Andy Ross wrote:
* Stated exactly: the silhouette is a subset of triangle pairs sharing
an edge where one triangle is front-facing with respect to the light
source, and the other is back-facing may be on the silhouette.
There is a pre process step where I look for the 2 triangles that share
an edge. For rendering we only consider
triangles facing the light. For each of its 3 egdes we check if the
neighbour triangle is facing the light or not.
The edge shared by 2 triangles facing the light is not a silouhette
edge, if the neighbour triangle is not facing the
light (or does not exist) then this edge is a silouhette.
The problem is that detecting this silhouette (or at least an
approximation) in a situation where the object and light source can
have any orientation is a big mess, and I never found a good way to do
it. Lots of really complicated code got me nowhere.
The idea is to use a light in model space, not in world space. So if the
object move it's like if the light was moving
in the opposite direction. When I compute the silouhette for an object I
climb the scene graph to find all the
ssgtransform node and then I use the inserve matrix to rotate my light
vector. The translation part of this
matrix is nullified since the sun if far way I do as if the sun was
moving with the object.
The silhouette computation is only done when my sun vector change
enought and I cache the list of silouhette
edges so the cpu is not too stressed every frame while nothing changes.
Basically, the whole experience convinced me that a shadow buffer
approach, which is *much* simpler conceptually (just draw the thing
into a texture from the point of view of the light source), was a
better idea. Shadow buffers don't need the really complicated
silhouette optimization work to make them run fast. It is true that
shadow buffers don't work for self-shadowing objects (shadow of the
vertical stabilizer on the wing, etc...), though.
Andy
I started a prototype before using hardware shadow maps. You are right
it's a lot simpler, but it can't run
on all hardware (and nvidia don't have the shadow ambient extension so
shadows would be black...)
and I think that shadows should be available to all users. And of course
shadow maps a problem of
pixelisation.
Harald.
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