Notices by Wojciech Jarosz (wjarosz@mathstodon.xyz)

#Mastodon question. It sometimes takes *minutes* to update my feed. How much of slow feed refresh, posting response is due to the particular instance I decided to join, vs. other servers, vs. just the decentralized nature of mastodon?

This project was the result of Zack doing an internship with #WDAS last summer. He explored improvements to volume rendering for the next generation of Disney’s Hyperion renderer, which they have used to render all their movies since Big Hero 6.

Hyperion uses null tracking to render volumes. Variants of these techniques go by many names like Woodcock tracking, delta scattering, delta tracking, ratio tracking, etc. There has been lots of work on these techniques over the last decade.

Heterogeneous media (where density and other properties vary across space) are notoriously hard to render using Monte Carlo. Null tracking techniques make this practical by pretending the volumes are actually homogeneous. You can think of this as injecting additional null (or “fictitious”) density where the real density is low, so that the sum of the null and real densities across space is homogeneous. This combined density is often called the “majorant”.

This majorant can theoretically be set to any positive value. Typically we choose a value that is larger than the density of the medium at any location in space (hence the name “majorant”). If we set it higher, it will just take longer to render. So we'd like to keep it low.

Theoretically, we can choose values that are smaller than the maximum density, but this quickly leads to really high variance.

This is one of the core challenges of null tracking methods: we’d like to set the majorant to the maximum density of the medium, and no higher -- so that both variance and render times are low.

If the medium is defined by a voxel grid, this is trivial: just iterate over all the voxels and compute the maximum value.

But in production, volumes are often the combination of several voxel grids, with additional procedural modifications (e.g. fractal noise) all authored as a complex node graph. This means we cannot generally find the exact maximum density.

A 🧵 about my PhD student (Zack Misso’s) upcoming #SIGGRAPH2023 #research #paper on progressive #nullTracking for #production #volumeRendering (in collaboration with @yiningkarlli and others at #DisneyAnimation).

The project page is now live at:

https://cs.dartmouth.edu/~wjarosz/publications/misso23progressive.html