30-bit X session and programs requiring 24-bit depth

Ilya Anfimov ilan at tzirechnoy.com
Mon Feb 17 08:51:50 UTC 2020

On Sat, Feb 15, 2020 at 12:32:15AM +0700, Antoine Martin wrote:
> On 14/02/2020 18:53, Marek Szuba wrote:
> > Hello,
> > 
> > I do quite a lot of photo editing on my box and with both my monitors
> > and my graphics card (amdgpu) supporting 10-bit colour channels, I tend
> > to run X at colour depth 30. Unfortunately some software, most notably
> > programs using OpenGL it seems, refuse to run in this mode - presumably
> > (I have seen this mentioned as the reason of problems with 30-bit mode
> > under Windows when the first cards supporting it came out) because the
> > software assumes 8-bit alpha channel but with the frame buffer still
> > being only 32-bit it is only 2-bit. Seeing as there seem to be no way of
> > switching colour depth on the fly, the best I have been able to come up
> > with is two separate X sessions - one at depth 30 and one at 24.
> > 
> > Is there, or perhaps will there be some way in the near future, to work
> > around this problem - either by increasing framebuffer BPP (tried it a
> > while ago but it didn't seem to accept anything more than 32), using a
> > virtual X server (tried using Xephyr but it complained about there being
> > no matching screen), or some other way I haven't thought of?
> If you don't mind the indirection this introduces:
> xpra start --start=xterm --attach=yes
> This will start your application (ie: xterm) on a 24-bit virtual
> framebuffer and display it on your local 30-bit display.

 And with software-only OpenGL.
 xpra itself is not really good at either proxying or accelerating

> The pixel depth upscaling will be done by your OpenGL driver, or failing
> that, in software.
> The reverse is also possible, with a local display at 24-bit:
> xpra start --start=xterm --attach=yes --pixel-depth=30
> Runs the app on a 30 bit virtual display.
> Apparently, there are apps that require a 30-bit display to use GPU
> image processing and some users don't care if they don't actually see
> the extra bit depth when doing so.
> Antoine
> > 
> > Thank you in advance for your comments.
> > 
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