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Bug#167719: xserver-xfree86: Display snow when moving windows

retitle 167719 xserver-xfree86: [ati/r128] transient 'snow' artifacts when moving windows at 1152x864 on Rage 128 Pro PF rev 0
tag 167719 + upstream

On Mon, Nov 04, 2002 at 01:22:02PM +0000, Mike Brodbelt wrote:
> There are visible display glitches, most obviously when moving windows. These
> take the form of display snow that occurs in narrow vertical lines. I'm
> guessing it's something to do with display memory alignment, as the problems
> are always found  in the same single pixel wide vertical line, and there are
> probably about 8 to 12 bad pixels visible at any time when windows are being
> moved, or text is being entered. The snow is most visible when using terminal
> windows - on a black background it's clear. I believe it is also occurring over
> lighter coloured backgrounds, but this is very difficult to see.
> This problem was evident in the XFree86 4.1 packages, and persists in the 4.2
> packages. It appears to be linked to the resolution - I'm unable to see it
> when I cycle through resolutions with Ctrl-Alt +/- in resolutions other than
> 1152x864 (though I've only tested the alternates that were in my XF86Config,
> and not added others).

I always thought that this sort of problem -- when it was transient as
you describe, and not "real" to the X server in that it doesn't cause
actual corruption of the framebuffer -- was due to the RAMDAC on the
card being pushed harder than it is supposed to go.

In theory you should be able to run a RAMDAC with a pixel clock at its
maximum rated speed (e.g., 300 MHz) and never see any transient
artifacts no matter what you're banging down the wire, but unfortunately
this doesn't appear to be true.  A casualty of marketroids getting their
hands on the notion of a RAMDAC speed specification, maybe.

Still, I don't actually know much about bit-banging at this level, so
perhaps this really is a software defect.

G. Branden Robinson                |    A celibate clergy is an especially
Debian GNU/Linux                   |    good idea, because it tends to
branden@debian.org                 |    suppress any hereditary propensity
http://people.debian.org/~branden/ |    toward fanaticism.    -- Carl Sagan

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