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Structured light and active ranging techniques Class 11

Structured light and active ranging techniques Class 11. last Tuesday: stereo. per-pixel optimization. per-scanline optimization. full image optimization. original image pair. planar rectification. polar rectification. Plane-sweep multi-view matching.

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Structured light and active ranging techniques Class 11

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  1. Structured light and active ranging techniquesClass 11

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

  3. original image pair planar rectification polar rectification

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

  6. A Taxonomy

  7. A taxonomy

  8. Unstructured light project texture to disambiguate stereo

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

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

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

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

  13. Triangulation

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

  15. Triangulation: Moving theCamera and Illumination

  16. Triangulation: Moving theCamera and Illumination (Rioux et al. 87)

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

  18. 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)

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

  20. Space-time analysis Curless ‘95

  21. Space-time analysis Curless ‘95

  22. Projector as camera

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

  24. Real-time system Koninckx and Van Gool

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

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

  27. Time-Coded Light Patterns • Assign each stripe a unique illumination codeover time [Posdamer 82] Time Space

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

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

  30. 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.

  31. 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)

  32. 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)

  33. 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

  34. 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)

  35. Shadow Moire

  36. Depth from focus/defocus Nayar’95 Nov. 8, don’t miss Distinguished lecture!

  37. Next class: structure from motion

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