Comment 12 for bug 242990

On Fri, Jul 11, 2008 at 11:52 AM, Dereck Wonnacott <email address hidden> wrote:
> lcampagn, how quickly were you moving the window at the time? I have to
> move the window pretty fast to get the distortion as bad as in the
> screenshot. I still notice it 'by eye' when moving windows normally, I
> can easily ignore it in daily use; does it tear to the degree in your
> screenshot during regular use?

It's not all that noticeable--I basically just kept swinging the mouse
around very quickly while snapping photos until I caught one that was
torn. If I take a screenshot using the same mouse motion, I get
roughly the same effect you saw. In general it's pretty easy to
ignore, but if you're aware of it, then even slow moving objects will
have a "rippling" effect on their edges. The distortion you see in
your screenshot occurs because the software that reads the contents of
the screen is too slow to take a full screenshot before the window has
moved several times, so what you see there isn't really related to
this bug. (I certainly never see tearing that bad)

The degree to which any system will show tearing will largely depend
on the vertical refresh rate of the monitor and the performance of the
system, but I doubt very much that anything like your screenshot is
actually visible on the screen--I've never caught a photo with more
than a single tear in it (although this is not impossible).

> Also, what hardware are your other
> computers that have the issue? Do you use compiz on them? And are they
> running any restricted drivers?

I have tried:
nVidia Corporation NV17 [GeForce4 MX 440] - restricted drivers, no compiz
nVidia Corporation NV43 [GeForce 6600] - restricted and OS drivers,
with and without compiz
Intel Corporation Mobile 945GM/GMS - "intel" driver, with and without compiz
Silicon Integrated Systems [SiS] 65x/M650/740 PCI/AGP - no compiz

> I would like to ask upstream about this just to have their input on it,
> but before that I want to try and isolate our issue a bit more. It
> really could be that we have totally different bugs with the same
> symptoms, but from your screenshot, it seems the same to me, I just
> happened to be moving my window faster and in circles producing more
> exaggerated tears.

I'd be interested to hear what upstream has to say, but I really
suspect Chris is correct--it is not reasonable for X to try to sync to
vblank unless it is rendering to an offscreen buffer, which is almost
the entire function of compiz. Once they have their sync issues sorted
out, this problem should be gone forever.

Q