"I have adapted to osgShadow the soft shadow map technique described in "Efficient Soft-Edged Shadows Using Pixel Shader Branching" by Yury Uralsky, Chapter 17 of GPU Gems 2 (Matt Pharr ed. Addison-Wesley).
Here is my code in attachment: basically, it works in the same way as osgShadow/ShadowMap (core code is copied from it) but implements a specific GLSL shader for the soft rendering of penumbra.
I have tested it under Linux with a NVidia graphic card, but there should be no dependency on platform nor on the graphics driver (as far as they support GLSL 2). Screenshots attached show the current results (frame rate bound to v-sync, but the shader takes actually not much time)."
