title: error while extracting images from a ROS bag
body: <p>i have been working on a localization via image processing localization project and the data sets we got was a compressed images in a .bag files so i have multiple ways to extract these images at first i tried the following method:</p>
  
  <pre><code>mkdir images
  cd ./images
  rosrun image_view extract_images _sec_per_frame:=0.01 image:=&lt;IMAGETOPICINBAGFILE&gt;
  run rosbag play &lt;BAGFILE&gt;
  </code></pre>
  
  <p>and i have tried to use the launch file as well</p>
  
  <pre><code>&lt;launch&gt;
     &lt;node pkg="rosbag" type="play" name="rosbag" args="-d 2 /home/devo/datasets/Ch2_002/HMB_1.bag"/&gt;
     &lt;node name="extract" pkg="image_view" type="extract_images" respawn="false" output="screen" cwd="ROS_HOME" args="compressed"&gt;
     &lt;remap from="image" to="/camera/image_raw"/&gt;
     &lt;param name="image_transport" value="compressed"/&gt;
     &lt;/node&gt;
  &lt;/launch&gt;
  </code></pre>
  
  <p>but it always ended up giving me the same error</p>
  
  <blockquote>
    <p>[ERROR] [1486685243.634766995]: Client [/extract_images_1486684806987962257] wants topic /can_bus_dbw/can_rx to have datatype/md5sum [sensor_msgs/Image/060021388200f6f0f447d0fcd9c64743], but our version has [dataspeed_can_msgs/CanMessageStamped/33747cb98e223cafb806d7e94cb4071f]. Dropping connection.</p>
  </blockquote>
  
  <p>so i just wanted to know what did i do wrong and how can i extract these images from the bag (knowing that my ROS version : 1.12.6)</p>