# Tutorial on Camera-IMU and Motion Capture Calibration with Realsense T265
In this tutorial we explain how to perform photometric and geometric calibration of the multi-camera setup and then calibrate the transformations between cameras, IMU and the motion capture marker setup. To make sure the calibration is successful we recommend to rigidly attach markers to the camera as shown on the figure above.
## Dataset
We provide a set of example datasets for the calibration. Even if you plan to record your own datasets (covered in the next section), we recommend to download and try the provided examples:
*`--dataset-path` specifies the location where the recorded dataset will be stored. In this case it will be stored in `~/t265_calib_data/<current_timestamp>/`.
*`--manual-exposure` disables the autoexposure. In this tutorial the autoexposure is disabled for all calibration sequences, but for the VIO sequence (sequence0) we enable it.
The GUI elements have the following meaning:
*`webp_quality` compression quality. The highest value (101) means lossless compression. For photometric calibration it is important not to have any compression artifacts, so we record these calibration sequences with lossless compression.
*`skip_frames` reduces the framerate of the recorded dataset by skipping frames.
*`exposure` controls the exposure time of the cameras.
*`record` starts/stops the recoding of the dataset. If you run the system on the slow PC pay attention to the number of messages in the queue. If it goes beyond the limit the recorder will start dropping frames.
*`export_calibration` exports factory calibration in the basalt format.
After recoding the dataset it is a good practice to verify the quality of the dataset. You can do this by running:
It will report the actual frequencies of the recorded sensor messages and warn you if any files with image data are missing.
Every sequence required for the calibration should have certain properties to enable successful calibration. Pay attention to the **Important for recording the dataset** subsections and inspect the provided examples before recoding your own sequences.
## Response calibration
In this project we assume a camera has a linear response function (intensity of the pixel is linearly proportional to the amount of light captured by the pixel). In this section we will verify this for the Realsense T265 cameras. We will need to record a static scene with different exposure times.
**Important for recording the dataset:**
* Keep the camera static and make sure that nothing in the scene moves during the recording.
* Move `webp_quality` slider to the highest value to enable lossless compression.
* Optionally set the `skip_frames` slider to 3-5 to speed up the dataset recoding.
* Start recoding and slowly move the exposure slider up and down. Stop recoding.
You should see the response function and the irradiance image similar to the one shown below. For the details of the algorithm see Section 2.3.1 of [[arXiv:1607.02555]](https://arxiv.org/abs/1607.02555). The results suggest that the response function used in the camera is linear.
*`detect_corners` detect corners. If the corners were detected before the cached detections will be loaded at the previous step so there is no need to re-run the detection.
*`init_cam_intr` initialize camera intrinsics.
*`init_cam_poses` initialize camera poses using the current intrinsics estimate.
*`init_cam_extr` initialize transformations between multiple cameras.
*`init_opt` initialize optimization.
*`opt_until_converge` optimize until convergence.
*`opt_cam_poses` some initial poses computed from the initialized intrinsics can be far from optimum and not converge to the right minimum. To improve the final result we can re-initialize poses with optimized intrinsics.
*`init_opt` initialize optimization with new initial poses.
*`opt_until_converge` optimize until convergence.
*`compute_vign` after optimizing geometric models compute the vignetting of the cameras.
*`save_calib` save calibration file to the `~/t265_calib_results/calibration.json`.
## IMU and Motion Capture Calibration
After calibrating cameras we can proceed to geometric and time calibration of the cameras, IMU and motion capture system. Setting up the motion capture system is specific for your setup.
For the motion capture recording we use [ros_vrpn_client](https://github.com/ethz-asl/ros_vrpn_client) with [basalt_capture_mocap.py](scripts/basalt_capture_mocap.py). We record the data to the `mocap0` folder and then move it to the `mav0` directory of the camera dataset. This script is provided as an example. Motion capture setup is different in every particular case.
**Important for recording the dataset:**
* Set the `skip_frames` slider to 1 to use the full framerate.
* Reduce the exposure time to reduce the motion blur.
* Move the setup such that all axes of accelerometer and gyro are excited. This means moving with acceleration along X, Y and Z axes and rotating around those axes.
*`detect_corners` detect corners. If the corners were detected before the cached detections will be loaded at the previous step so there is no need to re-run the detection.
*`init_cam_poses` initialize camera poses.
*`init_cam_imu` initialize the transformation between cameras and the IMU.
*`init_opt` initialize optimization.
*`opt_until_converge` optimize until convergence.
* Enable `opt_cam_time_offset` and `opt_imu_scale` to optimize the time offset between cameras and the IMU and the IMU scale.
*`opt_until_converge` optimize until convergence.
*`init_mocap` initialize transformation between the motion capture marker frame and the IMU and the transformation between the aprilgrid pattern and the motion capture system origin.
* Enable `opt_mocap`.
*`opt_until_converge` optimize until convergence.
*`save_calib` save calibration file to the `~/t265_calib_results/calibration.json`.
*`save_mocap_calib` save motion capture system calibration file to the `~/t265_calib_results/mocap_calibration.json`.
## Generating Time-Aligned Ground Truth
Since motion capture system and the PC where the dataset was recorded might not have the same clock we need to perform the time synchronization. Additionally we need to transform the coordinate frame of the GT data to the IMU frame (originally it is in the coordinate frame attached to the markers).
The raw motion capture data is stored in the `mav/mocap0/` folder. We can find the time offset by minimizing the error between gyro measurements and rotational velocities computed from the motion capture data. If you press the `save_aligned_dataset` button the resulting trajectory (time aligned and transformed to the IMU frame) will be written to `mav/gt/data.csv` and automatically loaded when available.
