Runtime-optimised intra-4×4 mode-decision for H.264/AVC video encoding

We describe a method that considerably improves the computational behaviour of H.264 intra-only encoders. Such intra-only encoders come to use in video-cutting and low-latency video coding where temporal prediction via using inter-frames is no feasible option. We identify the spatial prediction step as the computational bottleneck in intra-only encoders. In this step, the encoder tests various modes that represent predictions of the current macroblock´s or sub-macroblock´s texture from spatial neighbouring pixels in order to find the mode of lowest residuum. Unfortunately, testing the complete set of allowed modes is computational expensive. However, as is demonstrated by an analysis provided in this paper, it is reasonable to assume that a large percentage of blocks preserve their prediction modes over time. Based on this assumption we develop two algorithms that improve the computation time in the prediction step. These algorithms differ by their criteria used to decide whether a block´s coding mode can be propagated from a temporal preceding frame. Computational speed is enhanced, since we test the full set of modes only for blocks that fail these criteria. Experimental results show that our methods considerably improve the execution time of an intra-only encoder and only show small impact on data-rate and image quality.