Object-based texture coding of moving video in MPEG-4

This paper describes some of the most promising segment-based coding techniques which have been investigated in the course of the MPEG-4 standardization process. Padding methods aim at extending arbitrarily shaped image segments to a regular block grid such that common hybrid block-based coding techniques can be applied. A simple and efficient padding technique employing low-pass extrapolation is outlined which yields a signal extension with high energy concentration in the low-frequency area. Simulations indicate that this method is well suited for block-based video coding, and clearly outperforms other low-complexity extrapolation methods with respect to coding efficiency. In contrast to padding techniques, shape-adaptive methods take advantage of the shape information available at the decoder side. A well-known representative of this class is the SA-DCT. However, having been primarily designed for intraframe coding, it is shown that the transform is suboptimal when applied to interframe coding. Using a suitable covariance model, it is demonstrated that a rescaled, orthonormalized transform much closer approximates the optimal shape-adaptive eigentransform of motion-compensated frame difference images. Rate distortion curves verify that orthonormalization improves coding efficiency in interframe coding by up to 2 dB while not adding to complexity. In a comparison, it is finally shown that extrapolation and SA-DCT perform very closely in the case of low data rates, while there is a clear advantage for the shape-adaptive transform in the case of high-quality video coding.