how many points pair are needed for a correct homography calculation?

Question

in order to get a homography between two images, I wanted to know how many points I need at least to get a good estimation of the homography matrix?

Answer 1

You need a minimum of four to get anything at all, but I've found 7 to 10 to be a good number in practice, provided that they are chosen "manually". Using some kind of feature detector, you may need ten or 20 times that to find a suitable sub-set with few outliers.

It really depends on the distribution of the points - if you have 10 co-linear points, you're not going to get a happy result, for example. However, for a slightly distorted rectangle mapping, the four corners are all you need.