1. Structured light and active ranging techniques Class 8

2. per-pixel optimization per-scanline optimization full image optimization last Wednesday: stereo

3. polar rectification planar rectification original image pair

4. Plane-sweep multi-view matching Simple algorithm for multiple cameras No rectification necessary, but also no gain Doesn’t deal with occlusions Collins’96; Roy and Cox’98 (GC); Yang et al.’02/’03 (GPU)

5. 3D photography course schedule (tentative) LectureExercise Sept 26Introduction- Oct. 3Geometry & Camera modelCamera calibration Oct. 10Single View MetrologyMeasuring in images Oct. 17Feature Tracking/matching (Friedrich Fraundorfer) Correspondence computation Oct. 24Epipolar GeometryF-matrix computation Oct. 31Shape-from-Silhouettes (Li Guan) Visual-hull computation Nov. 7Stereo matchingProject proposals Nov. 14Structured light and active range sensing Papers Nov. 21Structure from motionPapers Nov. 28Multi-view geometry and self-calibration Papers Dec. 5Shape-from-XPapers Dec. 123D modeling and registrationPapers Dec. 19Appearance modeling and image-based rendering Final project presentations

6. Today’s class unstructured light structured light time-of-flight (some slides from Szymon Rusinkiewicz, Brian Curless)

7. A Taxonomy

8. A taxonomy

9. Unstructured light project texture to disambiguate stereo

10. Space-time stereo Davis, Ramamoothi, Rusinkiewicz, CVPR’03

11. Space-time stereo Davis, Ramamoothi, Rusinkiewicz, CVPR’03

12. Space-time stereo Zhang, Curless and Seitz, CVPR’03

13. Space-time stereo results Zhang, Curless and Seitz, CVPR’03

14. Light Transport Constancy Davis, Yang, Wang, ICCV05

15. Triangulation

16. Triangulation: Moving the Camera and Illumination Moving independently leads to problems with focus, resolution Most scanners mount camera and light source rigidly, move them as a unit

17. Triangulation: Moving the Camera and Illumination

18. (Rioux et al. 87)

19. Triangulation: Extending to 3D Possibility #1: add another mirror (flying spot) Possibility #2: project a stripe, not a dot Object Laser CameraCamera

20. Triangulation Scanner Issues Accuracy proportional to working volume (typical is ~1000:1) Scales down to small working volume (e.g. 5 cm. working volume, 50  m. accuracy) Does not scale up (baseline too large…) Two-line-of-sight problem (shadowing from either camera or laser) Triangulation angle: non-uniform resolution if too small, shadowing if too big (useful range: 15  -30  )

21. Triangulation Scanner Issues Material properties (dark, specular) Subsurface scattering Laser speckle Edge curl Texture embossing

23. Space-time analysis Curless ‘95

24. Space-time analysis Curless ‘95

25. Projector as camera

26. Multi-Stripe Triangulation To go faster, project multiple stripes But which stripe is which? Answer #1: assume surface continuity e.g. Eyetronics’ ShapeCam

27. Real-time system Koninckx and Van Gool

28. Multi-Stripe Triangulation To go faster, project multiple stripes But which stripe is which? Answer #2: colored stripes (or dots)

29. Multi-Stripe Triangulation To go faster, project multiple stripes But which stripe is which? Answer #3: time-coded stripes

30. Time-Coded Light Patterns Assign each stripe a unique illumination code over time [Posdamer 82] Space Time

31. Better codes… Gray code Neighbors only differ one bit

32. Poor man’s scanner Bouget and Perona, ICCV’98

33. Pulsed Time of Flight Basic idea: send out pulse of light (usually laser), time how long it takes to return

34. Pulsed Time of Flight Advantages: Large working volume (up to 100 m.) Disadvantages: Not-so-great accuracy (at best ~5 mm.) Requires getting timing to ~30 picoseconds Does not scale with working volume Often used for scanning buildings, rooms, archeological sites, etc.

35. Depth cameras 2D array of time-of-flight sensors e.g. Canesta’s CMOS 3D sensor jitter too big on single measurement, but averages out on many (10,000 measurements  100x improvement)

36. Depth cameras Superfast shutter + standard CCD cut light off while pulse is coming back, then I~Z but I~albedo (use unshuttered reference view) 3DV’s Z-cam

37. AM Modulation Time of Flight Modulate a laser at frequency m, it returns with a phase shift  Note the ambiguity in the measured phase!  Range ambiguity of 1 / 2 m n

38. AM Modulation Time of Flight Accuracy / working volume tradeoff (e.g., noise ~ 1 / 500 working volume) In practice, often used for room-sized environments (cheaper, more accurate than pulsed time of flight)

39. Shadow Moire

40. Depth from focus/defocus Nayar’95

41. Next class: structure from motion