On 24/11/18 3:22 am, Aurélien Pierre wrote:
Hi everyone,
my darktable is installed on Ubuntu Budgie (fork of Gnome 3), but it
was the same when I used Gnome Shell. I have a custom screen ICC
profile installed in gnome-color-manager, and loaded in darktable
through colord.
When I change the ICC profile on Gnome with darktable open, the colors
of the darkroom preview change too (no matter if darktable uses the
system display profile or one built-in profile, like Adobe RGB). That
is the contrast and white point of the picture, plus the color of the UI.
So that means that the OS is stacking another color transformation on
top of darktable's one.
Is it possible that you're just seeing the effects of the gamma ramps
changing when they're loaded from the VCGT of the profile that you
switch to?
If you can change the profile without loading the VCGT (or restore the
gamma ramps manually after the change) and the supposedly
non-colour-managed parts of the UI change, then I'd say that there's
some kind of double-correction happening, otherwise it's probably
working as intended.
Fromthis article
<http://libregraphicsworld.org/blog/entry/richard-hughes-on-color-management-in-linux-and-gnome>
(2011), I get that gnome expects apps to take care of themselves :
One of the things I tried to deliberately ignore designing colord
was actually flipping pixel values. Colord is a very high level
daemon that can say to apps like Krita or GIMP “Use this profile
for this device” rather than them supplying a 140Mb memory buffer
and an operation list. This means we can do the conversion on the
CPU using lcms2 for some programs and using the GPU using a shader
in things that use 3D. By not trying to wrap lcms we can let the
application do the pixel conversion in the right layer in the
right way.
Of course, the downside of this is that you have to patch
applications to actually do the right thing. We can make this
easier by doing some framework code for Clutter and Cairo, but
sooner or later the application has to know about color management
in one form or another. This is going to be my main focus for
GNOME 3.4; now we have all the framework installed and working and
we can say to application authors “It's already installed, so
don't worry about the additional dependency, just commit my patch
and it'll just work”.
But gnome-color-manager has no documentation, and even the Gnome color
dev documentation is pretty useless (a lot of "how to", no "what's
going on", but they found time to design a cheesy kindergarten theme).
Looking at GDK pixbuf doc, they don't have tags to explicitely say
"hey that's already color-corrected so bug off". The Wikipedia entry
ofLinux color management
<https://en.wikipedia.org/wiki/Linux_color_management> is as helpful
and factual as a marketing director motivational speech (let's
increase the leverage of color management by ensuring the quality of
good devices, with a pro-active method to supervise critical elements
in a proficent way — sure !).
As of now, I have seen no block diagram to describe the full color
pipe in Linux, nor any way to ensure the quality of the transform.
From the info I have gathered, the pipe I have put together is as follow:
|| darktable pipe -> LCMS/(Internal cmatrix color correction + TRC) ->
Cairo surface -> GDK pixbuff -> || -> Mutter compositor -> (OS color
correction ? TRC ?) -> Xorg -> Nvidia/Intel GPU driver -> (Color
correction ? VCGT ?) -> || -> HDMI DAC (gamma 2.2) -> Screen
So my question is : does anyone have any idea of what's going on with
color on Linux, or are we stacking ICC on top of shit just to pretend
it's color-managed magically, somehow ?
Thanks,
Aurélien.
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David Houlder
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