[xorg-xgamma] Xgamma and xrandr discrepancy

[Alex Deucher]

On Sat, Dec 21, 2013 at 11:14 AM, Adrian C. <anrxc at sysphere.org> wrote:
> Hello I have a small issue with xgamma for a very long time that nobody
> could explain to me, so now I'd like to ask the developers.
>
> To get decent colors on my workstation and external monitors I want to
> apply gamma correction. For me these magic values do it "Gamma 0.75 0.65
> 0.60", I set them in xorg.conf for each monitor.
>
> If it happens that an external monitor wasn't connected when I started X
> I can attach it with xrandr. Well when I do that the colors on the
> monitor are completely off. Even though xgamma reports "-> Red 0.750,
> Green 0.650, Blue 0.600" when I check, the colors are simply wrong. I
> have to kill X, start, and everything is back to normal. These forced
> restarts partly defeat the hotplugging ideas in xrandr.
>
> Here's what Xorg.0.log has to say:
>> [ 44816.100] (II) intel(0): Using default gamma of (1.0, 1.0, 1.0)  unless otherwise stated.
>> [ 44816.101] (II) intel(0): Output VGA1 wants gamma correction (0.8, 0.6, 0.6)
>
> Trying to correct colors my self, after using xrandr, isn't succesful.
> xgamma doesn't produce good colors. I have also tried to set gamma with
> xrandr, both when attaching the display or afterwords, i.e. by using
> this switch --gamma 0.75:0.65:0.60 or --gamma 0.8:0.6:0.6. This produces
> bad results as well. Colors are always wrong, always very washed out to
> my eye.
>
> Nothing but an X restart does it for years, across 5 portable computers
> and all forms and sizes of CRT and LCD monitors. Could anyone explain
> why? Thanks.

IIRC, xgamma doesn't work with xrandr 1.2+ capable drivers since they
support per display gamma correction and xgamma assumes only a single
display.  I believe the proper way to adjust it is to use xrandr.

Alex