2d and 3d joints positions

[Marlod390]

I tried to use the smpl parameter to calculate the position of 3d joints in order to compare with the data under the 3d_joints key, but found that the two data are different. At the same time, when I visualized the 2d joints on the original image, I also found that their positions did not match the actual ones. So I want to know how the 3d joints and 2d joints are calculated. Thanks in advance