Multimesh caching and hardware sampling for progressive and interactive rendering

1
Multi-mesh caching and hardware sampling for
progressive and interactive rendering

• Gabriel Fournier and Bernard Péroche

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Introduction

• Our Goal
• Interactive rendering
• a few images/sec
• walkthrough
• of
• triangulated scenes
• area light sources
• on a single PC with
• soft shadows
• indirect lighting
• no long preprocessing

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The problem

• Full lighting computations for each pixel
• ? Too long for interactive time
• Solutions
• Lighting computations for only a few pixels
• Speed up and adaptation of light sampling
• Progressive improvement of the image quality

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Outline of the presentation

• Previous work
• Our method
• Overview
• Multi mesh
• Mesh subdivision
• Hardware sampling
• Results and discussions
• Future work

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Cache of samples

• Lighting cached ? Re-used from frame to frame
• Object space partition cache
• Irradiance caching (Ward et al. 1988), Light
  vectors (Zaninetti and Péroche 1998)
• Complex interpolations
• Image cache
• Render cache (Walter et al. 1998),
• Missing pixels (Render cache),
• Tapestry (Simmons and Séquin - 2000)
• First frames with inexact geometry
• Object space cache
• On the geometry Shading cache (Tole et al
  2002)

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Lighting sampling

• What is sampled and interpolated ?
• Single global lighting value
• Shading cache (Tole et al. - 2002)
• Limited re-use of cached value
• Sampling density
• Multiple lighting values (direct, indirect,
  caustic radiance)
• Light vectors (Zaninetti and Péroche - 1998)
• No method with multiple values cached on the
  geometry

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Indirect lighting sampling

• Direct irradiance required on CPU
• Costly if many area lights (100 rays per light)
• Direct irradiance should be cached
• to be re-used during indirect irradiance sampling

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Overview

• Object space cache triangle mesh over the
  geometry
• ?hardware rendering, fast subdivisions and
  interpolations
• Multiple meshes separate storage of
• direct irradiance from each light source
• indirect irradiance
• ? Re-use of already computed values
• Direct diffuse irradiance for the indirect one
• Diffuse radiance from the previous frames
• Multi-pass rendering color / direct radiance /
  indirect radiance
• ? interactivity, progressiveness

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Framework
CPU
GPU
User interaction
Geometrical
Rendering of ID
Mesh
Visible triangles
set
Rendering of
construction
material reflectance
properties
Rendering of
radiance
Irradiance sampling
Rendering of the
final image
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Multiple meshes

• Triangular mesh
• Simple interpolations
• Fast subdivision
• Fast ray tracing
• OpenGL primitive

Geometry mesh
Indirect irradiance mesh
Direct irradiance meshes for each light source
Direct diffuse radiance mesh
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Mesh example
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Direct irradiance caching

• Each light source sampled and cached separately
• Reduce the number of samples

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Subdivision element choice
Element to subdivide

• Priority stored in the mesh tree leaves
• visible size of the triangle
• maximum radiance contrast on its edge
• 0 if the triangle should not be subdivided

Priority max(sons priority)
Radiance mesh

Geometry mesh

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Subdivision criteria
Radiance

• Subdivision of a triangle if
• it has more than 3 visible pixels
• and
• it is too big
• or
• it has at least 2 required vertices
• sampled ! interpolated
• or
• it has one edge with visible
• T-vertices

Sampled radiance
A
M required vertex
Interpolated radiance
B
A
B
M
T-vertex
N
D
C
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Subdivision radiance difference

• Subdivision only if there might be a visible
  difference
• Tone mapping of the values ?on screen color
• Use of an experimentally built map to get the
  maximum authorized difference

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Sampling density
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Area light source sampling

• Small light source bounding frustum
• Hardware rendering on a small image
• Texture mapping to weight each pixel
• Multiple passes to sum the image until it is
  small enough to be read back

• Faster than ray tracing
• Frees up the CPU

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Indirect lighting sampling

• Only 1 bounce indirect diffuse radiance being
  sampled
• Rendering of a low detail version of the diffuse
  direct radiance mesh
• Small part of the incoming irradiance is missing
• 160 field of view

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Final image generation
Screen
GPU
CPU
1x /frame
Scene description
Rendered 3x15x /s
Updated 20x /s
Irradiance of each light sources
Updated 3x /s
Indirect diffuse irradiance
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Results
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Discussion

• Pros
• Interactivity with exact geometry
• Exact soft shadows
• We save some computations compared to the Shading
  cache method
• Cons
• Indirect lighting quite limited, still slow
• High memory cost 200 MB for the 8000 triangles
  scene
• Non real time

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Conclusion and future work

• Include missing part of radiance (indirect
  specular, caustic and indirect with more bounces)
• Include a visual model to better tune the
  subdivision of the mesh (priority, precision)
• Optimize and increase the quality of indirect
  lighting computations

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Questions ?
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References

• Ward, Rubinstein, Clear. 1988. A ray tracing
  solution for diffuse interreflection. In Computer
  Graphics (Proceedings of SIGGRAPH 88), vol. 22,
  8592.
• Zaninetti, Serpaggi, Péroche. 1998. A vector
  approach for global illumination in ray tracing.
  Computer GRaphics Forum, 17(3)149-158, 1998
• Simmons, Séquin. 2000. Tapestry A dynamic
  mesh-based display representation for interactive
  rendering. In Rendering Techniques 2000 11th
  Eurographics Workshop on Rendering, 329340.
• Tole, Pellacini, Walter, P. Greenberg. 2002.
  Interactive global illumination in dynamic
  scenes. ACM transactions on graphics, 21(3)
  537-546, July 2002