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In H.264/AVC rate control algorithm, the bit allocation process and the QP determination are not optimal.
At frame layer, there is an implicit assumption considering that the video sequence is more or less stationary
and consequently the neighbouring frames have similar characteristics. So, the target Bit-Rate for each frame
is estimated using a straightforward process that allocates an equal bit budget for each frame regardless of its
temporal and spatial complexities. This uniform allocation is surely not suited especially for all types of video
sequences. The target bits determination at macroblock layer uses the MAD (Mean Absolute Difference) ratio
as a complexity measure in order to promote interesting macroblocks, but this measure remains inefficient in
handling macroblock characteristics. In a previous work we have proposed Rate-Quantization (R-Q) models
for Intra and Inter frames used to deal with the QP determination shortcoming. In this paper, we look to
overcome the limitation of the bit allocation process at the frame and the macroblock layers. At the frame
level, we enhance the bit allocation process by exploiting frame complexity measures. Thereby, the target bit
determination for P-frames is adjusted by combining two temporal measures: The first one is a motion ratio
determined from actual bits used to encode previous frames. The second measure exploits both the difference
between two consecutive frames and the histogram of this difference. At macroblock level, the visual saliency
is used in the bit allocation process. The basic idea is to promote salient macroblocks. Hence, a saliency map,
based on a Bottom-Up approach, is generated and a macroblock classification is performed. This classification
is then used to accurately adjust UBitsH264 which represents the usual bit budget estimated by H.264/AVC
bit allocation process. For salient macroblocks the adjustment leads to a bit budget which is always larger
than UBitsH264. The extra bits added to code these macroblocks are deducted from the bit budget allocated
to the non-salient macroblocks. Simulations have been carried out using JM15.0 reference software, several
video sequences and different target Bit-Rates. In comparison with JM15.0 algorithm, the proposed approach
improves the coding efficiency in terms of PSNR/PSNR-HVS (up to +2dB/+3dB). Furthermore, the bandwidth
constraint is always satisfied because the actual Bit-Rate is always lower than or equal to the target Bit-Rate.
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