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Epipolar lines. epipolar plane. epipolar lines. epipolar lines. Baseline. O’. O. Rectification.

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Presentation Transcript
epipolar lines
Epipolar lines

epipolar plane

epipolar lines

epipolar lines

Baseline

O’

O

rectification
Rectification
  • Rectification: rotation and scaling of each camera’s coordinate frame to make the epipolar lines horizontal and equi-height,by bringing the two image planes to be parallel to the baseline
  • Rectification is achieved by applying homography to each of the two images
rectification1
Rectification

Baseline

O’

O

cyclopean coordinates
Cyclopean coordinates
  • In a rectified stereo rig with baseline of length , we place the origin at the midpoint between the camera centers.
  • a point is projected to:
    • Left image: ,
    • Right image: ,
  • Cyclopean coordinates:
disparity
Disparity
  • Disparity is inverse proportional to depth
  • Constant disparity constant depth
  • Larger baseline, more stable reconstruction of depth (but more occlusions, correspondence is harder)

(Note that disparity is defined in a rectified rig in a cyclopean coordinate frame)

random dot stereogram
Random dot stereogram
  • Depth can be perceived from a random dot pair of images (Julesz)
  • Stereo perception is based solely on local information (low level)
compared elements
Compared elements
  • Pixel intensities
  • Pixel color
  • Small window (e.g. or ), often using normalized correlation to offset gain
  • Features and edges (less common)
  • Mini segments
dynamic programming
Dynamic programming
  • Each pair of epipolar lines is compared independently
  • Local cost, sum of unary term and binary term
    • Unary term: cost of a single match
    • Binary term: cost of change of disparity (occlusion)
  • Analogous to string matching (‘diff’ in Unix)
string matching
String matching
  • Swing → String

S t r i n g

Start

S w i n g

End

string matching1
String matching
  • Cost: #substitutions + #insertions + #deletions

S t r i n g

S w i n g

dynamic programming1
Dynamic Programming
  • Shortest path in a grid
  • Diagonals: constant disparity
  • Moving along the diagonal – pay unary cost (cost of pixel match)
  • Move sideways – pay binary cost, i.e. disparity change (occlusion, right or left)
  • Cost prefers fronto-parallel planes. Penalty is paid for tilted planes
dynamic programming2
Dynamic Programming

Start

,

Complexity?

probability interpretation viterbi algorithm
Probability interpretation: Viterbi algorithm
  • Markov chain
  • States: discrete set of disparity
  • Log probabilities: product sum
probability interpretation viterbi algorithm1
Probability interpretation: Viterbi algorithm
  • Markov chain
  • States: discrete set of disparity
  • Maximum likelihood: minimize sum of negative logs
  • Viterbi algorithm: equivalent to shortest path
dynamic programming pros and cons
Dynamic Programming: Pros and Cons
  • Advantages:
    • Simple, efficient
    • Achieves global optimum
    • Generally works well
  • Disadvantages:
dynamic programming pros and cons1
Dynamic Programming: Pros and Cons
  • Advantages:
    • Simple, efficient
    • Achieves global optimum
    • Generally works well
  • Disadvantages:
    • Works separately on each epipolar line, does not enforce smoothness across epipolars
    • Prefers fronto-parallel planes
    • Too local? (considers only immediate neighbors)
markov random field
Markov Random Field
  • Graph In our case: graph isa 4-connected gridrepresenting one image
  • States: disparity
  • Minimize energy of the form
  • Interpreted as negative log probabilities
iterated conditional modes icm
Iterated Conditional Modes (ICM)
  • Initialize states (= disparities) for every pixel
  • Update repeatedly each pixel by the most likely disparity given the values assigned to its neighbors:
  • Markov blanket: the state of a pixel only depends on the states of its immediate neighbors
  • Similar to Gauss-Seidel iterations
  • Slow convergence to (often bad) local minimum
graph cuts expansion moves
Graph cuts: expansion moves
  • Assume is non-negative and is metric:
  • We can apply more semi-global moves using minimal s-t cuts
  • Converges faster to a better (local) minimum
expansion
α-Expansion
  • In any one round, expansion move allows each pixel to either
    • change its state to α, or
    • maintain its previous state

Each round is implemented via max flow/min cut

  • One iteration: apply expansion moves sequentially with all possible disparity values
  • Repeat till convergence
expansion1
α-Expansion
  • Every round achieves a globally optimal solution over one expansion move
  • Energy decreases (non-increasing) monotonically between rounds
  • At convergence energy is optimal with respect to all expansion moves, and within a scale factor from the global optimum:

where

expansion 1d example8
α-Expansion (1D example)
  • Such a cut cannot be obtained due to triangle inequality:
common metrics
Common Metrics
  • Potts model:
  • Truncated :
  • Truncated squared difference is not a metric
reconstruction with graph cuts
Reconstruction with graph-cuts

Original Result Ground truth

a different application detect skyline
A different application: detect skyline
  • Input: one image, oriented with sky above
  • Objective: find the skyline in the image
  • Graph: grid
  • Two states: sky, ground
  • Unary (data) term:
    • State = sky, low if blue, otherwise high
    • State = ground, high if blue, otherwise low
  • Binary term for vertical connections:
    • If the state of a node is sky, the node above should also be sky (set to infinity if not)
    • If the state of a node is ground, the node below should also be ground
  • Solve with expansion move. This is a binary (two state) problem, and so graph cut can find the global optimum in one expansion move
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