Need help understanding Tango's functions related to coordinate systems - google-project-tango

Question

I am confused by parameters of those functions related to coordinate systems, for eample:
TangoSupport_getMatrixTransformAtTime(double timestamp,
TangoCoordinateFrameType base_frame,
TangoCoordinateFrameType target_frame,
TangoSupportEngineType base_engine,
TangoSupportEngineType target_engine,
TangoSupportDisplayRotation display_rotation_type,
TangoMatrixTransformData *matrix_transform)
(1)Base_engine: If I choose COORDINATE_FRAME_START_OF_SERVICE as base_frame . As described in the document, the coordinate system will use "Right Hand Local Level" . Then, what's the purpose of the base_engine parameter ? Is it meaningful here to choose something other than TANGO_SUPPORT_ENGINE_TANGO ?
(2) Target_engine: I choose COORDINATE_FRAME_START_OF_SERVICE as base_frame , and DEVICE as target. choose OPENGL for base_engine. then choose any value for target_engine. the result is always same

Answer 1

(1)Base_engine: If I choose COORDINATE_FRAME_START_OF_SERVICE as base_frame . As described in the document, the coordinate system will use "Right Hand Local Level" . Then, what's the purpose of the base_engine parameter ? Is it meaningful here to choose something other than TANGO_SUPPORT_ENGINE_TANGO ?
This really depends on your use case. It is rare that you use Tango coordinate as base frame, unless you have another set of transformation that transform start service to local origin.
Let's say you did a query like this: TangoSupport_getMatrixTransformAtTime(0.0, START_SERVICE, DEVICE, TANGO, TANGO,...); it is quavalant of doing a TangoService_getPoseAtTime query with start service and device frame pair.
More common case is that you want to transform something(i.e depth point) in to your local origin (i.e OpenGL origin) for render. What you will do is: TangoSupport_getMatrixTransformAtTime(0.0, START_SERVICE, DEPTH, OPENGL, TANGO,...);, the result of this call is opengl_T_depth_camera, you can then multiply this transform to the depth point returned from depth camera: P_opengl = opengl_T_depth_camera * P_depth_camera;. P_opengl is the point you can render out directly in OpenGL.
(2) Target_engine: I choose COORDINATE_FRAME_START_OF_SERVICE as base_frame , and DEVICE as target. choose OPENGL for base_engine. then choose any value for target_engine. the result is always same
This should be true for OPENGL and TANGO. There's a happy coincedent that opengl coordinate is same as the device frame coordinate. So if you put TANGO or OPENGL on the target_frame, the result will be the same. But if you put UNITY as target engine type, the result will be different.