Extract data from Image.msg

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I need to process the data from a webcam (i used USB_CAM Package).

Initially I created a listener that from the topic "/usb_cam/image_raw" extrapolates the array uint8 [] data ... but ...

Here's the python code:

# / usr / bin / env python
import roslib; roslib.load_manifest ('webcam_image')

import rospy
Image import from sensor_msgs.msg

def callback (date):
     print "I heard% s"% str (Image.data)

def listener ():
     rospy.init_node ('read_data', anonymous = True)

     rospy.Subscriber ("usb_cam / image_raw", Image, callback)

     rospy.spin ()

if __ name__ == '__main__':
     listener ()

And there the output on the shell when I start:

I heard < member'data' of'Image' objects>

If i write:

rostopic echo /usb_cam/image_raw

appears correctly:

header:
  seq: 491
  stamp:
    secs: 1372323952
    nsecs: 364679466
  frame_id: usb_cam
height: 4
width: 4
encoding: rgb8
is_bigendian: 0
step: 12
data: [103, 111, 98, 104, 112, 99, 102, 110, 104, 103, 111, 105, 100, 112, 100, 101, 113, 101, 103, 109, 107, 102, 108, 106, 101, 108, 108, 102, 109, 109, 102, 109, 109, 102, 109, 109, 103, 110, 102, 102, 109, 101, 105, 113, 100, 100, 108, 95]

What could be wrong?