1. A decoder for an image compression system, the decoder being configured to receive a sequence of frames encoded by a coder for decoding by the decoder, wherein the sequence of frames comprises a sequence of referenceable frames, wherein at least one picture region of at least one frame is based on weighted interpolations of two or more other frames, such weighted interpolations being based on at least one set of unequal weights, wherein at least the one set of unequal weights are derived based on a position of one or more of the referenceable frames in the sequence of frames.