Add multivariant output support to Membrane.Transcoder by khamilowicz · Pull Request #21 · membraneframework/membrane_transcoder_plugin

khamilowicz · 2026-05-21T12:24:07Z

Summary

On-request output pads — Membrane.Transcoder now exposes availability: :on_request output pads, each accepting independent output_stream_format, transcoding_policy, and native_acceleration options via via_out. Single-output backward-compatible usage is unchanged.
Fan-out via Tee — multi-output mode inserts a Membrane.Tee.Parallel between the connector and per-output transcoding chains, created atomically with all outputs in one spec to avoid the race where the Tee could receive data before any output is connected.
Example — examples/multivariant_output.exs demonstrates a single H264 input transcoded simultaneously to H264, H265, and VP8 outputs.
Tests — 5 new integration tests cover multivariant video and audio scenarios. Content correctness is verified by asserting each multivariant output is byte-identical to the equivalent single-output pipeline run.

Usage

# Multiple outputs
child(:transcoder, Membrane.Transcoder),
get_child(:transcoder)
|> via_out(Pad.ref(:output, 0), options: [output_stream_format: H264])
|> child(:hd_sink, Membrane.File.Sink),
get_child(:transcoder)
|> via_out(Pad.ref(:output, 1), options: [output_stream_format: VP8])
|> child(:vp8_sink, Membrane.File.Sink)

Test plan

mix test --exclude vk — 54 tests, 0 failures
Single-output backward-compatible usage unchanged (existing test suite covers this)
Multivariant output produces byte-identical results to equivalent single-output pipelines
Run elixir examples/multivariant_output.exs to verify the example produces three non-empty output files

🤖 Generated with Claude Code

varsill · 2026-05-22T09:35:44Z

I don't think we want to have this commited

varsill · 2026-05-22T09:37:35Z

I think we need to update existing examples as well

varsill · 2026-05-25T13:38:04Z

  end
+
+  defp child_name(nil, base), do: base
+  defp child_name(suffix, base), do: :"#{suffix}_#{base}"


I am slightly worried about creating atoms dynamically since they are not garbage collected. How about making child's name a String?

ChildSpec allows only atoms or tuples as names, I went with tuple {suffix, base}.

varsill · 2026-05-25T13:48:54Z


-  defp maybe_plug_encoder_and_parser(builder, %VP8{}) do
+  defp maybe_plug_encoder_and_parser(builder, %VP8{}, suffix) do
    cpu_quota = :erlang.system_info(:cpu_quota)


(I know it's unrelates but it brought my attention :D)
What :cpu_quota would return in contenerized environments (e.g. with docker)?
The problem is that if :cpu_quota doesn't reflect some cgroup limits, then we might end up in a situation that we will try to use all the cores available in the machine, and we will be given only fraction of it, which ends up in thrashing

I used :logical_processors_online to handle this case.

The output pad is now availability: :on_request, allowing a single transcoder bin to produce multiple independent output streams each with their own output_stream_format, transcoding_policy, and native_acceleration options. Single-output backward-compatible usage is unchanged — implicit via_out links inherit bin-level options and use the same internal child names, so existing tests and code require no modification. Multi-output pipelines use a Membrane.Tee.Parallel (new dep) placed between the connector and per-output transcoding chains within a single atomic spec, avoiding the race where the Tee could receive data before any output is connected. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

- use strings as children names - update examples

varsill

I think it's the last round of review :D
After addressing my comments, I think it's the right time to bump minor version of the plugin so that we can release it immediately afterwards

varsill · 2026-05-28T12:44:07Z

you can mention this new example in README.md

varsill · 2026-05-28T12:45:31Z

+  end
+
+  @tag :tmp_dir
+  test "multivariant output: two video outputs with different resolutions", %{tmp_dir: tmp_dir} do


these are more like two different codecs, not two different resolutions.

At the same time, I think it would be good to have a separate test that produces two video outputs with different resolution

varsill · 2026-05-28T12:48:32Z

+  defp pad_id_to_suffix(id) when is_integer(id), do: "output_#{id}"
+  defp pad_id_to_suffix(id) when is_atom(id), do: "output_#{id}"
+  defp pad_id_to_suffix(id), do: "output_#{:erlang.phash2(id)}"


I am not sure I like this function - it will generate output_nil if id == nil but output_<some hash> if id == "some_hash".
Do we really need to use this hash here?

varsill

LGTM, I just left one comment for clarification.
Except for that - as discussed, let's open a separate PR with a feature of providing a desired output resolution.

varsill · 2026-05-29T07:36:36Z

-    suffix = pad_id_to_suffix(pad_id)
-    funnel_name = :"funnel_#{suffix}"
+    suffix = {pad_id, :output}
+    funnel_name = {pad_id, :output, :funnel}


Why funnel_name cannot be created just as any other element's name i.e. "element type" + suffix?

varsill

🥇

khamilowicz requested a review from varsill May 21, 2026 12:25

khamilowicz added this to Smackore May 21, 2026

khamilowicz force-pushed the multivariant-output branch 3 times, most recently from 406c796 to 0eb8420 Compare May 22, 2026 08:33

varsill requested changes May 25, 2026

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khamilowicz force-pushed the multivariant-output branch 2 times, most recently from 5a6305d to 3b00a93 Compare May 26, 2026 08:55

khamilowicz requested a review from varsill May 26, 2026 09:06

khamilowicz force-pushed the multivariant-output branch from 3b00a93 to b29e223 Compare May 26, 2026 09:38

Apply review suggestions

be2620a

- use strings as children names - update examples

khamilowicz force-pushed the multivariant-output branch from b29e223 to be2620a Compare May 26, 2026 09:41

Handle cpu quota for cgroups v2 in docker container

948a200

varsill reviewed May 26, 2026

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Comment thread test/integration_test.exs Outdated

Use @tag :tmp_dir in integration tests

889c051

khamilowicz requested a review from varsill May 26, 2026 14:29

fix credo

76392e6

varsill requested changes May 28, 2026

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varsill assigned khamilowicz May 28, 2026

varsill moved this to In Review in Smackore May 28, 2026

Use tuples as children names

047b2db

varsill approved these changes May 29, 2026

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khamilowicz added 2 commits May 29, 2026 12:09

Fix children names

7eae6e1

Update README

b5029f0

khamilowicz requested a review from varsill May 29, 2026 11:03

varsill approved these changes May 29, 2026

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khamilowicz merged commit 23b0550 into master May 29, 2026
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github-project-automation Bot moved this from In Review to Done in Smackore May 29, 2026

khamilowicz deleted the multivariant-output branch May 29, 2026 12:01

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khamilowicz commented May 21, 2026 •

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