1. A method of identifying a displacement of a segment present in a first image and a second image, the displacement representing a relative change in position of the segment between the first and second images, the method comprising: deriving a first image frame by copying the first image and setting all pixel values to zero for pixels that do not fall within the segment; deriving an inverse image frame, wherein a pixel value in the inverse image frame is an inverse of a corresponding pixel value in the second image, with corresponding pixel values being for pixels located in corresponding positions; and determining the value of u that yields a minimum for the absolute value of the quantity j(u)−A, 9 j &af; ( u ) = &Sum; r &it; &it; f &af; ( r - u ) &it; h &af; ( r ) = A, r is a variable that ranges over the second image, f is a function that returns a pixel value at the pixel location of the first image frame, the pixel location being specified by the function argument, h is a function that represents the inverse image frame and returns a value that is the inverse of the value that a function g returns, g is a function that returns a pixel value at the pixel location of the second image specified by the function argument, u is a motion vector representing the correct displacement of the segment within g for a match, and A equals the area of the segment represented as a number of pixels.