How to create disparity images with ueye_cam and nodelets

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Hi there,

I'm currently trying to get a disparity image out of a pair of externally triggered UEye cameras from IDS. I'm using nodelets instead of nodes to avoid overhead and desynchronization caused by the transfer of data over TCP and the necessary (de)serialization. My cameras are working and I get rectified versions of the camera image, but the last nodelet in the launchfile advertises a "disparity" topic but does not publish any data for it.

Below you can see my launchfile:

<launch>

   <arg name="manager" default="nodelet_manager"/> 

   <node pkg="nodelet" type="nodelet" name="$(arg manager)" args="manager" output="screen" />

   <node pkg="nodelet" type="nodelet" name="ueye_cam_nodelet_left"
      args="load ueye_cam/ueye_cam_nodelet $(arg manager)">
      <param name="camera_name" type="str" value="left" /> <!-- == namespace for topics and services -->
      <param name="camera_topic" type="str" value="image_raw" />
      <param name="camera_id" type="int" value="1" /> <!-- 0 = any camera; 1+: camera ID -->
      <param name="camera_intrinsics_file" type="string" value="$(find ueye_cam)/resources/left.yaml" /> <!-- default: ~/.ros/camera_info/<camera_name>.yaml -->
      <param name="camera_parameters_file" type="string" value="" /> <!-- default: ~/.ros/camera_conf/<camera_name>.ini -->
      <param name="ext_trigger_mode" type="bool" value="True" /> <!-- if False, then camera will operate in free-run mode; otherwise, frames need to be triggered by hardware signal (falling-edge) on digital input pin of camera -->
[...]
</node>

<node pkg="nodelet" type="nodelet" name="ueye_cam_nodelet_right"
    args="load ueye_cam/ueye_cam_nodelet $(arg manager)">
      <param name="camera_name" type="str" value="right" /> <!-- == namespace for topics and services -->
      <param name="camera_topic" type="str" value="image_raw" />
      <param name="camera_id" type="int" value="2" /> <!-- 0 = any camera; 1+: camera ID -->
      <param name="camera_intrinsics_file" type="string" value="" /> <!-- default: ~/.ros/camera_info/<camera_name>.yaml -->
      <param name="camera_parameters_file" type="string" value="" /> <!-- default: ~/.ros/camera_conf/<camera_name>.ini -->
      <param name="ext_trigger_mode" type="bool" value="True" /> <!-- if False, then camera will operate in free-run mode; otherwise, frames need to be triggered by hardware signal (falling-edge) on digital input pin of camera -->
[...]
</node>

<node pkg="nodelet" type="nodelet" name="debayer_left"
    args="load image_proc/debayer $(arg manager)">
      <remap from="image_raw" to="/left/image_raw" />
      <remap from="image_mono" to="/left/image_mono" />
      <remap from="image_color" to="/left/image_color" />
</node>

<node pkg="nodelet" type="nodelet" name="debayer_right"
  args="load image_proc/debayer $(arg manager)">
      <remap from="image_raw" to="/right/image_raw" />
      <remap from="image_mono" to="/right/image_mono" />
      <remap from="image_color" to="/right/image_color" />
</node>

<node pkg="nodelet" type="nodelet" name="rectifier_right"
  args="load image_proc/rectify $(arg manager)">
      <remap from="camera_info" to="/right/camera_info" />
      <remap from="image_mono" to="/right/image_mono" />
      <remap from="image_rect" to="/right/image_rect" />
</node>

<node pkg="nodelet" type="nodelet" name="rectifier_left"
  args="load image_proc/rectify $(arg manager)">
      <remap from="camera_info" to="/left/camera_info" />
      <remap from="image_mono" to="/left/image_mono" />
      <remap from="image_rect" to="/left/image_rect" />
</node>

<node pkg="nodelet" type="nodelet" name="disparity"
args="load stereo_image_proc/disparity $(arg manager)" />

</launch>