Launch files in ROS can do multiple tasks in a single file. Launch files have an extension of .launch. The following code shows the definition of start_usb_cam.launch, which starts the usb_cam node for publishing the camera image as an ROS topic:

    <launch> 
      <node name="usb_cam" pkg="usb_cam" type="usb_cam_node" 
    output="screen" > 
        <param name="video_device" value="/dev/video0" /> 
        <param name="image_width" value="640" /> 
        <param name="image_height" value="480" /> 
        <param name="pixel_format" value="yuyv" /> 
        <param name="camera_frame_id" value="usb_cam" /> 
        <param name="auto_focus" value="false" /> 
        <param name="io_method" value="mmap"/> 
      </node> 
    </launch> 

Within the <node>...</node> tags, there are camera parameters that can be changed by the user. For example, if you have multiple cameras, you can change the video_device value from /dev/video0 to /dev/video1 to get the second camera's frames.

The next important launch file is start_tracking.launch, which will launch the face-tracker node. Here is the definition of this launch file:

    <launch> 
    <!-- Launching USB CAM launch files and Dynamixel controllers --> 
      <include file="$(find 
    face_tracker_pkg)/launch/start_usb_cam.launch"/> 
 
    <!-- Starting face tracker node --> 
       <rosparam file="$(find face_tracker_pkg)/config/track.yaml" 
    command="load"/> 
 
       <node name="face_tracker" pkg="face_tracker_pkg" 
    type="face_tracker_node" output="screen" /> 
    </launch> 

It will first start the start_usb_cam.launch file to get ROS image topics, then load track.yaml to get the necessary ROS parameters, and then load face_tracker_node to start tracking.

The final launch file is start_dynamixel_tracking.launch; this is the launch file we have to execute for tracking and Dynamixel control. We will discuss this launch file at the end of this chapter, after discussing the face_tracker_control package. Let's now learn how to run the face tracker node.