Using OpenCV's findHomography to align scanned documents; How do I avoid obviously-bad (non-planar/3D) transforms?

first: avoid this. present your inputs, and your matching results, and your code that does it. akaze and ratio test should have been robust enough. your images are probably to blame. show them.

then: eigenvectors and eigenvalues help evaluate such a matrix. there’s a few things one can try to evaluate how “ridiculous” the matrix is.

related: