How to do 2D landmark loss minimize?

[JohnFive5]

Thank you for your excellent code!I wonder why you strongly recommend fitting to 2D landmarks but implemented a 3D landmark loss.I tried to implement a 2D loss,using the camera calibration as you suggested in the paper,and transformed the 3D coordinates to pixel coordinates to make a loss function.But it seems that Chumpy can not get the gradient.What's the problem?How should I modify x0？