This paper proposes an improved rate control
algorithm for jointly encoding multiple arbitrarily shaped video
objects in the context of low-delay MPEG-4 compliant video
coding. The algorithm provides adequate mechanisms for dealing
with deviations between the ideal and the actual behavior of video
scene encoders, notably: 1) compensation mechanisms (e.g., rate
control decisions) that are able to track these deviations and
compensate them to allow a stable and efficient operation of
the encoder, and 2) adaptation mechanisms (e.g., estimation of
model parameters) that are able to instantaneously represent
the actual behavior of the encoder and its rate controller. The
proposed solution efficiently allocates the available resources, i.e.,
target bit rate and bitstream buffer space, aiming at maximizing
the average scene quality and minimizing quality fluctuations
along time and among the various video objects. The results
show that this solution outperforms the usual reference solutions,
notably those specified in the rate control informative annex of
the MPEG-4 visual standard.