Exporting your video

Pick what the render writes: an MP4, a GIF, an image sequence, or an alpha-capable overlay. Set Video.export to one of the four modes and Fluvie encodes that container. Here are all four:

Export sharable() => const Export.mp4(quality: Quality.max); // H.264 MP4, CRF 14
Export loopingThumb() => const Export.gif(fps: 12); // animated GIF at 12 fps
Export forCompositing() => const Export.imageSequence(); // one PNG per frame
Export overlay() => const Export.transparent(); // WebM with an alpha channel

With no export, a render writes an MP4 at the default quality. The rest of this page covers the four modes, the poster frame, and the command-line renderer.

MP4 quality

Export.mp4(quality:) writes an H.264 MP4. The quality picks the encoder’s constant-rate factor (CRF), where a lower CRF means a bigger file and less compression:

Export.mp4(quality: Quality.low), // CRF 28, smallest file, visible compression
Export.mp4(quality: Quality.medium), // CRF 23, a rough-cut trade-off
Export.mp4(quality: Quality.high), // CRF 18, the default for published videos
Export.mp4(quality: Quality.max), // CRF 14, near-lossless, the largest file

Quality.high is the default, so const Export.mp4() writes a published-grade file. Reach for Quality.low for quick draft renders and Quality.max for an archival master.

GIF

Export.gif(fps:) writes an animated GIF. The fps samples the frames, since a GIF rarely needs the full frame rate. A lower fps makes a smaller file:

Export.gif(fps: 15), // the default sample rate
Export.gif(fps: 12), // a smaller, choppier loop

The encoder builds an optimized 256-color palette from your frames in one pass, then maps the frames onto it with dithering, so a GIF keeps its color and stays small. A GIF carries no audio.

Image sequence

Export.imageSequence() writes one lossless PNG per frame, named frame_000000.png, frame_000001.png, and so on. Reach for it when a compositing pipeline downstream wants individual frames rather than a container:

const Export.imageSequence(); // one PNG per frame, into the output directory

The sequence carries no audio. Each still is lossless, so the pipeline starts from the exact captured pixels.

Transparent overlay

Export.transparent() writes a WebM with an alpha channel, so the video can layer over other footage. The captured RGBA alpha is preserved end to end:

const Export.transparent(); // VP9 WebM with a yuva420p alpha plane

Reach for it when you render a lower-third, a logo sting, or a caption pass that sits on top of a clip in another editor. An alpha-capable overlay carries no audio. ProRes .mov with alpha is forthcoming; WebM ships today.

A poster frame

Video.poster names a Time, and the render grabs that one frame as a still thumbnail alongside the main output. Set it on the same Video as the export:

Video exportedReel() => Video(
  size: VideoSize.reels,
  export: const Export.mp4(quality: Quality.max), // near-lossless, CRF 14
  poster: const Time.seconds(1.5), // the thumbnail frame
  scenes: [
    Scene.centered(
      duration: const Time.seconds(3),
      background: Background.color(const Color(0xFF0E1116)),
      child: const Text('Exported', style: TextStyle(fontSize: 72, color: Color(0xFFE6EDF3))),
    ),
  ],
);

The poster is a second output from the same frames, so it costs one extra encode of one frame. Pick a Time that lands on a representative moment, for example 1.5 seconds into a 3 second card.

The command-line renderer

fluvie render captures a registered composition and encodes it without opening the example app. The minimal call names a composition key and an output file:

fluvie render 01_hello_video --out hello.mp4

The flags map onto the export modes above:

fluvie render 01_hello_video --out reel.mp4 --aspect reels --quality high
fluvie render 01_hello_video --out clip.gif --format gif
fluvie render 01_hello_video --out frames/ --format imageSequence
fluvie render 01_hello_video --out overlay.webm --format transparent
fluvie render 01_hello_video --out hero.mp4 --poster 1.5s

The flags:

• --out <file> sets the output path. It is required.
• --aspect <name> picks the aspect: reels, square, landscape, or portrait45. With no --aspect the composition renders at its declared size; pass an aspect to re-frame it.
• --quality <name> picks the MP4 quality: low, medium, high, or max.
• --format <name> picks the export mode: mp4, gif, imageSequence, or transparent. It defaults to mp4.
• --poster <time> writes a poster still from a Time string, for example 1.5s, 30f, or 500ms.

A bad enum value exits with code 64 and names the valid set, so a typo fails fast rather than rendering the wrong thing. The other flags from earlier phases stay: --frames N for a draft render, --project <dir>, --ffmpeg <path>, --no-cache, --keep-temp, and --verbose.

To see every composition key you can render, run:

fluvie list

It prints one key per line.

Where to next

• Templates: render one definition per data row.
• Multi-aspect: one definition rendered to reels, square, landscape, and portrait.