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Computational Photography and Capture: Time-Lapse Video Analysis & Editing

Computational Photography and Capture: Time-Lapse Video Analysis & Editing. Gabriel Brostow & Tim Weyrich TAs: Clément Godard & Fabrizio Pece. Today’s schedule. Computational Time-Lapse Video Bennett & McMillan, Siggraph 2007 Factored Time-Lapse Video

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Computational Photography and Capture: Time-Lapse Video Analysis & Editing

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  1. Computational Photography and Capture: Time-Lapse VideoAnalysis & Editing Gabriel Brostow & Tim Weyrich TAs: Clément Godard & Fabrizio Pece

  2. Today’s schedule • Computational Time-Lapse Video • Bennett & McMillan, Siggraph 2007 • Factored Time-Lapse Video • Sunkavalli, Matusik, Pfister, Rusinkiewicz Siggraph 2007 • Video Synopsis & Indexing, • Pritch, Rav-Acha, Peleg, ICCV 2007

  3. Time-lapse Photography • Definition: when frames are captured at a lower rate than the rate at which they will ultimately be played back. • Compare to • Slow-motion • Bullet-time

  4. Time-lapse Filmmaker’s Challenges • Lighting changes (strobing) • High frequency motion (missed action) • Saturation & exposure • Camera motion: intentional or not • Triggering / data storage / camera safety • Useful? Editable?

  5. Does the old definition of time-lapse still hold?

  6. (Quick Overview of)Computational Time-Lapse Video Bennett & McMillan Siggraph 2007 (Project web page) (though link seems to have died, so see my archive here)

  7. Virtual Shutter

  8. Sampling in 1D • Uniform • Interpolate linearly+ piece-wise vs. • Same # of samples, but where needed! • Optimum polygonal approximation of digitized curves, Perez & Vidal, PRL’94 • Use Dynamic Programming to find global min.

  9. Video

  10. Completely User-controllable:Min-Change, Min-Error, Median, etc

  11. Factored Time-Lapse Video Sunkavalli, Matusik, Pfister, Rusinkiewicz SIGGRAPH 2007 Project web page

  12. Outdoors: More Than Just Color vs Illumination? • Wishlist • Remove shadows • Render new shadows • Change albedo • Change global illumination Original No Shadows Given: Daytime, under clear-sky conditions

  13. Outdoors: More Than Just Color vs Illumination? Original No Shadows Lighting due to sun? sky? Surface Normals?

  14. Formulation • size: width x height x time • F: xyt volume of frames over time • Isky: Accumulated intensity due to sky-light • Isun: Accumulated intensity due to sun-light • Ssun: Binary; is a pixel out of shadow?

  15. Intuition

  16. Separate Sky first • Photoshop! • Just to pick “non-surfaces” • When is sun’s contribution == 0? • Leaving just sky’s contribution

  17. “Definitely” shadow: median of darkest 20%, then threshold at 1.5x

  18. Bilateral Filter “Definitely” Shadow

  19. How Did Skylight Change? • Isky: Accumulated intensity due to sky-light • Wsky: Sky-light image • Hsky: Sky-light basis curve (1D function!!) • Whole scene got brighter/darker together

  20. Blue: skylight curve (same one)

  21. Factorization • Decompose appearance into • per-pixel W • H curve • Alternating Constrained Least Squares (ACLS): • H(t) is held fixed while W is optimized using least squares, then vice versa

  22. ACLS: Alternating Constrained Least Squares • Inverse shade trees for non-parametric material representation and editing, • Lawrence et al., Siggraph 2006, code online

  23. Original and Isky

  24. So Far - Explained Sky

  25. Now Decompose Sun’s Contributions • Isun: Accumulated intensity due to sun-light • Wsun: Sun-light image, per-pixel weights • Hsun: Sun-light basis curve • : Shift-map; per pixel offset in time

  26. Isun: Accumulated intensity due to sun-light • Wsun: Sun-light image, per-pixel weights • Hsun: Sun-light basis curve • : Shift-map; per pixel offset in time

  27. Total sunlight contribution Sunlight image (like on moon) Shift map Sunlight

  28. Factorize Again! • For each image, subtract off Isky(t) • Ssun: Sky-light mask, serves as C, confidence map • ACLS again • But add 3rd update phase, searching for that minimizes reconstruction error of scaled + offset H(t) vs. F(t)

  29. Green: sunlight curve (same one, but time-shifted)

  30. Isun(t) Wsun (time offset)

  31. Reconstruction

  32. Does It Work? (black curves are calculated sunlight basis curves)

  33. Video: FactoredTimeLapseV

  34. What have we gained? • Can • Remove shadows • Change albedo • Change amount of global illumination • Compression • Render new shadows: • We have 1 component of the per-pixel Normals, N

  35. Estimated Normals (1D)

  36. Manipulating Normals

  37. (View Results of)Video Synopsis & Indexing Pritch, Rav-Acha, Peleg ICCV 2007, PAMI 2008 (Project web page)

  38. What to do with blob-tracks? (Project web page)

  39. HDR Time-Lapse • Essentially started in 2006 (manually at first) • More are popping up online

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