Hi,
I reached some difficulties with handling of lights when I was
considering some improvements to osgShadow, like multiple light sources:
1. single light source object can be placed two times to the scene.
Because of multi-parenting, the light may have two different
transformations. Although one object and one unique pointer, it
identifies two light sources, thus it is not unique light
identification. When current implementation of
ShadowMap::setLight(light) sets the light for shadows, which one of the
two should cast the shadows? Definitely, not both of them, at least not
with the current light implementation.
Options:
- The most robust approach is to use "paths", identifying the node path
down in the scene graph to the light. This would allow proper identity
of multiple-parented objects and position of the light would be given by
the transformations on the path.
- Another approach may use just the first or the last light occurence in
the scene graph.
- cull-time approach may use first or last occurence of the light in the
RenderStage::PositionalAttributeList.
- any other options?
The first "most robust" approach is used by Open Inventor and it works
pretty good, although it may be considered overkill by some people.
My current preference is to use the first position of the light in the
scene graph (retrieved from RenderStage::PositionalAttributeList,
because it is used in cull-time), and to silently keep all the remaining
light occurrences in the scene graph on the same position. When an user
will need lights on different positions, he will just create copies of
the same light.
2. LightSource question: I was always wondering why there are two light
classes - osg::Light and osg::LightSource - and not just one. Is the
only purpose for having both osg::Light and osg::LightSource the ability
of LightSource to specify ReferenceFrame? Apparently, osg::Light as
Attribute can not have reference frame. Any other reasons, or am I
missing something?
Thanks,
John
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