docs: update readme of distortion_corrector (autowarefoundation#752) (a…

…utowarefoundation#192)

* update readme of distortion_corrector

* Update sensing/preprocessor/pointcloud/pointcloud_preprocessor/docs/distortion-corrector.md

Co-authored-by: Yukihiro Saito <yukky.saito@gmail.com>

* Update sensing/preprocessor/pointcloud/pointcloud_preprocessor/docs/distortion-corrector.md

Co-authored-by: Yukihiro Saito <yukky.saito@gmail.com>

* Update sensing/preprocessor/pointcloud/pointcloud_preprocessor/docs/distortion-corrector.md

Co-authored-by: Yukihiro Saito <yukky.saito@gmail.com>

* Update sensing/preprocessor/pointcloud/pointcloud_preprocessor/docs/distortion-corrector.md

Co-authored-by: Yukihiro Saito <yukky.saito@gmail.com>

Co-authored-by: Yukihiro Saito <yukky.saito@gmail.com>
Signed-off-by: wep21 <border_goldenmarket@yahoo.co.jp>

Co-authored-by: Taichi Higashide <taichi.higashide@tier4.jp>
Co-authored-by: Yukihiro Saito <yukky.saito@gmail.com>
Signed-off-by: Takagi, Isamu <isamu.takagi@tier4.jp>

sensing/pointcloud_preprocessor/docs/distortion-corrector.md

@@ -4,8 +4,21 @@
 
 The `distortion_corrector` is a node that compensates pointcloud distortion caused by ego vehicle's movement during 1 scan.
 
+Since the LiDAR sensor scans by rotating an internal laser, the resulting point cloud will be distorted if the ego-vehicle moves during a single scan (as shown by the figure below). The node corrects this by interpolating sensor data using odometry of ego-vehicle.
+
 ## Inner-workings / Algorithms
 
+- Use the equations below (specific to the Velodyne 32C sensor) to obtain an accurate timestamp for each scan data point.
+- Use twist information to determine the distance the ego-vehicle has travelled between the time that the scan started and the corrected timestamp of each point, and then correct the position of the point.
+
+The offset equation is given by
+$ TimeOffset = (55.296 \mu s _SequenceIndex) + (2.304 \mu s_ DataPointIndex) $
+
+To calculate the exact point time, add the TimeOffset to the timestamp.
+$ ExactPointTime = TimeStamp + TimeOffset $
+
+![distortion corrector figure](./image/distortion_corrector.jpg)
+
 ## Inputs / Outputs
 
 ### Input
@@ -30,11 +43,3 @@ The `distortion_corrector` is a node that compensates pointcloud distortion caus
 | `timestamp_field_name` | string | "time_stamp"  | time stamp field name |
 
 ## Assumptions / Known limits
-
-## (Optional) Error detection and handling
-
-## (Optional) Performance characterization
-
-## (Optional) References/External links
-
-## (Optional) Future extensions / Unimplemented parts