The Path-Buffer

With a small modification the z-buffer can be made flexible enough to be used in global illumination algorithms traditionally handled only by ray casting, but still enjoy the benefits of z-buffer algorithms including predictable performance, memory coherence, and acceleration using specialized z-buffer hardware. The fundamental difference between the path-buffer and a standard z-buffer is that the global near clipping plane is replaced with a per pixel near clipping value z0. The success of each pixel write depends on compares with both the current z and z0 values. We demonstrate the power of this additional test by showing how two global illumination algorithms can be adapted to use the path-buffer instead of ray casting. First we adapt Kajiya-style path tracing for finding view-dependent global illumination solutions. Since the scene database is streamed through the path-buffer, and no complicated meshing or per-element storage is needed, very large and complex models may be illuminated. Both diffuse and non-diffuse reflectance functions can be handled as well any complex geometric primitives which can be rasterized (e.g. displacement-mapped spline surfaces). Second we show how the path-buffer can be used to do an image gather from a coarse radiosity solution in two-pass radiosity methods