How to calibrate camera extrinsic parameters

[Skullface9512]

Hi, according to the readme, i know you can compute extrinsic parameters by L-frame method, but i don't realize the specific principles of this algorithm, and in calibration_extrinsic_Lframe.m, it needs (x,y,z) dimensions for each of the post tops on the L-frame device what i exactly don't know it represents, is this using checkerboard or other methods, otherwise, the calibration algorithm will convert the first frame to tiff of each video to compute extrinsic parameters, i don't know whether it needs other frames.