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Analyzing Impossible Images: 

Analyzing Impossible Images Steve Seitz University of Washington Computational Photography Symposium May 23, 2004

Imaging Breakthroughs: 

Imaging Breakthroughs photography, moving pictures xray, ultrasound, MRI, etc. Etienne-Jules Marey, falling cat

Imaging Desiderata: 

Imaging Desiderata Analyzing real images is a pain occlusions clutter shading focus fidelity Impossible images don’t have such problems can computational imaging make these problems go away?

Removing Occlusions: 

Rollout Photographs © Justin Kerr http://research.famsi.org/kerrmaya.html Removing Occlusions

Slide5: 

The Blue Marble, NASA satellite image composite

Slide6: 

The Blue Marble, NASA satellite image composite

Slide7: 

by David Dewey

Slide8: 

by Jiwon Kim

Open Questions: 

Open Questions How much visibility can we get? sensor design Many possible projections How do we process these images?

Removing Interreflection: 

Removing Interreflection Images by Ward et al., SIGGRAPH 88

Bounce Images: 

Bounce Images

Main Results: 

Main Results There exists a matrix C1 that removes all interreflections in a photograph (or lightfield) Works for any illumination There is a matrix Ck that retains only the kth bounce

Light transport: 

Light transport T

The transport matrix: 

The transport matrix = T Lin Lout Accounts for interreflections, shadows, refraction, subsurface scatter, ... [Dorsey 94] [Zongker 99] [Debevec 00] [Peers 03] [Goesele 05] [Sen 05] ...

The transport matrix: 

The transport matrix = T Lin Lout

Direct illumination rendering: 

Direct illumination rendering = T1 Lin L1out Single bounce from light to eye no interreflections

Inverse rendering: 

Inverse rendering = T-1 Lin Lout

Removing Interreflections: 

Removing Interreflections L1out Lout T-1 T1 C1 Second derivation: from the rendering equation [Kajiya 86] [Cohen 86]

Cancellation Operators: 

Cancellation Operators Recursively define other operators (I – C1) gives interreflected light C2 = C1 (I – C1) gives second bounce of light Ck = C1(I – C1)k-1 gives kth bounce of light Inverse ray tracing!

How to compute C1: 

How to compute C1 Simplified case Lambertian reflectance and fixed viewpoint Lin and Lout are 2D Can capture T by scanning a laser

Synthetic M Scene: 

Synthetic M Scene 1 2 3 4

Slide24: 

real data

Slide27: 

(x3) (x3) (x6) (x15) flash light illumination

Collaborators on Interreflections: 

Collaborators on Interreflections Kyros Kutulakos (U. Toronto) Yasuyuki Matsushita (MSR Asia) S. M. Seitz, Y. Matsushita, and K. Kutulakos, “A Theory of Inverse Light Transport,” Microsoft Technical Report MSR-TR-2005-66, May 2005.

Conclusions: 

Conclusions Impossible images no occlusions, no interreflections Better sensing techniques can they solve all analysis problems? shape tracking recognition What other kinds of “impossible” images do we want?