How to calculate distance_from_center using zed2i ros2 object centroid position

Software Python
1

hi,

I have a AGX orin + ZED2i + ros2 setup.

I’m subscribing to this topic /zed2i/zed_node/body_trk/skeletons. And I’m trying to calculate the distance between body centroid position and camera view center, see Python code below:
(obj_x and obj_z if from position array: position[0], position[2])

def calculate_camera_width(obj_depth):
    HFOV_deg = 104 # HD720P
    HFOV_rad = math.radians(HFOV_deg)

    # Calculate the width at the specified depth
    camera_width = 2 * obj_depth * math.tan(HFOV_rad / 2)

    return abs(camera_width)

distance_from_center = obj_x - calculate_camera_width(obj_z) / 2

However the obj_x seems much bigger than camera_width at specific depth. Can you spot anything I’m doing wrong?

Thanks.

2

Hi,

Have you checked that the values you get for position are changing as you would expect? While the default coordinate frame uses Z as depth, I believe that most messages are converted into the ROS frame which uses X as depth instead.

https://www.stereolabs.com/docs/positional-tracking/coordinate-frames/

I’m not an expert but its something worth checking. Besides that, your calculations look sound to me so I can’t be of much more help.

3

hi, thank you for the reply.

I can see ROS is indeed using a different coord frames
Right handed, z-up, x-forward (ROS)

I’ll test it out.