1. A method of compressing an image block comprising multiple image elements, said method comprising the steps of: a computer-implemented color quantizer determining a color codeword that is a representation of the colors of said multiple image elements; a computer-implemented alpha quantizer determining an alpha codeword that is a representation of the alpha values of said multiple image elements; providing an alpha modifying codeword that is a representation of a set of multiple alpha modifiers for modifying an alpha value generated based on said alpha codeword; and a computer-implemented index selector selecting, for each image element in said image block, an alpha modifier index associated with an alpha modifier from said alpha modifier set.