Image Sequence on Scroll

<script src="https://cdn.jsdelivr.net/npm/gsap@3.14.1/dist/gsap.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/gsap@3.14.1/dist/ScrollTrigger.min.js"></script>

<div data-sequence-wrap data-scroll-end="bottom top" data-scroll-start="top top" class="image-sequence__wrap">
  <div class="image-sequence__sticky">
    <div data-sequence-element class="image-sequence__element">
      <canvas data-sequence-canvas 
              data-desktop-src="https://osmo.b-cdn.net/resource-media/image-sequence-demo/desktop/highres" 
              data-mobile-src="https://osmo.b-cdn.net/resource-media/image-sequence-demo/mobile/mobile" 
              data-static-src="https://osmo.b-cdn.net/resource-media/image-sequence-demo/desktop/highres000.webp" 
              data-filetype="webp" 
              data-frames="120" 
              data-digits="3" 
              data-index-start="0" 
              class="image-sequence__canvas">
      </canvas>
    </div>
  </div>
</div>

.image-sequence__wrap {
  width: 100%;
  height: 300vh;
  position: relative;
}

.image-sequence__sticky {
  justify-content: center;
  align-items: center;
  width: 100%;
  height: 100vh;
  display: flex;
  position: sticky;
  top: 0;
}

.image-sequence__element {
  z-index: 0;
  width: 100%;
  height: 100%;
  position: absolute;
  inset: 0%;
}

.image-sequence__canvas {
  width: 100%;
  height: 100%;
  padding: 0;
  position: relative;
}

function initImageSequenceScroll() {
  const wraps = document.querySelectorAll('[data-sequence-wrap]');
  
  wraps.forEach((wrap) => {
    // Prevent double-initializing
    if (wrap.dataset.sequenceInit === 'true') return;
    wrap.dataset.sequenceInit = 'true';

    const element = wrap.querySelector('[data-sequence-element]');
    const canvas = element && element.querySelector('[data-sequence-canvas]');
    if (!element || !canvas) return;
    
    // Data attributes and their fallbacks
    const frames = parseInt(canvas.dataset.frames, 10) || 1;
    const digits = parseInt(canvas.dataset.digits, 10) || 3;
    const indexStart = parseInt(canvas.dataset.indexStart, 10) || 0;
    const desktopSrc = canvas.dataset.desktopSrc || '';
    const mobileSrc = canvas.dataset.mobileSrc || desktopSrc;
    const staticSrc = canvas.dataset.staticSrc;
    const filetype = canvas.dataset.filetype || 'webp';
    const startTrigger = wrap.dataset.scrollStart || 'top top';
    const endTrigger = wrap.dataset.scrollEnd || 'bottom top';
    const reduceMotion = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

    const isMobile = window.matchMedia('(max-width: 767px)').matches;
    const baseUrl = isMobile ? mobileSrc : desktopSrc;
    const lastIndex = indexStart + frames - 1;

    // Track last rendered scroll progress so we can redraw on resize
    let lastProgress = 0;

    // Canvas setup (size to the sticky element)
    const ctx = canvas.getContext('2d');
    function resizeCanvas() {
      const dpr = window.devicePixelRatio || 1;
      const width = element.clientWidth;
      const height = element.clientHeight;
      if (canvas.width !== width * dpr || canvas.height !== height * dpr) {
        canvas.width = width * dpr;
        canvas.height = height * dpr;
        canvas.style.width = `${width}px`;
        canvas.style.height = `${height}px`;
      }
    }
    resizeCanvas();

    // Image cache and loading queue
    const loaded = new Map();
    const queue = [];
    let processingQueue = false;
    let resizeTimer;

