Automatic process for detecting changes between two images

Abstract

In a method for detecting changes in a scene by detecting the differences between first and second images of the scene taken at different times the first and second images are registered and a distance value d representing the average displacement, in pixels, between the two images after registration is determined. A pixel in the first image (x.sub.0, y.sub.0) and a corresponding pixel (x.sub.1, y.sub.1) at the same location in the second image (x.sub.0 =x.sub.1 and y.sub.0 =y.sub.1) are defined. For each pixel (x.sub.0, y.sub.0) in the first image, a search through each of the pixels in a neighborhood of the second image, (x.sub.1 +.DELTA.x, y.sub.1 +.DELTA.y), where .DELTA.x=0, .+-.1, . . . , .+-.d and .DELTA.y=0, .+-.1, . . . , .+-.d is conducted to find the pixel that is closest in value to the value of pixel (x.sub.0, y.sub.0 ) by taking the absolute value of the difference between the value of the pixel at (x.sub.0, y.sub.0 ) and the value of each of the pixels, (x.sub.1 +.DELTA.x, y.sub.1 +.DELTA.y), from the second image and taking the minimum of these differences to be a first difference pixel value. For each pixel, (x.sub.1, y.sub.1) in the second image, a search through each of the pixels in the first image, (x.sub.0 +.DELTA.x, y.sub.0 +.DELTA.y), where .DELTA.x=0, .+-.1, . . . , .+-.d and .DELTA.y=0, .+-.1, . . . , .+-.d, is conducted to find the pixel that is closest in value to the pixel at (x.sub.1, y.sub.1) by taking the absolute value of the difference between the value of the pixel at (x.sub.1, y.sub.1) and the value of each of the pixels, (x.sub.0 +.DELTA.x, y.sub.0 +.DELTA.y), from the first image and taking the minimum of these differences to be a second difference pixel value. For each pixel location, the maximum of the first and second difference pixel values is taken to obtain a difference image representing the changes in the scene.