The Free-form The Free-form

Light StageLight Stage

Vincent Masselus

Philip Dutré

Frederik AnrysDepartment of Computer Science

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The Free-form Light StageThe Free-form Light Stage

• Capture appearance of an object– With hand–held light source

• Relight real objects

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Related workRelated work

• Image-based relighting– Combination of Basis Images

[Nimeroff et al.1994]

– Light Stage

[Debevec et al. 2000]

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Basis imagesIlluminant direction

estimationTake pictures and move hand-held light source

Incidentlight mapCreate weighted sum

…

· W1 + · W2 +

+ · Wn

Relit object

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Taking the picturesTaking the pictures

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RelightingRelighting

· W1 + · W2 + · Wn-1 + · Wn +…

=

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Take pictures and move hand-held light source

Illuminant direction estimationBasis images

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Illuminant direction estimationIlluminant direction estimation

Diffuse white spheres

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Illuminant direction estimationIlluminant direction estimation

Lamberts Law

Ii = ρ IL ( Ni · L)

L

N1

I1L

N2

I2

LN3I3

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Illuminant direction estimationIlluminant direction estimation

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Illuminant direction estimationIlluminant direction estimation

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A direction per basis imageA direction per basis image

…

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Accuracy of estimationAccuracy of estimation

Light source at infinity

Area light source

VirtualMaximum error:

1 degree

Maximum error:

2 degrees

Real N/AMaximum error:

7 degrees

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Illuminant direction estimationBasis imagesTake pictures and move

hand-held light source

Incidentlight mapCreate weighted sum

…

· W1 + · W2 +

+ · Wn

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Weight computationWeight computation

• Weight is based on incident light in neighborhood of light source direction

• Non-uniform sampling of light source directions

use an angular Voronoi diagram

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Plot directions on a hemispherePlot directions on a hemisphere

…

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Plot directions on hemispherePlot directions on hemisphere

30 light directions

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Create an angular Voronoi DiagramCreate an angular Voronoi Diagram

30 light directions

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Choose a parameterizationChoose a parameterization

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Filter the incident light mapFilter the incident light map

Filter each Voronoi cell Overlay incident light map with angular Voronoi diagram

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400 light directions

Using all illuminant directionsUsing all illuminant directions

30 light directions

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RelightingRelighting

…

· W1 +

· W2 +

· W3 +

· Wn

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· W1 +

…

· W2 +

· W3 +

· Wn

=

RelightingRelighting

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Illuminant direction estimationBasis imagesTake pictures and move

hand-held light source

Incidentlight map

… + · Wn

· W2 + · W1 +

Create weighted sumRelit object

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ResultsResults

400 basis images

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ResultsResults

400 basis images

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ResultsResults

500 basis images

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ResultsResults

500 basis images

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ResultsResults

500 basis images

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Results Results

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ResultsResults

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Augmented virtualityAugmented virtuality

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ConclusionConclusion• Relighting real objects

– Easy and cheap data acquisition– Portable– Scalable

• Future work– Test BIG objects– Alternative filters– Sampling density accuracy in illuminant

direction estimation

• For more info: see the EGWR ’02 paper

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AcknowledgementsAcknowledgements

• Frank Suykens and Pieter Peers

• Environment maps from: – http://www.debevec.org/Probes– http://www.cs.kuleuven.ac.be/~graphics

• FWO Grant #G.0354.00

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