libuvc_camera啓動報錯Invalid mode問題的解決辦法

libuvc_camera是ROS下的一個軟件包，用於將USB攝像頭的視頻內容發佈到Topic上，但是啓動時

roslaunch uvc.launch

經常會遇到以下報錯：

process[camera/mycam-1]: started with pid [4578]
unsupported descriptor subtype VS_COLORFORMAT
unsupported descriptor subtype VS_COLORFORMAT
uvc_get_stream_ctrl_format_size: Invalid mode (-51)
[ERROR] [1578882156.980703592]: check video_mode/width/height/frame_rate are available
DEVICE CONFIGURATION (0000:0000/[none]) ---
Status: idle
VideoControl:
	bcdUVC: 0x7273
[camera/mycam-1] process has died [pid 4578, exit code -11, cmd /opt/ros/kinetic/lib/libuvc_camera/camera_node __name:=mycam __log:=/home/jeffrey2019/.ros/log/b6a4ed00-35a9-11ea-be20-000c297f47e8/camera-mycam-1.log].
log file: /home/jeffrey2019/.ros/log/b6a4ed00-35a9-11ea-be20-000c297f47e8/camera-mycam-1*.log
all processes on machine have died, roslaunch will exit
shutting down processing monitor...
... shutting down processing monitor complete
done

其中

uvc_get_stream_ctrl_format_size: Invalid mode (-51)

給出了報錯的關鍵，指定的工作模式不被攝像頭支持，必須找到一種被支持的模式才行。
對於我的戴爾G3 3579筆記本攝像頭，我並不知道它支持哪些模式，只好不斷試錯，最終發現這種可以：

<launch>
  <group ns="camera">
    <node pkg="libuvc_camera" type="camera_node" name="mycam">
      <!-- Parameters used to find the camera -->
      <param name="vendor" value="0x0bda"/>
      <param name="product" value="0x568a"/>
      <param name="serial" value=""/>
      <!-- If the above parameters aren't unique, choose the first match: -->
      <param name="index" value="0"/>

      <!-- Image size and type -->
      <param name="width" value="640"/> <!-- 640 -->
      <param name="height" value="480"/> <!-- 480 -->
      <!-- choose whichever uncompressed format the camera supports: -->
      <param name="video_mode" value="uncompressed"/> <!-- or yuyv/nv12/mjpeg -->
      <param name="frame_rate" value="30"/>

      <param name="timestamp_method" value="start"/> <!-- start of frame -->
      <param name="camera_info_url" value="file:///tmp/cam.yaml"/>

      <param name="auto_exposure" value="3"/> <!-- use aperture_priority auto exposure -->
      <param name="auto_white_balance" value="false"/>
    </node>
  </group>
</launch>

試錯區域集中在Image size and type區段，包括width、height、video_mode、frame_rate，每一個參數都必須調整到攝像頭恰好支持的配置，不能高也不能低，例如frame_rate，目前大多數攝像頭都有30，那麼libuvc_camera的wiki頁面上的默認取值15就是不對的。

試錯成功後，啓動下面命令行，就能看到視頻流：

rosrun image_view image_view image:=/camera/image_raw