Catching up on WebKit GStreamer WebAudio backends maintenance

Over the past few months the WebKit development team has been working on modernizing support for the WebAudio specification. This post highlights some of the changes that were recently merged, focusing on the GStreamer ports.

My fellow WebKit colleague, Chris Dumez, has been very active lately, updating the WebAudio implementation for the mac ports in order to comply with the latest changes of the specification. His contributions have been documented in the Safari Technology Preview release notes for version 113, version 114, version 115 and version 116. This is great for the WebKit project! Since the initial implementation landed around 2011, there wasn’t much activity and over the years our implementation started lagging behind other web engines in terms of features and spec compliance. So, many thanks Chris, I think you’re making a lot of WebAudio web developers very happy these days :)

The flip side of the coin is that some of these changes broke the GStreamer backends, as Chris is focusing mostly on the Apple ports, a few bugs slipped in, noticed by the CI test bots and dutifully gardened by our bots sheriffs. Those backends were upstreamed in 2012 and since then I didn’t devote much time to their maintenance, aside from casual bug-fixing.

One of the WebAudio features recently supported by WebKit is the Audio Worklet interface which allows applications to perform audio processing in a dedicated thread, thus relieving some pressure off the main thread and ensuring a glitch-free WebAudio rendering. I added support for this feature in r268579. Folks eager to test this can try the GTK nightly MiniBrowser with the demos:

$ wget https://trac.webkit.org/export/270226/webkit/trunk/Tools/Scripts/webkit-flatpak-run-nightly
$ chmod +x webkit-flatpak-run-nightly
$ python3 webkit-flatpak-run-nightly --gtk MiniBrowser https://googlechromelabs.github.io/web-audio-samples/audio-worklet/

For many years our AudioFileReader implementation was limited to mono and stereo audio layouts. This limitation was lifted off in r269104 allowing for processing of up to 5.1 surround audio files in the AudioBufferSourceNode.

Our AudioDestination, used for audio playback, was only able to render stereo. It is now able to probe the GStreamer platform audio sink for the maximum number of channels it can handle, since r268727. Support for AudioContext getOutputTimestamp was hooked up in the GStreamer backend in r266109.

The WebAudio spec has a MediaStreamAudioDestinationNode for MediaStreams, allowing to feed audio samples coming from the WebAudio pipeline to outgoing WebRTC streams. Since r269827 the GStreamer ports now support this feature as well! Similarly, incoming WebRTC streams or capture devices can stream their audio samples to a WebAudio pipeline, this has been supported for a couple years already, contributed by my colleague Thibault Saunier.

Our GStreamer FFTFrame implementation was broken for a few weeks, while Chris was landing various improvements for the platform-agnostic and mac-specific implementations. I finally fixed it in r267471.

This is only the tip of the iceberg. A few more patches were merged, including some security-related bug-fixes. As the Web Platform keeps growing, supporting more and more multimedia-related use-cases, we, at the Igalia Multimedia team, are committed to maintain our position as GStreamer experts in the WebKit community.