Structured light and active ranging techniques Class 8

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Structured light and active ranging
techniquesClass 8
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last Wednesday stereo
per-pixel optimization
per-scanline optimization
full image optimization
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original image pair
planar rectification
polar rectification
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Plane-sweep multi-view matching

• Simple algorithm for multiple cameras
• No rectification necessary, but also no gain
• Doesnt deal with occlusions

Collins96 Roy and Cox98 (GC) Yang et
al.02/03 (GPU)
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3D photography course schedule(tentative)
Lecture Exercise
Sept 26 Introduction -
Oct. 3 Geometry Camera model Camera calibration
Oct. 10 Single View Metrology Measuring in images
Oct. 17 Feature Tracking/matching (Friedrich Fraundorfer) Correspondence computation
Oct. 24 Epipolar Geometry F-matrix computation
Oct. 31 Shape-from-Silhouettes (Li Guan) Visual-hull computation
Nov. 7 Stereo matching Project proposals
Nov. 14 Structured light and active range sensing Papers
Nov. 21 Structure from motion Papers
Nov. 28 Multi-view geometry and self-calibration Papers
Dec. 5 Shape-from-X Papers
Dec. 12 3D modeling and registration Papers
Dec. 19 Appearance modeling and image-based rendering Final project presentations
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Todays class

• unstructured light
• structured light
• time-of-flight

(some slides from Szymon Rusinkiewicz, Brian
Curless)
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A Taxonomy
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A taxonomy
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Unstructured light

• project texture to
• disambiguate stereo

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Space-time stereo
Davis, Ramamoothi, Rusinkiewicz, CVPR03
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Space-time stereo
Davis, Ramamoothi, Rusinkiewicz, CVPR03
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Space-time stereo
Zhang, Curless and Seitz, CVPR03
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Space-time stereo
Zhang, Curless and Seitz, CVPR03

• results

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Light Transport Constancy
Davis, Yang, Wang, ICCV05
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Triangulation
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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

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Triangulation Moving theCamera and Illumination
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Triangulation Moving theCamera and Illumination
(Rioux et al. 87)
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Triangulation Extending to 3D

• Possibility 1 add another mirror (flying spot)
• Possibility 2 project a stripe, not a dot

Object
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Triangulation Scanner Issues

• Accuracy proportional to working volume(typical
  is 10001)
• 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?)

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Triangulation Scanner Issues

• Material properties (dark, specular)
• Subsurface scattering
• Laser speckle
• Edge curl
• Texture embossing

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(No Transcript)
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Space-time analysis
Curless 95
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Space-time analysis
Curless 95
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Projector as camera
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Multi-Stripe Triangulation

• To go faster, project multiple stripes
• But which stripe is which?
• Answer 1 assume surface continuity

e.g. Eyetronics ShapeCam
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Real-time system
Koninckx and Van Gool
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Multi-Stripe Triangulation

• To go faster, project multiple stripes
• But which stripe is which?
• Answer 2 colored stripes (or dots)

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Multi-Stripe Triangulation

• To go faster, project multiple stripes
• But which stripe is which?
• Answer 3 time-coded stripes

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Time-Coded Light Patterns

• Assign each stripe a unique illumination
  codeover time Posdamer 82

Time
Space
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Better codes

• Gray code
• Neighbors only differ one bit

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Poor mans scanner
Bouget and Perona, ICCV98
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Pulsed Time of Flight

• Basic idea send out pulse of light (usually
  laser), time how long it takes to return

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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.

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Depth cameras

• 2D array of
• time-of-flight sensors
• e.g. Canestas CMOS 3D sensor
• jitter too big on single measurement,
• but averages out on many
• (10,000 measurements?100x improvement)

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Depth cameras
                                                                                                                                       
                                                                                                                                       
                                                                                                                                       
                                                                                                                                       
                                                                                                                                       
3DVs Z-cam

• Superfast shutter standard CCD
• cut light off while pulse is coming back, then
  IZ
• but Ialbedo (use unshuttered reference view)

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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

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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)

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Shadow Moire
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Depth from focus/defocus
Nayar95
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Next class structure from motion