Efficient Simulation of Light Transport in Scenes with Participating Media using Photon Maps - Henrik Wann Jensen Per H. Christensen

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Efficient Simulation of Light Transport in
Scenes with Participating Media using Photon
Maps- Henrik Wann Jensen Per H. Christensen

• 05 Digital Image Synthesis
• Presented by Jen-Yuan Chiang

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Issues addressed by the paper

• Realistic Volume Rendering
• The ability to simulate following effects
• Multiple Volume Scattering
• Color Bleeding between volumes and surfaces
• Volume Caustics

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Multiple Scattering

• Single Scattering
• Multiple Scattering

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Color Bleeding

• Without participating media With
  participating media

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Caustics

• Surface Caustics
• Light reflected from or
• transmitted through
• one or more specular
• surfaces strikes a
• diffuse surface.

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Caustics

• Volume Caustics
• Light reflected from or
• transmitted through
• specular surfaces and
• then scattered by a
• medium

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Issues addressed in this paper

• Extends the method of photon mapping to achieve
  the global illumination of scenes with
  participating media

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Outline

• Overview of Photon mapping for surfaces
• Light transport in participating media
• Extending Photon Mapping to Participating Media
• Results

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Overview of Photon Mapping for Surfaces

• Global Illumination technique
• Two-pass particle-tracing algorithm
• First pass
• Building the photon maps using photon tracing
• Second pass
• Rendering using these photon maps

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First pass

• Photons emitted from light sources
• Simulate the transport of each photon
• Store photon in photon maps when it hits
  nonespecular surfaces
• Direct map
• Caustics map
• Indirect map
• Balanced kd-tree is used to handle photons

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3 photon maps
Ex. LSSSDSSSSD
Caustic map
Indirect map
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Second Pass
Specular reflection
Direct Illumination
Indirect illumination
Caustics
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Second Pass

• Illumination at a point is divided into four
  parts
• Specular reflection ray tracing
• Direct illumination direct map or ray tracing
• Caustics caustics photon map
• Indirect illumination indirect photon map

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Radiance Estimate

• Information of Photons
• Position(p), power( ), incoming direction(
  )

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Outline

• Overview of Photon mapping for surfaces
• Light transport in participating media
• Extending Photon Mapping to Participating Media
• Results

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Light Transport in Participating media
p
q
Radiance L changes continuously from L(p,w) to
L(q,w)
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Volume Scattering

• Emission
• In-Scattering
• Absorption
• Out-Scattering

Scattering coefficient
Absorption coefficient
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Volume Rendering Equation
Extinction coefficient
Ray marching
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Ray Marching

• Computes the contribution from the medium by
  dividing the ray into smaller segments

X0
X1
X2
Xk
emission
in-scattering
extinction(assuming medium properties are the
same through )
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Outline

• Overview of Photon mapping for surfaces
• Light transport in participating media
• Extending Photon Mapping to Participating Media
• Results

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Extending Photon Mapping to participating media

• From for surfaces to for volumes
• Still 2 pass particle tracing algorithm
• First pass
• additional volume photon map
• Second pass
• rendering using ray marching

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Volume Radiance Estimate

• Estimate radiance using volume photon map

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Volume Radiance Estimate for Ray Marching

• For each ray through the volume, we can get the
  radiance caused by volume scattering by marching
  along the ray and cumulating every

in-scattered radiance
single scattering (direct)by ray tracing
Multiple scattering (indirect) by volume
radiance estimate
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Outline

• Overview of Photon mapping for surfaces
• Light transport in participating media
• Extending Photon Mapping to Participating Media
• Results

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Features of Volumetric Photon Mapping

• Can model-
• Homogeneous as well as non-homogeneous media.
• Isotropic as well as anisotropic media.
• Since decoupled from geometry (photons stored in
  kd-tree), so capable of handling complex scene.

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Some Results

• Anisotropic and non-homogeneous medium

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Underwater Scene with Volume Caustics
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Pseudo code for volume photon mapping

• http//www-graphics.stanford.edu/courses/cs348b-co
  mpetition/cs348b-05/abandoned/index.html

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• Thanks!