[osg-users] Supporting multiple frame rates

Robert Osfield robert.osfield at gmail.com
Thu Apr 28 08:59:21 PDT 2011

Hi Brad,

I'm afraid decoupling the rendering of two parts of graphics window so
that they can run at their own frame rate won't be a straight forward
task, and well off the standard viewer path used by the vast majority
of the community.  You are infact the first I recall wanting to do
exactly this combination of video at 30fps and man scene at it's own
frame rate.

Thoughts off the top of my head:

    You only need to do something special when 3D scene takes longer
than 30ms and would cause
    frame rate to drop below the min 30fps, so the follow suggestions
are for when frame rate would
    drop below 30.

    Could you render the video to the front buffer and just overlay
the background 3D scene?  This
    would enable you to remove the need for a swap buffers call at the
end of the render of teh video texture.

    Another approach would be to render the 3D scene into a texture
and then compose each frame
    from the 3D scene texture and the video texture and do the swap
buffers at video texture frame rate.

    In both cases you the 3D scene is taking too long to render, and
since you are using a single
    graphics context there isn't a natural way to split up and
decouple the rendering completely,
    so to be able to put out frames at 30fps you'll need to break the
render of the 3D scene into
    segments each of which can fit into with the time available when
rendering at 30fps.

    Splitting the scene won't be trivial, the easist method would be
something like depth partitioning,
    and render different partition in depth and balance the depth
range for each so that the load is
    as well balanced as you can get it.

That's my best suggestions so far...  Far from trivial to set up, very
much on the bleeding edge!


On Thu, Apr 28, 2011 at 4:39 PM, Brad Huber <bradh at procerus.com> wrote:
> Hello,
> We currently have an OSG based application which renders at 30-60+ fps on
> our development hardware with discrete gpus.  Unfortunately we have to be
> able to  deploy on a wide range of hardware including stuff that is
> significantly less capable (Intel GPUs, etc).  On some of these slower
> platforms we are seeing framerates that are 10-15 fps.  This is actually ok
> for us with one exception, live video.
> We show live video in the scene graph (for now strictly in the HUD).  We
> have a requirement to make the video render at native speed (~30 fps).  I am
> looking for a way to make the HUD/video render at 30 fps and let the rest of
> the scene graph render at a floating rate (whatever the machine can handle).
> How might this be accomplished?  Since this is a HUD, obviously it must
> ultimately render to the same graphics context, however it doesn’t really
> need to respect the depth buffer, etc of the rest of the scene (it should
> always render on top).
> PS Should I be thinking about render to texture?
> Related threads I came across:
> http://forum.openscenegraph.org/viewtopic.php?t=7858
> http://lists.openscenegraph.org/pipermail/osg-users-openscenegraph.org/2008-January/006000.html
> http://forum.openscenegraph.org/viewtopic.php?t=7117
> Thanks
> -Brad
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