Structured light and active ranging techniques Class 8.
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Transcript of Structured light and active ranging techniques Class 8.
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Structured light and active ranging techniques
Class 8
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per-pixel optimization
per-scanline optimization
full image optimization
last Wednesday: stereo
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polarrectification
planarrectification
originalimage pair
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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)
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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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Today’s 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 stereoDavis, Ramamoothi, Rusinkiewicz, CVPR’03
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Space-time stereoDavis, Ramamoothi, Rusinkiewicz, CVPR’03
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Space-time stereo
Zhang, Curless and Seitz, CVPR’03
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Space-time stereo
• resultsZhang, Curless and Seitz, CVPR’03
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Light Transport ConstancyDavis, 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
ObjectObject
LaserLaser
CameraCameraCameraCamera
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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)
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Triangulation Scanner Issues
• Material properties (dark, specular)• Subsurface scattering• Laser speckle• Edge curl• Texture embossing
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Space-time analysisCurless ‘95
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Space-time analysisCurless ‘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]
SpaceSpace
TimeTime
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Better codes…
• Gray codeNeighbors only differ one bit
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Poor man’s scanner
Bouget and Perona, ICCV’98
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Pulsed Time of Flight
• Basic idea: send out pulse of light (usually laser), time how long it takes to return
tcd 2
1tcd
2
1
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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. Canesta’s CMOS 3D sensor
jitter too big on single measurement,
but averages out on many(10,000 measurements100x
improvement)
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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
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AM Modulation Time of Flight
• Modulate a laser at frequencym , it returns with a phase shift
• Note the ambiguity in the measured phase! Range ambiguity of 1/2mn
2
2
2
1 n
ν
cd
m
2
2
2
1 n
ν
cd
m
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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
Nayar’95
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Next class: structure from motion