Hi Joakim,
On 04/11/2011 12:14, Joakim Low wrote:
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.
I have not experienced this exact problem, but have found some strange
things with CullingSet::computePixelSizeVector. See below:
article.gmane.org/gmane.comp.graphics.openscenegraph.user/70166/
If you find any more details about the implementation it might help.
rgds
jp
Thank you for any input, Joakim
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