Coding of arbitrarily-shaped image blocks based on a constrained quantization

Coding of arbitrarily-shaped image/video segments is important to achieving the object-based coding - which is a key technique in many multimedia applications. MPEG-4 suggested a low-pass-extrapolation (LPE) padding method to handle each boundary block of an arbitrary shape. In this paper, we develop a constrained quantization technique and apply it to each LPE-padded boundary block. This "smart" quantization is performed in such a way that the pixel-domain MSE distortion due to the DCT domain quantization is shaped as much as possible onto all padded pixels - as these pixels will be thrown away in the end. In the meantime, all pixels in each original boundary block would be coded with an improved quality. Compared to the normal rounding-based quantization used in the original MPEG-4's LPE scheme, our new quantization needs some extra computations at the quantization stage in the encoding procedure but keeps the decoding complexity unchanged, while offering a remarkable coding gain - as demonstrated by some simulation results. © 2008 IEEE