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Vectorized Production Path Tracing

Talk, High Performance Graphics 2017

This paper presents MoonRay, a high performance production rendering architecture using Monte Carlo path tracing developed at DreamWorks. MoonRay is the first production path tracer, to our knowledge, designed to fully leverage Single Instruction/Multiple Data (SIMD) vector units throughout. To achieve high SIMD efficiency, we employ Embree for tracing rays and vectorize the remaining compute intensive components of the renderer: the integrator, the shading system and shaders, and the texturing engine. Queuing is used to help keep all vector lanes full and improve data coherency. We use the ISPC programming language to achieve improved performance across SSE, AVX/AVX2 and AVX512 instruction sets. Our system includes two functionally equivalent uni-directional CPU path tracing implementations: a C++ scalar depth-first version and an ISPC vectorized breadth-first wavefront version. Using side by side performance comparisons on complex production scenes and assets we show our vectorized architecture, running on AVX2, delivers between a 1.3× to 2.3× speed-up in overall render time, and up to 3×, 6×, and 4×, speed-ups within the integration, shading, and texturing components, respectively.

Research Paper for Vectorized Production Path Tracing

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Multi-species simulation of porous sand and water mixtures

Paper, SIGGRAPH 2017

We present a multi-species model for the simulation of gravity driven landslides and debris flows with porous sand and water interactions. We use continuum mixture theory to describe individual phases where each species individually obeys conservation of mass and momentum and they are coupled through a momentum exchange term. Water is modeled as a weakly compressible fluid and sand is modeled with an elastoplastic law whose cohesion varies with water saturation. We use a two-grid Material Point Method to discretize the governing equations. The momentum exchange term in the mixture theory is relatively stiff and we use semi-implicit time stepping to avoid associated small time steps. Our semi-implicit treatment is explicit in plasticity and preserves symmetry of force linearizations. We develop a novel regularization of the elastic part of the sand constitutive model that better mimics plasticity during the implicit solve to prevent numerical
cohesion artifacts that would otherwise have occurred. Lastly, we develop an improved return mapping for sand plasticity that prevents volume gain artifacts in the traditional Drucker-Prager model.

Research Paper for Multi-species simulation of porous sand and water mixtures

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Lighting Grid Hierarchy for Self-illuminating Explosions

ACM Transactions on Graphics 2017

Rendering explosions with self-illumination is a challenging problem. Explosions contain animated volumetric light sources immersed in animated smoke that cast volumetric shadows, which play an essential role and are
expensive to compute. We propose an efficient solution that redefines this problem as rendering with many animated lights by converting the volumetric lighting data into a large number of point lights. Focusing on temporal coherency to avoid flickering in animations, we introduce lighting grid hierarchy for approximating the volumetric illumination at different resolutions. Using this structure we can efficiently approximate the lighting at any point inside or outside of the explosion volume as a mixture of lighting contributions from all levels of the hierarchy.

Research Paper for Lighting Grid Hierarchy for Self-illuminating Explosions

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Novel Algorithm for Sparse and Parallel Fast Sweeping: Efficient Computation of Sparse Signed Distance Fields

Talk, SIGGRAPH 2017

We present a new efficient algorithm for computing signed distance fields by means of the Fast Sweeping Method. Unlike existing algorithms ours is explicitly designed to explore the benefits of sparse (vs dense) grids as well as concurrency, i.e. mutli-threading.

Research Paper for Novel Algorithm for Sparse and Parallel Fast Sweeping: Efficient Computation of Sparse Signed Distance Fields

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FurCollide: Fast, Robust, and Controllable Fur Collisions with Meshes

Talk, SIGGRAPH 2017

We present FurCollide, a fast, robust, and artist friendly tool used for collision detection and collision resolution of fur curves with meshes. Œe tool helps artists interact with and control tens of
thousands of curves with ease while providing high €fidelity realistic and/or artistic collision results. This tool is in use at DreamWorks Animation and has been used in a wide variety of fur and/or grass collision situations in various films.

Research Paper for FurCollide: Fast, Robust, and Controllable Fur Collisions with Meshes

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Production ready MPM simulations

Talk, SIGGRAPH 2017

The Material Point Method has proven itself to be a versatile tool for animation of natural phenomena, like sand [Klar et al. 2016], snow [Stomakhin et al. 2013], and even lava [Stomakhin et al. 2014].
The main benefits of MPM in production relate to its ability to preserve overall dynamics over different resolutions, and its robustness to particle overlap. However, these advantages come at a cost that renders MPM impractical for all but hero shots.

Research Paper for Production ready MPM simulations

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DreamWorks Fabric Shading Model: from Artist Friendly to Physically Plausible

Talk, SIGGRAPH 2017

Since Shrek 2, DreamWorks artists have used the fabric model developed by [Glumac and Doepp 2004] extensively on cloth material shading. Even after we developed the physically based microcylinderical cloth model by [Sadeghi et al. 2013], they continued to prefer the intuitive control of the DreamWorks fabric shading model, which is also a cyindrical shading model, with easy to use artistic controls for highlights, and highlight directions. In this talk, we present our approach to make this much-loved, production-oriented fabric shading model physically plausible, so we can use it within the framework of a physically based renderer that requires its brd f models to be energy conserving and, ideally, reciprocal as well. We also present an accurate importance sampling algorithm for this fabric shading model that makes it efficient to render within a path tracer.

Research Paper for DreamWorks Fabric Shading Model: from Artist Friendly to Physically Plausible

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Hairy Effects in Trolls

Talk, SIGGRAPH 2017

Hair plays a feature role in the €film Trolls. It is a crucial part of the overall character design of the Trolls themselves, typically composing over half the silhouette of the character. However, the use of hair on the show went well beyond the standard coif and bled into acting beats, traditional effects, environments, and set pieces. is talk presents the wide variety of unique and challenging hair effects in the €film and the techniques used to create them.

Research Paper for Hairy Effects in Trolls

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