More details about default camera intrinsics in demo.py file

[AishwaryaPalli]

Can you provide explanation for why the default focal length, principal point are set in demo.py file as shown below. Also what does "focal_length_ndc " means?

if focal_length == 0:
focal_length_ndc = 4.0
focal_length = focal_length_ndc * h / 2

    if len(principal_point) == 0:
        px, py = w/2, h/2
    else:
        px, py = principal_point