update README

calibration/README.txt

@@ -1,12 +1,59 @@
-camx.yaml: Cameras intrinsic
-imu.yaml: IMU intrinsic
+# File Format
 
-Cam-lidar-imu.yaml:
+cam<n>.yaml: Cameras intrinsics obtained from Kalibr (https://github.com/ethz-asl/kalibr)
+- camera_matrix (3x3) 
+- distortion_coefficients ([k1, k2, k3, k4])
+- distortion_model (equidistant, see https://github.com/ethz-asl/kalibr/wiki/supported-models)
+- projection_matrix (3x4, same as camera matrix)
+- rectification_matrix
+- image_width, image_height
 
-T_cam_lidar: Camera-LiDAR extrinsic (homogeneous transformation matrix)
-C_r_CL: Camera-LiDAR extrinsic (translation: x, y, z)
-C_q_CL: Camera-LiDAR extrinsic (rotation quaternion: x, y, z, w)
-C_r_CI: Camera-IMU extrinsic (translation: x, y, z)
-C_q_CI: Camera-IMU extrinsic (rotation quaternion: x, y, z, w)
+imu.yaml: IMU intrinsics obtained from Allen Variance ROS (https://github.com/ori-drs/allan_variance_ros)
+- accelerometer_noise_density
+- accelerometer_random_walk
+- gyroscope_noise_density
+- gyroscope_random_walk
+- rostopic
+- update_rate
+
+cam-lidar-imu.yaml: Camera-LiDAR-IMU extrinsics obtained from DiffCal (https://github.com/ori-drs/diffcal)
+- T_cam_lidar: 4x4 homogeneous matrix that converts points in lidar frame to camera<n> frame
+- C_r_CL: translation (x,y,z) of T_cam<n>_lidar
+- C_q_CL: rotation quaternion (x,y,z,w) of T_cam<n>_lidar
+- C_r_CI: translation (x,y,z) of T_cam<n>_imu
+- C_q_CI: rotation quaternion (x,y,z,w) of T_cam<n>_imu
+- rostopic
 
 Intra-camera calibration and IMU-LiDAR can be inferred from the transformations above.
+
+# Base Frame
+An artificial frame that is located at the bottom of the device with the x-axis forward (same direction as the front camera), y-axis to the left, and z-axis to the top. It is designed to make the visualisation of the sensor pose more intuitive.
+
+The base frame is manually set with respect to the lidar as
+T_base_lidar_t_xyz_q_xyzw = [0.0, 0.0, 0.124, 0.0, 0.0, 1.0, 0.0]
+
+The processed (undistorted) point clouds from VILENS are provided in the base frame, not the lidar frame. 
+
+# Calibration sequences
+There are three sequences recorded for calibration
+- 2024-03-19
+- 2024-06-29 (although imu calibration is done on 2024-06-27)
+- 2024-10-29
+ 
+
+The default calibration files provided in this folder are obtained from these sequences.
+- cam<n>.yaml: 2024-03-19 sequence
+- imu.yaml: 2024-06-29 sequence
+- cam-lidar-imu.yaml
+  - cam-lidar: 2024-03-19 sequence
+  - cam-imu: 2024-06-29 sequence 
+  
+The rationale is that 
+- most sequences we have are collected in March (2024-03-xx)
+- for these sequences, the closest IMU calibration we did is in June (2024-06-29)
+
+
+We recommend using the calibration that is closest to the sequence that you are testing.
+- 2024-03-xx sequences (the majority): use the default calibration files
+- 2024-05-20 sequences (bodleian library-02): either calibration from 2024-03-19 or 2024-06-29 
+- 2024-07-09 sequences (new college): use calibration from 2024-06-29