01/18 lab meeting ucla vision lab department of computer science university of california at los...
TRANSCRIPT
01/18 Lab meeting
UCLA Vision Lab
Department of Computer Science
University of California at Los Angeles
Fabio Cuzzolin
Los Angeles, January 18 2005
… past and present PhD student, University of Padova, Department of Computer
Science (NAVLAB laboratory) with Ruggero Frezza
Visiting student, ESSRL, Washington University in St. Louis
Visiting student, UCLA, Los Angeles (VisionLab)
Post-doc in Padova, Control and Systems Theory group
Young researcher, Image and Sound Processing Group,
Politecnico di Milano
Post-doc, UCLA Vision Lab
3
… the research
research
Computer vision object and body tracking
data association
gesture and action recognition
Discrete mathematics
linear independence on lattices
Belief functions and imprecise probabilities
geometric approach
algebraic analysis
combinatorial analysis
1Upper and lower probabilities
5
Past work
Geometric approach to belief functions (ISIPTA’01, SMC-C-05)
Algebra of families of frames (RSS’00, ISIPTA’01, AMAI’03)
Geometry of Dempster’s rule (FSKD’02, SMC-B-04)
Geometry of upper probabilities (ISIPTA’03, SMC-B-05)
Simplicial complexes of fuzzy sets (IPMU’04)
The theory of belief functions
7
Uncertainty descriptions
A number of theories have been proposed to extend or replace classical probability: possibilities, fuzzy sets, random sets, monotone capacities, etc.
theory of evidence (A. Dempster, G. Shafer)
belief functions Dempster’s rule families of frames
8
Motivations
9
Axioms and superadditivity
probabilities additivity: if then
1)(.2 p0)(.1 p
BA )()()( BpApBAp
belief functions 3. superadditivity
0)(.1 s 1)(.2 s
i ji
nn
jiin AAsAAsAsAAs )...()1(...)()()...( 11
1
10
Example of b.f.
11
belief functions s: 2Θ ->[0,1]
AB
BmAs )( A
Belief functions
B2
B1
1)( B
Bm
..where m is a mass function on 2Θ s.t.
12
Dempster’s rule
b.f. are combined through Dempster’s rule'', ssss
ABBmABel)()(
Ai
Bj
AiBj=A
intersection of focal elements
ji
ji
BAji
ABAji
BmAm
BmAm
Am)()(1
)()(
)(21
21
13
Example of combination
s1: m({a1})=0.7, m({a1 ,a2})=0.3
a1
a2
a3
a4
s2: m()=0.1, m({a2 ,a3 ,a4})=0.9
s1 s2 : m({a1})=0.19, m({a2})=0.73
m({a1 ,a2})=0.08
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Bayes vs Dempster
Belief functions generalize the Bayesian formalism as:
1- discrete probabilities are a special class of belief functions
2 - Bayes’ rule is a special case of Dempster’s rule
3 - a multi-domain representation of the evidence is contemplated
15
My research
Theory of evidence
algebraic analysis geometric
analysis
categorial?
probabilistic analysis
combinatorial analysis
Algebra of frames
17
Family of frames
example: a function y [0,1] is quantized in three different ways
1
.0 .1
.00 .01 .10 .11
.0 .1 .2 .3 .4
0.00 0.09
0.90 0.99
0.49
0.25 0.750.50
refining
Common refinement
18
Lattice structure
minimal refinement
1F
maximal coarsening
F is a locally Birkhoff (semimodular with finite length) lattice bounded
below
order relation: existence of a refining
Geometric approach to upper and lower probabilisties
20
it has the shape of a simplex
),( APClS A
Belief space
the space of all the belief functions on a given frame
each subset A A-th coordinate s(A) in an Euclidean space
21
Geometry of Dempster’s rule
constant mass loci
foci of conditional subspaces
Dempster’s rule can be studied in the geometric setup too
Px
Py
P
F (s)x
F (s)y
y
x
y
x
22
),,( AACl A
the space of plausibilities is also a simplex
*sP
),(~*
yxs ppP
s
yyP
xxP
P
x
y
p
p,1
0,
x
y
p
p
S
Geometry of upper probs
23
PPs s *
Belief and probabilities
s
*sP
P)(sP
1xP 2xP
nxP
Ps
)(sP
}{ AX
study of the geometric interplay of belief and probability
24
}{
)()(
xA A
AmsP
Consistent probabilities
Each belief function is associated with a set of consistent probabilities, forming a simplex in the probabilistic subspace
the vertices of the simplex are the probabilities assigning the mass of each focal element of s to one of its points
the center of mass of P(s) coincides with Smets’ pignistic function
25
Possibilities in a geometric setup
possibility measures are a class of belief functions
they have the geometry of a simplicial complex
Combinatorial analysis
27
Total belief theorem
a-priori constraint
conditional constraint
generalization of the total probability theorem
28
Existence
candidate solution: linear system nn where the columns of A are the focal
elements of stot
bAx
problem: choosing n columns among m s.t. x has positive components
method: replacing columns through
i j
ji eeeee
29
Solution graphs
all the candidate solutions form a graph
Edges = linear transformations
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New goals...
algebraic analysis
combinatorial analysis
Theory of evidence
geometric analysis
?
probabilistic analysis
31
Approximations
compositional criterion the approximation behaves like s when
combined through Dempster
tpCs
C dttstss minarg
problem: finding an approximation of s
probabilistic and fuzzy approximations
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Indipendence and conflict
s1,…, sn are not always combinable
1,…, n are indipendent if
iinn AAA ,)(...)( 11
any s1,…, sn are combinable are defined on independent frames
33
Pseudo Gram-Schmidt
Vector spaces and frames are both semimodular lattices -> admit independence
mn
mn
ssss
FF
',...,',...,
',...,',...,
11
11
pseudo Gram-Schmidt
new set of b.f. surely combinable
34
Canonical decomposition unique decomposition of s into simple b.f.
