Efficient Rendering of Local Subsurface Scattering

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Efficient Rendering of Local Subsurface Scattering
Tom Mertens1, Jan Kautz2, Philippe Bekaert1,
Frank Van Reeth1, Hans-Peter Seidel2
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Overview

• Problem
• Related Work
• Local Subsurface Scattering
• Our Approach
• Implementation Results
• Discussion
• Summary Future Work

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Subsurface Scattering
translucent
opaque

• BRDF

BSSRDF
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BSSRDF model

• introduced by Jensen et al. (SIGGRAPH01)
• multiple scattering
• materials with high albedo marble, milk, wax,
  skin,

function of distance
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BSSRDF model

• introduced by Jensen et al. (SIGGRAPH01)
• multiple scattering
• materials with high albedo marble, milk, wax,
  skin,

function of distance
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Related Work

• Jensen et al. 02
• General scattering effects
• Offline rendering
• Mertens et al. 03
• Dynamic models
• General scattering effects
• Per vertex
• Our paper
• Dynamic models
• Local scattering effects
• Per pixel

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Local Subsurface Scattering

• Certain cases no global response
• Dense materials
• Large scale
• Distinct appearance!
• Rough surface
• Local sampling sufficient
• But accuracy is important!
• Rd decays exponentially
• Per vertex too coarse
• Apply to skin rendering

Global response
Only local response
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Local Subsurface Scattering
Local subsurface scattering
Diffuse
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Local Subsurface Scattering
Local
Full
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Our Approach

• High level description
• Employ importance sampling scheme for Rd
• Rendering algorithm
• Generate importance samples
• Render irradiance image
• Integrate irradiance image locally in tangent
  plane

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Importance Sampling of Rd

• Need to solve integral
• Idea sample according to Rd
• Result set of distances ri
• Issues
• Need samples on surface, not ris
• Need irradiance at sample

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Importance sampling of Rd

• Solution
• Pick a view e
• Render irradiance to image T
• Generate sample p in tangent plane
• Project p on surface ? p
• Project p into T
• to retrieve irradiance E(p)

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Importance sampling of Rd

• We take eye position for e
• p ? p implies a jacobian J
• ratio of solid angles
• Integral becomes

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Rendering Algorithm

• Generate importance samples in 2D

2D
Rd
ri
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Rendering Algorithm

• Render irradiance image

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Rendering Algorithm

• Integrate image locally in tangent plane

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Rendering Algorithm

• Store result in final image

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Implementation

• Variance reduction
• Stratified sampling
• Deterministic, pseudo random
• Interleaved sampling
• Noise ? dither pattern
• Combined sampling
• Importance uniform
• Irradiance discontinuties
• Software implementation
• Programmable Graphics Hardware

Combined sampling Uniform importance
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Implementation

• Programmable Graphics Hardware
• Overview
• generate 2D samples
• quick per-frame preprocess in software
• Render irradiance image T
• Bind E as texture
• For each sample
• Look up sample E in T (pixel shader)
• Accumulate E in temporary texture
• Output temporary texture

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Results

• ATI Radeon 9700 Pro
• 500x500 image, 4 to 5 frames/sec
• Some pictures

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Image Quality
Color bleeding (forehead)
Shadow smoothing
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Image Quality
nVIDIAs skin shader
Our method
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Complex lighting
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Demo video
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(No Transcript)
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Discussion

• No global effects
• E.g. backlit ears
• Prone to noise
• Irradiance discontinuities
• Shadow borders
• Geometric discontinuities
• Kills effect of importance sampling
• Ghosting artifacts
• Accumulation ? fill-rate limited

ghosting
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Summary

• Novel technique for local subsurface scattering
• Amenable for hardware implementation
• Interactive frame rates
• Dynamic models
• Application skin rendering

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Future Work

• Hybrid algorithm
• Global response per vertex
• Local response per pixel
• Eliminate ghosting
• Apply technique in texture space
• Combine with skin BRDF
• Take into account varying blood concentrations

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Acknowledgments

• Head model courtesy of nVIDIA
• Funding
• European Regional Development Fund