What are valid codec strings to be used in the web codecs API?

Question

I want to know exactly which strings I can pass as codec to VideoEncoder.isConfigSupported({ codec, width, height }) but so far I have not found a definite answer:

• The TS declaration file only precises that it must be a string (I wish I could have found a union of string constants there ^^)
• This webcodec article from google only precises that vp8 is a valid codec string
• The specs are a rabbit hole that I have trouble navigating
  • They describe what a codec string should do but don't include an extensive list
  • They precise that a codec string must match either av01.*, avc1.*, vp8, or vp09.*. From there, there are some links to find more, but I couldn't find how to navigate them. For example, I wanted to know which codecs start with av01., so I went to its registration page that links there that link there, there, and there. The two first links don't say anything about fully qualified codec strings, and the last link links to the web codecs specs I started from originally :/
• This MDN page about the codec in mime types is quite complete and explains the rules to create codec strings, but lot of what I've tried with didn't seem to work

Since, at the moment, only chromium-based browsers support this, I decided to lookup the chromium source code and I found places where more codec strings are listed:

• Tests from the webcodecs directory are apparently run via TEST.run that is not found triggered from the JS layer but rather from there, in this python file, where some more codec strings are mentionned just above: avc1.42001E, vp8, vp09.00.10.08
• Some codec strings seem to be mentionned here, along with other things: video, h264, vp8, hevc, vp9, audio, aac, opus
• This test file list a bunch of codecs to be used with canPlayType (not part of web codecs, but related)

All of this is quite frustrating because I have quite a big list of codecs that can sometimes be passed to web codecs, but I am not certain that this is the complete list of it. Where else should I look?

const maybeCodecs = ["aac","ac-3","audio","avc1","avc1, mp4a.40","avc1, mp4a.40.02","avc1, mp4a.40.2","avc1.42001E","avc1.42101E","avc1.42701E","avc1.42E01E","avc1.42E01E, mp4a.40","avc1.42E01E, mp4a.40.02","avc1.42E01E, mp4a.40.2","avc1.42F01E","avc3","avc3, mp4a.40","avc3, mp4a.40.02","avc3", "mp4a.40.2","avc3.42801E","avc3.42C01E","avc3.42E01E","avc3.42E01E, mp4a.40","avc3.42E01E, mp4a.40.05","avc3.42E01E, mp4a.40.29","avc3.42E01E, mp4a.40.5","ec-3","flac","h264","hev1.1.6.L93.B0","hev1.1.6.L93.B0,mp4a.40.5","hevc","hvc1.1.6.L93.B0","hvc1.1.6.L93.B0,mp4a.40.5","mp4a.40","mp4a.40.02","mp4a.40.2","mp4a.66","mp4a.67","mp4a.68","mp4a.69","mp4a.6B","mp4a.A5","mp4a.A6","mp4a.a5","mp4a.a6","opus","theora","video","vp09.00.10.08","vp8","vp9"];

(async () => {
  if (typeof VideoEncoder === "undefined") return console.log('web codecs is not supported');
  const supportedCodecs = [];
  const unsupportedCodecs = [];
  const codecsThatTriggerError = [];
  for (const codec of maybeCodecs) {
    try {
      const { supported } = await VideoEncoder.isConfigSupported({ codec, width: 256, height: 256 });
      if (supported) supportedCodecs.push(codec);
      else unsupportedCodecs.push(codec);
    } catch(e) {
      codecsThatTriggerError.push(codec);
    }
  }
  console.log({ supportedCodecs, unsupportedCodecs, codecsThatTriggerError });
})()

Answer 1

For consistency, the format strings described in the WebCodecs codec registry have been designed to match the strings used in ISOBMFF. Depending on the codec there may be a fair amount of metadata included in the string, so while the list is finite is is extremely large.

Taking AV1 as an example, the ISOBMFF binding specifies the format to be

av01.<profile>.<level><tier>.<bitDepth>.<monochrome>.<chromaSubsampling>.
<colorPrimaries>.<transferCharacteristics>.<matrixCoefficients>.<videoFullRangeFlag>

with most of those fields being optional.

In general, a codec string is valid if it can be parsed according to the definition in the codec registration.