The making of black-hole and nebula clouds for the motion picture “Sphere” with volumetric rendering and the f-rep of solids

Ray tracing through a volume of gas computed the sum of the densities and density-weighted colors, called samples. The contribution of the samples attenuates with the cumulative density as the ray advances away from the camera. The ray tracing proceeds until the opacity clips or the ray leaves the complete volume. The volume rendering approach taken for this project is computationally intensive. The early-version volume renderer developed at Cinesite supports point-like and directional light sources, with full self shadowing and asymmetric scattering. Ray tracing consists of traversal of voxels and cells, accumulating radiance and attenuation. For the purposes of this project however, the gas was lit internally, so it acted as a glowing and attenuating material with a variety of colors and complex structures. To accomodate the artistic design process and be as flexible as possible, the renderer accesses material properties through method calls in an API. This allows artists and technical directors to build volumetric data in procedural codes without dealing directly with the rendering process. The ISM mainly provides the point samples as the ray tracing takes place, and it manages the primitives, sampling and interpolation of these samples, and loading and evaluation of the appropriate shaders for these primitives. The volumetric shaders used to represent nebula-like structures require varying densities and colors. Noise patterns were used in both shading and displacing the isosurfaces. Briefly, the fractals and random patterns are associated with the f-rep of the solids when inside of the clouds (defined as f(x, y, z, t) > = 0) and another atmospheric (ambient) shading when outside of the shape (defined as f(x, y, z, t) < 0). The properties of implicit surfaces were also used to enhance the known shading techniques. We have optimized the f-rep evaluation. The cloudy shapes we needed in order to achieve the desired nebula effects required large databases. A "realistic" look for these gas clouds requires certain shapes based on storyboards from pre-production, as well as examples from real counterparts. Independent of the choice of the implicit functions, the algorithm to solve the f-rep and evaluate the solids was designed to be capable of localizing the data in a smaller volume to reduce the volume of operations. Executing the shader for the whole ray trace was very expensive , and in some cases such as evaluation of gas motion in vectoral fields, it was impossible directly. So we investigated methods …