上一篇博客里介绍Udacity’s无人驾驶仿真环境的社区时，同时也说明了，Udacity‘s simulator是基于Unity 3D，针对每个任务专门构建的封闭式环境（里面场景已经固定）。想要利用这些封闭的仿真环境，首要的一个问题就是：怎么与之进行数据交换（通信）。因此，本篇将进一步介绍Udacity 's无人驾驶仿真环境的python与c++数据接口代码。

系统:ubuntu 18.04 / gcc 7.4.0 + g++ 7.4.0 / anaconda2 + python2.7

1. 测试通讯的仿真环境下载与配置

• 下载Term 2 Simulator到本地
  进入下载页面，选择Term 2 SImulator最新版本，下面有对应系统的安装ZIP文件（此处系统为ubuntu，因而选term2_sim_linux.zip）。
  
• 配置

1. 手动解压，或查找相应命令解压文件；
2. cd到解压得到的文件目录，例如我的为cd /home/xxx/Downloads/term2_sim_linux;
3. 手动或利用命令将与系统对应版本(32位还是64位)term2_sim.x86_64[64位]或term2_sim.x86[32位]变成可执行文件,例如在终端执行sudo chmod +x term2_sim.x86_64;

• 运行

cd /home/xxx/Downloads/term2_sim_linux
./term2_sim.x86_64

出现如下界面（如果电脑性能般，就选分辨率低的，例如下图），点击ok：

进入环境选择（有些封装好的仿真器只有一个环境就没有这个界面），选择对应的环境 ，点击SELECT正式进入仿真环境。

2. C++通讯接口代码

参照DhruvaKumar/model-predictive-contro/src/main.cpp.

• 安装uWebSockets
  参照model-predictive-control/install-ubuntu.sh的步骤进行安装，或者下载该文件，然后运行sudo bash install-ubuntu.sh

#! /bin/bash
sudo apt-get update
sudo apt-get install git libuv1-dev libssl-dev gcc g++ cmake make
git clone https://github.com/uWebSockets/uWebSockets 
cd uWebSockets
git checkout e94b6e1
mkdir build
cd build
cmake ..
make 
sudo make install
cd ../..
sudo ln -s /usr/lib64/libuWS.so /usr/lib/libuWS.so
sudo rm -r uWebSockets

• 通讯C++代码

udacity’s simulator采用json数据格式进行传输，以下代码是完整的通信骨架，里面需要根据自己的需要添加相应的代码到TODO list 1,2,3。

#include <uWS/uWS.h>
#include <iostream>
#include "json.hpp"
// Checks if the SocketIO event has JSON data.
// If there is data the JSON object in string format will be returned,
// else the empty string "" will be returned.
string hasData(string s) {auto found_null = s.find("null");auto b1 = s.find_first_of("[");auto b2 = s.rfind("}]");if (found_null != string::npos) {return "";} else if (b1 != string::npos && b2 != string::npos) {return s.substr(b1, b2 - b1 + 2);}return "";
}
int main() {uWS::Hub h;h.onMessage([&mpc](uWS::WebSocket<uWS::SERVER> ws, char *data, size_t length,uWS::OpCode opCode) {// "42" at the start of the message means there's a websocket message event.// The 4 signifies a websocket message// The 2 signifies a websocket eventstring sdata = string(data).substr(0, length);cout << sdata << endl;if (sdata.size() > 2 && sdata[0] == '4' && sdata[1] == '2') {string s = hasData(sdata);if (s != "") {auto j = json::parse(s);string event = j[0].get<string>();if (event == "telemetry") {// j[1] is the data JSON object/* TODO list 1*//* data parsing code should be putted here!*//* TODO list 2*//* Algorithm that generating feedback control commands code should be putted here!*/json msgJson;/* TODO list 3*//* json datas construction code should be putted here!*/// add header to the json dataauto msg = "42[\"steer\"," + msgJson.dump() + "]"; std::cout << msg << std::endl;// send new constrol command to simulatorthis_thread::sleep_for(chrono::milliseconds(100));ws.send(msg.data(), msg.length(), uWS::OpCode::TEXT);}} else {// Manual drivingstd::string msg = "42[\"manual\",{}]";ws.send(msg.data(), msg.length(), uWS::OpCode::TEXT);}}});// We don't need this since we're not using HTTP but if it's removed the// program// doesn't compile :-(h.onHttpRequest([](uWS::HttpResponse *res, uWS::HttpRequest req, char *data,size_t, size_t) {const std::string s = "<h1>Hello world!</h1>";if (req.getUrl().valueLength == 1) {res->end(s.data(), s.length());} else {// i guess this should be done more gracefully?res->end(nullptr, 0);}});h.onConnection([&h](uWS::WebSocket<uWS::SERVER> ws, uWS::HttpRequest req) {std::cout << "Connected!!!" << std::endl;});h.onDisconnection([&h](uWS::WebSocket<uWS::SERVER> ws, int code,char *message, size_t length) {ws.close();std::cout << "Disconnected" << std::endl;});int port = 4567;if (h.listen(port)) {std::cout << "Listening to port " << port << std::endl;} else {std::cerr << "Failed to listen to port" << std::endl;return -1;}h.run();
}

