Assuming conditions 1 and 3a or 3b, then most accurate gamut check is 
when the TRC is the labl trc, but the sRGB trc is also pretty accurate. 
Both of these TRCs are close to gamma=2.2, and profiles such as 
AdobeRGB1998 (which has the gamma=2.2 TRC) will also show an accurate 
gamut check.

If you change the precision to high bit depth floating point precision, 
and/or if the source color gamut doesn't entirely encompass the soft 
proofing profile color gamut, and/or if the soft proofing profile 
supports unbounded ICC profile conversions, then a whole other set of 
LCMS soft proofing issues comes into play. For example, assign 
sRGB-elle-V4-srgbtrc.icc to the "all colors" test image. Change the 
precision to 32-bit floating point. Do "Colors/Saturation" and set the 
slider to 2. Now almost all the colors are exceedingly out of gamut with 
respect to the sRGB color space. And yet the gamut checks will have 
disappeared.

As an added complication, what you see partly depends on what version of 
LCMS2 is installed, but I'm not sure if the first relevant change came 
before or after LCMS 2.7, which is the minimum version of LCMS2 that 
will work when compiling GIMP 2.9.

Best,
Elle