- ② The GridMedium and RGBGridMedium classes use a relatively large amount of memory for complex volume densities. Determine their memory requirements when used with complex medium densities and modify their implementations to reduce memory use. One approach might be to detect regions of space with constant (or relatively constant) density values using an octree data structure and to only refine the octree in regions where the densities are changing. Another possibility is to use less memory to record each density value—for example, by computing the minimum and maximum densities and then using 8 or 16 bits per density value to interpolate between them. What sorts of errors appear when either of these approaches is pushed too far?
- ② Improve GridMedium to allow specifying grids of arbitrary Spectrum values to define emission. How much more memory does your approach use for blackbody emission distributions than the current implementation, which only stores floating-point temperatures in that case? How much memory does it use when other spectral representations are provided? Can you find ways of reducing memory use—for example, by detecting equal spectra and only storing them in memory once?
- ③ One shortcoming of the majorants computed by the RGBGridMedium is that they do not account for spectral variation in the scattering coefficients—although conservative, they may give a loose bound for wavelengths where the coefficients are much lower than the maximum values. Computing tighter majorants is not straightforward in a spectral renderer: in a renderer that used RGB color for rendering, it is easy to maintain a majorant grid of RGB values instead of Floats, though doing so is more difficult with a spectral renderer, for reasons related to why RGBUnboundedSpectrum values are stored in the grids for and and not RGB. (See the discussion of this topic before the <<Initialize majorantGrid voxel for RGB and >> fragment.) Investigate this issue and develop an approach that better accounts for spectral variation in the scattering coefficients to return wavelength-varying majorants when RGBGridMedium::SampleRay() is called. You might, for example, find a way to compute RGBUnboundedSpectrum values that bound the maximum of two or more others. How much overhead does your representation introduce? How much is rendering time improved for scenes with colored media due to more efficient sampling when it is used?
- ② The Medium implementations that use the MajorantGrid all currently use fixed grid resolutions for it, regardless of the amount of variation in density in their underlying media. Read the paper by Yue et al. (2011) and use their approach to choose those resolutions adaptively. Then, measure performance over a sweep of grid sizes with a variety of volume densities. Are there any cases where there is a significant performance benefit from a different grid resolution? Considering their assumptions and pbrt’s implementation, can you explain any discrepancies between grid sizes set with their heuristics versus the most efficient resolution in pbrt?
- ② Read Wrenninge’s paper (2016) on a time-varying density representation for motion blur in volumes and implement this approach in pbrt. One challenge will be to generate volumes in this representation; you may need to implement a physical simulation system in order to make some yourself.