We need four pairs of corresponding points between two images to uniquely identify the projective homography matrix by solving the following system of equations. (8 unknows and equations)
Once we have the matrix , we can apply it on the project an image onto the reference surface by having . More on this on StackExchange
Below are examples that shows projective transformation in use.
By taking two pictures with different point of views but overlapped field of view, say pic1 and pic2. Then warp pic1 using the homography matrix defined between the two pictures.