Perspective Shadow Maps: Care and Feeding

Shadow generation has always been a big problem in real-time 3D graphics. Determining whether a point is in shadow is not a trivial operation for modern GPUs, particularly because GPUs work in terms of rasterizing polygons instead of ray tracing. Today's shadows should be completely dynamic. Almost every object in the scene should cast and receive shadows, there should be self-shadowing, and every object should have soft shadows. Only two algorithms can satisfy these requirements: shadow volumes (or stencil shadows) and shadow mapping. The difference between the algorithms for shadow volumes and shadow mapping comes down to object space versus image space: ● Object-space shadow algorithms, such as shadow volumes, work by creating a polygo-nal structure that represents the shadow occluders, which means that we always have pixel-accurate, but hard, shadows. This method cannot deal with objects that have no polygonal structure, such as alpha-test-modified geometry or displacement mapped geometry. Also, drawing shadow volumes requires a lot of fill rate, which makes it difficult to use them for every object in a dense scene, especially when there are multiple lights.