写在前面：

系统：ubuntu18.04 LST
ros 版本：melodic
自行安装usb_cam
依赖：

sensor_msgs
opencv2
cv_bridge
rospy
std_msgs

代码

读取usb_cam摄像头图像，画个圆圈，再发布出去。

#!/usr/bin/env python
from __future__ import print_function

# import roslib
# roslib.load_manifest('my_package')
import sys
import rospy
import cv2
from std_msgs.msg import String
from sensor_msgs.msg import Image
from cv_bridge import CvBridge, CvBridgeError

class image_converter:

  def __init__(self):
    self.image_pub = rospy.Publisher("image_topic_2",Image)

    self.bridge = CvBridge()
    self.image_sub = rospy.Subscriber("/usb_cam/image_raw",Image,self.callback)

  def callback(self,data):
    try:
      cv_image = self.bridge.imgmsg_to_cv2(data, "bgr8")
    except CvBridgeError as e:
      print(e)

    (rows,cols,channels) = cv_image.shape
    if cols > 60 and rows > 60 :
      cv2.circle(cv_image, (50,50), 10, 255)

    cv2.imshow("Image window", cv_image)
    cv2.waitKey(3)

    try:
      self.image_pub.publish(self.bridge.cv2_to_imgmsg(cv_image, "bgr8"))
    except CvBridgeError as e:
      print(e)

def main(args):
  ic = image_converter()
  rospy.init_node('image_converter', anonymous=True)
  try:
    rospy.spin()
  except KeyboardInterrupt:
    print("Shutting down")
  cv2.destroyAllWindows()

if __name__ == '__main__':
    main(sys.argv)

相关链接：
Converting between ROS images and OpenCV images (Python)
Ros利用usb_cam进行opencv测试