计算机视觉课件_LC07-CamLecture5.ppt

Computer Vision Semester BLecture 7Shape Representation  Shape Representation - what for? Chain Codes Encode start point and relative direction around the region boundary Chain Codes Encode start point and relative direction around the region boundary Differential Chain Codes Encode start point and the change in relative direction around the region boundary Differential Chain Codes Encode start point and the change in relative direction around the region boundary Chain Codes Chains and Rotation What happens if the object is rotated? (Suppose for simplicity that rotations are in increments of 90o) Chain: 0700776545543322212 Rotate 90o cw: 0007065665543233211 Differential: 0710707771077070071 Rotate 90o cw: 0007167107077710770 How can we eliminate the differences when rotating differential chain codes? Shape Numbers View the differential chain code as an n-digit number Normalize the number by rotating the code to get the smallest value Chain: 0700776545543322212 Differential: 6710707771077070071 (rotation adjusted) Shape number: 0071071670777107707 Shape Numbers Steps for Generating Shape Numbers Chain Codes: Smoothing and Resampling Problem: Pixel grid and noise cause change in code content and length Usual Solution: Smooth the shape/code Resample to some fixed number of points (code length) Tangential Representations: ψ - s curves Encodes the tangent angle as a function of arc length Similar to differential chain codes, but doesn’t have to be pixel-to-pixel Radial Representations: r - s curves Encodes the distance from the “center” as a function of arc length Curvature Representations: k - s curves Encodes the curvature k (s) as a function of arc length Differentiates the tangent vector (not quite the same as differentiating the ψ - s curve, but close) Bending Energy Idea: How much work do you have to do to bend a straight line into the shape? Calculation: Signatures In general, a signature is a 1-dimensional function describing a shape