  Rate-control plays an important role in video coding. However, in the
conventional rate-control algorithms, the number and position of Macroblocks
(MBs) inside one basic unit for rate-control is inflexible and predetermined.
The different characteristics of the MBs are not fully considered. Also, there
is no overall optimization of the coding of basic units. This paper proposes a
new region-based rate-control scheme for H.264/AVC to improve the coding
efficiency. The inter-frame information is explored to objectively divide one
frame into multiple regions based on their rate-distortion behaviors. The MBs
with the similar characteristics are classified into the same region, and the
entire region instead of a single MB or a group of contiguous MBs is treated as
a basic unit for rate-control. A linear rate-quantization stepsize model and a
linear distortion-quantization stepsize model are proposed to accurately
describe the rate-distortion characteristics for the region-based basic units.
Moreover, based on the above linear models, an overall optimization model is
proposed to obtain suitable Quantization Parameters (QPs) for the region-based
basic units. Experimental results demonstrate that the proposed region-based
rate-control approach can achieve both better subjective and objective quality
by performing the rate-control adaptively with the content, compared to the
conventional rate-control approaches.
