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Shmdata

Shmdata, developed at the SAT Metalab, is a Unix-oriented protocol for sharing memory between applications easily.
It is mainly used to share video frames.

Thanks to it, score can easily send and receive video data from and to GStreamer for instance.

Compiling and installing shmdata

Refer to the latest instructions on the shmdata Readme. Score comes built-in with shmdata support on Linux and macOS (Windows does not support the necessary Unix primitives).

Test that it works correctly with the gst-launch commands given in that Readme.

Using shmdata in score

Shmdata can currently be used as a video input and output device. Simply create the devices you need, and set your video input / output to the shmdata device’s name.

These examples assume that the shmdata GStreamer plug-in is installed in /opt/shmdata:

Sending data from GStreamer to score:

$ gst-launch-1.0 --gst-plugin-path=/opt/shmdata videotestsrc pattern=snow ! queue ! videoconvert ! shmdatasink socket-path=/tmp/score_shmdata_input

Sending data from score to GStreamer:

$ gst-launch-1.0 --gst-plugin-path=/opt/shmdata shmdatasrc socket-path=/tmp/score_shmdata_output ! videoconvert ! xvimagesink

Here is a score that would process the input and write it to the output.

  • Input device (receives the GStreamer feed): Shmdata input window

  • Output device (sends the feed to GStreamer): Shmdata output window

  • The score and the GStreamer output which shows the processing done by score: Score and shmdata output

Recording score’s video output

Here is a GStreamer command-line which will encode the shmdata output as .mkv with relatively low quality:

$ gst-launch-1.0 -e \
      shmdatasrc socket-path=/tmp/score_shm_video \
    ! queue \
    ! videoconvert \
    ! videorate \
    ! video/x-raw,framerate=60/1 \
    ! x264enc \
    ! matroskamux \
    ! filesink location=foo.mkv

Or as .mov encoded in Apple ProRes (warning: very, very CPU hungry):

$ gst-launch-1.0 -e \
      shmdatasrc socket-path=/tmp/score_shm_video \
    ! queue \
    ! videoconvert \
    ! videorate \
    ! video/x-raw,framerate=60/1 \
    ! avenc_prores_ks \
    ! qtmux \
    ! filesink location=foo.mov