ROS通过话题发布订阅Image类型的视频帧（python）
前言：
本文中，主要是关于OpenCV格式图片（或视频帧）和ROS数据格式图片（或视频帧）之间的转换。或者直白点书，通过ROS发送图片（Image）数据类型的消息（message）。
本文其实是为下一篇博文“YOLO在ROS下的使用”打下基础。
1、使用环境和平台
ubuntu 18.04+ python2.7+opencv3注意：使用python3的话提示报错，还是用python2吧
2、示例代码
其实，下述代码完全可以在一个脚本中完成，而且不需要结合ROS；本文为了讲述通过ROS发送Image的方法，故而拆分开来。一个脚本中，只进行图像捕捉；另一个订阅之后，只进行图像现实。
(1)通过调用webcam捕捉视频，然后经过ROS的Topic发布出去：
#!/usr/bin/env python
#!coding=utf-8
#write by leo at 2018.04.26
#function:
#1, Get live_video from the webcam.
#2, With ROS publish Image_info (to yolo and so on).
#3, Convert directly, don't need to save pic at local.
import rospy
from sensor_msgs.msg import Image
import cv2
import numpy as np
from cv_bridge import CvBridge, CvBridgeError
import sys
def webcamImagePub():
# init ros_node
rospy.init_node('webcam_puber', anonymous=True)
# queue_size should be small in order to make it 'real_time'
# or the node will pub the past_frame
img_pub = rospy.Publisher('webcam/image_raw', Image, queue_size=2)
rate = rospy.Rate(5) # 5hz
# make a video_object and init the video object
cap = cv2.VideoCapture(0)
# define picture to_down' coefficient of ratio
scaling_factor = 0.5
# the 'CVBridge' is a python_class, must have a instance.
# That means "cv2_to_imgmsg() must be called with CvBridge instance"
bridge = CvBridge()
if not cap.isOpened():
sys.stdout.write("Webcam is not available !")
return -1
count = 0
# loop until press 'esc' or 'q'
while not rospy.is_shutdown():
ret, frame = cap.read()
# resize the frame
if ret:
count = count + 1
else:
rospy.loginfo("Capturing image failed.")
if count == 2:
count = 0
frame = cv2.resize(frame,None,fx=scaling_factor,fy=scaling_factor,interpolation=cv2.INTER_AREA)
msg = bridge.cv2_to_imgmsg(frame, encoding="bgr8")
img_pub.publish(msg)
print '** publishing webcam_frame ***'
rate.sleep()
if __name__ == '__main__':
try:
webcamImagePub()
except rospy.ROSInterruptException:
pass(2)通过ROS订阅Image类型的视频帧，然后在窗口显示出来：
#!/usr/bin/env python
#!coding=utf-8
#right code !
#write by leo at 2018.04.26
#function:
#display the frame from another node.
import rospy
import numpy as np
from sensor_msgs.msg import Image
from cv_bridge import CvBridge, CvBridgeError
import cv2
def callback(data):
# define picture to_down' coefficient of ratio
scaling_factor = 0.5
global count,bridge
count = count + 1
if count == 1:
count = 0
cv_img = bridge.imgmsg_to_cv2(data, "bgr8")
cv2.imshow("frame" , cv_img)
cv2.waitKey(3)
else:
pass
def displayWebcam():
rospy.init_node('webcam_display', anonymous=True)
# make a video_object and init the video object
global count,bridge
count = 0
bridge = CvBridge()
rospy.Subscriber('webcam/image_raw', Image, callback)
rospy.spin()
if __name__ == '__main__':
displayWebcam()当然，上面话题发布之后，也可以使用RVIZ工具箱的image工具进行显示
3、代码解释（函数讲解）
从代码中可以看出：
from cv_bridge import CvBridge, CvBridgeError导入了一个模块下的两个类，然后实例化一个对象：
bridge = CvBridge()接下来，调用该对象下的方法（函数）：
msg = bridge.cv2_to_imgmsg(frame, encoding="bgr8")
img_pub.publish(msg)发布信息的脚本（上程序（1）中）里，利用此方法将OpenCV类型的图片转化为ROS类型，然后通过话题发布出去；
然后：
cv_img = bridge.imgmsg_to_cv2(data, "bgr8")订阅话题的脚本（上程序（2）中）里，利用此方法将订阅到的ROS类型的数据转化为OpenCV格式的图片，然后通过imshow函数在窗口显示出图像。
PS：上边的程序中，不论发布还是订阅，都可以跳过一些帧（通过改变count的值即可）。
4、参考链接
ROS初学者教程：
http://wiki.ros.org/cn/ROS/Tutorials
OpnCV-Python教程：
https://docs.opencv.org/3.0-beta/doc/py_tutorials/py_tutorials.html
Converting between ROS images and OpenCV images (Python) https://answers.ros.org/question/226819/trouble-converting-cv2-to-imgmsg/ http://wiki.ros.org/cv_bridge/Tutorials/ConvertingBetweenROSImagesAndOpenCVImagesPython
https://github.com/pirobot/ros-by-example/blob/master/rbx_vol_1/rbx1_vision/nodes/cv_bridge_demo.py
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