Walt Disney Animation Studios has transitioned to path-traced global illumination as part of a progression of brute-force physically based rendering in the name of artist efficiency. To achieve this without compromising our geometric or shading complexity, we built our Hyperion renderer based on a novel architecture that extracts traversal and shading coherence from large, sorted ray batches. In this article, we describe our architecture and discuss our design decisions. We also explain how we are able to provide artistic control in a physically based renderer, and we demonstrate through case studies how we have benefited from having a proprietary renderer that can evolve with production needs.
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Our goal was to create realistic digital pyrotechnic effects for the film "Reign of Fire". The specific effects we needed were fire, smoke, fog, flying embers and dust kicked up from dragons flying overhead. We also needed the effects to interact with the environment as well as the dragons. To make the challenge more difficult, we realized that it wasn't good enough to create real world physical interactions; the film needed imagery that was larger than life and exagerated. We needed to be able to art direct the simulations. SimulationThe basic motion of all our effects and their interactions were simulated using computational fluid dynamics (CFD). Generally, phenomena such as smoke and fire have a characteristic flow, but their underlying motions are similar. However, physical simulations are difficult to control, and even after many iterations the results still may not tell the story. Therefore, our workflow had the basic philosophy of allowing viewing and editing of the data at each stage of the pipeline. Our digital pyro effects pipeline had the following stages:• CFD scene setup and voxelization • CFD simulation • CFD simulation viewing and editing • Particle simulation using CFD results • Particle viewing and editing • Rendering Setup, viewing, and editing were accomplished using custom tools built on top of the Houdini animation package. Our CFD simulator, smaug, generated a description of the air flow at each frame of the simulation. Its computational model was based on the work of Nick Foster described in "Modeling the Motion of a Hot, Turbulent Gas" and the work of Jos Stam described in his various papers on modelling gaseous phenomena. Smaug used three kinds of data as input:• Configuration information which specified the regular grid and physical parameters that are used for the simulation. • A description of the initial conditions for the simulation. • Animating velocity grids (one per time step) representing moving objects.Except for the configuration file, input and output data was generated in a Houdini-native data format to facilitate viewing and editing. Each output file contained all the same information as the next frame's initial condition, letting us edit simulations that have become numerically unstable and restart them at the previously unstable frame using the newly edited conditions.After generating the flow field, we used Houdini to create particles to visualize the basic look of the effects. For fog, the particles were distributed throughout the flow field. For the others, the particles were generated at descriptive sources in the scene. The quantity of particles varied depending upon the effect. Sometimes secondary motion was explicitly added to the particles to create motion features that were too fine for the CFD simulation to generate. RenderingSmoke, fog, dust and fire were volume rendered using in-house custom shaders written for Steamboat Software's Jig renderer, and the embers were rendered using Houdini's Mantra renderer. The rendering technique varied depending upon the effec...
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