Point-Based Approximate Color Bleeding

Per H. Christensen

(Copyright Disney and Jerry Bruckheimer; image courtesy of ILM)

Abstract: This technical memo describes a fast point-based method for computing diffuse global illumination (color bleeding). The computation is 4--10 times faster than ray tracing, uses less memory, has no noise, and its run-time does not increase due to displacement-mapped surfaces, complex shaders, or many complex light sources. These properties make the method suitable for movie production. The input to the method is a point cloud (surfel) representation of the directly illuminated geometry in the scene. The surfels in the point cloud are clustered together in an octree, and the power from each cluster is approximated using spherical harmonics. To compute the indirect illumination at a receiving point, we add the light from all surfels using three degrees of accuracy: ray tracing, single-disk appoximation, and clustering. Huge point clouds are handled by reading the octree nodes and surfels on demand and caching them. Variations of the method efficiently compute area light illumination and soft shadows, final gathering for photon mapping, HDRI environment map illumination, multiple diffuse reflection bounces, ambient occlusion, and glossy reflection. The method has been used in production of more than a dozen feature films, for example for rendering Davy Jones and his crew in two of the ``Pirates of the Caribbean'' movies.

One-line summary: Point-based color bleeding is faster and uses less memory than ray-traced methods.

Published as: Pixar Technical Memo #08-01. Pixar, July 2008.

Download the tech memo here: paper.pdf.

This method is based on an earlier method by Michael Bunnell (then at NVIDIA) and was developed and tested in collaboration with Rene Limberger (Sony) and Christophe Hery (ILM).

An overview plus updated information can be found in the slides from my talk at the Annecy Animation Film Festival (June, 2009).

Here's an interview with Christophe Hery describing how the method was used on the "Pirates of the Caribbean" movies: story.

Back to Per's publication page.