How to use image_proc/crop_decimate to crop an image?

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Hi, I used usb_cam driver to publish a camera raw image, and I'm trying to crop the published image using image_proc/crop_decimate. when I viewed the output of image_proc/crop_decimate, I did not see any difference between the original image and the cropped image. it seems that the image_proc/crop_decimate node is bypassing the parameters of my launch file. the size of the original image 640x480 and I want to crop the image to 640x300 This is my launch file script:

<node pkg="image_proc" type="image_proc"
      ns="usb_cam"
      name="image_proc">
</node>

<node pkg="nodelet" type="nodelet"
      args="standalone image_proc/crop_decimate"
      name="my_decimator">
  <param name="x_offset" type="int" value="0" />
  <param name="y_offset" type="int" value="0" />
  <param name="width" type="int" value="640" />
  <param name="height" type="int" value="250" />

  <!-- remap input topics -->
  <remap from="usb_cam/image_rect_color" to="camera/image_raw"/>
  <remap from="usb_cam/camera_info" to="camera/camera_info"/>

  <!-- remap output topics -->
  <remap from="camera_out/image_raw" to="camera_crop/image_rect_color"/>
  <remap from="camera_out/camera_info" to="camera_crop/camera_info"/>
</node>

Does any one has an idea how to use image_proc/crop_decimate node?

Thanks