MPEG-4 rate control for multiple video objects

This paper describes an algorithm which can achieve a constant bit rate whe n coding multiple video objects, The implementation is a nontrivial extensi on of the MPEG-4 rate control algorithm for single video objects which empl oys a quadratic rate-quantizer model. The algorithm is organized into two s tages: a pre- and a postencoding stage. In the preencoding stage, an initia l target estimate is made for each object. Based on the buffer fullness, th e total target is adjusted and then distributed proportional to the relativ e size, motion, and variance of each object. Based on the new individual ta rgets and rate-quantizer relation for texture, appropriate quantization par ameters are calculated. After each object is encoded, the model parameters for each object are updated, and if necessary, frames are skipped to ensure that the buffer does not overflow. A preframeskip control is exercised to avoid buffer overflow when the motion and shape information occupies a sign ificant portion of the bit budget. The rate control algorithm switches betw een two operation modes so that the coder can reduce the spatial coding acc uracy for an improved temporal resolution, A shape-coding control mechanism is also proposed, which provides a tradeoff between texture and shape codi ng accuracy, Overall, the algorithm is able to successfully achieve the tar get bit rate, effectively code arbitrarily shaped objects, and maintain a s table buffer level. These techniques have been adopted hy the MPEG committe e in July 1997 as part of the video Verification Model (VM8).