Unable to retrieve sensor data from .svo file

LoicFerrot commented 2 years ago

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Description

I'm trying to access the IMU data when playing back a .svo file, but calling imu_data.get_linear_acceleration() always returns the same vector of [4.585188705117234e-41, 0., 0.] across different svo files. I also tried to get the temperature data, but it always returns -1. From what I understood of the documentation, only get_linear_acceleration_uncalibrated shouldn't be available in svo mode...

Steps to Reproduce

from pyzed import sl
init_parameters = sl.InitParameters()
init_parameters.set_from_svo_file("/path/to/recording.svo")
zed = sl.Camera()
zed.open(init_parameters)
sensors_data = sl.SensorsData()
for i in range (100):
    if zed.grab() == sl.ERROR_CODE.SUCCESS:
        zed.get_sensors_data(sensors_data, sl.TIME_REFERENCE.IMAGE)
        imu_data = sensors_data.get_imu_data()
        temperature_data = sensors_data.get_temperature_data()
        print("imu data: ", imu_data.get_linear_acceleration())
        print("temperature data: ", temperature_data.get(sl.SENSOR_LOCATION.ONBOARD_LEFT))

Expected Result

Real changing values, linear acceleration of norm 9.81 (camera is static)

Actual Result

Constant values, not making sense

ZED Camera model

ZED2i

Environment

Latest docker `stereolabs/zed:3.6-gl-devel-cuda11.4-ubuntu20.04`
`torch==1.10.0+cu113`

NVIDIA GeForce RTX 3080
Intel® Core™ i9-10900K CPU @ 3.70GHz × 20

Anything else?

No response

Myzhar commented 2 years ago

Hi @LoicFerrot the SVO does not have acceleration and angular velocity data from the IMU, but only the fused attitude information. That's because the IMU samples are provided at a higher framerate with respect to the camera frames (400 Hz vs max 100Hz) and saving them at a reduced sampled rate could generate unwanted effects like aliasing.

LoicFerrot commented 2 years ago

Hey @Myzhar, thanks for your answer! Do you have any idea on how I could retrieve the gravity vector of a static camera ?

I used the example code snippet from the documentation on Camera.get_position() (which has to be slightly adapted), and as it seems expected since it gives the pose relative to the first frame, I only get a constant 0 translation and unit quaternion...

Myzhar commented 2 years ago

That's not possible by using the SVO. You must write your own recorder for IMU data to save the information that you require at maximum rate retrieving them with the command get_sensors_data and the parameter TIME_REFERENCE.CURRENT. Please note that the sensors' data grabbing must run in a separated thread with respect to the image data grabbing because the grab call is blocking.

stereolabs / zed-python-api