deformable models segmentation methods until now (no knowledge of shape: thresholding edge based...
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Deformable ModelsDeformable Models
Segmentation methods until now (no Segmentation methods until now (no knowledge of shape:knowledge of shape:
ThresholdingThresholding
Edge basedEdge based
Region based Region based
Deformable modelsDeformable modelsKnowledge of the shape of the objectKnowledge of the shape of the object
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Deformable ModelsDeformable Models
Initial shape (curve or surface) Initial shape (curve or surface)
Move the shape according:Move the shape according:Internal forces (curve/surface properties)Internal forces (curve/surface properties)E.g.: Curvature to keep the object smoothE.g.: Curvature to keep the object smooth
External forces (image properties)External forces (image properties)E.g.: To track the object to the boundaryE.g.: To track the object to the boundary
2D Snakes 2D Snakes Kass, Witkin and Terzopoulos 1987Kass, Witkin and Terzopoulos 1987
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ExampleExample
Animation with a 2D countour Animation with a 2D countour adapting to the edgeadapting to the edge
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Deformable ModelsDeformable Models
Various names for the same:Various names for the same:2D snakes, active contours, and 2D snakes, active contours, and deformable contours ...deformable contours ...
3D ballons, active surfaces, and 3D ballons, active surfaces, and deformable surfaces ...deformable surfaces ...
Two main groupsTwo main groupsParametric deformable models (based Parametric deformable models (based on parametric form of models)on parametric form of models)
Geometric deformable models (curve Geometric deformable models (curve evolution or level sets)evolution or level sets)
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Parametric ContourParametric Contour
CurveCurveExample:Example:
Internal Energy Functional (internal forces)Internal Energy Functional (internal forces)
Potential Energy Functional (external Potential Energy Functional (external forces) forces)
Moves to minimize the energy functional: Moves to minimize the energy functional:
( ) ( ( ), ( )), [0,1]s X s Y s s X
1
2
( ) ( 2 ,sin( 2 ))
( ) (cos( 2 ),sin( 2 ))
s s s
s s s
X
X
0s
0.25s
0.5s
0.75s
1s
( ) ( ) ( ) ...elasticity curvatureS S S X X X
( ) ( ) ...bordersP P X X
( ) ( ) ( )E S P X X X
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Internal Energy FunctionalInternal Energy Functional
Elasticity, avoids stretchingElasticity, avoids stretching
CurvatureCurvature
are often defined as constants are often defined as constants
21
0
( ) ( )elasticityS s dss
X
X
F F
21 2
20
( ) ( )curvatureS s dss
X
XF
( ), ( )s s
( ) weighting param eters
( ) weighting param eters
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Potential Energy FunctionalPotential Energy Functional
Energy based on features of the Energy based on features of the imageimage
Potential field based on borders. We Potential field based on borders. We want small values close to borders, want small values close to borders, e.g.:e.g.:
1
0
( ) ( ( ))P p s dsX X
2( ) ( ) borderp x f x
( )f x
positive weighting parameter
y
fx
f
G
G
y
xf
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Potential Field ExamplePotential Field Example
* ( )G f x
( )f x
Image smoothed by convolving with a Gaussian
222 2/)(
2
1),(
yxeyxG
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Potential fieldPotential field
( ) ( * ( ))p x G f x ( ) * ( )p x G f x
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Dynamic Force FormulationDynamic Force Formulation
Formulation as a dynamic problemFormulation as a dynamic problem
Allows definition of extra external Allows definition of extra external forcesforces
Based on Newton’s second law Based on Newton’s second law : :
( , )s tX2
2( ) ( ) ( )damp int extt
X
F X F X F X
0the mass unit, often then equal to previous sec tion
damp t
X
F damping (viscosity) coefficient 2
2inertial r e fo c
t
X
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External ForcesExternal Forces
Combination of different forcesCombination of different forces
Pressure Force (ballon)Pressure Force (ballon)1 2( ) ( ) ( ) ... ( )ext N F X F X F X F X
( ) ( ) ( )where
is the inward
Normal t op pF X N X N X
X0p
0p
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Problems with CavitiesProblems with Cavities
Gradient Vector FlowGradient Vector FlowDiffuse the vector field according to Diffuse the vector field according to strength of the edgesstrength of the edges
Dynamic Distance ForceDynamic Distance ForceUse of signed distancesUse of signed distances
Interactive VariationsInteractive VariationsUser interaction modeledUser interaction modeledas force termsas force terms
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ImplementationImplementation
We work with in a discrete form:We work with in a discrete form:
The derivatives need to be The derivatives need to be approximated by, for example, finite approximated by, for example, finite differencesdifferences
( ( ), ( )), {1,... }X i Y i i n X
X
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Problems of Active ContoursProblems of Active Contours
Initial contour needs to be close to final Initial contour needs to be close to final resultresult
Reparameterization is needed (3D is difficult)Reparameterization is needed (3D is difficult)
Topological adaptation (3D is Topological adaptation (3D is complicated)complicated)
Forces definition, parameter settingForces definition, parameter setting
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