Python OpenCV namedWindow and imshow freeze

edit

The intendation issue was really just a copy-paste error, I corrected it. I've also verified that the image is actually being published on this topic. When I leave out the line with cv2.namedWindow(window_name), the image is properly shown in a OpenCV-Window, caused by the line cv2.imshow(window_name, cv_img). But like I said, I want to declare and initialize the namedWindow outside of the callback function to prevent the above mentioned error, which may be an overflow caused by the recursive call of the callback function.

Updated Answer

Here is how it works for me. I can now close the cv2 windows with the Escape-Button. I've also added trackbars to update image detection parameters during runtime, maybe that is also helpful for somebody.

#!/usr/bin/env python
import sys, rospy, traceback, math, cv2, numpy as np, std_msgs.msg
from sensor_msgs.msg import Image
from cv_bridge import CvBridge, CvBridgeError


class objectDetection():
    def __init__(self):
        self.bridge = CvBridge()
        self.sub = rospy.Subscriber('/camera/rgb/image_rect_color', Image, self.callback)
        self.orig_img = None
        self.params = None

    def callback(self, data):
        self.params = self.getTrackbarParams()
        try:
            cv_img = self.bridge.imgmsg_to_cv2(frame, "bgr8")
        except CvBridgeError as exc:
            print(traceback.format_exc())
        if 'cv_img' in locals():
            self.orig_img = cv_img

    def getTrackbarParams(self):
        return [cv2.getTrackbarPos('Param', 'Parameter-Trackbars')]


def showImages(obj):
    if obj.orig_img is not None:
        cv2.imshow('Original', obj.orig_img)

def handleTrackbarChanges(obj):
    params = obj.getTrackbarParams()

def trackbar_callback(x):
    pass

def createWindowsAndTrackbars():     
    cv2.namedWindow('Parameter-Trackbars')
    cv2.namedWindow('Original')

    cv2.createTrackbar('Param', 'Parameter-Trackbars', 0, 179, trackbar_callback)
    # some more stuff

def main(args):
    createWindowsAndTrackbars()
    od = objectDetection()
    rospy.init_node('objectdetection', anonymous=True)

    while (1):
        [...]
        cv2.imshow('Parameter-Trackbars', cv2.cvtColor(trackbar_img, cv2.COLOR_HSV2BGR))
        showImages(od)

        k = cv2.waitKey(1) & 0xFF
        if k == 27:
            break

        handleTrackbarChanges(od)

        try:
            rospy.spin()
        except KeyboardInterrupt:
            print('Shutting down...')
        cv2.destroyAllWindows()

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