How to subscribe and publish images in ROS

Question

I am trying to subscribe to the "/camera/image_color" topic which is data from my camera. I then want to do some voodoo on these images in opencv and publish them at a specific frequency. So that I can subscribe to them with another node.

I have sofar tried the below code, and many many variations thereof. At this point the code is doing nothing. No images are being published to the "/imagetimer" topic. I get an output "Timing images" then nothing happens further.

#!/usr/bin/env python
# -*- encoding: utf-8 -*-
import rospy
from sensor_msgs.msg import Image
from cv_bridge import CvBridge
import cv2
import os
import numpy as np


class Nodo(object):
    def __init__(self):
        # Params
        self.image = None
        self.br = CvBridge()
        # Node cycle rate (in Hz).
        self.loop_rate = rospy.Rate(1)

        # Publishers
        self.pub = rospy.Publisher('imagetimer', Image,queue_size=10)

        # Subscribers
        rospy.Subscriber("/camera/image_color",Image,self.callback)


    def callback(self, msg):
        self.image = self.br.imgmsg_to_cv2(msg)


    def start(self):
        rospy.loginfo("Timing images")
        rospy.spin()
        br = CvBridge()
        while not rospy.is_shutdown():
            rospy.loginfo('publishing image')
            #br = CvBridge()
            self.pub.publish(br.cv2_to_imgmsg(self.image))
            rate.sleep()

if __name__ == '__main__':
    rospy.init_node("imagetimer111", anonymous=True)
    my_node = Nodo()
    my_node.start()

Solution

Once you run rospy.spin() the code doesn't go forward. In rospy as soon as you have the rospy.Subsriber() line it spins off another thread for the callback. A rospy.spin() essentially keeps the node alive so the callbacks and keep chugging. Here you are using a while loop for keeping the node alive, so shouldn't need rospy.spin(). This version should work:

import rospy
from sensor_msgs.msg import Image
from cv_bridge import CvBridge
import cv2
import os
import numpy as np

class Nodo(object):
    def __init__(self):
        # Params
        self.image = None
        self.br = CvBridge()
        # Node cycle rate (in Hz).
        self.loop_rate = rospy.Rate(1)

        # Publishers
        self.pub = rospy.Publisher('imagetimer', Image,queue_size=10)

        # Subscribers
        rospy.Subscriber("/camera/image_color",Image,self.callback)

    def callback(self, msg):
        rospy.loginfo('Image received...')
        self.image = self.br.imgmsg_to_cv2(msg)


    def start(self):
        rospy.loginfo("Timing images")
        #rospy.spin()
        while not rospy.is_shutdown():
            rospy.loginfo('publishing image')
            #br = CvBridge()
            if self.image is not None:
                self.pub.publish(br.cv2_to_imgmsg(self.image))
            self.loop_rate.sleep()

if __name__ == '__main__':
    rospy.init_node("imagetimer111", anonymous=True)
    my_node = Nodo()
    my_node.start()

Comments

[Vik]

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