why you subtract self._principle_point in project_2d_to_3d and add self._principle_point in project_3d_to_2d?

[admyxs]

Thank for your great work

but I have a problem about the Class Camera function "project_2d_to_3d" and "project_3d_to_2d"
https://github.com/RoboticImaging/RectConv/blob/main/scripts/projection.py#L202
https://github.com/RoboticImaging/RectConv/blob/main/scripts/projection.py#L196

1、I want to konw why you subtract self._principle_point in project_2d_to_3d
and add self._principle_point in project_3d_to_2d?

2、Does it corrlate to other function?

3、What will happen if i delete the two lines?

Thank for your great work again!