Scroll-Based Animation Frame-by-Frame with JavaScript

index.html

<canvas id="animation" /></canvas>
<main>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer eros arcu, feugiat ut cursus sit amet, sagittis et massa. Ut non rhoncus tellus. Mauris tincidunt, tellus ut tempus imperdiet, est arcu ultrices elit, id cursus arcu leo sed tortor. Suspendisse et maximus mauris. Sed sit amet gravida metus. Vestibulum vel augue mattis velit hendrerit accumsan ac ut enim. Duis id sem metus. Nullam tristique mattis nulla, at scelerisque massa vulputate a. Integer convallis pellentesque turpis quis interdum. Integer non commodo lectus. Mauris tincidunt, tellus ut tempus imperdiet.</p>
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</main>

script.js

const html = document.documentElement;
const canvas = document.getElementById("animation");
const context = canvas.getContext("2d");

const frameCount = 40;
const currentFrame = index => (
  `https://cybersandbox.ca/assets/frames/frame_${index.toString().padStart(2, '00')}.jpg`
)

const preloadImages = () => {
  for (let i = 1; i < frameCount; i++) {
    const img = new Image();
    img.src = currentFrame(i);
  }
};

const img = new Image()
img.src = currentFrame(1);
canvas.width=539;
canvas.height=475;
img.onload=function(){
  context.drawImage(img, 0, 0);
}

const updateImage = index => {
  img.src = currentFrame(index);
  context.drawImage(img, 0, 0);
}

window.addEventListener('scroll', () => {  
  const scrollTop = html.scrollTop;
  const maxScrollTop = html.scrollHeight - window.innerHeight;
  const scrollFraction = scrollTop / maxScrollTop;
  const frameIndex = Math.min(
    frameCount - 1,
    Math.ceil(scrollFraction * frameCount)
  );
  
  requestAnimationFrame(() => updateImage(frameIndex + 1))
});

preloadImages()

scroll-based-animation-frame-by-frame-with-javascript.markdown

Scroll-Based Animation Frame-by-Frame with JavaScript

A Pen by Carter Lovelace ⚙️ on CodePen.

License.

style.css

* {
  box-sizing: border-box;
}

html {
  height: 100vh;
  scroll-behavior: smooth;
}

body {
  background: #000;
}

canvas {
  position: fixed;
  top: 0;
  left: 30px;
  max-width: 100vw;
  height: 100vh;
  width: 50vw;
  object-fit: contain;
  filter: invert(1);
}

main {
  width: 50%;
  float: right;
  color: white;
  font-family: sans-serif;
  padding: 0 50px;
}

main p {
  margin: 22vh 0;
  line-height: 1.6em;
}

@media only screen and (max-width: 550px) {

main {
  width: 100%;
  position: relative;
  z-index: 2;
  padding-top: 75vh;
}
  main p {
  background: rgba(0,0,0,0.5);
    padding: 15px 0;
    margin: 15vh 0;
}

canvas {
  width: calc(100% - 100px);
  left: 50px;
  }
}