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Multiple View Geometry. Marc Pollefeys University of North Carolina at Chapel Hill. Modified by Philippos Mordohai. Outline. Fundamental matrix estimation Image rectification Chapter 10 of “Multiple View Geometry in Computer Vision” by Hartley and Zisserman.

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multiple view geometry

Multiple View Geometry

Marc Pollefeys

University of North Carolina at Chapel Hill

Modified by Philippos Mordohai

outline
Outline
  • Fundamental matrix estimation
  • Image rectification
  • Chapter 10 of “Multiple View Geometry in Computer Vision” by Hartley and Zisserman
epipolar geometry basic equation
Epipolar geometry: basic equation

separate known from unknown

(data)

(unknowns)

(linear)

the singularity constraint
The singularity constraint

SVD from linearly computed F matrix (rank 3)

Compute closest rank-2 approximation

the minimum case 7 point correspondences
The minimum case – 7 point correspondences

one parameter family of solutions

but F1+lF2 not automatically rank 2

the minimum case impose rank 2

3

F7pts

F

F2

F1

The minimum case – impose rank 2

(obtain 1 or 3 solutions)

(cubic equation)

Compute possible l as eigenvalues of

(only real solutions are potential solutions)

the not normalized 8 point algorithm

~10000

~100

~10000

~100

~10000

~10000

~100

~100

1

Orders of magnitude difference

between column of data matrix

 least-squares yields poor results

!

The NOT normalized 8-point algorithm
the normalized 8 point algorithm
Transform image to ~[-1,1]x[-1,1]

(-1,1)

(1,1)

(0,0)

(-1,-1)

(1,-1)

normalized least squares yields good results(Hartley, PAMI´97)

The normalized 8-point algorithm

(0,500)

(700,500)

(0,0)

(700,0)

geometric distance
Geometric distance

Gold standard

Sampson error

Symmetric epipolar distance

gold standard
Gold standard

Maximum Likelihood Estimation

(= least-squares for Gaussian noise)

Initialize: normalized 8-point, (P,P‘) from F, reconstruct Xi

Parameterize:

(overparametrized F=[t]xM)

Minimize cost using Levenberg-Marquardt

(preferably sparse LM, see book)

slide12

Gold standard

Alternative, minimal parametrization (with a=1)

(note (x,y,1) and (x‘,y‘,1) are epipoles)

  • problems:
  • a=0

 pick largest of a,b,c,d to fix to 1

  • epipole at infinity

 pick largest of x,y,w and of x’,y’,w’

4x3x3=36 parametrizations!

reparametrize at every iteration, to be sure

first order geometric error sampson error
First-order geometric error(Sampson error)

(one eq./point

JJT scalar)

(problem if some x is located at epipole)

advantage: no subsidiary variables required (coordinates of world points)

slide18

Some experiments:

Solid: 8-point algorithm

Dashed: geometric error

Dotted: algebraic error

Residual error:

(for all matches, not just n)

recommendations
Recommendations:
  • Do not use unnormalized algorithms
  • Quick and easy to implement: 8-point normalized
  • Better: enforce rank-2 constraint during minimization
  • Best: Maximum Likelihood Estimation (minimal parameterization, sparse implementation)
special cases
Special cases:

Enforce constraints for optimal results:

Pure translation (2dof),

Planar motion (6dof),

Calibrated case (5dof) – Essential matrix

the envelope of epipolar lines
The envelope of epipolar lines

What happens to an epipolar line if there is noise?

Monte Carlo

n=50

n=25

n=15

n=10

Notice that narrower part of envelope is not the correct match

other entities
Other entities?

Lines give no constraint for two view geometry

(but will for three and more views)

Curves and surfaces yield some constraints related to tangency

automatic computation of f
Automatic computation of F
  • Interest points
  • Putative correspondences
  • RANSAC
  • (iv) Non-linear re-estimation of F
  • Guided matching
  • (repeat (iv) and (v) until stable)
feature points
Extract feature points to relate images

Required properties:

Well-defined

(i.e. neigboring points should all be different)

Stable across views

Feature points

(i.e. same 3D point should be extracted

as feature for neighboring viewpoints)

feature points25
Feature points

(e.g.Harris&Stephens´88; Shi&Tomasi´94)

Find points that differ as much as possible

from all neighboring points

homogeneous

edge

corner

Mshould have large eigenvalues

Feature = local maxima (subpixel) of F(1,2)

feature points26
Feature points

Select strongest features (e.g. 1000/image)

feature matching
Evaluate NCC for all features with

similar coordinates

?

Feature matching

Keep mutual best matches

Still many wrong matches!

feature example

3

3

2

2

4

4

1

5

1

5

Feature example

Gives satisfying results

for small image motions

wide baseline matching
Requirement to cope with larger variations between images

Translation, rotation, scaling

Foreshortening

Non-diffuse reflections

Illumination

Wide-baseline matching

geometric

transformations

photometric

changes

ransac
Step 1. Extract features

Step 2. Compute a set of potential matches

Step 3. do

Step 3.1 select minimal sample (i.e. 7 matches)

Step 3.2 compute solution(s) for F

Step 3.3 determine inliers

until (#inliers,#samples)<95%

(generate

hypothesis)

(verify hypothesis)

RANSAC

Step 4. Compute F based on all inliers

Step 5. Look for additional matches

Step 6. Refine F based on all correct matches

finding more matches
Finding more matches

restrict search range to neighborhood of epipolar line

(1.5 pixels)

relax disparity restriction (along epipolar line)

degenerate cases
Degenerate cases

Planar scene

Pure rotation

No unique solution

Remaining DOF filled by noise

Use simpler model (e.g. homography)

Model selection (Torr et al., ICCV´98, Kanatani, Akaike)

Compare H and F according to expected residual error (compensate for model complexity)

Degenerate cases:
more problems
Absence of sufficient features (no texture)

Repeated structure ambiguity

More problems:
  • Robust matcher also finds
  • support for wrong hypothesis
  • solution: detect repetition

(Schaffalitzky and Zisserman,

BMVC‘98)

outline35
Outline
  • Fundamental matrix estimation
  • Image rectification
  • Chapter 10 of “Multiple View Geometry in Computer Vision” by Hartley and Zisserman
image pair rectification

e

Image pair rectification

simplify stereo matching

by warping the images

Apply projective transformation so that epipolar lines

correspond to horizontal scanlines

e

map epipole e to (1,0,0)

try to minimize image distortion

problem when epipole in (or close to) the image

planar rectification

~ image size

(calibrated)

Planar rectification

(standard approach)

Distortion minimization

(uncalibrated)

Bring two views

to standard stereo setup

(moves epipole to )

(not possible when in/close to image)

polar rectification
Polar rectification

(Pollefeys et al. ICCV’99)

Polar re-parameterization around epipoles

Requires only (oriented) epipolar geometry

Preserve length of epipolar lines

Choose  so that no pixels are compressed

original image

rectified image

Works for all relative motions

Guarantees minimal image size

example b guinage of leuven
Example: Béguinage of Leuven

Does not work with standard

Homography-based approaches