About combine v4l2 streams into one

[Jerryfy]

     Hi everybody!
     I'm trying to combine two v4l2 streams into one with the latest
gstreamer-1.4.3. First of all, I want to test the videomixer function with
gst-launch-1.0 use the follow command:
     gst-launch-1.0 v4l2src device=/dev/video0 ! video/x-raw ! videobox
left=0 border-alpha=0 ! alpha ! queue ! videomixer name=mix ! videoconvert !
xvimagesink sync=false \
v4l2src device=/dev/video1 ! video/x-raw ! alpha ! videobox left=-704
border-alpha=0 ! queue ! mix.

      Unfortunately it not worked and appeared the follow error:
      Setting pipeline to PAUSED ...
      Pipeline is live and does not need PREROLL ...
      Setting pipeline to PLAYING ...
      New clock: GstSystemClock
      ERROR: from element /GstPipeline:pipeline0/GstV4l2Src:v4l2src0:
Internal data flow error.
      Additional debug info:
      gstbasesrc.c(2933): gst_base_src_loop ():
/GstPipeline:pipeline0/GstV4l2Src:v4l2src0:
      streaming task paused, reason not-negotiated (-4)
      Execution ended after 0:00:00.000593582
      Setting pipeline to PAUSED ...
      Setting pipeline to READY ...
      Setting pipeline to NULL ...
      Freeing pipeline ...

      But if I test this function with gst-launch-0.10 as the follow
commands:
      gst-launch-0.10 v4l2src device=/dev/video0 ! video/x-raw-yuv !
videobox left=0 border-alpha=0 ! alpha ! queue ! videomixer name=mix !
ffmpegcolorspace ! xvimagesink sync=false v4l2src device=/dev/video1 !
video/x-raw-yuv ! alpha ! videobox left=-704 border-alpha=0 ! queue ! mix.

      Miraculously, the video wall works well!

      So I guess maybe I should setup some parameters when use
gst-launch-1.0 but I was not do it, so appeared the error. 
       I need everyone's help! Tell me how to use gst-launch-1.0 to combine
v4l2 streams into one! 
       Thanks very much!

 



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