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Lecture 6 : Level Set Method
Introduction
• Developed by– Stanley Osher (UCLA)– J. A. Sethian (UC Berkeley)
• Books– J.A. Sethian: Level Set Methods and Fast
Marching Methods, 1999– S. Osher, R. Fedkiw, Level Set Methods and
Dynamic Implicit Surfaces , 2002
Evolving Curves and Surfaces
Geometry Representation
Explicit Techniques for Evolution
Explicit Techniques - Drawbacks
Implicit Geometries
Discretized Implicit Geometries
Level Set Method: Overview
• Generic numerical method for evolving fronts in an implicit form– Handles topological changes of the evolving interface– Define problem in 1 higher dimension
• Use an implicit representation of the contour C as the zero level set of higher dimensional function - the level set function
Level Set Method: Overview
• Move the level set function, so that it deforms in the way the user expects
• contour = cross section at z=t
Implicit Curve Evolution
Level Set Evolution
• Define a speed function F, that specifies how contour points move in time– Based on application-specific physics such as time,
position, normal, curvature, image gradient magnitude
• Build an initial level set curve
• Adjust over time
• Current contour is defined as
Equation for Level Set Evolution
• Indirectly move C by manipulating
where F is the speed function normal to the curve
Level set equation
Example: an expanding circle
• Level Set representation of a circle
– Setting F=1 causes the circle to expand uniformly
– Observe everywhere– We obtain
• Explicit solution: – meaning the circle has radius r+t at time t
Example: an expanding circle
Motion under curvature
• Complicated shapes?
– Each piece of the curve moves perpendicular to the curve with speed proportional to the curvature
– Since curvature can be either positive or negative , some parts of the curve move outwards while others move inwards
– Example movie file• Setting F = curvature
Level Set Segmentation
• We may think of as signed distance function– Negative inside the curve– Positive outside the curve– Distance function has unit gradient almost
everywhere and smooth
• By choosing a suitable speed function F, we may segment an object in an image
Level Set Segmentation
• Evolving Geometry : (X,t)=0
– Intuitively, move a lot on low intensity gradient area and move little on high intensity gradient area along normal direction
– F : speed function , k : curvature , I : intensity
Segmentation Example
• Arterial tree segmentation
Discretization
• Use upwinded finite difference approximations (first order)
Acceleration Techniques
• Acceleration for fast level set method– Narrow band level set method– Fast marching method
Narrow band level set method
• The efficiency comes from updating the speed function
• We do not need to update the function over the whole image or volume
• Update over a narrow band (2D or 3D)
Fast Marching Method
• Assume the front (level set) propagates always outward or always inward
• Compute T(x,y)=time at which the contour crosses grid point (x,y)
• At any height T, the surface gives the set of points reached at time T
Fast Marching Algorithm
Fast Marching Algorithm
Fast Marching Method
Applications
• Segmentation
• Level Set Surface Editing Operators
• Surface Reconstruction
Segmetation
• 2D
• 3D
Level Set Surface Editing Operators
• SIGGRAPH 2002
Level Set Surface Editing Operators
Surface Reconstruction
• zhao, osher, and fedkiw 2001
A painting interface for interactivesurface deformations