Structured light and active ranging techniques

Class 11

per-pixel optimization

per-scanline optimization

full image optimization

last Tuesday: stereo

polarrectification

planarrectification

originalimage pair

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)

Today’s class

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

(some slides from Szymon Rusinkiewicz, Brian Curless)

A Taxonomy

A taxonomy

Unstructured light

project texture to disambiguate stereo

Space-time stereoDavis, Ramamoothi, Rusinkiewicz, CVPR’03

Space-time stereoDavis, Ramamoothi, Rusinkiewicz, CVPR’03

Space-time stereo

Zhang, Curless and Seitz, CVPR’03

Space-time stereo

• resultsZhang, Curless and Seitz, CVPR’03

Triangulation

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

Triangulation: Moving theCamera and Illumination

Triangulation: Moving theCamera and Illumination

(Rioux et al. 87)

Triangulation: Extending to 3D

• Possibility #1: add another mirror (flying spot)

• Possibility #2: project a stripe, not a dot

ObjectObject

LaserLaser

CameraCameraCameraCamera

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)

Triangulation Scanner Issues

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

Space-time analysisCurless ‘95

Space-time analysisCurless ‘95

Projector as camera

Multi-Stripe Triangulation

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

continuity

e.g. Eyetronics’ ShapeCam

Real-time system

Koninckx and Van Gool

Multi-Stripe Triangulation

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

Multi-Stripe Triangulation

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

Time-Coded Light Patterns

• Assign each stripe a unique illumination codeover time [Posdamer 82]

SpaceSpace

TimeTime

An idea for a project?

Bouget and Perona, ICCV’98

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

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.

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)

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

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

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)

Shadow Moire

Depth from focus/defocus

Nayar’95

Nov. 8, don’t miss Distinguished lecture!

Next class: structure from motion