A face annotation object contains the results of face detection.
The fd_bounding_poly bounding polygon is tighter than the
boundingPoly, and encloses only the skin part of the face.
Typically, it is used to eliminate the face from any image
analysis that detects the "amount of skin" visible in an
image. It is not based on the landmarker results, only on the
initial face detection, hence the fd (face detection) prefix.
Roll angle, which indicates the amount of clockwise/anti-
clockwise rotation of the face relative to the image vertical
about the axis perpendicular to the face. Range [-180,180].
Pitch angle, which indicates the upwards/downwards angle that
the face is pointing relative to the image's horizontal plane.
Range [-180,180].