Global illumination for PIXAR movie production

KAIST Computer Science
Global illumination for
PIXAR® movie production
20100121
Jaehyun Jang

• Global illumination for movie production
• Current Lighting Computation Methods
• Point-based Global Illumination for Movie Production
• Physically-based Lighting at PIXAR
• Global illumination researches in PIXAR®
• Rendering Pipeline in Production (as a Rendering TD)
Contents

• The first 3D animation used global illumination - Shrek 2 (2004)
Global illumination for movie production
Fig 1. Shrek 2.
[Photograph]. Retrieved from http://www.awn.com/vfxworld/illuminating-global-illumination
Courtesy of Paramount Pictures.

• Faking it : adding extra light sources
labour intensive
• Ray Tracing : requires many rays + shader evaluations
Too much slow (at 2010!)
• Radiosity
Requires entire scene data on memory
• Point-based
Little memory, no shader evaluations
Current Lighting Computation Method

Current : Physically-based Ray Tracing
Fig 2. Monster University
[Photograph]. Retrieved from http://neogaf.com/forum/showthread.php?t=569821
Courtesy of Disney-PIXAR
• PIXAR has been rendering their movie using physically based ray-tracing after
Monster University (2013)

Past : Point-based Global Illumination Techniques
Fig 3. Toy Story 3
[Photograph]. Retrieved from http://graphics.pixar.com/library/PointBasedGlobalIlluminationForMovieProduction/
• PIXAR uses point-based global illumination for rendering at 2010. At that time, ray-
tracing cost is too high to render images.

• First introduced this technique in ‘Point-Based Approximate Color Bleeding’, (2008).
• Background : Ray-tracing based global illumination is still expensive.
• Advantages using point-based :
1. Fast Computation
2. Noisy-Free
• Disadvantages :
1.Multi-pass approach, it cannot guarantee results as precise as ray tracing.
Point-Based GI for Movie Production (2010)

• Point Cloud
Each point : position, normal, radius, color = a colored disk
Color : the direct illumination at that surface position
How to generate? : PRMan to tessellate the surfaces into small micropolygons,
compute direct illumination from all light sources by using surface shader.
—> This approach is “Baking the direct illumination” (TD Terminology)

• Compute octree
A bottom-up computation
Consisting a node similar normals
Compute a spherical harmonic from leaf-node
Leaf-node : the sums of the coefficients for the surfels in the node.
Non-leaf-node : the sums of the coefficients of its child nodes.

Fig 4. Rasterization onto six raster cube faces
[Photograph]. Retrieved from http://
graphics.pixar.com/library/
PointBasedGlobalIlluminationForMovieProducti
on/
• Compute diffuse, and glossy global illumination
Traverse octree, visiting a node and rasterize the
illumination from node.
Rasterize colors contributing to a point : world
“as seen” by that point (a low resolution fish-eye
image)
Convolution with the BRDF
1. loop over raster pixels and multiplying
colors with the BRDF for the direction
corresponding to that pixel.
2. Can deal with glossy global illumination

Fig 5. Rendering with PRMan, baked radiosity value of point cloud (700,000 points) (left), glossy reﬂection (right)
[Photograph]. Retrieved from http://penguin.ewu.edu/RenderMan/RMS_2.0/pointbased.html

• Variation and extensions
Area light sources and soft shadows
Environment illumination
Multiple diffuse bounces
Final gathering for photon mapping
Ambient occlusion and reflection occlusion
etc.

Fig 6. Textured and displaced area light sources

Fig 7. Point cloud, Ambient occlusion, Reﬂection Occlusion, additional environment reﬂection

• The lighting pipeline at PIXAR was completely rewritten
and switched to a physically based and ray-traced system.
• Rendering Equation
L : radiance, f : BRDF
2 parts working in tandem :
1.Physically correct lights emitting energy in the scene
2.Physically correct BRDFs bouncing energy in the
scene
Physically based rendering pipeline (2013)

• Break down the rendering equation, each part will be solved by different
coshaders(defined in RSL 2.0) called integrators (http://renderman.pixar.com/view/
coshaders)
•
!
!
• direct lighting integrator + indirect diffuse integrator + indirect specular integrator

Fig 8. Light reﬂection
[Photograph]. Retrieved from http://content.gpwiki.org/index.php/D3DBook:
(Lighting)_Foundation_and_theory
Courtesy of Wikimedia

• Direct Lighting Integrator (Coshader)
1.Sample from lobes using BRDF coshaders
2.Sample from lights using light coshaders
3.Combine samples using Multiple Importance Sampling (Ryusuke Villemin, et al.
2013).

Fig 9. Structures communicates data between three coshaders consisting of Direct Light Integrator
[Photograph]. Retrieved from http://graphics.pixar.com/library/PhysicallyBasedLighting/paper.pdf

Algorithm 1. (Light Integration)
Several algorithms in the paper : http://graphics.pixar.com/library/PhysicallyBasedLighting/paper.pdf

Fig 10. Various luminaries in this shot, The Blue Umbrella
[Photograph]. Retrieved from http://graphics.pixar.com/library/PhysicallyBasedLighting/paper.pdf

Fig 11. Physically-based lighting in Monster University
[Photograph]. Retrieved from http://blog.selfshadow.com/publications/s2013-shading-course/pixar/
s2013_pbs_pixar_slides.pdf

Fig 12. Physically-based lighting in Monster University
[Photograph]. Retrieved from http://blog.selfshadow.com/publications/s2013-shading-course/pixar/
s2013_pbs_pixar_slides.pdf

• Based on PIXAR online library (from 2011 - now)
• Sampling
Importance sampling
1. A statistical framework for comparing importance sampling methods, and an
application to rectangular lights (Leonid Pekelis, et al. 2014).
2. Multiple Importance Sampling for Emissive Effects (Ryusuke Villemin, et al.
2013).
3. Importance Sampling of Reflections from Hair Fibers (Christophe Hery, et al.
2011).
Stratified Sampling
1. Correlated Multi-Jittered Sampling (Andrew Kensler, 2013).
Global Illumination Researches in PIXAR

• Caching
1. Multiresolution Radiosity Caching for Efficient Preview and Final Quality
Global Illumination in Movies (Per H. Christensen, et al. 2012)
• Summary
PIXAR researches physically-based ray tracing, their rendering pipeline running on
monte-carlo ray tracing, and it needs to develop proper sampling techniques for
BRDF and etc.
Global Illumination Researches in PIXAR

• Physically-based rendering is production standard.
• Current PRMan® pipeline, RSL 2.0 supports physically plausible shaders.
• For rendering technical directors, these kind of techniques are tremendously
demanded.
• — All is commented by technical director in PIXAR.
Rendering Pipeline in Production

Global illumination for PIXAR movie production

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