Shadow Volumes Revisited

We present a method to utilize the Shadow Volume Algorithm by Crow and Williams without using a stencil buffer. We show that the shadow mask can be generated in the alpha channel or even in the screen buffer, if a hardware-accelerated stencil buffer is not available. In comparison to the original stencil buffer method, a small speed up can be achieved, if the shadow mask is computed in the alpha buffer. The method using the screen buffer requires the scene to be rendered a second time after the shadow mask has been computed. Both methods are less restrictive with respect to hardware requirements, since we use only standard color blending and depth testing. In general, rasterization bandwidth is the main bottle neck when generating the shadow mask at high screen resolutions. In order to overcome this bottle neck we propose a way to compute the shadow mask at a resolution that is lower than the resolution of the screen buffer. Then the shadow mask is applied to the scene by utilizing texture mapping. The latter method might be reasonable especially in interactive entertainment, where rendering speed is traded in favour of image quality.