Hi,
I'm experiencing some problems with disappearing terrain tiles close to camera,
which I believe have to do with a combination of PagedLOD range handling and
pixel-size-on-screen computation.
The problem:
At certain combinations of distance and angle from camera, PagedLOD tiles are
completely culled, causing a hole in the terrain. The problem seems to appear
only for tiles close to camera, when using large coordinates.
My understanding:
I've pinpointed the source of my problem to the function:
float CullingSet::pixelSize(const Vec3& v, float radius) const { return
radius/(v*_pixelSizeVector); }
Occasionally, the scalar product in the denominator becomes zero, causing the
function to return an inf-value. This means no LOD-range will cover the
resulting pixelSize, which in turn means no tile is shown. The _pixelSizeVector
is computed by the function CullingSet::computePixelSizeVector. I don't have
much understanding of how this function works, but after looking at the
components of _pixelSizeVector (a Vec4 vector), my guess is that the problem
appears due to numerical cancellation. In my case, the x,y,z components of
_pixelSizeVector are very small compared to the w-component, while the
components of v (a Vec3 vector representing the bounding sphere center here)
are large. The scalar product returns zero due to limited precision, where it
probably should return some very small number instead.
Questions:
I can think of two ways to deal with this:
1. Find another, more stable, way to compute the pixel-size-on-screen. If I
understood the idea/algorithm used in the CullingSet::computePixelSizeVector, I
could try to figure a different way to do it, or predict the cases where the
problem occurs. I'm not sure this would utlimately solve the problem though,
since pixel-size-on-screen may be extremely large (?), causing the tile pixel
size to fall out of all LOD-ranges. Can anyone point to an explanation of the
idea/algorithm used to compute in CullingSet::computePixelSizeVector? I would
really like to know more.
2. Accept that the CullingSet::pixelSize may return extremely large values,
even inf in some cases, and handle this in PagedLOD. This is the work-around
I'm currently using.
Has anyone else experienced similar problems? Is there something I've
missed/overlooked? I'm currently on OSG version 2.8.2, but as far as I can
tell, the code in trunk still works the same way.
Thank you for any input,
Joakim
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http://forum.openscenegraph.org/viewtopic.php?p=43712#43712
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