University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Photon Mapping and Irradiance Caching

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Bidirectional Paths

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x2

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x1€

x0

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y1

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y2

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y0

Light path

Eye path

Shadow ray

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Bidirectional Paths

flux (photons)

radiance

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L(x i → x i−1) = Φ(y j ) f r(y j−1 → y j → x i)G(y j , x i) f r (y j → x i → x i−1)

shadow ray

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y j€

y j−1

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x i€

x i−1

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G(y j , x i) =cos(θ j )cos(θ i)

x i − y j

2

cos(θ j ) = (y j → x i) •r n j

cos(θ i) = (y j → x i) •r n i

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Weighting the paths

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Lp = wi, jLi, jj= 0

N j

∑i= 0

N i

∑

Pixel estimate is weighted sum of path radiances

where

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wi,N−ii= 0

N

∑ =1 for N = 0,1,2,3,K

Choice of weights is critical for low variancesee Veach for details

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Advantages and disadvantages

Bidirectional path tracingGood for small sources of strong indirect illumination (caustics)

Path tracingGood for outdoor scenes with only a small part of the model visible

Both have trouble sampling mirror images of causticsBoth need lots of samples to avoid objectionable high frequency noise

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Photon mapping

Cache light paths for efficiency?Decouple geometry and illumination

Don’t use illumination maps where number of samples needed is dependent on primitive size and scene complexity

Get rid of high-frequency noise efficientlyDensity estimation

However, density estimation produces biased (but consistent) samples

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Two pass approach

Pass one: photon tracingBuild the photon map data structure

Pass two: renderingTrace eye paths using photon map to produce radiance estimates efficiently

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell