Ray Tracing Solution for Film Production

RenderingMarcos Fajardo

marcos@solidangle.com

SIGGRAPH 2010 Global Illumination Across Industries

SIGGRAPH 2010

Distributed ray tracing, Cook et al.1984

Image courtesy of LucasfilmSIGGRAPH 2010

The Rendering Equation, Kajiya 1986

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CGI Studio

• Blue Sky Studio’s proprietary renderer• Physically-based Monte Carlo ray tracer• Created by Carl Ludwig, Eugene

Troubetzkoy, Michael Ferraro et al.

• ex-MAGI/SynthaVision team (TRON etc)

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Image courtesy of Blue Sky Studios, Inc.

Carl Ludwig 1990 (!)

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Image courtesy of Blue Sky Studios, Inc.

uncredited,1991

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Image courtesy of Blue Sky Studios, Inc.

John Kars 1992

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Image courtesy of Blue Sky Studios, Inc.

“Bunny”, Best Animated Short 1998

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Arnold

• Physically-based Monte Carlo ray tracer• Unbiased, uni-directional path tracing• Co-developed by Solid Angle SL and

Sony Pictures Imageworks, Inc.

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“Pepe” 1999Image courtesy of Daniel Martinez Lara. © 1999

Daniel Martinez Lara. All rights reserved.

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Image courtesy of Ruairi Robinson. © 2001 Zanita Films. All rights reserved.

“Fifty Percent Grey” 2001

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Image courtesy of Columbia Pictures. © 2006 Columbia Pictures Industries, Inc. All rights reserved.

“Monster House” 2006

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Image courtesy of Sony Pictures Animation. © 2009 Sony Pictures Animation, Inc. All rights reserved.

“Cloudy with a Chance of Meatballs” 2009

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Image courtesy of Columbia Pictures. © 2009 Columbia Pictures Industries, Inc. All rights reserved.

“2012” 2009

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“Alice in Wonderland” 2010

Lots of furry characters ...

SIGGRAPH 2010 Images courtesy of Walt Disney Pictures

What is Arnold?

• Unbiased Monte Carlo ray tracer• Uni-directional path tracer• 200k lines of highly optimized C/C++

code

• Fully multi-threaded + SIMD architecture

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Production features• Motion blur (xform and deformation)• Networked, programmable shaders• Hundreds of millions of triangles and

hair splines

• Deferred/procedural loading of geometry

• Texture cachingSIGGRAPH 2010

Production features• Motion blur (xform and deformation)• Networked, programmable shaders• Hundreds of millions of triangles and

hair splines

• Deferred/procedural loading of geometry

• Texture cachingSIGGRAPH 2010

Pros• Single-pass: dramatically simpler

pipeline (no extra files or caches)

• Only one quality knob (number of samples)

• Shadows are always perfect (no shadowmaps etc)

• Interactive (via progressive refinement)• No memory used to store lighting/GI

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Cons

• Slow?• Noise• Indoors are hard• Geometry must reside in memory at all

times

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Address the cons!

• Three orthogonal axes:• make rays faster• reduce memory use (cost per

polygon)

• variance reduction

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Address the cons!

• Aim for optimal multi-threading performance (linear scalability)

• 8-core machines are the norm at the workstation

• 32- and 64-core machines are in the horizon

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Address the cons!

• Explore new ray acceleration structures• BVH, kd-tree etc• Build time is less important than

traversal

• Geometry quantization

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Variance reduction• Good 2D (pixel, lens, light, BRDF) and 1D

(motion blur, volume) sample sets

• BRDF importance sampling• Solid angle-based light source importance

sampling

• Multiple importance sampling• Volume importance sampling• Careful with correlation between

dimensionsSIGGRAPH 2010

Geometry instancing• Easy to generate billions of visible

polygons

• Greatly helps in many common scenarios:

• cities• crowds• debris

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Geometry instancing

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Chew and Swallow city from Cloudy with a Chance of Meatballs1 million separate instanced objects, 2.5 GB

Geometry instancing

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Geometry instancing

Geometry instancing

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Interactivity

• Why is interactive feedback so important?

