Calibration from vanishing points

[kvuong2711]

Hello,

Thanks for your great work. I have a question about "Calibrating from three orthogonal vanishing points". This is a classic problem and it's not the main point of your paper, but I also read about it in your blog (https://yichaozhou.com/post/20190402vanishingpoint/) and my question is, the first element (S_{11}) of the matrix S = K^-T K^-1 is 1/f^2, which will be very small when f is sufficiently large, leading to numerical instability. Do you have any idea how to handle this? Thanks in advance.

[zhou13]

Not sure if I understand correctly, could you list your S matrix here? The condition number should be only related to the relative scale.

[kvuong2711]

Sorry for the confusion. My goal is to estimate the focal length f of the camera, which will involve solving for the matrix S, especially its first element which is equal to 1/f^2 (in order to find f). My concern is that this number can very small. For example, if the ground truth f is 1000, then 1/f^2 = 1e-6. This number is very small and with small numerical error, the estimated focal length f will be way off from the ground truth f. Hope it clarifies things.

[zhou13]

Since the 1/f^2 is before the whole matrix, it should not matter. Please post the value of your matrix S and its condition number so I can better understand your problem.

[zhou13]

Anyway, I guess your problem is ox^2 + oy^2 + f^2 is too large. You may want to rescale the unit so that they are around 1.

[zhou13]

If you still have issues, feel free to reopen this post.