FAQ - shaded, ray-traced images of very large data sets

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Q. What is a recommended method to make shaded, ray-traced images of very large data sets?

A. Use compute shade - a combined colorizer/compute gradient/gradient shade module

This module combines the functions of the colorizer, compute gradient, and gradient shade modules into a single, memory efficient module. These modules are used primarily to make shaded, ray-traced images. The problem is that they are highly inefficient in terms of memory allocation:

colorizer takes in a 1 byte per voxel and outputs 4 bytes per voxel
compute gradient takes in 1 byte per voxel and outputs 12 (3 floats)
gradient shade outputs 4 bytes per voxel

These three modules produce 20 bytes for every input data set byte. It is for this reason that some people have experienced problems trying to render ray castings of large data sets--the tracing code is pretty computationally efficient--it's just that most of the system resources go to swapping data, rather than computing the image.

The compute shade module does gradient computation, colorizing, and shading on a per slice basis. All in all, it takes less time than running the original three modules in sequence. However, it does take longer than running gradient shade alone.

Therefore, it is most useful for extremely large data sets (> 100 * 100 * 100 voxels) which would normally choke a system's memory rather than small data sets.