[Fix] Converting point with specified rt_matrix (#556)

* [Fix] Converting point with specify rt_matrix

* boundary condition

* more readability and comprehensibility

mmdet3d/core/bbox/structures/coord_3d_mode.py

@@ -221,10 +221,15 @@ class Coord3DMode(IntEnum):
         # TODO: LIDAR
         # only implemented provided Rt matrix in cam-depth conversion
         if src == Coord3DMode.LIDAR and dst == Coord3DMode.CAM:
+            if rt_mat is None:
                 rt_mat = arr.new_tensor([[0, -1, 0], [0, 0, -1], [1, 0, 0]])
         elif src == Coord3DMode.CAM and dst == Coord3DMode.LIDAR:
+            if rt_mat is None:
                 rt_mat = arr.new_tensor([[0, 0, 1], [-1, 0, 0], [0, -1, 0]])
         elif src == Coord3DMode.DEPTH and dst == Coord3DMode.CAM:
+            # LIDAR-CAM conversion is different from DEPTH-CAM conversion
+            # because SUNRGB-D camera calibration files are different from
+            # that of KITTI, and currently we keep this hack
             if rt_mat is None:
                 rt_mat = arr.new_tensor([[1, 0, 0], [0, 0, -1], [0, 1, 0]])
             else:
@@ -238,8 +243,10 @@ class Coord3DMode(IntEnum):
                 rt_mat = rt_mat @ rt_mat.new_tensor([[1, 0, 0], [0, 0, 1],
                                                      [0, -1, 0]])
         elif src == Coord3DMode.LIDAR and dst == Coord3DMode.DEPTH:
+            if rt_mat is None:
                 rt_mat = arr.new_tensor([[0, -1, 0], [1, 0, 0], [0, 0, 1]])
         elif src == Coord3DMode.DEPTH and dst == Coord3DMode.LIDAR:
+            if rt_mat is None:
                 rt_mat = arr.new_tensor([[0, 1, 0], [-1, 0, 0], [0, 0, 1]])
         else:
             raise NotImplementedError(