A GPU Accelerated Volumetric Ray Tracer for Incandescent Gas

[1] lays out a basic algorithm for volumetric ray tracing: it renders a given volume based on local data specified at each voxel within the volume without the use of geometric primitives. In his method, voxels are treated as samples on a continuous function for color/opacity, and the specific values at a point are derived with local operators. Front-to-back eye rays are modeled as vectors of color/opacity values sampled from a given number of evenly spaced points along the ray. These sample values, in turn, are created from a trilinear interpolation of the locally derived values from the 8 voxels surrounding the one which the ray is passing through. The final returned from a given eye ray is the composite of all the values in the vector, combined with a completely opaque bacground. Optimization is achieved by stopping eye rays once a high enough opacity value is encountered, and by hierarchical spatial enumeration.