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

[Elle Stone <ellestone ninedegreesbelow com>]

On 01/12/2017 01:12 PM, Elle Stone wrote:
> 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.

Above is true.

Below is wrong:

> 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.


Below is true:

> 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.


And my apologies for the confusion!

Elle