Advances in hybrid video coding Hybrid video coding is known for a long time and is applied in all video coding standards such as MPEGx or H.26x. This paper shows that there is still enough potential for further coding efficiency improvements. The paper starts with an overview of stateoftheart hybrid video coding schemes such as H.264AVC. Thereafter, our advances on main building blocks of H.264AVC are presented that significantly improve the coding efficiency. For instance, intra prediction is improved by changing scan directions and thus providing better reference pixels for specific prediction directions. Adaptive filtering and high precision motion compensation improves the motion compensated prediction. Furthermore, the combination of transformation, quantization, and entropy coding of the prediction error is improved using an advanced frequency selective coding technique. With the transmission of postfilter hints it is possible to design a Wiener postfilter that significantly enhances the picture quality. Finally, texture synthesis techniques are used to improve the subjective quality for specific textures with homogenous statistical characteristics. This paper presents our abovementioned techniques in detail. Depending on the input sequence and the bit rate, the objective andor subjective gains compared to H.264AVC are quite significant.

Adaptive block transforms for hybrid video coding Todays standard video coders employ the hybrid coding scheme on a macroblock basis. In these coders blocks of 16xc3x9716, and 8 xc3x97 8 pixel are used for motion compensation of noninterlaced video. The Discrete Cosine Transform (DCT) is then applied to the prediction error on blocks of size 8xc3x978. The emerging coding standard H.26L employs a set of seven different block sizes for motion compensation. The size of these blocks varies from 4 x 4 to 16 x 16. The block sizes smaller than 8x8 imply that the 8x8 DCT cannot be used for transform coding of the prediciton error. In the current test model an integer approximation of the 4xc3x974 DCT matrix is employed. In this paper the concept of Adaptive Block Transforms is proposed. In this scheme, the transform block size is adapted to the block sizes used for motion compensation. The transform exploits the maximum possible signal length for transform coding without exceeding the compensated block boundaries. The proposed scheme is integrated into the H.26L test model. New integer approximations of the 8xc3x978 and the 16 x 16 DCT matrices are introduced. Like the TML 4xc3x974 transform the coefficient values of these matrices are restricted to a limited range. The results presented here are based on an entropy estimation. They reveal an increased ratedistortion performance of approximately 1.1 dB for high rates on the employed test sequences.