• CPU time: $0.1 / hour• artist time: $40 / hour• Final “beauty” render time is not as

important as reducing lighting overhead

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1 million polys, area light, sky, 2-bounce GI, reflections(rendered on a single quad-core CPU, 4 threads)

Interactivity

• Can’t afford to have lighters waiting for shadowmaps, pointclouds and caches everytime they need to move a light

• Scene updates are interactive: lights, shaders, camera, moving geometry

• Inmediate feedback via progressive refinement

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1.5 M displaced tris3 area lights + sky

diffuse + glossy + sss10x10 pixel samples

1920x1080

00:14

model by Glassworks LondonSIGGRAPH 2010

Low-overheaddeformationmotion blur

1.5 M displaced tris3 area lights + sky

diffuse + glossy + sss10x10 pixel samples

1920x1080

00:16

model by Glassworks LondonSIGGRAPH 2010

Low-overheaddeformationmotion blur

Less artistic freedom?• Often mentioned as a criticism for global

illumination renderers

• ... but in practice we are not getting many complaints, specially in photoreal projects

• Many optional controls:• light groups (e.g. lights for eyes only)• per-light spec/diffuse switches• hide/substitute objects for certain ray

typesSIGGRAPH 2010

Less artistic freedom?

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"The initial expectation was that combined specular and reflection parameters would result in reduced ability to art-direct and dial the looks. In practice however, the imagery

looked more correct out of the box, we almost never received direction to dial specular illumination and reflection

independently."

Adam Martinez on Alice in Wonderland

Initial reluctance• Traditionally too slow for production use

...

• But both algorithms and hardware have improved dramatically in recent years

• GI is now “always on”, rarely turned off• By having it on by default, first-time

artists don’t have to press extra buttons and get up to speed more quickly

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Some tricks

• Noisy caustics: turn off certain light paths

• Noisy caustics: broaden specular lobes for secondary bounces

• Clamp 1/d^2 term near the lights

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Broaden specular lobes for direct lighting in secondary

bounces

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phong_exponent = 2000 phong_exponent = 50

same render time!

Conclusion

• Path tracing for film production is here to stay

• Radically changes the rendering pipeline at film studios (after an initial period of scepticism)

• Still lots of room for improvement (sampling, new accels etc)

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Thanks!

• Blue Sky: Carl Ludwig• Imageworks: Cliff Stein, Chris Kulla, Larry Gritz, Alex

Conty, Rene Limberger, Rob Bredow

• Solid Angle: Angel Jimenez, Borja Morales, Luis Armengol

• Glassworks: Andreas Byström

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Ray Tracing Solution for Film Production RenderingSlide Number 2Slide Number 3CGI StudioImage courtesy of Blue Sky Studios, Inc.Image courtesy of Blue Sky Studios, Inc.Image courtesy of Blue Sky Studios, Inc.Image courtesy of Blue Sky Studios, Inc.Arnold“Pepe” 1999��Image courtesy of Daniel Martinez Lara. © 1999 Daniel Martinez Lara. All rights reserved.Image courtesy of Ruairi Robinson. © 2001 Zanita Films. All rights reserved.Image courtesy of Columbia Pictures. © 2006 Columbia Pictures Industries, Inc. All rights reserved.Image courtesy of Sony Pictures Animation. © 2009 Sony Pictures Animation, Inc. All rights reserved.Image courtesy of Columbia Pictures. © 2009 Columbia Pictures Industries, Inc. All rights reserved.Slide Number 15What is Arnold?Production featuresProduction featuresProsConsAddress the cons!Address the cons!Address the cons!Variance reductionGeometry instancingGeometry instancingGeometry instancingGeometry instancingGeometry instancingInteractivitySlide Number 31InteractivitySlide Number 33Slide Number 34Less artistic freedom?Less artistic freedom?Initial reluctanceSome tricksBroaden specular lobes for direct lighting in secondary bouncesConclusionThanks!

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