WebM to MP4

Transcode a WebM to MP4. VP8/VP9/AV1 and Opus/Vorbis are not MP4-native codecs, so this decodes and re-encodes every frame through WebCodecs, in the browser only, rather than repackaging the bytes.

WebM to MP4 re-encodes a clip whose VP8, VP9, AV1, Opus or Vorbis streams have no home inside an MP4 container. Every frame decodes and a fresh, MP4 compatible codec gets encoded in its place, at a quality preset, medium by default. WebCodecs does the work, so this page needs a browser tab.

Questions

Does this repackage or re-encode?

Re-encode. VP8, VP9 and AV1 with Opus or Vorbis are not MP4-native codecs, so this decodes every frame and encodes it fresh into an MP4-storable codec through WebCodecs, rather than copying the compressed bytes.

What is lost?

One generation of re-encoding quality, at the low, medium or high preset chosen, medium by default. There is no lossless path from WebM to MP4 here.

Which engine handles this, and where can it run?

The mediabunny Conversion API drives WebCodecs decode and encode, so this is browser only. The command line and MCP refuse it and point to the web page, where the work runs in a Web Worker with nothing uploaded.

What size should I expect?

A real MP4 with a genuine mdat box holding sample data, not an empty shell. A two-second, 320 by 240 source converts to a similarly short MP4 whose exact size tracks the quality preset chosen.

What is the reverse page?

MP4 to WebM runs the opposite direction, the reverse pair of this one, with the same re-encode and quality trade-off in the other direction.

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