Rate and distortion models can play a very important role in real-time video encoding, since they can be used to obtain near optimal operation performance in terms of the RD tradeoff without the drawback of having to encode multiple times the same VOP to find the best combination of coding parameters. In the context of object-based video encoding, notably in MPEG-4 video encoding, rate and distortion models characterize the relation between the average number of bits/pixel to code a given Video Object Plane (VOP), the average VOP distortion, and the relevant coding parameters. These models are usually defined in terms of rate-quantization (RQ), distortion-quantization (DQ), and rate-distortion (RD) functions.
This paper addresses the problem of rate and distortion modeling for Intra and Inter coding in the context of object-based MPEG-4 video encoding. In the case of Intra coding, the VOP to encode does not depend on other past or future VOPs; therefore, its rate and distortion characteristics depend exclusively on the current quantizer parameter(s) and VOP statistics. In the case of Inter coding, the rate and distortion functions depend not only on the current VOP but also on its reference VOP(s); therefore the rate and distortion functions become bidimensional and consequently more difficult to estimate during encoding. In this paper, a new approach is proposed where the rate and distortion functions for Inter coding are modeled as one-dimensional functions plus an adaptation term.