Building a Scroll-Based Image Sequencer with GSAP

I’ve always loved those silky-smooth, scroll-driven animations you see on high-end sites — like a video tied to scroll that plays out like a mini movie.

Normally I’d use the Vimeo API + GSAP scrolltrigger to handle it. But I wanted to know if a canvas based approach would be more performant for smaller animations.

In this article my goal is to explain the concept and exploration as I would have liked to learn it.

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Breaking Down the Video

I started with a short clip I exported from premiere pro and wanted to turn into individual frames.

I’ve used ffmpeg before to compress videos and found that it had a command to split videos into it’s frames.

1ffmpeg -i file-location.mp4 -vf fps=30 file-output/frame_%03d.png

I went with 30fps, which gave me about 150 frames. Probably overkill, but I was curious to see how far I could push the smoothness.

Note: Later on I tried 60fps and the performance was horrendous

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Converting to AVIF

PNG is WAAAY to heavy. So I needed to compress all the files to AVIF.

9/10 times I use the Webflow compression as it’s faster and very reliable but I didn’t have that luxury here. So I used avif-cli via npm:

1npx avif --input="file-locations/*.webp" --output="output-folder"

The command above assumes you already have WebPs, but you can also point it at PNGs by swapping the *.webp to *.png or *.jpg

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Hosting the Frames

So the next question is. how would you host 150 images so you can add them to the canvas in order.

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Option 1: Webflow

This was my first attempt. I uploaded all the frames into a Webflow folder and thought I’d be clever by scripting the asset retrieval.

The issue: Webflow renames everything and gives each file a unique hosted URL.

For example:

1https://cdn.prod.website-files.com/67f8ebcae6e93dd5ea335c00/681cabb31fb837fcfb503530_your-file-name.webp

So I tried grabbing the URLs via the Webflow API:‍


1for id in $(
2  curl -s https://api.webflow.com/v2/asset_folders/123123123123123 \
3    -H "Authorization: Bearer token" |
4  jq -r '.assets[]'
5); do
6  curl -s https://api.webflow.com/v2/assets/$id \
7    -H "Authorization: Bearer token" |
8  jq -r '.hostedUrl'
9done

This mostly worked… except it randomly returned null for about 30 of the images. I don’t know why. After a few retries I dropped it and went with option B

I did all this in the terminal because I was lazy. Looking back it would have been a lot easier to just create a JS file and run it with node.

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Node JS‍


1const fetch = (...args) =>
2  import('node-fetch').then(({ default: fetch }) => fetch(...args));
3
4const folderId = '6806535e20562ef894dcdbd1';
5const token = 'YOUR_WEBFLOW_API_TOKEN';
6
7const headers = {
8  'Authorization': `Bearer ${token}`,
9  'Content-Type': 'application/json'
10};
11
12async function run() {
13  const res = await fetch(`https://api.webflow.com/v2/asset_folders/${folderId}`, { headers });
14  const data = await res.json();
15  const assetIds = data.assets;
16
17  for (const assetId of assetIds) {
18    const res = await fetch(`https://api.webflow.com/v2/assets/${assetId}`, { headers });
19    const data = await res.json();
20    console.log(data.hostedUrl);
21  }
22}
23
24run();

Then run the file with node

1node your-file-name.js

If you don’t have node-fetch installed, you’ll need that

1npm install node-fetch

If you don’t have NPM installed. I’d recommend googling that, there’s a sea of guides better than I can write

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Option 2: AWS S3

Eventually, I switched to using an AWS S3 bucket. Mostly because I wanted predictable URLs like:

1https://my-bucket.s3.amazonaws.com/frames/frame_001.avif

That made the GSAP integration way easier.

If I was doing this for a personal project I’d use AWS but for a client it wouldn’t be worth the setup and hassle. I’d use Webflow and the node script to get the urls and sort them.

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Webflow Setup

This part was nice and simple. On the page you’ll need a HTML Embed that holds your canvas that’s position sticky.

