Symmetry-based recognition of vehicle rears

Abstract

Symmetry is an effective feature for locating vehicle rears. Three symmetry criteria are presented: contour symmetry, gray level symmetry and horizontal line symmetry. The application of these criteria to an image yields three histograms which are analyzed to find the vehicle centerline. The mean error in the vehicle centerline location is generally less than five percent. The contours of the vehicle are followed outward from the centerline to find the vehicle width. The calculated width averages 87 percent of the true width. A nonlinear filter for edge extraction and an automatic thresholding scheme are also presented. The complete algorithm requires approximately 20 seconds on a 25 MHz 80386 based computer for a 512 by 512 image.