Shadow mapping works in that it checks if a point is visible from the light or not. If a point is visible from the light then it’s obviously not in shadow, otherwise it is. The basic shadow mapping algorithm can be described as short as this:
- Render the scene from the lights view and store the depths as shadow map
- Render the scene from the camera and compare the depths, if the current fragments depth is greater than the shadow depth then the fragment is in shadow
It’s the implementation of it that is hard.
The two big problem areas with shadow mapping:
- Hard to select an appropriate bias (epsilon)
- Hard to get rid of artifacts at shadow edges
Projective texturing ( the method used to transform the fragment depth to the light space (where the shadow map is) for comparision)
OpenGL fixed-function pipeline implementation of shadow mapping:
A GLSL implementation of shadow mapping (in one of the posts)
Another GLSL shadow mapping shader:
DirectX9 shadow mapping example with source
Nvidias implementation of shadow mapping with source for both OpenGL and DirectX.