Re: Can you use 10 bit RAMDAC cards under X?

From: Dances With Crows (danSPANceswitTRAPhcrows_at_gmail.com)
Date: 03/30/05 

Date: 30 Mar 2005 21:35:53 GMT

On Wed, 30 Mar 2005 15:09:55 -0600, Ron Howes staggered into the Black
Sun and said:
> I know there are video cards that support 10 bit RAMDAC technology for
> high resolution graphics,

? 10 bits/channel should be orthogonal to the display resolution. I
think... ICBW.

> 10 bits implies 30 bits for color depth, which implies 32 BPP for
> running these under X. However, I have not been able to completely
> verify if X can support actual 30 bit color graphics when in 32BPP.
> Some of what I have seen seems to imply that 32BPP actually defaults
> to a real 24BPP color depth, or 8 bits per color channel.

The documentation for the latest stable Xorg says that 24 bits/pixel is
padded to 32 bits/pixel for speed if there's enough VRAM. The extra 8
bits is not usable for anything right now. I think the Xorg developers
were going to try using those extra 8 bits for alpha channel, not for
additional color depth. Remember that all the applications you're using
on a 10 bits/channel card would have to be aware of the extra bits and
calculate colors accordingly. I don't think there are very many
applications that do that right now.

> can X support an actual 30 bit color depth for such video cards when
> in 32BPP mode?

Not AFAICT. Fixing this would require a fair bit of banging on the bare
metal and full tech specs for the graphics chipset in question. I'm
really not sure how much 10 bits/channel would help in the real world,
since most humans can only discern about 6 or 7 bits of RGB on a CRT.
YMMV though. 

-- 
Matt G|There is no Darkness in Eternity/But only Light too dim for us to see
Brainbench MVP for Linux Admin /    mail: TRAP + SPAN don't belong
http://www.brainbench.com     /                Hire me! 
-----------------------------/ http://crow202.dyndns.org/~mhgraham/resume