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Embedding a pipeline (programmatic construction)

A gst-launch-1.0 string is the fastest way to try a pipeline, but an application that embeds these elements builds the graph in code instead — so it can set properties from config, read metadata off a pad, react to bus messages, and change state at runtime. The construction is plain core-GStreamer API and is identical whether the elements are host-only (nvmmtracker) or Jetson (nvmminfer); only the factory names and properties differ from the demo below.

The worked example is examples/programmatic_pipeline.cpp. It is core-GStreamer only (videotestsrc → capsfilter → queue → identity → fakesink), so it builds and runs everywhere and is exercised in CI as a meson test. The six steps are the whole pattern:

  1. Make each element by its registered factory name with gst_element_factory_make (the same names you would type in a launch string), and a gst_pipeline_new to hold them.
  2. Configure with g_object_set — this is where engine-file, conf-threshold, leaky, and the rest go, by property name.
  3. Add every element to the bin (gst_bin_add_many) and link them in order (gst_element_link_many); check the link return, it stops at the first pair that fails to negotiate caps.
  4. Read results off a pad with gst_pad_add_probe — the example counts buffers; a real graph reads GstNvmmDetMeta / GstNvmmTrackMeta off the same kind of probe.
  5. Watch the bus (gst_bus_add_watch) so EOS and ERROR end the run instead of blocking forever.
  6. Run — gst_element_set_state(PLAYING), drive a GMainLoop, then tear down to NULL.

Turning it into an inference graph

Swap the core elements for the real ones — the surrounding code (steps 3–6) does not change:

  • videotestsrc → your source, plus nvvidconv into video/x-raw(memory:NVMM), format=NV12 (the caps nvmminfer expects).
  • identity → nvmminfer (set engine-file), optionally followed by nvmmtracker / nvmmfusekf.
  • fakesink → your sink (encoder + filesink/udpsink, or an appsink you pull metadata from).
  • Put a queue between the heavy stages for pipeline parallelism, and a queue leaky=2 max-size-buffers=2 at a live source for freshest-frame behaviour — see the tracker pipeline.

The element must be on GST_PLUGIN_PATH for gst_element_factory_make to resolve it, exactly as for gst-launch-1.0.