initial_world_transform() and zed.get_position() have different results.

[jinho0309]

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Description

this is my code

  c_translation = sl.Translation()
    c_translation.init_vector(0.039138934265215745, 0.33897992297324625, 3.7656952005014612)
    c_rotation = sl.Rotation()
    c_rotation.set_rotation_vector(0.05210374780239872, 2.6160165564830247, -1.37881240459529)
    c_transform = sl.Transform()
    c_transform.init_rotation_translation(c_rotation, c_translation)
 

    c_pose = sl.Pose()
    c_pose.init_transform(c_transform)
    rotation = c_pose.get_rotation_vector()
    print(rotation)
   


    positional_tracking_parameters = sl.PositionalTrackingParameters()
    positional_tracking_parameters.set_as_static = True
    positional_tracking_parameters.enable_area_memory= False
    positional_tracking_parameters.set_initial_world_transform(c_transform)
    zed.enable_positional_tracking(positional_tracking_parameters)



    t_transform = positional_tracking_parameters.initial_world_transform()
    print(t_transform.get_rotation_vector())



    if zed.grab() == sl.ERROR_CODE.SUCCESS : # Grab an image and computes the tracking
        camera_pose = sl.Pose()
        zed.get_position(camera_pose, sl.REFERENCE_FRAME.WORLD)
        rotation = camera_pose.get_rotation_vector()


        print(rotation)

result : [ 0.05210375 2.61601663 -1.37881243] [ 0.05210375 2.61601663 -1.37881243] [-0.34887791 2.80270791 0.80921131]

Why diffrent?

ZED Camera model

ZED2

Environment

OS : Window

Anything else?

No response

[obraun-sl]

Hi, This is expected as the initial world transform is adjusted with the IMU (gravity only) orientation of the camera to match the real world pose. There will be a setting in a future version to be able to de-activate that.

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[haavarpb]

Hi, sorry for necroing the issue but I just encountered the same phenomenon.

Following is copied from PositionalTrackingParameters specification:

bool set_as_static

This setting allows you define the camera as static. If true, it will not move in the environment. This allows you to set its position using initial_world_transform. Camera::getPosition() will return the value set as initial_world_transform for the PATH, and identity as the POSE.

Also, from coordinate frames documentation:

World Frame

Describing the position of the camera in absolute real-world space requires a reference point that remains stationary as the device moves. This global reference frame is called World Frame. If no initial parameters are set, the World Frame is located at the place where the ZED first started motion tracking, and is oriented in the direction where the device was looking. To get the position of the camera in real-world space, use getPosition(zed_pose, REFERENCE_FRAME::WORLD).

Firstly, I think the API should be cleaned up by renaming getPosition to getPose. As I understand it, in robotics, a pose is both a rotation and a position. The getPosition function is therefore misleading since the function actually gives you a pose.

Secondly, I can't see any mentions of world frame being adjusted using the IMU. If this is behavior is indeed intended, the documentation should reflect that.