How can you tell if a homography matrix is acceptable or not?

Question

When using OpenCV\'s findHomography function to estimate an homography between two sets of points, from different images, you will sometimes get a bad homograph

Answer 1

The best way to tell if the homography is acceptable is.

1- Take the points of one image and reproject them using the computed homography.

//for one 3D point, this would be the projection
px' = H * px;
py' = H * py;
pz' = H * pz;

2- Calculate the euclidean distance between the reprojected points and the real points in the image.

Reprojection error for one point. p is the projected point and q is the real point.

3- Establish a treshold that decides if the reprojection error is acceptable.

For example, an error greater than one pixel wouldn't be acceptable for many tracking applications.