[Intel-gfx] 3.16, i915: less colors in X?

[Thomas Richter]

Hi Martin,

>> I'm trying to figure out how to ask X what color depth it is using...?
>
> I think:
>
> martin at merkaba:~> xdpyinfo | grep -i "depth of root"
>    depth of root window:    24 planes
>
> but am not completely sure.
>
>> This is thinkpad x60 with Debian 6.0.9.

AFAIK the 830GM chipset does not offer any support for hardware 
dithering. Whether the panel in the x60 does I do not know, though.

However, what is remarkable is that graphics on a 16 bit(!) screen may 
look more pleasing than graphics on a 24 bit screen, at least for such 
ancient machines. The reason is that the panel cuts the bitdepth down 
from 8 to 6 bits, without any dithering, just by cutting off the LSBs. 
However, if you select a 16bpp mode to begin with, some desktop 
environments (specifically gnome) apply a dithering of their own, even 
though the output is only 5 bit per component.

This is at least what I see here on the IBM R31 and the Fujitsu S6010: 
Gnome desktop at 16bpp looks better than the desktop at 8bpp, due to the 
lack of hardware dithering.

The X11 intel driver had an option "Dac6Bit" to signal the 6 bit panel 
resolution to X (even though the display pipeline operates in 8 bit 
mode), but I have never seen this working in the past time. It seems not 
to be supported anymore. Probably that's the culprit.

Martin, you should probably test Ville's alm_fixes5 kernel branch, its 
support for the 830GM chipset of the X30 is in my experience much better 
than that of the drm-intel-nightly or official kernels.

Greetings,
	Thomas