Webcam doesn't work in Hydro

edit

As i used the $ roslaunch rbx1_vision uvc_cam.launch device:=/dev/video0 in the book ros-by-example for hydro , i find the output like this :

exbot@ubuntu:~/catkin_ws$  roslaunch rbx1_vision uvc_cam.launch device:=/dev/video0
... logging to /home/exbot/.ros/log/bda49708-6edb-11e4-b665-44334c160d17/roslaunch-ubuntu-4099.log
Checking log directory for disk usage. This may take awhile.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

started roslaunch server http://ubuntu:34919/

SUMMARY
========

PARAMETERS
 * /rosdistro
 * /rosversion
 * /uvc_cam_node/device
 * /uvc_cam_node/exposure
 * /uvc_cam_node/frame_rate
 * /uvc_cam_node/gain
 * /uvc_cam_node/height
 * /uvc_cam_node/width

NODES
  /
    uvc_cam_node (uvc_cam/uvc_cam_node)

ROS_MASTER_URI=http://localhost:11311

core service [/rosout] found
process[uvc_cam_node-1]: started with pid [4117]
[ INFO] [1416292448.855562602]: opening uvc_cam at 320x240, 20.000000 fps
opening /dev/video0
capabilities 84000001
pixfmt 0 = 'YUYV' desc = 'YUV 4:2:2 (YUYV)'
  discrete: 640x480:   1/30 1/30 
  discrete: 320x240:   1/30 1/30 
camera formats supported
format 0 = 'YUYV' desc = 'YUV 4:2:2 (YUYV)'
  discrete: 640x480:   1/30 1/30 
  discrete: 320x240:   1/30 1/30 
[ WARN] [1416292448.856026405]: Reconfigure callback failed with exception couldn't set format:

and when i used :

$ rosrun image_view image_view image:=/camera/rgb/image_color

it only show a blank window;

why ??

i can see the picture form my webcam from cheese, but cant use it in ROS hydro

thinks a lot !