1. A method of measuring the translation and scale change in a sequence of video images, the method comprising: storing a first image in said sequence to form a stored image; forming a prediction of the translation and scale change from said stored image to a further image in said sequence; transforming said stored image by said prediction of the translation and scale change to form a transformed first image; comparing a further image in said sequence with said transformed first image; deriving measurements of translation and scale change between said stored image and said further image from said comparison and said prediction; and replacing said first image with a new incoming image when the image area common to both said first image and incoming image falls below a given proportion of the whole image area of said incoming image.