# Why is findHomography() so fast?

Hey there people,

maybe I got a silly question for you, but I am quite interested in the answer and have not found out myself yet:

Why is findHomography() so fast? I mean okay, with good inlier/outlier ratio and RANSAC we will find a very good initial solution for a homography. But still with 300< features the final refinement is so fast, I got computation times of less than 10 ms total, while testing with Googles Ceres Solver my Estimations are easily 10 times slower. Is there a smart logic behind the algorithm that reduces the computational costs of homography refinement or is the chance high, my Ceres Code is just bad?

Thanks in advance for any good hint!

Have you compare numerical results?

I want to use Ceres as global refiner. Therefore my algorithm wasn't meant to compare 2 homographies but 10 or more. However as I was testing around in a sequence of images I calculated a Homography by findHomography() and afterwards let my Ceres refinement optimize them. The solution was pretty much the same (just slightly better with Ceres) but as I said at least 10 times slower. No scientific comparison, but it seemed a little suprising to me.

yes I think there is a problem ...