Extending HEVC by an affine motion model

Digital video coding standards apply hybrid coding. Each frame is partitioned into blocks of various sizes. For each block, motion compensated prediction with a translational motion model and prediction error coding is applied. The maximum block size of standards preceding HEVC is 16 × 16 samples, for HEVC it is increased to 64 × 64 samples. For non-translational motion the translational model leads to high data rate due to inaccurate prediction. Although an affine motion model can describe non-translational motion, the data rate of its additional parameters is often larger than the data rate reduction achieved by more accurate prediction, especially if small blocks are used. This paper investigates if HEVC´s increased block size benefits the affine motion model. HEVC is extended such that for each block, either the translational or the affine motion model is used, selected by minimization of Lagrangian costs of data rate and summed absolute values of prediction error coefficients. For a set of ten test sequences of non-translational motion, the data rate is reduced by 6.3% in average and by 23.7% in maximum compared to HEVC at the same PSNR. Limiting the maximum block size to 16 × 16 samples, the average data rate reduction decreases to 0.1%, which proves synergy between the block size increase and the affine motion model.