Access Image via OpenCV in python (ROS Kinetic) - python-2.7

Question

I want to access my camera using OpenCV in ros kinetic,
this is my code
#!/usr/bin/env python2.7
import rospy
from sensor_msgs.msg import Image
import cv2
from cv_bridge import CvBridge, CvBridgeError
rospy.init_node('opencv_example', anonymous=True)
bridge = CvBridge()
def show_image(img):
cv2.imshow("Image Window", img)
cv2.waitKey(3)
def image_callback(img_msg):
try:
cv_image = bridge.imgmsg_to_cv2(img_msg, "passthrough")
except CvBridgeError, e:
rospy.logerr("CvBridge Error: {0}".format(e))
show_image(cv_image)
sub_image = rospy.Subscriber("/raspicam_node/image/compressed", Image, image_callback)
cv2.namedWindow("Image Window", 1)
while not rospy.is_shutdown():
rospy.spin()
all I get after this code is a blank image window
before you ask the topic address, I can access the camera with this command rostrum image_view image_view image:=/raspicam_node/image/ _image_transport:=compressed
currently, I am working with
Ubuntu 16.04
Ros Kinetic
Open CV 3.3.1

Answer 1

You've answered your own question, the image is coming in compressed. Because of this your subscriber is also wrong there is a specific sensor_msgs type for Compressed Images.
Using numpy you can decode the image directly in your subscriber like so:
def image_callback(img_msg):
np_arr = np.fromstring(img_msg.data, np.uint8)
image_np = cv2.imdecode(np_arr, cv2.CV_LOAD_IMAGE_COLOR)
show_image(cv_image)
As stated earlier, your subscriber also needs to look like
sub_image = rospy.Subscriber("/raspicam_node/image/compressed", CompressedImage, image_callback)
And you need to include from sensor_msgs.msg import CompressedImage
Edit:
Based on your comment you can also republish the compressed image as an uncompressed image to use with your original code using image_transport.
If you want to use rosrun in the terminal: rosrun image_transport republish compressed in:=/raspicam_node/image/compressed raw out:=/raspicam_node/image/uncompressed
Or if you're using a launch file:
<node name="republish" type="republish" pkg="image_transport" output="screen" args="compressed in:=/raspicam_node/image/compressed raw out:=/raspicam_node/image/uncompressed" />
Then you need to change your original code to
#!/usr/bin/env python2.7
import rospy
from sensor_msgs.msg import Image
import cv2
from cv_bridge import CvBridge, CvBridgeError
rospy.init_node('opencv_example', anonymous=True)
bridge = CvBridge()
def show_image(img):
cv2.imshow("Image Window", img)
cv2.waitKey(3)
def image_callback(img_msg):
try:
cv_image = bridge.imgmsg_to_cv2(img_msg, "passthrough")
except CvBridgeError, e:
rospy.logerr("CvBridge Error: {0}".format(e))
show_image(cv_image)
sub_image = rospy.Subscriber("/raspicam_node/image/uncompressed", Image, image_callback)
cv2.namedWindow("Image Window", 1)
while not rospy.is_shutdown():
rospy.spin()