computer and robot vision ii
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Computer and Robot Vision II. Chapter 17 The Consistent Labeling Problem. Presented by: 傅楸善 & 陳相廷 0935 681 486 [email protected] 指導教授 : 傅楸善 博士. 17.1 Introduction. - PowerPoint PPT PresentationTRANSCRIPT
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Computer and Robot Vision II
Chapter 17The Consistent Labeling
ProblemPresented by: 傅楸善 & 陳相廷
0935 681 [email protected]指導教授 : 傅楸善 博士
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17.1 Introduction In this class of problems we are given a set of
objects and a set of possible names or labels for those objects.
We are also given a set of constraints that limits the possible labels for each object.
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17.1 Introduction
Unary constraints: Certain features of an object limit the allowable
labels for that object.
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17.1 Introduction
Binary constraints: A particular label for one object limits the possible
set of labels for a related object. e.g. N-Queens problem
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17.1 Introduction
N-ary constrains: The sets of labels allowed for sets of N mutully co
nstrained objects is limited.
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17.1 Introduction N-ary consistent-labeling problem (CLP):
4-tuple ),,,(CLP RTLU
pairs label-unit of set over therelation ary - :relation constraint label-unit :
units of set over therelation ary - :relation constraint-unit :
labels possible ofset :units ofset :,...,1
LUNRR
UNTTL
MMU
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17.1 Introduction
. toˆ from ˆ: mapping a is of ,...,,ˆsubset of labeling
.set unit of subsets -size ordered of labeling allowable : ofelement
.,...,, labels ingcorrespond assigned ,...,, units :),(),...,,(),,(
another. oneconstrain mutually ,...,, units :),...,,(
21
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LULUfUuuuU
UNR
llluuuRlululu
uuuTuuu
N
NNN
NN
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17.1 Introduction
labelings.-consistent all moreor one find to :problem labeling consistent of Goal
.in is ))(,()),...,(,()),(,(then ,in is ),...,,( tuple- theand ˆin are ,...,,whenever
if consistent is units theof ˆsubset a of labelingA
2211
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RufuufuufuTuuuNUuuu
Uf
NN
NN
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17.2 Examples of Consistent-Labeling Problems
consistent-labeling problems arise in: computer vision artificial intelligence science engineering
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17.2.1 The N-Queens Problem N-queens problem:
given chessboard and N queens. queens placed on chessboard:
no queen captures any other queen. no two queens in same row, same column, or same diag
onal of chessboard. N-queens problem:
modeled as consistent-labeling.
NN
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17.2.1 The N-Queens Problem unit set
set of rows on chessboard label set
set of columns on chessboard exactly one queen per row:
labeling specifies the column where queen placed unit-constraint relation
set unit-label constraint relation:
:,..,3,2,1 NU
:,..,3,2,1 NL
:T jijiji uuUuuuuT and,|),(
||||,,,,),(|)],(),,[( jijijijijijjii lluuLllllTuululuR
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17.2.1 The N-Queens Problem
e.g. pair [(1,1), (2,4)] is in R since two queens do not capture each other. e.g. pair [(1,1), (2,2)] and [(1,1), (3,3)] are not in R
consistent labeling: solve N-queens problem with constraints satisfied
1
2
1 2
34
3 4
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17.2.2 The Latin-Square Puzzle The problem is to arrange the objects such that each
row, each column, and each of the two main diagonals of the matrix contains exactly one object of each color and exactly of each shape.
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17.2.2 The Latin-Square Puzzle
Latin-square puzzle: n x n matrix, n2 objects arranged on matrix, one
per square e.g. consider 4 x 4 puzzle for ease of
illustration object is one of four colors
C = { red, blue, green, yellow } object has one of four shapes
S = { circle, square, triangle, octagon }
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17.2.2 The Latin-Square Puzzle set of units U = { 1, 2,…, 16 }: 16 squares of matrix labels L: objects to be placed on squares, e.g. red
square, blue triangle, … Cartesian product set L = C x S model T as quaternary constraint:
}diagonalor column, row, same in the lie all ,,,|),,,({ 43214321 uuuuuuuuT
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17.2.2 The Latin-Square Puzzle The unit-label constraint relation R would then
consist of quadruples of unit-label pairs of the form
. and
, , whereand ,4,...,1for ),( ,in is ) ,,,( where
)]},,(,[)],,(,[)],,(,[)],,(,[{
4321
444333222111
ji
ji
ii
ss
ccjiiLscTuuuu
scuscuscuscu
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17.2.3 The Edge-Orientation Problem
If a local edge operator has been applied to an image and has determined, for each pixel, the strength of an edge passing through it in each of eight possible directions.
