line_follower.py

#!/usr/bin/python

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


class Follower:
    def __init__(self):
        rospy.init_node('line_follow')
        self.sub = rospy.Subscriber('/camera/rgb/image_raw', Image, self.on_image, queue_size=1)
        self.pub = rospy.Publisher('/cmd_vel', Twist, queue_size=1)
        self.cv_bridge = CvBridge()
        rospy.on_shutdown(self.stop)

    def run(self):
        rospy.spin()

    def on_image(self, msg):
        image = self.cv_bridge.imgmsg_to_cv2(msg, "mono8")
        h, w = image.shape
        target = w / 2 - 450
        center = np.argmax(image[-1] > 100) + (w - np.argmax(image[-1, ::-1] > 100)) / 2

        vel_msg = Twist()
        vel_msg.linear.x = 0.2
        vel_msg.angular.z = 0.001 * (target - center)
        self.pub.publish(vel_msg)

    def stop(self):
        vel_msg = Twist()
        self.pub.publish(vel_msg)


def main():
    follower = Follower()
    follower.run()


if __name__ == '__main__':
    main()