The Double Z Item Buffer for Volume Rendering Non-Regular Grids

摘要

The Double Z Item Buffer is a technique for improving the speed of volume rendering data sampled over irregular grids. Volume rendering of data sampled over regular grids can be done at interactive speeds and hardware has been designed to increase the size of the grids that can be volume rendered at these speeds. Volume rendering of data sampled on structured (also called array connected), but geometrically irregular, grids and unstructured (also called cell connected) grids is usually done by ray casting (ray tracing limited to initial rays from the eye through the pixels). Ray casting is used because it is a straight-forward method for finding a front-to-back ordering of the cells covering each pixel of the desired image. However, ray casting is numerically intensive and not well supported by the graphics hardware found in workstations. A small modification in graphics microcode and some additional memory (or a small special purpose hardware unit) can be used to generate the front-to-back ordering required for volume rendering data on these inconvenient grids. The same technique can be used to generate a back-to-front ordering, which may be used in compositing operations. The same second Z buffer and front-to-back ordering (or back-to-front ordering) can also be used to implement a more accurate transparency shading for surface modeled objects than is ordinarily available in Z buffer rendering.