The output of the edge operator is noisy due to image noise.
Most meaningful edges in the real world are highly continuous with low curvature.
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17.2.3 The Edge-Orientation Problem
maximum bending angle of any small edge segment: limited to some maximum e.g.
:upward) (direction toiprelationshnortheast in the is ' xx
:upward) (direction toiprelationshnorthwest in the is ' xx
horizon from 90 ,45between :' x
horizon from 135 ,90between : ' x
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Return true if l, l’ compatible for x, x’, false otherwise.
U: set of pixels of the image. L: set of possible edge orientations, including special
value none. E(x): set of possible edge orientations of pixel x,
based on the results of the local edge operator. Nbd(x): set of neighboring pixels to pixel x.
17.2.3 The Edge-Orientation Problem
:),,,( predicate '' lxlx
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When do pairs of neighboring pixels have compatible labels? At least one label is none. Predicate is satisfied.
17.2.3 The Edge-Orientation Problem
edge orientation of given pixels: constrained only by neighborhood edge
)',',,( lxlx
)}('|)',{( xNbdxxxT
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17.2.3 The Edge-Orientation Problem
unit-label constraint relation:
)}',',,()'or (
and ),'(' ),(,)',( | )]','(),,{[(lxlxnonelnonel
xElxElTxxlxlxR
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Jooooooooooke!
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17.2.4 The Subgraph-Isomorphism Problem
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17.2.4 The Subgraph-Isomorphism Problem
vertices V: set of vertices edges E: nonreflexive, symmetric binary
relation over V graph G: pair (V, E) It is often necessary to determine whether tw
o graphs representing two different entities are identical, except for the labels of the vertices, indicating that the two objects have the same structure.
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17.2.4 The Subgraph-Isomorphism Problem
G = (V, E) is isomorphic to G’=(V’, E’): if there is one-to-one, onto mapping f from V to V’,
satisfying that f: graph isomorphism
')](),([),( EvfufEvu
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17.2.4 The Subgraph-Isomorphism Problem
unit-set U: set of vertices V of G label-set L: set of vertices V’ of G’ unit-constraint relation T: edge set E of G unit-label constraint relation R:
}'' and,')','(,),(|)]',(),',{[(
vuvuEvuEvuvvuuR
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17.2.4 The Subgraph-Isomorphism Problem
Graph-isomorphism problem is a dual consistent-labeling problem f: VV’ and its inverse must be consistent labelin
gs
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17.2.5 The Relational-Homomorphism Problem
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17.2.5 The Relational-Homomorphism Problem
Subgraph-isomorphism is a special case of relational-homomorphism problem.
Relational-homomorphism is defined on N-ary relations, instead of just binary.
Homomorphism is also a structure-preserving mapping, but not necessarily to be one-to-one.
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17.2.5 The Relational-Homomorphism Problem
},...,1,)( with ),...,(|),...,{(:: with ofn compositio
set into set of elements mappingfunction ::set over relation ary :
sets two:,
11 NibafTaaBbbfTfTfT
BABAfANAT
BA
iiNN
N
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17.2.5 The Relational-Homomorphism Problem
Relational homomorphism from T to S:
mapping f: AB satisfying Relational homomorphism applied to N-tuple of T:
result is N-tuple of S
SfT
relationary second : NBS N
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17.2.5 The Relational-Homomorphism Problem
Finding relational homomorphism: sometimes called relational matching
Relational homomorphism: maps elements of A to B with same relationships
Relational monomorphism: is a relational homomorphism that is one-to-one. A monomorphism indicates a stronger match than
homomorphism.
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17.2.5 The Relational-Homomorphism Problem
Relational isomorphism f from N-ary relation T to N-ary relation S: is an one-to-one relational homomorphism from T to S. f-1 is a relational homomorphism from S to T.
Relational isomophism: A, B have same number of elements each primitive in A maps to unique primitive in B each primitive in A mapped to by a primitive of B each tuple in T has corresponding one in S, vice versa A strongest kind of match: symmetric match
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17.2.5 The Relational-Homomorphism Problem
Graph isomorphism is a binary-relational isomorphism. Relational-homomorphism fits consistent-labeling model in
much the same way that the graph-isomorphism problem did. A: set of units B: set of labels Unit-constraint relation: relation T of the relational homomorphism
problem. Unit-label constraint:
consistent labeling solution: relational homomorphism from A to B
}),...,( and ),...,(|)],(),...,,{[( 1111 SllTuululuR NNNN
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Joooooooke!
