[visionegg] Re: screen position and graphics artifacts

  • From: Andrew Straw <astraw@xxxxxxxxxxx>
  • To: visionegg@xxxxxxxxxxxxx
  • Date: Fri, 04 Nov 2005 17:01:42 -0800

Hi Adam,

Adam Light wrote:
> Hello All
> I'm just starting to use vision egg and it looks like a nice package. 
> I'm having difficulties with two things:
> 1.  How do I specify where on my screen the display should go?  If I
> create a window, I know how to direct the graphics to a certain location
> within that window, but how do I set the position of the window itself?

The screen position is determined automatically by the OS, I think. SDL
(which is wrapped by pygame, which is wrapped by the Vision Egg) has
some environment variables which are supposed to allow specification of
screen position. see:


Unfortunately, I think these environment variables aren't "officially
supported" in SDL, and thus are subject to future non inclusion.  (In
fact, a recent build of pygame/SDL on Windows XP seems to ignore them,
which I was planning on following up... Wait, it looks like it's
discussed here:
http://www.devolution.com/pipermail/sdl/2005-April/068395.html )

> > 2.  When using certain stimuli, including the demo versions of the
> moving grating and put_pixels, I sometimes see an artifact that travels
> from the bottom of the window to the top.  It usually looks like a
> slightly jagged line, and usually travels across the entire screen
> slowly to the top.  I'm not sure if this is a programming issue or
> something that is specific to my monitor.  It's an LCD monitor running
> at 60Hz, and that's what vision egg is set to use.

This is the well-known 'tearing' artifact. Googling that should give you
a better idea of what the problem is. You can either try to fix it by
clicking the "sync buffer swaps to vertical retrace" button on the
Vision Egg initial GUI popup window, or by forcing this (often called
"VSync") in your video drivers. If you need further help, let us know,
and be sure to specify your OS and video card.

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