Hi,
I need to check whether an object occludes another object behind it. To
do so, I currently use a class derived from osgShadow::OccluderGeometry
to compute the occlude silouhette from the camera point of view, and
create an osg::Polytope from that silouhette.
Going through osgShadow::OccluderGeometry code, I noticed that the
silouhette is computed via two odd looking functions. For example:
inline bool isLightPointSilhouetteEdge(const osg::Vec3& lightpos, const
Edge& edge) const
{
if (edge.boundaryEdge()) return true;
float offset = 0.0f;
osg::Vec3 delta(lightpos-_vertices[edge._p1]);
delta.normalize();
float n1 = delta * _triangleNormals[edge._t1] + offset;
float n2 = delta * _triangleNormals[edge._t2] + offset;
float angle_offset = 0.0f;
n1 = cos(acosf(n1) + angle_offset);
n2 = cos(acosf(n2) + angle_offset);
if (n1==0.0f && n2==0.0f) return false;
return n1*n2 <= 0.0f;
}
What puzzles me is the double calls to cos() and acosf(), as this is
very expensive. Having worked a long time ago on stencil shadows, I
looked back at my old code. It appears I was using the triangles plane
equation and then doing a dot product with the light position. Since the
plane equations need only to be computed once, this was a very fast way
of doing it (and works for both directional and point light). I wonder
what are the advantages of the current osg implementation.
My old code looked something like this:
void ComputeVisibility()
{
for (size_t i = 0; i < m_Planes.size(); ++i) {
const plane_eq & Plane = m_Planes[i];
double Side = 0.0;
Side += Plane.a * LightPosition[0];
Side += Plane.b * LightPosition[1];
Side += Plane.c * LightPosition[2];
Side += Plane.d * LightPosition[3];
if (Side > 0)
m_Triangles[i].mark();
else
m_Triangles[i].unmark();
}
}
Cheers,
Tanguy
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