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17.3 Search Procedures for Consistent Labeling
Given a consistent-labeling problem CLP = (U,L,T,R), then find the set of all consistent labelings.
If there is no consistent labeling, then returns the empty set.
Method: Backtracking tree search Backtracking tree search with speedup:
Forward checking Discrete relaxation
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17.3.1 The Backtracking Tree Search
backtracking tree search: begin with first unit of U select second unit of U, begins to construct
children of first node process continues to level |U| of the tree
path from root to any successful nodes at level |U|: consistent labeling
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Simple digraph-matching problem][outdegree)]([outdegree],[indegree)]([indegree uufuuf
in out
A 3 2
B 0 2
C 2 2
D 2 2
E 2 3
F 3 1
in out
1 0 2
2 2 1
3 2 1
4 2 2
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portion of the tree search for solving the graph-matching problem
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17.3.2 Backtracking with Forward Checking
backtracking tree search: has exponential time complexity
forward checking: once a unit-label pair (u,l) is instantiated at a node in the tree, the constraints imposed by the relations cause instantiation of some future unit-label pairs (u’,l’) to become impossible
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if 1=A then 2 can either be D or F,…
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17.3.2 Backtracking with Forward Checking
FTAB: future-error table FTAB(u’,l’)=1: still possible to instantiate (u’,l’) FTAB(u’,l’)=0: (u’,l’) already been ruled out FTAB(u’,l’)=X: (u’,l’) impossible from previous leve
l of recursion
one future-error table for each level of recursion in tree search
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17.3.3 Backtracking with Discrete Relaxation
forward-checking algorithm: prunes search tree of nodes ruled out
discrete relaxation: iterative polynomial complexity procedure greatly reduces search for tightly-constrained
problems constrains search further for not tightly
constrained tree
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17.3.4 Ordering the Units
Better to choose the unit that has the fewest labels left as the next unit.
We can modify the algorithms to keep track of how many labels remain for each unit and which unit has the least number of labels left.
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17.3.5 Complexity
Consistent-labeling problem: NP-complete problem
Forward checking and look-ahead: Drastically reduce number of nodes searched. But do not change overall complexity.
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17.3.6 The Inexact Consistent-Labeling Problem
Extracted line from images: some missing, partially missing, extra, distorted
Inexact consistent-labeling problem: allows some error for real-life problems
recursion of level previous from impossible )','( :)','(
1 of edges 6for ddistributeequally errors :61)','(
penalty no error thus no :0)','(
luXluerror
Gluerror
luerror
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17.3.6 The Inexact Consistent-Labeling Problem
tree search: initially called with past_error=0 past_error: never allowed to exceed error thr
eshold ε
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part of the inexact forward checking procedure for digraph matching
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17.4 Continuous Relaxation
discrete algorithms: calling a label either possible or impossible
continuous procedure: associate probability or certainty that u assigned l
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17.5 Vision Applications- Line Labeling with Discrete Relaxation
Assumption: simple blocks world scenes of planar polyhedra no shadows or cracks and trihedral vertices polygonal planar surfaces
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Line Labeling with Discrete Relaxation
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Line Labeling with Discrete Relaxation
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Line Labeling with Discrete Relaxation
+: convex interior line segments -: concave interior line segments >, <: boundary line segments with visible
surface on right
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Line Labeling with Discrete Relaxation
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Line Labeling with Discrete Relaxation
each face is a plane, not a curved surface
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Zollner Illusion: diagonals are parallelPoggendorf Illusion: two diagonal segments collinearHelmholtz’s Squares: two squares appear rectangularMuller-Lyer Illusion: both lines have the same lengthHering Illusion: two horizontal and parallel straight lines appear bowedWundt Illusion: two horizontal and parallel straight lines appear bowed
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mermaid’s tail fin is Poseidon’s moustache
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Young Girl/Old Woman: young girl’s chin is old woman’s noseVase/Faces: black region is background or foregroundPop In/Pop Out: spontaneous reversal of perceived concavities and convexitiesSix Cubes/Seven Cubes: reversal of perceived concavities and convexities
