Skip to content

Latest commit

 

History

History
187 lines (107 loc) · 4.67 KB

18_about_point_layout.md

File metadata and controls

187 lines (107 loc) · 4.67 KB
Raw

18 Point Type and Point Layout

18.1 Overview

This document describes:

  • the member variables of point and point cloud, and
  • what they really means

18.2 Point And Point Cloud

18.2.1 Point

The member variables of point are as below.

struct PointXYZIRT
{
  float x;
  float y;
  float z;
  uint8_t intensity;
  uint16_t ring;
  double timestamp;
};

18.2.2 Point Cloud

The member variables of point cloud are as below. Its member points is a vector of points.

template <typename T_Point>
class PointCloudT
{
public:
  typedef T_Point PointT;
  typedef std::vector<PointT> VectorT;

  uint32_t height = 0;    ///< Height of point cloud
  uint32_t width = 0;     ///< Width of point cloud
  bool is_dense = false;  ///< If is_dense is true, the point cloud does not contain NAN points
  double timestamp = 0.0; ///< Timestamp of point cloud
  uint32_t seq = 0;       ///< Sequence number of point cloud

  VectorT points;
};

18.3 Member ring of Point

18.3.1 Mechanical LiDAR

In design of mechanical LiDAR, its channels are numbered by scan order. However, this order is not a ascending/descending order, so vertical angles of channels are not either.

rs_driver sorts the channels by their vertical angels in ascending. The member ring is such a sorted number.

18.3.2 MEMS LiDAR

MEMS LiDAR scan 5 zone per scan, and get 5 points. Their timestamps are same.

The member ring is the number of zone.

18.4 Point Layout

18.4.1 Mechanical LiDAR

Take RS32 as an example.

It gets 32 points per scan, 1 point per channel. These 32 points is packed into a Block, and written into MSOP packet.

After a round of scan, LiDAR get a point set on a loop surface.

MSOP packet consists of Block. rs_driver parses Blocks one by one, and saves their points group by group. The points are in the member points of point cloud. LiDARs scans the channels, and rs_driver save their points in the same order.

To traverse all points in a channel, just skip 32 points every time.

18.4.2 MEMS LiDAR

MEMS LiDAR scans 5 zones per scan, and get 5 points, and packs them into a Block, write into MSOP packet.

rs_driverparses Blocks one by one, 5 points in a Blockis saved as a group in the member points of point cloud.

Here is the point cloud in each zone.

The zone is 126 lines x 125 columns. LiDAR scan from up to down in Z order. rs_driver same these points in the same order.

18.5 Coordinate of points

rs_drivercomply with the right-handed coordinate system.

To change this, change mapping relationship of (x, y, z) as below.

//.../decoder/decoder_RSM1.hpp

    int pitch = ntohs(channel.pitch) - ANGLE_OFFSET;
    int yaw = ntohs(channel.yaw) - ANGLE_OFFSET;

    float x = distance * COS (pitch) * COS (yaw);
    float y = distance * COS (pitch) * SIN (yaw);
    float z = distance * SIN (pitch);

18.6 Member timestamp of Point

Mechanical LiDAR (600 rpm） and MEMS LiDAR finish a frame in 0.1 second. Its points scatter into this time range.

To get higher resolution of timestamp than point cloud's, please use the point type of XYZIRT. T is the timestamp of point.

18.7 member timestamp of Point Cloud

timestamp of point cloud may be from from its first point or last point.

18.7.1 From LiDAR, or From Host ?

rs_drivermay get timestamp of point cloud from LiDAR or from host.

  • From MSOP packet. LiDAR set set the timestamp based on its own system clock. Generally, user synchronizes LiDAR's clock with the PTP protocol.
  • Invoke Operation System's API. With it, rs_driver gets the local system time. This is the default.

Use the option use_lidar_clock to change this.

struct RSDecoderParam  ///< LiDAR decoder parameter
{
  ...
  bool use_lidar_clock = false;  ///< true: use LiDAR clock as timestamp; false: use system clock as timestamp
  ...
}

18.7.2 The first point, or The last point ?

The default value is from the last point.

Use the option ts_first_point to change this.

struct RSDecoderParam  ///< LiDAR decoder parameter
{
  ...
  bool ts_first_point = false;   ///< true: time-stamp point cloud with the first point; false: with the last point;
  ...
}

18.7.3 UTC time or Local time ?

LiDAR writes UTC time into MSOP packets.

In CMakeLists.txt, enable the macro ENABLE_STAMP_WITH_LOCAL to convert it to local time.

option(ENABLE_STAMP_WITH_LOCAL    "Enable stamp point cloud with local time" OFF)