I am trying to find one image (needle) within another (haystack). The method is to find some important points in the needle and try to match them in the haystack. Then to use homography to map the needle image into the haystack. The method is explained here.
I noticed that when the needle isn't in the haystack I can still get a very large number of matched points, almost as many as when it is found. However the determinant of the homography matrix is several orders of magnitude smaller, a very clear distinction.
So what does the determinant of a homography matrix represent?