I think I have a pretty good idea of the cause.
The CullVisitor gathers near & far info during traversal but doesn't actually
compute the final projection matrix until after the traversal completes. This
means, obviously, that the projection matrix for the current frame hasn't been
computed at the time the CullVisitor encounters my post-render Camera. Instead,
the CullVisitor inserts last frame's projection matrix into the render graph,
which is used for the box rendering. Then it completes traversal, computes the
*correct* projection matrix, and uses this new matrix for the viewer root
Camera's cylinder rendering. Thus, the two projection matrices are off just
slightly, and the result is incorrect z interaction.
But knowing this, I haven't been able to come up with a workaround. (Okay, well,
short of editing the render graph after cull, which seems like I'd be asking for
trouble.) So, still hoping for input from the community on how to make this work
without visual artifacts. Thanks.
-Paul
On 12/14/2012 11:28 AM, Paul Martz wrote:
Hi all -- I'm attempting to render with two Cameras, where one inherits the
projection matrix of the other. But there appears to be some latency such that
the subordinate Camera doesn't get the projection matrix until one frame late.
You can reproduce the issue with the attached example and animation path.
The example configures the viewer's root Camera to render to an FBO with color
and depth textures attached. It renders a cylinder into these textures. A
post-render Camera then splats the color texture into the window with a full
screen quad.
The problem occurs when a final post-render Camera draws the checkerboard box.
It uses the depth texture an input to a fragment shader that performs a manual
depth test. I have taken steps to ensure that this post-render Camera has
RELATIVE_RF ref frame, and set it to disable near & far computation, so that it
will inherit the projection matrix of the viewer root Camera. This works quite
well except for the slight lag, indicating that this post-render Camera doesn't
get the correct projection matrix until about a frame too late.
The problem is definitely related to the fact that the viewer root Camera is
configured to auto-compute near & far, because if I disable this and set a fixed
projection matrix on the root Camera, the problem goes away (uncomment lines 49
and 50).
Can anyone explain the one-frame lag with the projection matrix? I'll be digging
into the code over the weekend to figure this out myself, but would appreciate
any input while I dig...
Thanks,
-Paul
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