Rendering to MP4

neko render videos/launch.md                 # → videos/launch.mp4
neko render videos/launch.md -o launch.mp4 -q high
neko render videos/ -o out/                  # every video page in the folder
neko render videos/launch.md --still 4.5     # one PNG of that moment

What happens

1. Build. Neko builds the page exactly as neko build would — the same parser, the same timeline compiler, the same standalone HTML — but only that page, and in memory.
2. Open. Headless Chromium (through Playwright) opens it at the video's own size with a device scale factor of 1, in render mode: no player chrome, the canvas at 1:1. The page is served from memory; anything it asks for that it did not inline (a clip) is served from the project folder. Neko waits for the fonts, every image's decode and every clip.
3. Capture. For frame n Neko calls window.nekoVideo.seek(n / fps) and takes a screenshot. Several browser pages capture at once, each a slice of the timeline (--workers).
4. Encode. ffmpeg turns the frames into H.264 in BT.709 (yuv420p, +faststart), and mixes the soundtrack: the music bed and each scene's voice line, each delayed to its time, the music faded in and out.

The page protocol is small and public, so anything that can load a page and call a function can drive it:

await window.nekoVideo.ready;          // fonts, images and clips are in
window.nekoVideo.duration;             // seconds
window.nekoVideo.renderMode();         // strip the chrome, canvas at 1:1
await window.nekoVideo.seek(12.5);     // paint 12.5 s (resolves once clips have seeked)

Options

ffmpeg

Neko does not bundle ffmpeg. It looks, in order, for:

1. --ffmpeg;
2. the NEKO_FFMPEG environment variable;
3. the Curiosity.FFmpeg.Runtimes.<rid> layout (runtimes/<rid>/native/ffmpeg-dist) next to the Neko binaries;
4. ffmpeg on the PATH;
5. a copy it downloaded before, in ~/.neko/ffmpeg;

and failing those it downloads the official build into ~/.neko/ffmpeg once, with Xabe.FFmpeg's downloader. Neko drives ffmpeg through Xabe.FFmpeg.

Chromium

neko render uses Playwright's Chromium. When Playwright's own build is missing it uses any Chromium Playwright already installed (PLAYWRIGHT_BROWSERS_PATH, ~/.cache/ms-playwright), and otherwise installs it once. --browser points at any other Chromium or Chrome — the way to get H.264 playback for ::: media clips, which open-source Chromium does not decode.

Speed

Capture is most of the time; the encode is a few seconds. On four cores the 35-second sample above captures its 1,062 frames in about 50 seconds with four workers. --jpeg and -q draft make a quick check faster; --from/--to and --still render only what you are working on.

In CI

- run: dotnet tool install -g Neko
- run: npx playwright install --with-deps chromium   # or let neko render install it
- run: sudo apt-get install -y ffmpeg                  # or let neko render download it
- run: neko render videos/ -o site/assets/videos -q high

Render before neko build and embed the files with [!video]:

[!video src="/assets/videos/launch.mp4" link="/videos/launch" title="Launch" poster="/assets/videos/launch.png"]