| Along with digital AV and multimedia technology developing,large numbers of video datum need to be compressed.At present,the International Coding Standards,such as MPEG-1,MPEG-2,MPEG-4, H.26x,have been established and applied to different application areas.Rate control serves as an important technique to moderate the bit rate of video transmission over a channel of limited bandwidth and to control the bit allocations within a video sequence to maximize its overall visual quality.How to find a set of coding parameters to achieve the video quality as good as possible while subject to a rate constraint is a subject of rate control.H.264/AVC is the latest international video coding standard that adopts JVT-G012 as its rate control scheme.However,there are two limitations in the scheme.Firstly,the original linear MAD prediction model performs poorly due to high motion or scene changes in the test sequence.Secondly,in contrast to the previous standards,there exist much more complex motion compensation strategies in the H.264 standard,and a higher percentage of bits is required to encode notexture information.Thus the method that simply predicts notexture bits of the current frame through that of previous coded frames is no longer efficient.The limitations mentioned above are especially outstanding at low bit rates.First of all,a macroblock-based histogram of difference is introduced,and bits are allocated rationally among coding units according to the encoding complexity of the video image.Furthermore,the thesis presents a novel rate control scheme named Spatio-Temporal MAD prediction model,and a more flexible prediction scheme of nontexture bits was derived.As shown in the experiment,the proposed spatio-temporal rate control algorithm achieves a bit rate closer to the target,provides an improved visual quality and the higher PSNR especially in unstable video sequences at low bit rate compared with JVT-G012 algorithm. |
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