1、Motion Estimation in x264
x264's ME methods in order from best quality to worst:
SATD Exhaustive (tesa)
Exhaustive (esa/full)
Uneven Multi-Hex (umh)
Hexagon (hex)
Diamond (dia/epzs)
x264 has 5 fullpel motion estimation modes:
radius-1 diamond,
radius-2 hex (default),
UMH (somewhat similar to JM),
exhaustive (implemented fast via sequential elimination),
and Hadamard exhaustive (implemented approximately via thresholded sequential elimination).
参考：X264的运动估计算法研究 作者： 丁超， 陈涛， DING Chao， CHEN Tao
2、Motion Estimation in HM
HM 中8PointDiamondSearch 和8PointSquareSearch 算法，默认使用是8PointDiamondSearch，但是Diamond算法的搜索点要比Square要多，Square算法不论半径是多大时一圈都是8个点，而根据代码中的Diamond算法，只有在半径小于等于8时一圈才是8个点，而当半径大于8时，一圈要搜索4+3*4 =16个点，除了搜索菱形的四个顶点外，还搜索了每条边1/4、2/4、3/4处的点。