How many points needed to compute the Homography?

Question

I'm working on a project where i'm using planar homography.

As seen in the above image, every point gives two equations and since the homography matrix has 8 degree of freedom, 4 points are enough to solve the H-matrix.

However, if if look at the two equations every point gives, we see that we also need to know x,y of the corresponding point.

So my question is in this case:

Do we need four points or four pairs to compute the homography?? and isn't it more precise to say four pair of points are needed instead of four points??

Thanks in Advance

Answer 1

If you don't know the transformed points, you have no equation at all !

Indeed, you need four pairs of corresponding points.