Animate a still image into video

Generate the still first

Video is the most expensive operation on the platform and the slowest, so never let a user burn a clip on a composition they have not seen. Generate a still, let them approve it, then animate that exact frame.

This also gives you a much better product: users iterate on the cheap step and commit on the expensive one.

curl https://api.fastgencloud.com/v1/images/generations \
  -H "Authorization: Bearer $FASTGEN_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
      "model": "general-image",
      "prompt": "a lighthouse on a cliff at dusk, low cloud, long lens, cinematic",
      "size": "1216x832"
    }'

Animate it

The prompt here describes motion, not content — the content is already fixed by the frame. Aspect ratio is preserved from your start frame.

curl https://api.fastgencloud.com/v1/videos/generations \
  -H "Authorization: Bearer $FASTGEN_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
      "model": "image-to-video",
      "image": "https://…/approved-frame.png",
      "prompt": "slow push in, low cloud drifting, beam sweeping across the frame",
      "duration": 5,
      "webhook_url": "https://your.app/hooks/fastgen"
    }'

Writing motion prompts

• Camera first: "slow push in", "slow pan left", "static locked-off shot"
• Then subject motion: "hair moving in the wind", "steam rising from the cup"
• Then ambient: "drifting clouds", "rain streaks on glass", "flickering neon"
• Less is more — asking for several simultaneous large movements is the fastest way to get warping
• An empty prompt is valid and gives subtle ambient motion, which is often exactly right for backgrounds

Use a webhook, not a poll loop

A 5 second clip takes on the order of three minutes on a warm worker, and longer from cold. That is far past any serverless request timeout, so the job must be a background job in your architecture too.

Register webhook_url on submit and verify the signature on receipt. The header is X-FastGen-Signature: t=<unix>,v1=<hex>, where v1 is an HMAC-SHA256 over `${t}.${rawBody}` using your webhook secret.

import { createHmac, timingSafeEqual } from "node:crypto";

function verify(secret, header, rawBody) {
  const parts = Object.fromEntries(header.split(",").map((kv) => kv.split("=")));
  const expected = createHmac("sha256", secret).update(`${parts.t}.${rawBody}`).digest();
  const given = Buffer.from(parts.v1, "hex");
  if (expected.length !== given.length) return false;
  // reject anything older than 5 minutes to prevent replays
  if (Math.abs(Date.now() / 1000 - Number(parts.t)) > 300) return false;
  return timingSafeEqual(expected, given);
}

Cost and retention

Video bills per second of output, so an 8 second clip costs 8x the per-second rate and a 5 second clip 5x. Failed jobs refund automatically.

Output URLs live for up to 7 days. Download the file and re-host it if it is going anywhere durable — do not hand a 7-day URL to a customer.