Shadowing (Direct3D 9)
Stencil buffers are a depth-buffer technique that can be updated as geometry is rendered, and used again as a mask for drawing more geometry. Common effects include mirrors, shadows (an advanced technique), dissolves, and so on. The ShadowVolume Sample uses stencil buffers to implement real-time shadows. Additional information is contained in Two-Sided Stencil (Direct3D 9).