nees ...1
convex geometry can be used to find it
xx PPs
yy PPs
s
xe
ye
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Tracking of rigid bodies
m-1m
past and present target association
rigid motion constraints can be written as conditional belief functions total belief needed
Am-1
past targets - model
associations
m-1m
Am-1
= Am-1 m-1
m
Am-1 ()
old estimates
rigid motion constraints
Kalman filters
Am
current targets – model association
Am
new estimates
data association of points belonging to a rigid body
36
Total belief problem and combinatorics
relation with positive linear systems
homology of solution graphs
matroidal interpretation
general proof, number of solutions, symmetries of the graph
2Computer vision
38
Vision problems
HMM and size functions for gesture recognition (BMVC’97)
object tracking and pose estimation (MTNS’98,SPIE’99, MTNS’00, PAMI’04)
composition of HMMs (ASILOMAR’02)
data association with shape info (CDC’02, CDC’04, PAMI’05)
volumetric action recognition (ICIP’04,MMSP’04)
Size functions for gesture recognition
40
Size functions for gesture recognition Combination of HMMs (for dynamics) and
size functions (for pose representation)
41
Size functions “Topological” representation of contours
42
Measuring functions
Functions defined on the contour of the shape of interest
real image
measuring function
family of lines
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Feature vectors a family of measuring functions is chosen
… the szfc are computed, and their means form the feature vector
44
Hidden Markov models
Finite-state model of gestures as sequences of a small number of poses
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Four-state HMM
Gesture dynamics -> transition matrix A
Object poses -> state-output matrix C
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EM algorithm
feature matrices: collection of feature vectors along time
EM A,C
learning the model’s parameters through EM
two instances of the same gesture
Compositional behavior of Hidden Markov models
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Composition of HMMs Compositional behavior of HMMS: the
model of the action of interest is embedded in the overall model
Example: “fly” gesture in clutter
49
State clustering Effect of clustering on HMM topology
“Cluttered” model for the two overlapping motions
Reduced model for the “fly” gesture extracted through clustering
50
Kullback-Leibler comparison
We used the K-L distance to measure the similarity between models extracted from clutter and in absence of clutter
Model-free object pose estimation
52
Model-free pose estimation Pose estimation: inferring the configuration of a moving object from one or
more image sequences
Most approaches in the literature are model-based: they assume some knowledge about the nature of the body (articulated, deformable, etc) and some sort of model Qtq k ˆT=0 t=T
53
Model-free scenario Scenario: the configuration of an uknown object
is desired, given no a-priori information about the nature itself of the body
The only info available is carried by the images
We need to build a map from image measurements to body poses
This can be done by using a learning technique, based on training data
54
“Evidential” model
The evidential model is built during the training stage, when the feature-pose maps are learned
1
23
approximate feature spaces
training set of sample poses
feature-pose maps (refinings)
55
32
1
Feature extraction
From the blurred image the region with color similar to the region of interest is selected, and the bounding box is detected.
1
2
3
56
Estimates from the combined model
Ground truth versus estimates...
... for two components of the pose
JPDA with shape information for data association
58
JPDA with shape info
YX
Z
XY
Z
robustness: clutter does not meet shape constraints
occlusions: occluded targets can be estimated
JPDA model: independent targets
Shape model: rigid links
Dempster’s fusion
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Triangle simulation
the clutter affects only the standard JPDA estimates
60
Body tracking
Application: tracking of feature points on a moving human body
Volumetric action recognition
62
Volumetric action recognition
problem: recognizing the action performed by a person viewed by a number of cameras
step 1: modeling the dynamics of the motion
step 2: extracting image features
2D approaches: features are extracted from single views -> viewpoint dependence
volumetric approach: features are extracted from a volumetric reconstruction of the moving body
63
Multiple sequences
synchronized views from different cameras, chromakeying
64
Volumetric intersection
silhouette extraction of the moving object from all views
3D object shape reconstruction through intersection of occlusion cones
more views -> more details
65
3D feature extraction
locations of torso, arms, and legs of the moving person
k-means clustering to
separate bodyparts
66
Feature matrices
two instances of the action “walking”
TORSO COORDINATES
ABDOMEN COORDINATES
RIGHT LEG COORDINATES
LEFT LEG COORDINATES
XY
Z
67
Modeling and recognition
• model of the “walking” action
…
classification: each new feature matrix is fed to all the learnt models, generating a set of likelihoods
HMM 1
HMM 2
HMM n
3Combinatorics
69
Independence on lattices
three distinct independence relations
modularity equivalent formulations
LI3
LI2=LI3
LI1
semimodular lattice modular lattice
LI2
LI1
70
Scheme of the proof
4Conclusions
72
from real to abstract
OBJECT TRACKING DATA ASSOCIATION
MEASUREMENTCONFLICT
POINTWISE ESTIMATE
CONDITIONALCONSTRAINTS
ALGEBRAICANALYSIS
GEOMETRICAPPROACH TOTAL BELIEF
the solution of real problems stimulates new theoretical issues
73
…concluding
the ToE comes from a strong critics to the Bayesian framework
useful for sensor fusion problems under incomplete information
real problem solutions stimulate the extension of the formalism
complex objects mathematically rich Young theory need completion
74
In the near future..
search for a metric on the space of dynamical systems – stochastic models
sistematic description of the geometric approach to non-additive measures
understand the intricate relations between probability and combinatorics