Intel 2.4.0 dithering problem

Peter Clifton pcjc2 at
Sun Aug 10 09:36:14 PDT 2008

On Sun, 2008-08-10 at 17:27 +0100, Peter Clifton wrote:
> Hi,
> I've just spent about a day playing with the git head Intel driver,
> mesa, drm (kernel patches etc..). (After the drm-gem merge)
> I have things basically working, EXA acceleration enabled (not UXA, that
> appeared to break things).
> What I immediately noticed was a problem with colour banding across my
> display. I tried forcing 24bit LVDS signalling, which turned out not to
> be the cause (and gave pretty colour patterns), and then started digging
> into the 8->6 bit dithering.

By "colour banding", I probably don't mean "colour".. the display just
looked as if it had low BPP when displaying a gradient.

> I've no idea why, but the i830_bios.c detects that the panel shouldn't
> be dithered. Forcing dithering on:

One important piece of information I forgot.. the chipset is 945GM.
Probably unrelated.. I also can't get X to restart / switch to VT.
Screen blanks and won't return after a VT switch.

> diff --git a/src/i830_bios.c b/src/i830_bios.c
> index fe55d23..58e818c 100644
> --- a/src/i830_bios.c
> +++ b/src/i830_bios.c
> @@ -100,7 +100,7 @@ parse_panel_data(I830Ptr pI830, struct bdb_header
> *bdb)
>      if (!lvds_options)
>         return;
> -    pI830->lvds_dither = lvds_options->pixel_dither;
> +    pI830->lvds_dither = 1;// lvds_options->pixel_dither;
>      if (lvds_options->panel_type == 0xff)
>         return;
> Returns the display to it's previous level of banding on gradients
> (display looks pretty good, no hugely obvious banding).

Peter Clifton

Electrical Engineering Division,
Engineering Department,
University of Cambridge,
9, JJ Thomson Avenue,

Tel: +44 (0)7729 980173 - (No signal in the lab!)

More information about the xorg mailing list