1. A method for processing graphical data representing a sequence of images for compression, including: analysing a plurality of adjacent areas of an image from the sequence to determine: (i) a first set of areas, each of which includes changes when compared with a corresponding area in a previous image from the sequence; and (ii) a second set of areas, each of which does not include a change when compared with a corresponding area in the previous image; for each area of the second set, replacing a section of the area with a block chosen to reduce the size of the graphical data once compressed, while leaving the original image adjacent any part of the perimeter of the area that adjoins an area of the first set; and providing a dataset allowing identification of the areas of the second set.