3. python通讯接口代码

• 安装socketio与eventlet

conda install -c conda-forge python-socketio# 建议用anaconda
conda install -c conda-forge eventlet

• 通讯python代码
  以下是利用python与ubacity‘s simulator通信的核心代码，同样的TODO list 1,2需要添加自己的代码对数据进行解析，处理。

import argparse
import base64
from datetime import datetime
import os
import numpy as np
import socketio
import eventlet
import eventlet.wsgi
from flask import Flasksio = socketio.Server()
app = Flask(__name__)@sio.on('telemetry')
def telemetry(sid, data):if data:# TODO list 1*/# data parsing code should be putted here!*/# TODO list 2*/# Algorithm that generating feedback control commands code should be putted here!*/send_control(steering_angle, throttle)except Exception as e:print(e)else:# NOTE: DON'T EDIT THIS.sio.emit('manual', data={}, skip_sid=True)@sio.on('connect')
def connect(sid, environ):print("connect ", sid)send_control(0, 0)def send_control(steering_angle, throttle):sio.emit("steer",data={'steering_angle': steering_angle.__str__(),'throttle': throttle.__str__()},skip_sid=True)if __name__ == '__main__':model = load_model(args.model)# wrap Flask application with engineio's middlewareapp = socketio.Middleware(sio, app)# deploy as an eventlet WSGI servereventlet.wsgi.server(eventlet.listen(('', 4567)), app)

4. json语法

udacity’s simulator采用Json格式对数据进行传输,Json是一个标记符的序列，这套标记符包含六个构造字符、字符串、数字和三个字面名。Json是一个序列化的对象或数组。

• 六个构造字符

构造字符	符号	意义
begin-array = ws %x5B ws	‘[’	表示一个数组的开始
begin-object = ws %x7B ws	‘{’	表示一个对象的开始
end-array = ws %x5D ws	‘]’	表示一个数组的结束
end-object = ws %x7D ws	‘}’	表示一个对象的结束
name-separator = ws %x3A ws	‘：’	字面值与数值的分隔符
value-separator	‘，’	各值的分隔符

在这六个构造字符的前或后允许存在无意义的空白符(ws)， 例如：
*（%x20 /; 空间
%x09 /; 水平标签
%x0A /; 换行或换行
%x0D）; 回程

• 值可以是对象、数组、数字、字符串或者三个字面值(false、null、true)中的一个。值中的字面值中的英文必须使用小写，例如：

{"name": "John Doe", "age": 18, "address": {"country" : "china", "zip-code": "10000"}}

其它一些示例

{"a": 1, "b": [1, 2, 3]}
[1, 2, "3", {"a": 4}]
3.14
"plain_text"

5. udacity’s simulator Json数据解析示例

针对第一节介绍的term2_sim_linux.zip中的MPC仿真环境的数据给出c++与python解析代码。

5.1 查看数据

我的方式是先利用通信骨架代码，将完整的数据打印出来：

• C++

string sdata = string(data).substr(0, length);
cout << sdata << endl;

42["telemetry",{"ptsx":[-43.49173,-61.09,-78.29172,-93.05002,-107.7717,-123.3917],"ptsy":[105.941,92.88499,78.73102,65.34102,50.57938,33.37102],"psi_unity":4.059185,"psi":3.794796,"x":-55.03409,"y":97.48677,"steering_angle":-0.003226886,"throttle":1,"speed":28.66575}]

