ros sensor_msgs::laserscan 数据格式及velodyne_laserscan.cpp文件解析

laserscan数据格式如下（摘自wiki):
每个成员根据注释容易看出表示什么意思，强调一个容易理解错误的地方，ranges[]数组表示雷达旋转时，记录从angle_min到angle_max 角度范围内的距离数据，数组的大小并不是固定的360个，与激光雷达转速、方向角分辨有关，即以多少角度为间隔采集数据，也就是消息里面的angle_increment，(angle_max-angle_min) /angle_increment 即是数组的大小，序号从angle_min开始，即angle_min对应的序号为0，intensities[]与ranges对应，记录每个数据点的强度或者是激光雷达的反射率。
下面通过分析velodyne官方源码中的由pointCloud转为laserscan的文件 velodyne_laserscan.cpp为例，说明如何发布laserscan消息。在注释中说明，请依序看完代码。

// Copyright (C) 2018, 2019 Kevin Hallenbeck, Joshua Whitley
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#include <sensor_msgs/point_cloud2_iterator.h>namespace velodyne_laserscan
{VelodyneLaserScan::VelodyneLaserScan(ros::NodeHandle &nh, ros::NodeHandle &nh_priv) :nh_(nh), srv_(nh_priv), ring_count_(0)
{ros::SubscriberStatusCallback connect_cb = boost::bind(&VelodyneLaserScan::connectCb, this);// 注册并绑定收到pointcloud2消息的回调函数 pub_ = nh.advertise<sensor_msgs::LaserScan>("scan", 10, connect_cb, connect_cb);//声明发布的话题srv_.setCallback(boost::bind(&VelodyneLaserScan::reconfig, this, _1, _2));//动态配置参数的回调函数，当设置的参数改变时回调调用}void VelodyneLaserScan::connectCb()
{boost::lock_guard<boost::mutex> lock(connect_mutex_);if (!pub_.getNumSubscribers()){sub_.shutdown();}else if (!sub_){sub_ = nh_.subscribe("velodyne_points", 10, &VelodyneLaserScan::recvCallback, this);}
}//收到点云数据消息的回调实现，即转化数据并发布scan话题
void VelodyneLaserScan::recvCallback(const sensor_msgs::PointCloud2ConstPtr& msg)
{// Latch ring count// 一开始ring_count为零，有动态参数配置后或者第二次收到数据后，即可正常工作// 此块逻辑不够清晰if (!ring_count_){// Check for PointCloud2 field 'ring'// 检查点去数据中是否有激光ID值，如果没有，则此点云数据无效，不处理数据。bool found = false;for (size_t i = 0; i < msg->fields.size(); i++){if (msg->fields[i].datatype == sensor_msgs::PointField::UINT16){if (msg->fields[i].name == "ring"){found = true;break;}}}if (!found){ROS_ERROR("VelodyneLaserScan: Field 'ring' of type 'UINT16' not present in PointCloud2");return;}//再次检查ring的值，感觉没鸟用，废代码for (sensor_msgs::PointCloud2ConstIterator<uint16_t> it(*msg, "ring"); it != it.end(); ++it){const uint16_t ring = *it;if (ring + 1 > ring_count_){ring_count_ = ring + 1;}}if (ring_count_){ROS_INFO("VelodyneLaserScan: Latched ring count of %u", ring_count_);}else{ROS_ERROR("VelodyneLaserScan: Field 'ring' of type 'UINT16' not present in PointCloud2");return;}}// Select ring to use// 通过配置，获取需要抽取的激光ID,即需要取哪一条线（比如VLP16中抽第8号线）来发布scan，  // 并且给ring_count_赋值uint16_t ring;if ((cfg_.ring < 0) || (cfg_.ring >= ring_count_)){// Default to ring closest to being level for each known sensorif (ring_count_ > 32){ring = 57;  // HDL-64E}else if (ring_count_ > 16){ring = 23;  // HDL-32E}else{ring = 8;  // VLP-16}}else{ring = cfg_.ring;}//如果没有配置参数，则按默认的线来运行，VLP16的8号laser id 最接近水平ROS_INFO_ONCE("VelodyneLaserScan: Extracting ring %u", ring);// Load structure of PointCloud2//获取 PointCloud2 点云数据中x,y,z,强度,laserid的位置偏移，相当于数组索引//初始值为-1,当获取到后，变为相应位置的正数，判断是否存在int offset_x = -1;int offset_y = -1;int offset_z = -1;int offset_i = -1;int offset_r = -1;for (size_t i = 0; i < msg->fields.size(); i++){if (msg->fields[i].datatype == sensor_msgs::PointField::FLOAT32){if (msg->fields[i].name == "x"){offset_x = msg->fields[i].offset;}else if (msg->fields[i].name == "y"){offset_y = msg->fields[i].offset;}else if (msg->fields[i].name == "z"){offset_z = msg->fields[i].offset;}else if (msg->fields[i].name == "intensity"){offset_i = msg->fields[i].offset;}}else if (msg->fields[i].datatype == sensor_msgs::PointField::UINT16){if (msg->fields[i].name == "ring"){offset_r = msg->fields[i].offset;}}}// Construct LaserScan message// 是否存在x,y,z值，如果不存在就报错返回，如果存在就初始化/scan消息// 初始化信息就可以看出/scan消息的结构组成if ((offset_x >= 0) && (offset_y >= 0) && (offset_r >= 0)){// /scan 数据的精度，即range[]数组的大小，//  size 是一圈对应的精度，因为angle_min,angle_max 在后面已经固定写死了//  因此size 是（pi-(-pi))/resolutionconst float RESOLUTION = std::abs(cfg_.resolution);const size_t SIZE = 2.0 * M_PI / RESOLUTION;sensor_msgs::LaserScanPtr scan(new sensor_msgs::LaserScan());scan->header = msg->header;//可以看到angle_increment 的值即为RESOLUTION     scan->angle_increment = RESOLUTION;//从-pi 到 pi ，程序已经写死了scan->angle_min = -M_PI;scan->angle_max = M_PI;// 截取0 到  200 米范围的数据，已经超VLP16量程了，所有数据都有效scan->range_min = 0.0;scan->range_max = 200.0;scan->time_increment = 0.0;//按分辨率推出的，ranges的大小在此定义scan->ranges.resize(SIZE, INFINITY);// 标准的点云数据中各个数据的序号是有规律的，否则是不规则的数据// 然而两者处理方式一样的，此处区分意义不大	if ((offset_x == 0) &&(offset_y == 4) &&(offset_i % 4 == 0) &&(offset_r % 4 == 0)){scan->intensities.resize(SIZE);const size_t X = 0;const size_t Y = 1;const size_t I = offset_i / 4;const size_t R = offset_r / 4;for (sensor_msgs::PointCloud2ConstIterator<float> it(*msg, "x"); it != it.end(); ++it){const uint16_t r = *((const uint16_t*)(&it[R]));  // ring// 对比laser id 是否与需要取的一致，如果不一致，则不更新range[]以及intensities[]if (r == ring){const float x = it[X];  // xconst float y = it[Y];  // yconst float i = it[I];  // intensity//求某一角度对应的序号，反正切值刚好是从-pi 到 pi//序号从angle_min开始升序,angle_min 为-pi,因此实际为：// bin=(atan2f(y,x)-angle_min)/angle_increment		 const int bin = (atan2f(y, x) + static_cast<float>(M_PI)) / RESOLUTION;if ((bin >= 0) && (bin < static_cast<int>(SIZE))){//range[] 的值是平面距离scan->ranges[bin] = sqrtf(x * x + y * y);scan->intensities[bin] = i;}}}}else{ROS_WARN_ONCE("VelodyneLaserScan: PointCloud2 fields in unexpected order. Using slower generic method.");if (offset_i >= 0){scan->intensities.resize(SIZE);sensor_msgs::PointCloud2ConstIterator<uint16_t> iter_r(*msg, "ring");sensor_msgs::PointCloud2ConstIterator<float> iter_x(*msg, "x");sensor_msgs::PointCloud2ConstIterator<float> iter_y(*msg, "y");sensor_msgs::PointCloud2ConstIterator<float> iter_i(*msg, "intensity");for ( ; iter_r != iter_r.end(); ++iter_x, ++iter_y, ++iter_r, ++iter_i){const uint16_t r = *iter_r;  // ringif (r == ring){const float x = *iter_x;  // xconst float y = *iter_y;  // yconst float i = *iter_i;  // intensity            const int bin = (atan2f(y, x) + static_cast<float>(M_PI)) / RESOLUTION;if ((bin >= 0) && (bin < static_cast<int>(SIZE))){scan->ranges[bin] = sqrtf(x * x + y * y);scan->intensities[bin] = i;}}}}else{sensor_msgs::PointCloud2ConstIterator<uint16_t> iter_r(*msg, "ring");sensor_msgs::PointCloud2ConstIterator<float> iter_x(*msg, "x");sensor_msgs::PointCloud2ConstIterator<float> iter_y(*msg, "y");for (; iter_r != iter_r.end(); ++iter_x, ++iter_y, ++iter_r){const uint16_t r = *iter_r;  // ringif (r == ring){const float x = *iter_x;  // xconst float y = *iter_y;  // yconst int bin = (atan2f(y, x) + static_cast<float>(M_PI)) / RESOLUTION;if ((bin >= 0) && (bin < static_cast<int>(SIZE))){scan->ranges[bin] = sqrtf(x * x + y * y);}}}}}//发布 消息pub_.publish(scan);}else{ROS_ERROR("VelodyneLaserScan: PointCloud2 missing one or more required fields! (x,y,ring)");}
}//动态参数回调，设置ring 以及revolution
void VelodyneLaserScan::reconfig(VelodyneLaserScanConfig& config, uint32_t level)
{cfg_ = config;
}}  // namespace velodyne_laserscan