Extract 60 fps image from bag file [closed]

edit

I have a bag file with a 60 fps camera image stream. I want to extract all the image frames. However, even if I specified _sec_per_frame to a very small value, I still got very low fps image files: there should be ~20k images for my 5min bag file, but now I only get ~2k images.

Here's the launch file I used:

<launch>
  <node pkg="rosbag" type="play" name="rosbag" args="-d 2 /home/hangchu/MITLL/testTrackLoop_ii_2015-06-26-14-43-04.bag"/>
  <node name="extract" pkg="image_view" type="extract_images" respawn="false" cwd="node">
    <remap from="image" to="/camera/image_raw"/>
    <param name="filename_format" value="/home/hangchu/MITLL/images/frame%06i.jpg"/>
    <param name="_sec_per_frame" value="0.001"/>
  </node>
</launch>