ROS通过话题发布订阅Image类型的视频帧（python）

「爱情、让人受尽委屈。」 2023-03-01 10:30 32阅读 0赞

## 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][http_wiki.ros.org_cn_ROS_Tutorials]

OpnCV-Python教程：

[https://docs.opencv.org/3.0-beta/doc/py\_tutorials/py\_tutorials.html][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/][https_answers.ros.org_question_226819_trouble-converting-cv2-to-imgmsg]  
[http://wiki.ros.org/cv\_bridge/Tutorials/ConvertingBetweenROSImagesAndOpenCVImagesPython][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][https_github.com_pirobot_ros-by-example_blob_master_rbx_vol_1_rbx1_vision_nodes_cv_bridge_demo.py]

[http_wiki.ros.org_cn_ROS_Tutorials]: http://wiki.ros.org/cn/ROS/Tutorials
[https_docs.opencv.org_3.0-beta_doc_py_tutorials_py_tutorials.html]: https://docs.opencv.org/3.0-beta/doc/py_tutorials/py_tutorials.html
[https_answers.ros.org_question_226819_trouble-converting-cv2-to-imgmsg]: https://answers.ros.org/question/226819/trouble-converting-cv2-to-imgmsg/
[http_wiki.ros.org_cv_bridge_Tutorials_ConvertingBetweenROSImagesAndOpenCVImagesPython]: 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]: https://github.com/pirobot/ros-by-example/blob/master/rbx_vol_1/rbx1_vision/nodes/cv_bridge_demo.py