rosbag image to cv image

1.use ros.wiki launch method

roswiki link

2.convert compressed image of bag to cv image

#!/usr/bin/python

# Extract images from a bag file.

#PKG = 'beginner_tutorials'
import roslib;   #roslib.load_manifest(PKG)
import rosbag
import rospy
import cv2
#from sensor_msgs.msg import Image
from sensor_msgs.msg import CompressedImage
from cv_bridge import CvBridge
from cv_bridge import CvBridgeError
import numpy as np
# Reading bag filename from command line or roslaunch parameter.
import os
import sys


class ImageCreator():


	def __init__(self):
		self.bridge = CvBridge()
       	with rosbag.Bag('/home/xxx/rosbag_images/zero.bag', 'r') as bag:  
            for topic,msg,t in bag.read_messages():
                if topic == "/camera/rgb/image_raw_L/compressed": 
					print 'received image of type: "%s"' % msg.format
					np_arr = np.fromstring(msg.data,np.uint8)
					cv_image = cv2.imdecode(np_arr,cv2.IMREAD_COLOR)
					img_file = "/home/xxx/rosbag_images/zero"
					timestr = "%.6f" %  msg.header.stamp.to_sec()
                        
					image_name = timestr+ ".png"
					cv2.imwrite(img_file +"/"+ image_name, cv_image)

if __name__ == '__main__':

    #rospy.init_node(PKG)

    try:
        image_creator = ImageCreator()
    except rospy.ROSInterruptException:
        pass

3. convert compressed image of bag to .avi video

#!/usr/bin/env python

import cv2
import numpy as np
import os
import rosbag
from sensor_msgs.msg import CompressedImage
from cv_bridge import CvBridge
from cv_bridge import CvBridgeError


# SETTINGS FOR SAVING OUTPUT VIDEO
out_file = 'outdoors01.avi'
fourcc_setings = cv2.VideoWriter_fourcc(*'XVID')
out_vid_dims = (640, 480)
fps = 30
out = cv2.VideoWriter(out_file,
                      fourcc=fourcc_setings,
                      fps=fps,
                      frameSize=out_vid_dims,
                      isColor=True)

# ROSBAG SETTINGS
bag_file = 'outdoors01_compressedimage.bag'

# OPEN BAG
bag = rosbag.Bag(bag_file, "r")
messages = bag.read_messages(topics=["/usb_cam/image_raw/compressed"])
num_images = bag.get_message_count(topic_filters=["/usb_cam/image_raw/compressed"])

for topic, msg, t in  bag.read_messages():
    if topic == "/usb_cam/image_raw/compressed":

        # CONVERT MESSAGE TO A NUMPY ARRAY
        np_arr = np.fromstring(msg.data, np.uint8)
        img = cv2.imdecode(np_arr, cv2.IMREAD_COLOR)

        if img is not None:
            print ('received image of type: "%s"' % msg.format)
            img = cv2.resize(img, out_vid_dims)

            # SHOW THE IMAGE
            cv2.imshow("rosbag_video", img)

            # SAVE THE FRAME TO THE OUTPUT VIDEO
            out_vid = cv2.resize(img, out_vid_dims)
            out.write(out_vid)

out.release()
cv2.destroyWindow('rosbag_video')
print("done!")