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Ray tracing for the movie

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Ray tracing for the movie Cars – PowerPoint PPT presentation

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Title: Ray tracing for the movie


1
Ray tracing for the movie Cars
  • Per Christensen
  • Pixar Animation Studios

Ayia Napa Seminar, June 2006
2
Cars challenges
  • Animation cars that move, talk, think
  • Rendering
  • geometric complexity
  • ray tracing reflections, shadows, ambient
    occlusion

3
Overview
  • Why ray tracing?
  • How to deal with overwhelming complexity?
  • Examples

4
Why ray tracing for Cars?
  • All previous Pixar movies were rendered with
    scanline rendering (shadow maps, reflection maps,
    )
  • But cars are very shiny reflective!
  • Shadows ambient occlusion
  • We were adding ray tracing to RenderMan anyway

5
Why ray tracing?
Environment map
Ray-traced reflections
6
Why ray tracing?
Ray-traced shadows (shadow maps hard)
7
Why ray tracing?
Ambient occlusion
8
Ray tracing effects summary
sharp shadow
self inter- reflection
soft shadow
mirror reflection
9
Ray tracing is easy or is it?
  • Yes but only if the scene fits in memory!
  • Further complications
  • Displacement shaders
  • Motion blur

10
Typical scene at Pixar
  • 100s of lights
  • 1,000s of textures too many to fit in mem!
  • 10,000s of objects
  • 100,000,000s of polygons too many to fit!
  • Shaders with 10,000s lines of code

11
Rendering requirements
  • Render at hi-res (2000 pixels)
  • Motion blur
  • Depth of field
  • No spatial or temporal aliasing (staircase
    effects, crawlies, popping, )

12
Scanline rendering (Reyes)
  • Advantages
  • Fast
  • One image tile at a time only needs small
    fraction of objectstextures
  • Can deal with very complex scenes
  • Limitations
  • Shadow maps (limited resolution)
  • Reflection maps (no interreflections)

13
Ray tracing
  • Advantages
  • Interreflections
  • Fine shadow details
  • Ambient occlusion
  • Disadvantage rays fly all over the scene
  • Needs all objectstextures all the time
  • Can not deal with very complex scenes

14
Goal best of both
  • Ray tracing
  • Very complex scenes (as scanline)
  • So augment RenderMans Reyes scanline with ray
    tracing

15
Main question
  • Some rays fly all over
  • Some rays require high geometric / texture
    precision
  • But not all rays fly all over and require high
    precision!
  • Which rays require which precision?

16
Ray differentials to the rescue
  • Keep track of differences between neighbor rays
  • Trace rays each ray represents a beam Igehy
    1999

17
Ray differentials and ray beam
ray
ray beam
neighbor rays
  • Narrow ray ray beam cross-section is small
  • Wide ray ray beam cross-section is large

18
Ray differentials use
  • Ray differentials tell us
  • Required tessellation rate of geometry
  • Quad sizes ray beam cross-section
  • Required texture resolution
  • Pixel sizes ray beam projected onto surface

19
Multi-resolution geometry cache
  • Split objects into patches (as usual)
  • Tessellate each patch on demand
  • Use ray width to determine which tessellation to
    use

1 quad 4x4 quads 16x16 quads
20
Multi-resolution geometry cache
  • Store tessellation in coarse, medium, or fine
    sub-cache
  • Same size (e.g. 10MB) but different capacity
  • Coherent lookups in medium fine sub-cache
  • Result can render scenes 100 x larger than cache
    size !

21
Example parking lot
15 cars 240M quads 80M rays
22
Parking lot cache stats
  • 1 billion geometry cache lookups
  • No cache run time gt 4 days
  • Single-resolution cache
  • hit rate 97.7
  • run time 11 hours
  • Multi-resolution cache
  • hit rate 99.9
  • run time 6 hours

23
Example 94 dragons
textures
displacements
sharp shadows
mirror reflection
24
94 dragons cache stats
  • 18 million geometry cache lookups
  • 3MB multi-res. cache performs well less than
    1/200 of the fully tessellated scene
  • Single-res. vs. multi-res. geometry cache
  • 1MB multi-res. cache beats 100MB single-res.
    cache (recomputed vertices)

25
Final car images
26
Final car images
27
Final car images
28
Final car images
29
More information
  • Book Advanced RenderMan
  • Ray differentials and multiresolution geometry
    caching for distribution ray tracing in complex
    scenes, Eurographics 2003

30
Conclusion (part 1)
  • Use multi-resolution geometry cache
  • Use multi-resolution texture cache
  • Use ray differentials to select resolution

31
Conclusion (part 2)
  • Result Can now ray trace production scenes
    same complexity as scanline !
  • Was used extensively in the rendering of Cars
    movie
  • Also used by other studios

32
Acknowledgments
  • Thanks to
  • Pixar RenderMan team
  • You for listening

33
Questions?
34
(No Transcript)
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