Testing in-browser video transcoding with MediaRecorder

A few months ago I made a quick test transcoding video from MP4 (or whatever else the browser can play) into WebM using the in-browser MediaRecorder API.

I’ve updated it to work in Chrome, using a <canvas> element as an intermediary recording surface as captureStream() isn’t available on <video> elements yet there.

Live demo: https://brionv.com/misc/browser-transcode-test/capture.html

There are a couple advantages of re-encoding a file this way versus trying to do all the encoding in JavaScript, but also some disadvantages…

Pros

• actual encoding should use much less CPU than JavaScript cross-compile
• less code to maintain!
• don’t have to jump through hoops to get at raw video or audio data

Cons

• MediaRecorder is realtime-oriented:
  • will never decode or encode faster than realtime
  • if encoding is slower than realtime, lots of frames are dropped
  • on my MacBook Pro, realtime encoding tops out around 720p30, but eg phone camera videos will often be 1080p30 these days.
• browser must actually support WebM encoding or it won’t work (eg, won’t work in Edge unless they add it in future, and no support at all in Safari)
• Firefox and Chrome both seem to be missing Vorbis audio recording needed for base-level WebM (but do let you mix Opus with VP8, which works…)

So to get frame-rate-accurate transcoding, and to support higher resolutions, it may be necessary to jump through further hoops and try JS encoding.

I know this can be done — there are some projects compiling the entire ffmpeg  package in emscripten and wrapping it in a converter tool — but we’d have to avoid shipping an H.264 or AAC decoder for patent reasons.

So we’d have to draw the source <video> to a <canvas>, pull the RGB bits out, convert to YUV, and run through lower-level encoding and muxing… oh did I forget to mention audio? Audio data can be pulled via Web Audio, but only in realtime.

So it may be necessary to do separate audio (realtime) and video (non-realtime) capture/encode passes, then combine into a muxed stream.