Willison built a video compressor to demo a map animation

Willison had GPT-6 Astra write a D3 animated transition between the Mercator and Equal Earth map projections on September 7. Publishing a demo of it meant producing a compressed video. FFMPEG's encoding flags are fiddly enough that most people get them right by trial and error. Willison skipped that by having a second model, Claude Fable 5.1 in Claude Code, build a browser-based video compressor on FFMPEG's WebAssembly build.

The compressor renders five presets at once. Each varies resolution, CRF between 22 and 28, audio bitrate between 64 and 128 kbps, encoder speed, H.264 profile, frame rate cap, and metadata handling. All five came back in 11.8 seconds, at file sizes between 145 KB and 264 KB.

The tool also prints the equivalent ffmpeg command for each preset. Once you have that command, you can run it straight from a terminal, no browser tool or WebAssembly build required.

Willison's post doesn't say how long the Astra animation took to write, only the compressor's render time.