GStreamer in OpenCV does not send video data over UDP - c++

Question

I'm trying to use GStreamer on Mac (10.12.6) to stream a video encoded from OpenCV frames via UDP. For some reason this does not work and I'm not getting any error output from GStreamer or OpenCV. This is how I open the writer in OpenCV:
cv::VideoWriter writer(
"appsrc ! videoconvert ! x264enc byte-stream=true threads=4 ! mpegtsmux ! udpsink host=localhost port=9999",
cv::CAP_GSTREAMER,
0,
(double) 5,
cv::Size(320, 240),
true);
This writer opens and I can feed frames into it but I don't get any output when listening to the port. The same pipeline works when compiled from (c++) source using the GStreamer API or when being launched via
/gst-launch-1.0 videotestsrc ! x264enc byte-stream=true threads=4 ! mpegtsmux ! udpsink host=localhost port=9999
I don't think OpenCV or GStreamer itself is at fault because I am able to stream video to the autovideosink in OpenCV when opening the writer via
cv::VideoWriter writer(
"appsrc ! autovideosink",
cv::CAP_GSTREAMER,
0,
(double) 5,
cv::Size(320, 240),
true);
For reference - I installed GStreamer via Brew and built OpenCV myself using the following CMake Flags:
-DOPENCV_ENABLE_NONFREE=ON
-DWITH_OPENGL=ON
-DWITH_OPENVX=ON
-DWITH_OPENCL=ON
-DBUILD_PNG=ON
-DBUILD_TIFF=ON
-DOPENCV_EXTRA_MODULES_PATH=~/src/opencv_contrib/modules
-DWITH_1394=OFF
-DWITH_CUDA=OFF
-DWITH_GSTREAMER=ON.
I'm relatively new to GStreamer so I could be missing something obvious. Any idea what I could be doing wrong here or what would help to track down the issue?

Answer 1

Finally got it. Turning on GStreamer Debug Output via setting GST_DEBUG=2 yielded that x264enc was not found by OpenCV. I didn't notice that as it was found when compiling the pipeline via the GStreamer C++ API.
The reason for that was that while the GStreamer install packages on OSX installed the x264enc element, brew on mac didn't. Having compiled OpenCV myself - it used the brew install of GStreamer and not the framework.
Solved the problem by reinstalling the x264enc element for GStreamer via brew (brew install gst-plugins-ugly --with-x264).