Harald JOHNSEN wrote: > I have started to add some volumetric shadows in Flightgear. It > uses the standard stencil method to count shadow volume

Wow, nice work. How are you handling silhouette optimization? For those interested, the basic idea behind stencil shadows is that, for every triangle in the input mesh, you render three infinitely long triangles from the sides of the original to a "point" along the line leading away from the light source. So the number of vertices you need to push goes up by a factor of four, and the number of pixels you need to fill goes up by a *lot* -- each small original triangle gets smeared all the way out to the edge of the screen in the shadow edges. I did a prototype stencil shadow engine about two years ago, and saw something like a 15x slowdown when stenciling was enabled. The "standard" way to optimize this involves detecting which triangles are on the "silhouette" of the object*, because those are the only ones which define the shadow volume. Since the silhouette is a 1D feature, and the polygon mesh is 2D, the number of edges on the silhouette goes roughly as the square root of the object's polygon count; so this can be a really important optimization. * 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. 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. 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 _______________________________________________ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d