ORB 是一种具有局部不变形的特征点特征，是FAST特征点和BRIEF描述子的结合。
FAST特征：
1.从图片中选择像素点P，亮度L
2.设定threshold T
3.consider a circle with R=3 , meaning 16 points in total
4. if there are n consective points on the circle, > I+T or <I-T , it is a corner point. n can be 12 or 9
we can use high speed test to fastly eliminate large unsuitable areas:
we check points No.1 9 5 and 13. if 1 and 9 meet requirement 4, then check 5 and 13. if it is a corner point ,there 3/4 of the circle points will meet 4. if not, it is not a corner point.
Note: we cannot use HST if n<12
BRIEF descriptor:
1. find a patch around a feature point
2. find Nd point pairs inside the patch
3. for every point pairs, if I(1)>I(2) , its corresponding binary value=0 , else 1
4.we can thus generate a binary string with length=Nd
5. we can then use hammering distance to describe these descriptors.
ORB 特征点 （Oriented FAST and Rotated BRIEF）
原文：http://www.cnblogs.com/SongHaoran/p/6650512.html