3-D Computer Vision Using3-D Computer Vision UsingStructured LightStructured Light

Prepared by Burak Borhan

Machine VisionMachine Vision

For many applications three dimensional For many applications three dimensional (3-D) descriptors of the scene are (3-D) descriptors of the scene are required.required.

Passive Approaches

Using two or more cameras in binocular and photometric stereo

Active Approaches

Use specialized illumination sources and detectors

Overcome the fundamental ambiguities associated with passive approaches.

STRUCTURED LIGHT

Structured Light SystemStructured Light System

Structured Light systems use triangulation to acquire range measurements. Structured Light systems use triangulation to acquire range measurements. In the upper system the ranging geometry is formed by a laser emission, the In the upper system the ranging geometry is formed by a laser emission, the reflected light observed by the camera and by a rigid backbone. Optical reflected light observed by the camera and by a rigid backbone. Optical measurements capture range data in a plane. A conveyer produces the measurements capture range data in a plane. A conveyer produces the necessary motion for 3-D range data.necessary motion for 3-D range data.

Three Main Challenges in Three Main Challenges in Developing a S.L. SystemDeveloping a S.L. System

CalibrationCalibration

AccuracyAccuracy

Acquisition SpeedAcquisition Speed

CalibrationCalibration

Calibration models are Calibration models are needed to relate image to needed to relate image to world coordinates.world coordinates.

Challenges in calibration Challenges in calibration also arise from the also arise from the complexity of models that complexity of models that are required for sensor are required for sensor kinematics, which kinematics, which describe the geometrical describe the geometrical relationship between the relationship between the camera and the laser camera and the laser plane.plane.

Ranging AccuracyRanging Accuracy

Ranging accuracy is highly dependent on Ranging accuracy is highly dependent on calibration models. However even the calibration models. However even the most careful calibration effort can be most careful calibration effort can be fruitless if the ranging geometry is fruitless if the ranging geometry is unfavorable.unfavorable.

Acquisition SpeedAcquisition Speed

Range Acquisition involves locating the laser Range Acquisition involves locating the laser profile within camera imagesprofile within camera images

This requires pixel examination and processing This requires pixel examination and processing in order to precisely locate image coordinates in order to precisely locate image coordinates that reside at the center of the laser profile.that reside at the center of the laser profile.

Analysis of an Existing S.L. SystemAnalysis of an Existing S.L. System

The camera is outfitted with an optical filter that is matched to the laser optical frequency. These matched optics produce very distinct imagery of the laser profile.

Block Diagram of Processing StepsBlock Diagram of Processing Steps

Image Processing Operations (1)Image Processing Operations (1)

The rough position estimates are achieved The rough position estimates are achieved by thresholding and then run length by thresholding and then run length coding.coding.

The RLC array provides a brief description The RLC array provides a brief description of the rough location of the laser line.of the rough location of the laser line.

Image Processing Operations (2)Image Processing Operations (2)

The upper figure illustrates typical pixel intensities, as sampled along a path roughly orthogonal to the laser line. In this system the pixel intensity profiles are 5 - 7 pixels wide with roughly Gaussian shape.

The main processor examines gray scale pixel values in a 20x1 window, W, centered at locations given by the RLC array.

Image Processing Operations (3)Image Processing Operations (3)

The mean and the variance of the row at The mean and the variance of the row at the center of the laser line are found with,the center of the laser line are found with,

The points above the threshold are used The points above the threshold are used to calculate the mean and variance.to calculate the mean and variance.

Calibration Process (X)Calibration Process (X)

z = f(r,c) calibration. Generate examples of |zi ri ci| matrix.

Calibration Process (Y)Calibration Process (Y)

y = f(r,c) calibration. Generate examples of |yi ri ci| matrix.

Image Processing Operations (4)Image Processing Operations (4)

A calibration model is used to convert the image A calibration model is used to convert the image coordinates (r,c) to world coordinates (y,z)coordinates (r,c) to world coordinates (y,z)

..

And the X components is found by monitoring And the X components is found by monitoring the motion of the conveyor belt using a wheel the motion of the conveyor belt using a wheel encoder.encoder.

Calibration ProcessCalibration Process

n

nnnnnn

iiiiii

iiiiiii

m

m

crcrcr

crcrcr

crcrcr

z

zz

zz

z

crcrcrz

1

22

22

111121

21

6

5

4

3

2

1

22

1

1

1

1

.parametersmodelcontains:)1(

.scoordinatepixelaugmentedcontains:)(

s.coordinateworldcontains:)1(

pM

pnF

nZ

FMZ

3- models3- models

3-D Image Reconstruction3-D Image Reconstruction

Geometric model of a polyhedral object is represented byGeometric model of a polyhedral object is represented by

tmmmm

kkkkk

jtjjj

ffftzyxv

ffvve

teeef

whereVEFG

,......,,,,,,

vertices,visibleallaboutninformatiorecordsVlistvertextheand

,,,

edges,visibleallaboutninformatiorecordsElistedgetheand

,,.......,,

faces,visibleallaboutninformatiorecordsFlistfaceThe

,:,,

21

2121

21

QUESTIONS ?QUESTIONS ?QUESTIONS ?QUESTIONS ?