Hi All,
The current Gimp Preferences/Color Management doesn't allow the user to
choose whether or not to use black point compensation for the display
monitor or for proofing. Instead:
* If the user chooses the relative colorimetric intent, they
automatically get black point compensation.
* If the user chooses perceptual intent, they automatically do not get
black point compensation.
I made a patch (attached) that adds the user option to choose whether to
use black point compensation for the display and also for soft proofing.
Also I revised a few of the "on hover" tool-tips and modified a couple
of the default settings.
I've tested pretty carefully and everything seems to act like it should.
But there are a lot of moving parts in the relevant code (well, to me, a
patch involving four files seems like a lot of moving parts), so it
wouldn't surprise me to find out I made mistakes.
Would anyone be willing to give the patch a try and see if they like it,
have suggestions for rewording the blurbs, changing defaults, etc, and
especially confirm whether or not the code looks right and everything
works as expected?
To test the patch, you need some LUT RGB profiles and some test images:
* The color.org website has some interesting non-floss LUT profiles for
experimenting with.
* Non-floss Chromira and Frontier RGB LUT printer profiles for
experimenting with can be downloaded from here (pick a state from the
left-hand column): http://www.drycreekphoto.com/icc/
* This page has many excellent (non-floss) black and white and color
test images:
http://www.northlight-images.co.uk/article_pages/test_images.html
Some relevant color management background:
* Most people's monitor profiles are matrix profiles. The current Gimp
default perceptual intent requires that the profile actually have a
perceptual intent lookup table. So for most people, when they select
perceptual intent for their display profile, what they really get with
the present Gimp Color Management options is relative colorimetric
without black point compensation.
* Gimp users who use a custom matrix monitor profile with a non-zero
black point, and also use Gimp's current default perceptual intent, do
see crushed shadows (falsely crushed on the screen, which they probably
compensate for while image editing, producing an image with lighter
shadows than they intended).
Related to the defaults:
* Real LCD monitors never have a zero black point (they always emit some
light even when displaying solid black), but if the monitor TRC curves
rapidly enough away from zero it doesn't matter too much. Unfortunately
sRGB as an LCD monitor profile does crush the shadows.
* For people who use custom monitor profiles with non-zero black points,
black point compensation as an option with relative colorimetric intent
is pretty important because sometimes you really do want the option to
disable black point compensation.
* The oft-repeated claim that perceptual intent is the "photographic"
intent and/or the "safe" intent is at best misleading:
* Perceptual intent only makes sense for destination profiles that
actually support perceptual intent (eg when using a LUT monitor profile
or when soft proofing to a LUT printer profile), and even when
supported, perceptual intent doesn't necessarily give the best results.
The best thing to do when soft proofing to a printer or other output
profile is experiment with all the (supported) intents on an image by
image basis.
* The advice to use perceptual intent because it "preserves all the
colors" gives people the false security that they really can "preserve
all the colors" even when the destination profile is a matrix profile
like sRGB. What really happens when they ask for perceptual intent from
a matrix destination profile is they get relative colorimetric, and out
of gamut colors are clipped.
* Not every profile accomodates every intent. For example, matrix
profiles only (and always) have relative and absolute colorimetric. LUT
profiles may or may not have a table for every possible LUT profile
intent, those being relative, absolute, perceptual, and saturation. For
example, some LUT profiles only have a perceptual table.
* Not every intent accomodates black point compensation. Absolute never
does. Relative always does (so you can use bpc or not, your choice, if
your software allows you to make the choice). Apparently perceptual
isn't supposed to, but some LUT profiles do accomodate black point
compensation for perceptual intent. If you ask LCMS for black point
compensation for an intent+profile that doesn't support it, that flag is
just ignored.
* If you ask for an intent that the profile doesn't have, according to
the LCMS documentation it uses perceptual intent. Unfortunately the LCMS
documentation doesn't specify what happens if you ask for perceptual
intent and the profile doesn't support it. But pretty much every image
editor I've ever used (including Gimp) defaults to relative colorimetric
if you ask for perceptual intent for a matrix profile.
As an aside, the user option to choose an "out of gamut" color doesn't
work anymore (not related to this patch). It also doesn't work in Gimp
2.8.10 installed from Gentoo portage, at least not on my computer.
Whatever color I choose in "Preferences/Color Management/Mark out of
gamut colors", the out of gamut color is black.
Elle
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