1<canvas id="sequence" width="1920" height="1080"></canvas>

Make sure the sticky parent is 200vh+ so there’s some scroll distance for your animation.

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GSAP

GSAP has a helper function for the image sequencer that I took from the docs. You can read more about it here.

I’ll explain the code in the comments

1// Registering the ScrollTrigger plugin
2gsap.registerPlugin(ScrollTrigger);
3
4// Creating an array of our images
5let imgs = new Array(151)
6  .fill()
7  .map((o, i) =>
8    `https://bucket.s3.eu-west-3.amazonaws.com/your-files/frame_${(i + 1)
9      .toString()
10      .padStart(3, '0')}.avif`
11  );
12
13// Sort images by frame number
14imgs.sort((a, b) => {
15  const frameA = parseInt(a.match(/frame_(\d+)/)[1]);
16  const frameB = parseInt(b.match(/frame_(\d+)/)[1]);
17  return frameA - frameB;
18});
19
20// The main animation function
21function heroAnimation() {
22  // Calling the helper function we have below
23  // (functions can be called before they're written because of JS hoisting).
24  imageSequence({
25    // Attaching our URLS
26    urls: imgs,
27
28    // Connecting the canvas
29    canvas: '#sequence',
30
31    // Creating the scroll trigger object
32    scrollTrigger: {
33      // Our trigger is the parent element
34      trigger: '.sticky-parent',
35      // Start when the top of the element hits the top of the viewport
36      start: 'top top',
37      // End when the bottom of the element hits the bottom of the viewport
38      end: 'bottom bottom',
39      // Animate while scrolling
40      scrub: true,
41    },
42
43    // This is quite self explanatory
44    fps: 30,
45  });
46}
47
48// Run our animation
49heroAnimation();
50
51// This was from GSAP. I'm not going to explain everything as it's very well documented on the docs 
52// but essentially it's redrawing the image on the canvas each time the GSAP engine updates
53function imageSequence(config) {
54  let playhead = { frame: 0 },
55    canvas = gsap.utils.toArray(config.canvas)[0] || console.warn("canvas not defined"),
56    ctx = canvas.getContext("2d"),
57    curFrame = -1,
58    onUpdate = config.onUpdate,
59    images,
60    updateImage = function () {
61      let frame = Math.round(playhead.frame);
62      if (frame !== curFrame) { // only draw if necessary
63        config.clear && ctx.clearRect(0, 0, canvas.width, canvas.height);
64        // Calculate scaling to cover the canvas
65        const img = images[frame];
66        const scale = Math.max(canvas.width / img.width, canvas.height / img.height);
67        const x = (canvas.width - img.width * scale) / 2;
68        const y = (canvas.height - img.height * scale) / 2;
69        ctx.drawImage(img, x, y, img.width * scale, img.height * scale);
70        curFrame = frame;
71        onUpdate && onUpdate.call(this, frame, images[frame]);
72      }
73    };
74
75  images = config.urls.map((url, i) => {
76    let img = new Image();
77    img.src = url;
78    i || (img.onload = updateImage);
79    return img;
80  });
81
82  return gsap.to(playhead, {
83    frame: images.length - 1,
84    ease: "none",
85    onUpdate: updateImage,
86    duration: images.length / (config.fps || 30),
87    paused: !!config.paused,
88    scrollTrigger: config.scrollTrigger
89  });
90}

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Canvas nuances.

Some canvas quirks I ran into:

• You need to set width and height inline in the HTML so the canvas resolution matches your image. Otherwise things look blurry

1<canvas id="sequence" width="1920" height="1080"></canvas>

• Then you can style the canvas with CSS for responsiveness:‍

1#sequence {
2	width: 100%;
3	height: 100%;
4}

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‍That way the canvas content fits the viewport but doesn’t have bad res

Accessibility considerations

Canvas doesn’t expose content to screen readers — it’s just pixels. But in this case, the animation was decorative, so I was comfortable leaving it as-is. If your animation conveys important info, you’ll need to provide an accessible fallback.

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