[argyllcms] Re: Black crush on macOS - some advise?
- From: Graeme Gill <graeme@xxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Tue, 26 Mar 2019 11:43:27 +1100
I have made a discovery! The problem is made by Night Shift!
I'm not quite sure how "Night Shift" works on OS X, but in general
all these types of programs will wreck color accuracy, and
maybe worse, as you have discovered.
Traditionally they work by taking control of the display hardware
VideoLUTs, and setting curves that alter the white point in some way.
Typically this is not a precise color transformation, but a rough approximation
of a chromatic transformation.
The trouble is that in a color managed workflow the VideoLUTs are
assumed to be for the sole purpose of display calibration (linearizing
the display behavior, setting the brightness and white point, making R=G=B
be neutral). So at best programs like "Night Shift" are altering the
color display that the profile doesn't account for, and at worst they
fighting with the color management.
I did a quick check on my MacBook, and it seems that _if_ Night Shift
is using the VideoLUTs, it's doing so in a way that is hidden from
the VideoLUT API. So it's either doing its modifications in a way
that is using ColorSync, or (more probably from your report), compounding
the NightShift curves with the ones set by the color management in some way.
This might be regarded as an Apple bug (i.e. NightShift should work
in a way that is colorimetrically valid), or as a user error
(if you want accurate color, make sure NightShift is off).
Note that having Night Shift on while you calibrate or profile
will wreck things as well!
[ It's not clear if Apple documents any API's for applications to
monitor or alter Night Shift. ]
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