Scientific Visualizations with POV-Ray

With a little work, the Persistence of Vision Raytracer (POV-Ray) can be adapted to create stunning three-dimensional imagery from floating-point scientific data files.
The Scene Description File

All of the pieces are now in place to construct a scene description file to be interpreted by POV-Ray. I used the example found on Suzuki's density file extension Web page as a template and modified it to fit my needs. Listing 3 contains the full scene description file used to render an isosurface of cloud concentration with a sky-blue background and a tiled surface, shown in Figure 1. Starting at the top, the #version statement is required in order for this unofficial version of POV-Ray to function. The following nine #declare statements specify the Cartesian coordinates that bound the box containing the isosurface, as well as scaling factors.

Continuing through the scene description file, the color and finish parameters are declared, and the camera and lighting parameters are set. The lines that follow contain the important bits for creating the isosurface. QCFUNC is declared as a function that uses the HDF file supercell.ck10990.hdf as a source of data; it chooses the variable QC (representing cloud concentration) within the file to render. Tricubic spline interpolation is chosen, and the entire domain is scaled so that all spatial indices, such as camera location and light location, coincide with array index values of the data. By default, POV-Ray's domain ranges from 0.0 to 1.0 in all three directions.

I created a macro called QCISOSFC, which takes as arguments the value of the isosurface I wanted to render and the level of transparency of the isosurface. Transparency is a useful isosurface property when rendering two isosurfaces where one exists inside another. For example, it is useful to render a transparent cloud that contains an isosurface of hail concentration, because hail often is contained within a thunderstorm cloud. QCFUNC, defined above, is selected as the isosurface function to render. The chosen isosurface binds a volume of cloud concentration greater than the chosen isosurface value of 0.0002.

The max_gradient parameter basically tells POV-Ray how much work it needs to do to find the isosurface. Technically, it tells POV-Ray what maximum gradient (largest change over distance) the function representing the isosurface data contains in the vicinity of the chosen isosurface. It is a number that must be chosen carefully. Too low a value produces an isosurface with holes or one that does not render at all; too large a value causes POV-Ray to run for a much longer time than is necessary. Some experimentation is required to find an appropriate value for max_gradient. I chose a value of 0.0002, which may seem small; however, cloud concentration ranges from 0.0 to about 0.01. POV-Ray warns you after rendering an isosurface with too large or small a value of max_gradient and suggests a value it deems appropriate after rendering.


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