An image comprises columns and rows of blocks of pixels. In each row and/or column, a series of pairs of blocks of pixels comprise first and second adjacent blocks of pixels, with the second block in a previous pair of blocks corresponding to the first block in a next pair of blocks. For at least a part of the pairs of blocks in at least a part of the rows and/or columns of blocks in the image, a method determines an indication of a block boundary between the first and second blocks. The method decides based on the determined indication whether the block boundary is a visible or invisible block boundary. These steps are then repeated for a next pair. If at least one invisible block boundary is present between two visible block boundaries, it is decided that a uniform image distortion zone has been detected.