[sldev] Re: Viewer with University of Michigan Stereoscopic Viewer patch released

Dale Glass dale at daleglass.net
Thu Nov 29 11:47:45 PST 2007


On Thursday 29 November 2007 17:54:20 Dale Mahalko wrote:
> I don't know if it was mentioned before, but users of this
> stereoscopic client are probably going to take a performance hit,
> since for each frame it has to render the scene twice and then figure
> out the anaglyph coloring difference between the two scenes. If your
> regular framerate isn't very good, this is only going to make it much
> worse.
It involves a performance hit, but it's not that horrible.

I just tested.
Normal: 34 fps
Anaglyph: 26 fps

So about a 25% slowdown in my case. Not all the time spent per frame is 
rendering, so doubling the rendering workload doesn't necessarily mean 
performance will be horrible. People whose bottleneck is in the CPU should 
suffer the lowest performance loss.


I'll soon include an option to toggle anaglyph rendering with a hotkey, as 
it should be useful for people who need to build or script (currently a 
bit inconvenient with glasses, will have to see if it can be improved)

> Also, if it renders the frame as a full scene for one eye, and then a
> second full scene for the other, then I assume vertical sync is forced
> on and can't be turned off since it needs each raster line rendered
> twice, to figure out the stereoscopic differential. You can't do that
> if the scene for the other eye hasn't updated yet.
I imagine vsync may be needed for active mode, but I don't see why would it 
be needed for anaglyph (red/cyan glasses), which is what most people will 
use. And vsync is quite meaningless on LCD monitors.

Anaglyph mode creates images like this and needs red/cyan glasses:
http://www.2ndlook.org/thumbnails.php?album=222


> Turning vertical sync off to allow tearing can be permitted if it
> renders the frame only one raster line at a time across both eyes.
> This allows the overall image to tear, but at least stereoscopic
> effect would be preserved across each individual scanline.
I assume you mean the active mode. This needs either two projectors (about 
$1300 each, plus $400 video card), or a 3D monitor ($4000). People who can 
afford this sort of setup probably will have top of the line hardware, so 
I don't think we have to worry about them.


> Is there a hit on video memory usage? Since it has to render the scene
> twice for each eye then I'd assume that is two frame buffers, plus
> possibly a third buffer to merge them together.
Can't see any in the stats. KTris doubles as expected, the rest stays the 
same. But it's not like it's a big deal anyway, 1600x1200 at 32 bits is 
just about 7MB, out of the 256MB on my card.
 



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