mpeg2，mpeg4，h264编码标准的异同
1、 宏块匹配像素精度 ： MPEG2中，运动估计的精度是 1/2 的像素，通过线性插实现（可能有简单修正）； H264和MPEG4 都可以支持 1/4 像素的精度 2、 参考帧的数量 ：MPEG2，MPEG4的P帧只能有一帧参考帧，B帧可以有两帧参考帧；H.264可以最多有15帧参考帧，一
1、宏块匹配像素精度： MPEG2中，运动估计的精度是1/2的像素，通过线性插值实现（可能有简单修正）； H264和MPEG4 都可以支持1/4像素的精度
2、参考帧的数量：MPEG2，MPEG4的P帧只能有一帧参考帧，B帧可以有两帧参考帧 ；H.264可以最多有15帧参考帧，一般情况下选择3-5帧作为参考帧。
3、滤波器的结构：插值滤波器不同，在MPEG4里面是简单的线性插值，而H264里面是5抽头的滤波器；MPEG2没有环内deblocking工具，MPEG4有deblocking工具，但是属于postprocessing; H264有环内deblocking工具 。
4、I帧的编码： MPEG2没有INTRA预测，但是有DC参数预测；MPEG4没有INTRA预测，支持DC预测，并且支持两个AC参数的预测。H264支持INTRA预测，4*4共有9种模式。
5、匹配宏块大小的区别：MPEG2最小匹配块尺寸为8*8，MPEG4同样也为8*8，H.264最小可以到4*4尺寸。
6、DCT变换的不同：MPEG系列为8*8的DCT变换，逆变换时会出现漂移现象；H264为整数4*4的DCT变换，而且直流系数经过HADMARD变换之后做的DCT变换，整数DCT变换可以防止逆变换时出现漂移现象。
7、量化之后的编码：MPEG2/MPEG4则都是基于huffman，并且头部的语法相对比较简单，很少使用变长码； H264提供基于上下文的CAVLC和CABAC 。
8、 编码之后的结构：MPEG2/MPEG4都是分层结构；H264则是提供NAL单层结构，提供参数集合的形式。
9、量化参数的区别：MPEG4提供31个量化参数（量化模式：常量化，系统默认量化矩阵，自定义量化矩阵），H264提供51个量化参数（量化模式：常量化，量化矩阵，自定义有否还不确定）；
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