Coding of arbitrarily shaped objects with binary and greyscale alpha-maps: what can MPEG-4 do for you?

MPEG-4 Visual, that part of the upcoming MPEG-4 standard describing the coding of natural and synthetic video signals, allows the encoding of video objects using motion, texture and shape information. In this paper, the shape coding algorithms and related texture coding algorithms are described. Whereas binary shapes are coded using a context-based arithmetic encoder with motion compensation, the coding of greyscale alpha maps requires coding of the values of the alpha maps inside of the object in addition to the binary shape information. In order to allow for an efficient coding of the texture at the boundary of arbitrarily shaped objects, padding techniques are used in the encoder and decoder.