1. Structured light and active ranging techniques Class 8

2. Geometric Computer Vision course schedule (tentative)
Lecture
Exercise
Sept 16
Introduction -
Sept 23
Geometry & Camera model
Camera calibration
Sept 30
Single View Metrology
(Changchang Wu)
Measuring in images
Oct. 7
Feature Tracking/Matching
Correspondence computation
Oct. 14
Epipolar Geometry
F-matrix computation
Oct. 21
Shape-from-Silhouettes
Visual-hull computation
Oct. 28
Stereo matching
papers
Nov. 4
Stereo matching (continued)
Project proposals
Nov. 11
Structured light and active range sensing
Papers
Nov. 18
Structure from motion and visual SLAM
Nov. 25
Multi-view geometry and self-calibration
Dec. 2
3D modeling, registration and range/depth fusion (Christopher Zach?)
Dec. 9
Shape-from-X and image-based rendering
Dec. 16
Final project presentations

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

4. A Taxonomy

5. A taxonomy

6. Unstructured light
project texture to
disambiguate stereo

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

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

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

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

11. Triangulation

12. 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

13. Triangulation: Moving the Camera and Illumination

14. Triangulation: Moving the Camera and Illumination
(Rioux et al. 87)

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

16. 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)

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

19. Space-time analysis
Curless ‘95

20. Space-time analysis
Curless ‘95

21. Projector as camera

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

23. Real-time system
Koninckx and Van Gool

24. Real-time scanning system
Rusinckiewicz et al. Siggraph02
Szymon Rusinckiewicz talk Friday 20/11 at 11:15 in ETZ E9
(in context of PhD defense Thibaut Weise,
also 20/11 at 15:00 in ETF C109)

25. In-hand modeling
Weise et al. CVPR08

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

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

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

29. Bouget and Perona, ICCV’98
An idea for a project?
Bouget and Perona, ICCV’98

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

31. Pulsed Time of Flight Advantages: Disadvantages:
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.

32. 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)

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

34. 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mn

35. 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)

36. Shadow Moire

37. Depth from focus/defocus
Nayar’95

38. Next class: structure from motion