Joint shape and texture rate control for MPEG-4 encoders
This paper describes an algorithm which can achieve a constant bit rate when coding multiple video objects. First, an initial target estimate is made for each object. Based on the buffer fullness, the total target is adjusted and then distributed proportional to the relative size, motion, and variance of each object. Based on the new individual target rates and rate distortion model for coding the texture of each object, appropriate quantization parameters are calculated. After each object is encoded, the texture model parameters for each object are updated, and if necessary, frames are skipped to ensure that the buffer does not overflow. To provide a proper trade-off between spatial and temporal coding, the proposed rate control algorithm switches between a high rate coding mode and a low rate one. In the low rate mode, a mechanism to control the parameters for shape coding is included. Overall, the algorithm is able to successfully achieve the target bit rate, effectively code arbitrarily shaped objects, and maintain a stable buffer level.