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Sh4lt

Sh4lt , is a Unix-oriented protocol for sharing data frames between applications easily. It is developped by the Lab148 coop, and is the successor of the Shmdata protocol originally developed at the SAT Metalab. Its integration in Ossia supports video frames sharing.

It can exchange video with GStreamer and other Sh4lt-enabled software. See the Sh4lt projects for available integrations.

Compiling and installing Sh4lt

Sh4lt support depends on the score build and is intended for Unix systems, notably Linux. Check that Sh4lt Input / Sh4lt Output appear in the device chooser. External GStreamer examples also require the Sh4lt GStreamer elements; support in score does not install those elements for other applications.

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

Using Sh4lt in score

Create a Sh4lt Input, select the producer, and assign it to a texture inlet. For Sh4lt Output, set Sh4lt label, Width, Height and Rate, then assign it as the destination of the final texture outlet. The producer/consumer labels must agree.

The output publishes raw RGBA frames via shared memory after GPU readback. This is not a compressed stream or a zero-copy GPU transport. Input caps can describe different supported pixel formats; do not infer output format support from the input converter.

See GStreamer, PipeWire video and Video formats and color for other routes and their limitations.

These examples assume that the Sh4lt GStreamer plug-in is installed in /opt/sh4lt:

Sending data from GStreamer to score:

$ gst-launch-1.0 --gst-plugin-path=/opt/sh4lt videotestsrc pattern=snow ! queue ! videoconvert ! sh4ltsink label=to_score

Sending data from score to GStreamer:

$ gst-launch-1.0 --gst-plugin-path=/opt/sh4lt sh4ltsrc label=score_output ! videoconvert ! autovideosink

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

  • Input device (receives the GStreamer feed): Sh4lt input window

  • Output device (sends the feed to GStreamer): Sh4lt output window

  • The score and the GStreamer output which shows the processing done by score: Score and sh4lt output

Recording score’s video output

Here is a GStreamer command-line which will encode the sh4lt output as .mkv with relatively low quality:

$ gst-launch-1.0 -e \
      sh4ltsrc label=score_output \
    ! 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 \
      sh4ltsrc label=score_output \
    ! queue \
    ! videoconvert \
    ! videorate \
    ! video/x-raw,framerate=60/1 \
    ! avenc_prores_ks \
    ! qtmux \
    ! filesink location=foo.mov