Rate-scalable object-based wavelet codec with implicit shape coding

In this paper, we present an embedded approach for coding image regions with arbitrary shapes. Our scheme takes a different approach by separating the objects in the transform domain instead of the image domain so that only one transform for the entire image is required. We define a new shape-adaptive embedded zerotree wavelet coding (SA-EZW) technique for encoding the coefficients corresponding to specific objects in gray-scale and color-image segments by implicitly representing their shapes, thereby forgoing the need for separately coding the region boundary. At our decoder, the shape information can be recovered without separate and explicit shape coding. The implicit shape coding enables the bit stream for the object to be fully rate scalable, since no explicit bit allocation is needed for the object shape. This makes it particularly suitable when content-based functionalities are desired in situations where the user bit rate is constrained and enables precise bit-rate control while avoiding the problem of contour coding. We show that our algorithm sufficiently addresses the issue of content-based scalability and improved coding efficiency when compared with the "chroma keying" technique, an implicit shape-coding technique which is adopted by the current MPEG-4 standard.