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Belvedere: seven geometrical inconsistencies not readily apparent
1. cross front to rear, and vice versa
2. ladder’s base inside the building, but its top outside
4. examined by person on bench: geometrically impossible
3. Topmost level at right angles to middle level
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Penrose Triangle: if it’s polyhedron, it could not form closed loopPenrose staircase: decends (ascends) all the way around to starting stepEscher Cube: top inconsistent with its baseTwo-pronged trident: three prong terminals with two prong base
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consistent line labels necessary but not sufficient for physical realizability
(a) line drawing with no consistent labeling
(b) consistent labeling of line drawing of impossible polyhedron
(c) physically realizable line drawing with unrealizable consistent labeling
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interpretation changes when image turned upside down
Implicit assumption: scene is lit from above
right-side up: two lava cones with cratersupside down: craters with mounds
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DC & CV Lab.DC & CV Lab.CSIE NTU
Line Labeling with Discrete Relaxation
set of units U: set of line segments of the line drawing
set of labels L: label set {+, -, >, <} only lines that meet at a junction constrain one anot
her T={(ui,uj)|ui and uj meet at a junction}
unit-label constraint relation R: defined according to legal labeling
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DC & CV Lab.DC & CV Lab.CSIE NTU
Inexact Matching in 2D Shape Recognition
2D shape matching: recognition task in many machine vision applications
2D shape primitives: simple, near-convex pieces and intrusions into boundary
primitive: allowed to overlap
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DC & CV Lab.DC & CV Lab.CSIE NTU
Inexact Matching in 2D Shape Recognition
intrusion relation Ri: (simple part 1, intrusion, simple part 2)
Ri: two simple parts touch or nearly touch and form boundary of intrusion
protrusion relation Rp: (intrusion 1, simple part, intrusion 2) Rp: simple part protrudes between two intrusion
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DC & CV Lab.DC & CV Lab.CSIE NTU
17.5.1 Image Matching Using Continuous Relaxation
initial strong matches: provide context for matching less well-defined
objects islands of confidence
features: region size, intensity, location, texture, shape
measures relationships among primitives:
adjacency, relative position, distance
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DC & CV Lab.DC & CV Lab.CSIE NTU
17.6 Characterizing Binary-Relation Homomorphism
S HR bbHbaHba
BHHbaB
AHSRH
BAHBBSAAR
3.)'imply )',( and ),((
on valued-single is 2.)),(such that ,bA,a(
on everywhere defined is 1.iff into of smhomomorphi a is
,,
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DC & CV Lab.DC & CV Lab.CSIE NTU
17.6.1 The Winnowing Process)()'( implies )',(,),( bSaHRaaHba
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DC & CV Lab.DC & CV Lab.CSIE NTU
17.6.1 The Winnowing Process)()'( implies ,),(,),'( 1 bSaHHbaRaa
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DC & CV Lab.DC & CV Lab.CSIE NTU
17.6.1 The Winnowing Process
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DC & CV Lab.DC & CV Lab.CSIE NTU
T: check if there is outgoing or incoming arrows
domain rangeR {1, 3, 4} {2, 3, 4, 5}S {a, b, c, d} {b, c, d, e}
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DC & CV Lab.DC & CV Lab.CSIE NTU
from table at left, derive T2
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DC & CV Lab.DC & CV Lab.CSIE NTU
repeat above process using T2
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a
aa
aa
a
T
TT
TdcT
aaT
13
12
12
12
11
11
generate toprocess aboverepeat
generate of left table from
of left table generate thus,or beonly can 4,3:
else any thing benot can thus1 from choose:
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DC & CV Lab.DC & CV Lab.CSIE NTU
17.6.2 Binary-Relation Homomorphism Characterization
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DC & CV Lab.DC & CV Lab.CSIE NTU
17.6.3 Depth-First Search for Binary-Relation Homomorphism
similarly derived others above; as 13
1 aa TT
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tree determined by depth-first search: range {a, b, c, d, e} allowed to repeat
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DC & CV Lab.DC & CV Lab.CSIE NTU
IPPR Conference on Computer Vision, Graphics, and Image Processing
Title
Abstract
1. Introduction: what is dominant point? Why important 2.
Background: previous research, drawback, advantage Motivation: why your approach? Contribution: what’s new, original, or novel?
3. Approach, theory, method, steps 4. Experiments: setup, assumption, equipment 5.
Results: pictures with dominant points, working prototype on real-world images Performance measure: time, space, probability of detection/misdetection, false
alarm comparison with other work 6. Discussion, Conclusion, Future Work 7. Reference