Why swap x and y when project points from 3d to 2d

[gold-mango]

According to Objectron Geometry Tutorial , project points from 3d to 2d have two steps x = p_2d_ndc[:, 1] y = p_2d_ndc[:, 0] x comes from 1 dim, y comes from 0 dim, why swap x and y here?

[ahmadyan]

images are recorded in portrait mode, but the camera poses are in landscape.