• python

print(data)

{u'ptsy': [113.361, 105.941, 92.88499, 78.73102, 65.34102, 50.57938], u'ptsx': [-32.16173, -43.49173, -61.09, -78.29172, -93.05002, -107.7717], u'psi': 3.733651, u'throttle': 0, u'psi_unity': 4.12033, u'y': 108.73, u'x': -40.62, u'steering_angle': 0, u'speed': 0.8760166}

c++与python读出来的数据是一致的，除了数据位置不一样，以及c++将web socket的头数据也加进去了。刨掉头数据，只看Json格式部分，两者读出来的数据格式是一样的。

5.2 解析数据

• c++
  TODO list 1处代码如下，可对以上格式的数据进行解析。

// j[1] is the data JSON object
vector<double> ptsx = j[1]["ptsx"];  // waypoints' position in x axis
vector<double> ptsy = j[1]["ptsy"]; // waypoints' position in y axis
double px = j[1]["x"]; // ugv's position in x axis
double py = j[1]["y"]; // ugv's position in y axis
double psi = j[1]["psi"]; // ugv's headding angle
double v = j[1]["speed"];  // ugv's speed
double delta = j[1]["steering_angle"]; // ugv's steering angle
double acceleration = j[1]["throttle"]; // ugv's throttle

• python

ptsx = [float(d) for d in data["ptsx"]]
ptsy = [float(d) for d in data["ptsy"]]
px = float(data["x"])
py = float(data["x"])
psi = float(data["psi"])
v = float(data["speed"])
delta = float(data["steering_angle"])
acceleration = float(data["throttle"])

5.3 发送数据到simulator

• c++
  C++的发送代码在第二部分的通讯骨架代码中，大致思路是，然构建Json数据，然后加上数据头部，最后发送。

一个例子：

以下代码应放在TODO list 3处.
分成三块：1.构建Json数据；2.加上web socket数据头；3.发送。

/*  构建Json数据 */
json msgJson;  //
msgJson["steering_angle"] = steer_value;
msgJson["throttle"] = throttle_value;//Display the MPC predicted trajectory
//.. add (x,y) points to list here, points are in reference to the vehicle's coordinate system
// the points in the simulator are connected by a Green linemsgJson["mpc_x"] = mpc.x_pred_vals;
msgJson["mpc_y"] = mpc.y_pred_vals;//Display the waypoints/reference line
//.. add## (x,y) points to list here, points are in reference to the vehicle's coordinate system
// the points in the simulator are connected by a Yellow line
vector<double> next_x_vals;
vector<double> next_y_vals;
int n_waypoints = 25;
int step = 2.5;
for (int i = 1; i<n_waypoints; ++i)
{next_x_vals.push_back(step*i);next_y_vals.push_back(polyeval(coeffs, step*i));
}
msgJson["next_x"] = next_x_vals;
msgJson["next_y"] = next_y_vals;

/*  加上web socket数据头 */
auto msg = "42[\"steer\"," + msgJson.dump() + "]";
std::cout << msg << std::endl;
// Latency// The purpose is to mimic real driving conditions where
// the car does actuate the commands instantly.
//
// Feel free to play around with this value but should be to drive
// around the track with 100ms latency.
//
// NOTE: REMEMBER TO SET THIS TO 100 MILLISECONDS BEFORE
// SUBMITTING.

/*  发送 */
this_thread::sleep_for(chrono::milliseconds(100));
ws.send(msg.data(), msg.length(), uWS::OpCode::TEXT);

• python
  调用以下函数

def send_control(steering_angle, throttle):sio.emit("steer",data={'steering_angle': steering_angle.__str__(),'throttle': throttle.__str__()},skip_sid=True)

总结

本篇详细介绍了Udacity’s simulator的c++与python通讯接口，并且以term2_sim_linux.zip的数据为例介绍了Json数据解析与构造。这之后，我们可以与udacity’s simulator进行通信，获取仿真环境的数据，利用算法处理、学习得到控制指令后，发送给仿真环境中的无人车执行。

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以及