title: Video capture doesn't work with all supported caps
body: <p>I am using Python 3.6 on Ubuntu 18.04 on an NVIDIA Jetson TX2. All software is up to date. The Linux kernel is <strong>4.9.140-tegra</strong>. GStreamer is version <strong>1.14.1</strong>.</p>
  
  <p>I have a UVC Capable webcam that shows the following capabilities and I need to stream with the Y16 ( GRAY16_LE ) format:</p>
  
  <pre><code>root@nvidia:/tmp# v4l2-ctl --list-formats-ext -d /dev/video1
  ioctl: VIDIOC_ENUM_FMT
          Index       : 0
          Type        : Video Capture
          Pixel Format: 'UYVY'
          Name        : UYVY 4:2:2
                  Size: Discrete 160x120
                          Interval: Discrete 0.111s (9.000 fps)
  
          Index       : 1
          Type        : Video Capture
          Pixel Format: 'Y16 '
          Name        : 16-bit Greyscale
                  Size: Discrete 160x120
                          Interval: Discrete 0.111s (9.000 fps)
                  Size: Discrete 160x122
                          Interval: Discrete 0.111s (9.000 fps)
  
          Index       : 2
          Type        : Video Capture
          Pixel Format: 'GREY'
          Name        : 8-bit Greyscale
                  Size: Discrete 160x120
                          Interval: Discrete 0.111s (9.000 fps)
  
          Index       : 3
          Type        : Video Capture
          Pixel Format: 'RGBP'
          Name        : 16-bit RGB 5-6-5
                  Size: Discrete 160x120
                          Interval: Discrete 0.111s (9.000 fps)
  
          Index       : 4
          Type        : Video Capture
          Pixel Format: 'BGR3'
          Name        : 24-bit BGR 8-8-8
                  Size: Discrete 160x120
                          Interval: Discrete 0.111s (9.000 fps)
  </code></pre>
  
  <p>With GStreamer from the command line, I can successfully get the device into the <strong>PLAYING</strong> state with gst-launch in the format I want:</p>
  
  <pre><code>root@nvidia:/tmp# gst-launch-1.0 v4l2src device=/dev/video1 ! video/x-raw,width=160,height=122,format='GRAY16_LE',framerate=9/1 ! fakesink
  Setting pipeline to PAUSED ...
  Pipeline is live and does not need PREROLL ...
  Setting pipeline to PLAYING ...
  New clock: GstSystemClock
  </code></pre>
  
  <p>I am trying to open the device with the GRAY16_LE format with Python OpenCV:</p>
  
  <pre><code>import cv2
  cap = cv2.VideoCapture("v4l2src device=/dev/video1 ! video/x-raw, format=(string)GRAY16_LE, width=(int)160, height=(int)120, pixel-aspect-ratio=(fraction)1/1, framerate=(fraction)9/1 ! appsink")
  ret, frame = cap.read()
  </code></pre>
  
  <p>During elevated GStreamer debug, the capabilities are probed and printed below:</p>
  
  <pre><code>0:00:00.260383826 15768      0x49a9850 INFO                    v4l2 gstv4l2object.c:4136:gst_v4l2_object_probe_caps:&lt;v4l2src0:src&gt; probed caps: video/x-raw, format=(string)UYVY, width=(int)160, height=(int)120, pixel-aspect-ratio=(fraction)1/1, framerate=(fraction)9/1; video/x-raw, format=(string)BGR, width=(int)160, height=(int)120, pixel-aspect-ratio=(fraction)1/1, framerate=(fraction)9/1; video/x-raw, format=(string)RGB16, width=(int)160, height=(int)120, pixel-aspect-ratio=(fraction)1/1, framerate=(fraction)9/1; video/x-raw, format=(string)GRAY8, width=(int)160, height=(int)120, pixel-aspect-ratio=(fraction)1/1, framerate=(fraction)9/1; video/x-raw, format=(string)GRAY16_LE, width=(int)160, height=(int)122, pixel-aspect-ratio=(fraction)1/1, framerate=(fraction)9/1; video/x-raw, format=(string)GRAY16_LE, width=(int)160, height=(int)120, pixel-aspect-ratio=(fraction)1/1, framerate=(fraction)9/1
  </code></pre>
  
  <p>With GStreamer debug on, the errors are shown as below:</p>
  
  <pre><code>0:00:00.260385266 15768      0x4744c00 INFO              GST_STATES gstelement.c:2579:_priv_gst_element_state_changed:&lt;v4l2src0&gt; notifying about state-changed PAUSED to PLAYING (VOID_PENDING pending)
  0:00:00.260529393 15768      0x49a9850 WARN                 basesrc gstbasesrc.c:3055:gst_base_src_loop:&lt;v4l2src0&gt; error: Internal data stream error.
  0:00:00.260531697 15768      0x4744c00 INFO              GST_STATES gstbin.c:2952:gst_bin_change_state_func:&lt;pipeline0&gt; child 'v4l2src0' changed state to 4(PLAYING) successfully
  0:00:00.260589009 15768      0x49a9850 WARN                 basesrc gstbasesrc.c:3055:gst_base_src_loop:&lt;v4l2src0&gt; error: streaming stopped, reason not-negotiated (-4)
  0:00:00.260660881 15768      0x4744c00 INFO              GST_STATES gstbin.c:2087:gst_bin_get_state_func:&lt;pipeline0&gt; getting state
  0:00:00.260748913 15768      0x49a9850 INFO        GST_ERROR_SYSTEM gstelement.c:2145:gst_element_message_full_with_details:&lt;v4l2src0&gt; posting message: Internal data stream error.
  0:00:00.260762833 15768      0x4744c00 INFO              GST_STATES gstelement.c:2392:gst_element_get_state_func:&lt;pipeline0&gt; waiting for element to commit state
  0:00:00.260909520 15768      0x49a9850 INFO        GST_ERROR_SYSTEM gstelement.c:2172:gst_element_message_full_with_details:&lt;v4l2src0&gt; posted error message: Internal data stream error.
  0:00:00.261064816 15768      0x4744c00 INFO              GST_STATES gstbin.c:2504:gst_bin_element_set_state:&lt;appsink0&gt; current PAUSED pending VOID_PENDING, desired next READY
  0:00:00.261189072 15768      0x49a9850 INFO                    task gsttask.c:316:gst_task_func:&lt;v4l2src0:src&gt; Task going to paused
  </code></pre>
  
  <p>I am using the identical format that the probe returns but the error indicates "format negotiation error". How can I fix this so my frames appear in Python? Thanks. </p>