Regarding requirement that 3D joint location is given at the axis system of the train cameras

[aviralchharia]

Can the author elaborate on the following lines from the Conclusions and limitations section of the article? "One limitation of our approach is the dependency on 3D joint location ground-truth, and in particular, the requirement that it is given at the axis system of the train cameras." As far as I understand, the dependency of the 3D joint location ground-truth is due to the learning based approach that optimizes the Loss Functions. Not sure if I understand the second limitation.