Bit allocation for MPEG-4 video coding with spatio-temporal tradeoffs
This paper describes rate-control algorithms that consider the tradeoff between coded quality and temporal rate. We target improved coding efficiency for both frame-based and object-based video coding. We propose models that estimate the rate-distortion characteristics for coded frames and objects, as well as skipped frames and objects. Based on the proposed models, we propose three types of rate-control algorithms. The first is for frame-based coding, in which the distortion of coded frames is balanced with the distortion incurred by frame skipping. The second algorithm applies to object-based coding, where the temporal rate of all objects is constrained to be the same, but the bit allocation is performed at the object level. The third algorithm also targets object-based coding, but in contrast to the second algorithm, the temporal rates of each object may vary. The algorithm also takes into account the composition problem, which may cause holes in the reconstructed frame when objects are encoded at different temporal rates. We propose a solution to this problem that is based on first detecting changes in the shape boundaries over time at the encoder, then employing a hole detection and recovery algorithm at the decoder. Overall, the proposed algorithms are able to achieve the target bit rate, effectively code frames and objects with different temporal rates, and maintain a stable buffer level.