Papers by Christophe Hery

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RenderMan, Theory and Practice

Dana Batali, Byron Bashforth, Chris Bernardi, Per H. Christensen, David Laur, Christophe Hery, Guido Quaroni, Erin Tomson, Thomas Jordan, Wayne Wooten
July 2003

Siggraph 2003 course notes.

Paper (PDF)

Importance Sampling of Reflections from Hair Fibers

Christophe Hery, Ravi Ramamoorthi
December 2011

Hair and fur are increasingly important visual features in production rendering, and physically-based light scattering models are now commonly used. In this paper, we enable efficient Monte Carlo rendering of reflections from hair fibers by describing a simple and practical importance sampling strategy for the reflection term in the Marschner hair model. Our ... more

Paper (PDF)

Available as Pixar Technical Memo #11-07

Porting RSL to C++

Ryusuke Villemin, Christophe Hery
November 2012

In a modern renderer, relying on recursive ray-tracing, the number of shader calls increases by one or two order of magnitude compared to a straighforward rasterizer dealing only with camera visible objects. Recognizing the potential overhead of RSL parsing in all these shader calls, this report evaluates different C++ pre-compiled ... more

Paper (PDF)

Available as Pixar Technical Memo #12-08

Texture mapping for the Better Dipole model

Christophe Hery
December 2012

We derive an exact diffuse integral for Eugene D"Eon"s new Better Dipole model and we provide the code for numerically inverting it, leading to easy texture mapping for translucency effects.

Paper (PDF)

Available as Pixar Technical Memo #12-11

Multiple Importance Sampling for Emissive Effects

Ryusuke Villemin, Christophe Hery
January 2013

We present a method for creating and sampling volumetric light sources directly using the volumetric data, obtaining high quality results with any mirror, glossy or diffuse objects, and integrating with any global illumination framework.

Paper (PDF)

Additional materials: [], []

Published version available as

Available as Pixar Technical Memo #13-02

Physically Based Lighting at Pixar

Christophe Hery, Ryusuke Villemin
July 2013

We recently participated in the Siggraph 2013 Physically Based Shading course, with all notes and documents stored at We provide here direct access to our own chapter, describing the Physically Based System we designed at Pixar (on top of RenderMan) for the movie Monsters University and the short film ... more

Paper (PDF)

A statistical framework for comparing importance sampling methods, and an application to rectangular lights

Leonid Pekelis, Christophe Hery
February 2014

Importance sampling methods for ray tracing are numerous. One reason is due to the lack of a robust, quantitative method for comparison. This tech memo takes strides to provide a framework for comparing sampling methods on fair ground. We define a simple, mathematical notion of strictly preferring one method to ... more

Paper (PDF)

Available as Pixar Technical Memo #14-01

Geometry into Shading

Christophe Hery, Michael Kass, Junyi Ling
October 2014

Sometimes, when 3D CG objects are viewed from a distance, a great deal of geometry can end up inside a single pixel. When this happens, full-sized facets, or pixels in a displacement map should play the role of traditional micro-facets. Here we describe how to alter traditional shading calculations to ... more

Paper (PDF)

Additional materials: [Bump2Roughness.tar.gz]

Available as Pixar Technical Memo #14-04

A Data-Driven Light Scattering Model for Hair

Leonid Pekelis, Christophe Hery, Ryusuke Villemin, Junyi Ling
May 2015

We present an implementation of the [Marschner et al. 2003] model for importance sampling light reflected from hair. The implemen- tation makes use of a version of Adaptive Importance Sampling (AIS), specialized to fit easily sampled distributions to BCSDFs. Our model is novel among importance sampling implementations in that it ... more

Paper (PDF)

Additional materials: [], []

Available as Pixar Technical Memo #15-02

Art and Technology at Pixar, from Toy Story to Today

Ryusuke Villemin, Christophe Hery, Sonoko Konishi, Takahito Tejima, David Yu
November 2015

Technology has always played an important part in Pixar's movie making process, starting with Toy Story over 20 years ago. This course will take you on a journey through that technical evolution, focusing on how story drove our technical designs and how we utilized new technical advancements to tell more appealing stories. Finally ... more

Paper (PDF)

SIGGRAPH Asia 2015 Course Notes.

Towards Bidirectional Path Tracing at Pixar

Christophe Hery, Ryusuke Villemin, Florian Hecht
July 2016

On Finding Dory and Piper, we were faced with rendering a lot of water, or creatures and sets seen through water and glass. We worked with production artists to give controls for resolving these difficult light transports. We also learned to cheat these effects where appropriate. In these course notes, ... more

Paper (PDF)

Importance Resampling for BSSRDF

Ryusuke Villemin, Christophe Hery, Per Christensen
November 2016

We present a method to improve BSSRDF rendering, which reduces variance due to sampling of the scattering exit point. By using importance resampling, we are able to sample any arbitrary diffusion model, taking into account the geometry of the object at the same time. The new method is trivial to implement ... more

Paper (PDF)

Available as Pixar Technical Memo #16-05

Building an Orthonormal Basis, Revisited

Tom Duff, James Burgess, Per Christensen, Christophe Hery, Andrew Kensler, Max Liani, Ryusuke Villemin
March 2017

Frisvad [2012b] describes a widely-used computational method for augmenting a given single unit vector with two other vectors to produce an orthonormal frame in three dimensions, a useful operation for any physically based renderer. The implementation has a precision problem: as the z component of the input vector approaches -1, floating point cancellation causes ... more

Paper (PDF)

Path Traced Subsurface Scattering using Anisotropic Phase Functions and Non-Exponential Free Flights

Magnus Wrenninge, Ryusuke Villemin, Christophe Hery
July 2017

With the recent move to path tracing for both surface and volume rendering, subsurface scattering has been one of the last light transport modes to rely on empirical or approximate models. Although rendering of subsurface scattering using path tracing is conceptually simple, making the model artist friendly is not. Our new ... more

Paper (PDF)

Additional materials: [PTSSSAlbedoInversionCoeffs.txt]

Available as Pixar Technical Memo #17-07

Pixar's Foundation for Materials

Christophe Hery, Ryusuke Villemin, Junyi Ling
August 2017

Though the philosophies and main principles behind our setups date back to our work on Physically Based Lighting for Monsters University, we present the mental models and more modern features and usages covered by PxrSurface and PxrMarschnerHair.

Paper (PDF)

SIGGRAPH 2017 Course Notes

Caustic Connection Strategies for Bidirectional Path Tracing

Sebastien Speierer, Christophe Hery, Ryusuke Villemin, Wenzel Jakob
March 2018

We propose a new type of sampling strategy for connection-based path tracing algorithms such as bidirectional path tracing. Classical bidirectional path tracing generally exhibits poor performance when sampling light paths involving specular transport (e.g. refraction through dielectrics). We therefore introduce specialized connection strategies that connect through chains of specular events. ... more

Paper (PDF)

Available as Pixar Technical Memo #18-01

MCQMC 2018 Illumination 101

Christophe Hery
July 2018

We talked at MCQMC 2018 in Rennes (France) about the usage of control variates for direct ligting computation.

Paper (PDF)

MCQMC 2018 Talk Slides