make-video

Make a video with Helios

A Helios video is one HTML page with one function, window.renderAt(t). It draws the frame at t seconds. The Helios renderer loads the page in headless Chrome, calls renderAt(t) once per frame, captures each frame and encodes the MP4. There is no bundler, no framework and no capture script to write.

1. Write the page

Every frame must be a pure function of t. The renderer may ask for frames in any order, and it renders the same frame identically every time.

• Derive everything from t: positions, opacity, which scene is showing. Never use counters, x += speed, physics stepped frame by frame, or setTimeout/setInterval.
• Randomness must be seeded. Use rand(seed) below, not a bare Math.random() inside renderAt.
• Load assets before the first frame. Keep a ready promise for fonts and images, and make renderAt async and await ready. The renderer waits for any promise that renderAt returns.
• Size the page to the video. A canvas the size of the video, with html,body{margin:0;overflow:hidden}.

<!doctype html>
<html>
<head>
<meta charset="utf-8">
<style>html,body{margin:0;background:#0b0b10;overflow:hidden}</style>
</head>
<body>
<canvas id="c" width="1920" height="1080"></canvas>
<script>
const W = 1920, H = 1080;
const ctx = document.getElementById('c').getContext('2d');

const clamp = (x, a = 0, b = 1) => Math.min(b, Math.max(a, x));
const ease = (x) => 1 - Math.pow(1 - clamp(x), 3);              // easeOutCubic
const span = (t, start, dur) => clamp((t - start) / dur);         // 0→1 over [start, start+dur]
const rand = (seed) => { const x = Math.sin(seed * 12.9898) * 43758.5453; return x - Math.floor(x); };

const ready = document.fonts.ready;                               // plus any images you load

window.renderAt = async (t) => {
  await ready;
  ctx.fillStyle = '#0b0b10';
  ctx.fillRect(0, 0, W, H);

  // Scene 1 (0–3s): title rises in.
  const p = ease(span(t, 0.2, 1.2));
  ctx.globalAlpha = p;
  ctx.fillStyle = '#f5b700';
  ctx.font = '800 140px system-ui, sans-serif';
  ctx.textAlign = 'center';
  ctx.fillText('Hello, video', W / 2, H / 2 + 60 * (1 - p));
  ctx.globalAlpha = 1;

  // Particles: a seeded position per particle, animated by t.
  for (let i = 0; i < 80; i++) {
    const x = (rand(i) * W + t * 60 * (0.5 + rand(i + 99))) % W;
    const y = rand(i + 7) * H;
    ctx.fillStyle = `rgba(255,255,255,${0.15 + 0.35 * rand(i + 3)})`;
    ctx.fillRect(x, y, 3, 3);
  }
};
</script>
</body>
</html>

DOM, CSS, SVG and WebGL pages work too. Set styles from t inside renderAt, or let CSS animations and GSAP/motion timelines run. The renderer seeks those to each frame's time. See references/pages.md.

2. Render it

npx -y @helios-project/cli@latest render video.html -o video.mp4 --duration 12

Flag	Use
--duration <s>	Length in seconds. Fractions are fine. Required for a renderAt page.
--width 1080 --height 1920	Frame size; the default is 1920×1080. Match your canvas. Vertical 9:16 is 1080x1920.
--fps <n>	Frame rate; the default is 30.
--audio song.mp3	Soundtrack, trimmed to the video.
--mode canvas	Faster for a page that is only one full-frame <canvas>. The default, dom, captures the page as it looks.
--preset medium	For the final file. It's several times smaller at the same quality and takes longer to encode. The default, ultrafast, is quick for drafts.

Rendering is frame-exact. Frame n is exactly renderAt(n / fps), with no dropped or duplicated frames and no dependence on machine speed.

For a GIF, render the MP4 and then convert it. See references/pages.md.

3. Look at the frames before you say it's done

Don't hand over a video you haven't looked at. Render stills and contact sheets from the page itself; they're fast and need no encode:

npx -y @helios-project/cli@latest still video.html --at 0.5,3,7.25            # full-size PNGs
npx -y @helios-project/cli@latest sheet video.html --every 1 --duration 12   # one labelled grid
npx -y @helios-project/cli@latest sheet video.html --strip 2:3 --fps 30      # every frame in 2–3s
npx -y @helios-project/cli@latest sheet video.html --at 4 --crop 600,300,720,480  # zoom into a region

Before a long render, check that the page really is a function of t:

npx -y @helios-project/cli@latest verify video.html --duration 12   # exits 1 and names the frames that depend on history
npx -y @helios-project/cli@latest verify video.html --duration 12 --cues lyrics.srt   # also: every lyric or caption on screen at its time

Pick the moments for a sheet from the video's structure, not at random: the first, middle and last frame of each scene, every big beat or hit, every entrance, the middle of every transition.

Open the PNGs with your image-viewing tool and check:

• text is legible and inside the frame
• transitions actually happen
• there are no empty or black frames
• the pacing matches the brief

Fix the page, check again, and render again. Then check the file itself:

npx -y @helios-project/cli@latest check video.mp4   # flashing above the WCAG 2.3.1 limit, colour tags, frame count

4. Music and sound

--audio muxes an audio file. To sync visuals to the music, analyse it first:

npx -y @helios-project/cli@latest analyze track.mp3   # writes track.beats.json: beats, downbeats, drum hits, big moments, loudness per frame

Load that file in the page and read it at t: cut on downbeats, punch on kicks, land big moves on hits. Real songs drift in tempo, so use its beats list rather than k * 60 / bpm. Lyrics and captions go on screen at their sung time, not snapped to the beat. See references/music.md.

When something is wrong

Symptom	Fix
"How long should the video be? Pass --duration"	Add --duration <seconds>.
window.renderAt(2.5) threw: …	Your page errored at that time. The message includes your stack trace.
The first render pauses to download Chromium	That's expected, and it happens once. Don't run npx playwright install yourself: it fetches a build for the wrong Playwright version.
"The page defines no window.helios, window.renderAt(t) …"	Your script never defined renderAt. Check the page for a load error, such as a syntax error or a failed import.
Frames differ between two renders, or verify fails	Something isn't a function of t: an unseeded Math.random(), a counter or a timer. On a canvas, also a pixel readback without willReadFrequently, or context state left from the last frame. See references/pages.md.
verify --cues says a cue is not on screen	Show each cue for its whole time. Text drawn on a canvas must be declared: window.heliosDrawnText?.add(text).
check reports flashing	More than three flashes in a second can trigger seizures. Slow the strobe, shrink the flashing area, or lower its contrast.
Blank WebGL frames	Draw inside renderAt (or a rAF loop), not once at load.

Bigger projects (React/Vue components, studio editing, distributed cloud rendering) use the Helios packages directly: https://github.com/BintzGavin/helios.