I/P/B frame type decision by collinearity of displacements

State-of-art encoders in real life applications typically select the number of B frames to be coded between each I or P frame to be equal to one or two by default. The few research works that have addressed the problem of encoder optimization by frame type decision rely on measures of motion magnitude and amount to select the number of B frames. In contrast to such solutions, in this paper we advocate the idea of motion similarity in terms of speed and direction as the basis for deciding how many B frames to encode between any two stored frames. Such similarity is evaluated by the collinearity of the displacements in successive frames. Experimental results using the proposed decision method integrated in our H.264 compliant codec show that the bit rates achieved in the compressed bitstreams are optimal and near-optimal for the tested sequences, and are lower by up to 26% than those obtained using the default number of B frames of typical encoders. Furthermore, such bit rate savings are obtained with no subjective loss in the visual quality of the decoded sequences. Last but not least, our method provides a simple form of temporal scalability and can be employed in any coding system that makes use of I, P and B frames.