    // Draw helper (canvas equivalent of object-fit: cover)
    function drawCover(img) {
      if (!img) return;
      resizeCanvas();
      const canvasWidth = canvas.width;
      const canvasHeight = canvas.height;
      const scale = Math.max(canvasWidth / img.width, canvasHeight / img.height);
      const x = (canvasWidth - img.width * scale) / 2;
      const y = (canvasHeight - img.height * scale) / 2;
      ctx.clearRect(0, 0, canvasWidth, canvasHeight);
      ctx.drawImage(img, x, y, img.width * scale, img.height * scale);
    }

    window.addEventListener('resize', () => {
      clearTimeout(resizeTimer);
      resizeTimer = setTimeout(() => {
        resizeCanvas();
        if (loaded.size) render(lastProgress);
        ScrollTrigger.refresh();
      }, 200);
    });

    function pad(num) {
      return String(num).padStart(digits, '0');
    }
    
    function getUrl(i) {
      return `${baseUrl}${pad(i)}.${filetype}`;
    }
    
    function loadFrame(i, onDone) {
      if (loaded.has(i) || i < indexStart || i > lastIndex) return;
      const img = new Image();
      img.src = getUrl(i);

      img.onload = () => {
        loaded.set(i, img);
        if (typeof onDone === 'function') onDone();
      };

      // Log when a frame is missing or fails to load
      img.onerror = () => {
        console.warn('[ImageSequence] Failed to load frame', {
          index: i,
          url,
          wrap: wrap
        });
      };
    }

    // Daybreak-style progressive loader (binary midpoint / "wave" fill)
    function processQueue() {
      if (processingQueue) return;
      const next = queue.shift();
      if (!next) return;
      processingQueue = true;
      const [a, b] = next;
      if (b - a <= 1) {
        processingQueue = false;
        processQueue();
        return;
      }
      const m = Math.floor((a + b) / 2);
      loadFrame(m, () => {
        queue.push([a, m], [m, b]);
        processingQueue = false;
        setTimeout(processQueue, 0);
      });
    }
    
    function startLoading() {
      loadFrame(indexStart, () => {
        drawImageAt(indexStart); // Show the first frame right away
        loadFrame(lastIndex); // Preload the last frame
        queue.push([indexStart, lastIndex]);
        processQueue();
        ScrollTrigger.refresh();
      });
    }
    
    function findNearestLoaded(i) {
      for (let r = 1; r <= 10; r++) {
        if (loaded.has(i - r)) return i - r;
        if (loaded.has(i + r)) return i + r;
      }

      const keys = Array.from(loaded.keys());
      if (keys.length === 0) return null;
      let nearest = keys[0];
      let minDiff = Math.abs(i - nearest);
      for (const k of keys) {
        const diff = Math.abs(i - k);
        if (diff < minDiff) {
          nearest = k;
          minDiff = diff;
        }
      }
      return nearest;
    }
    
    function drawImageAt(i) {
      const img = loaded.get(i);
      if (!img) return;
      drawCover(img);
    }
    
    function render(progress) {
      const relative = progress * (frames - 1);
      const index = indexStart + Math.round(relative);
      if (loaded.has(index)) {
        drawImageAt(index);
      } else {
        const nearest = findNearestLoaded(index);
        if (nearest !== null) drawImageAt(nearest);
      }
    }

    // Reduced motion: draw a single static image (or first frame fallback)
    if (reduceMotion) {
      if (staticSrc) {
        const staticImage = new Image();
        staticImage.src = staticSrc;
        staticImage.onload = () => {
          drawCover(staticImage);
        };
        staticImage.onerror = () => {};
        return;
      }
      loadFrame(indexStart, () => {
        drawImageAt(indexStart);
      });
      return;
    }
    
    // Begin loading frames in the background
    startLoading();
    
    // Set up ScrollTrigger
    const st = ScrollTrigger.create({
      trigger: wrap,
      start: startTrigger,
      end: endTrigger,
      scrub: true,
      onUpdate: (self) => {
        lastProgress = self.progress;
        render(self.progress);
      }
    });

    // Draw once immediately
    lastProgress = st.progress || 0;
    render(lastProgress);

  });
}

// Init Image Sequence on Scroll
document.addEventListener("DOMContentLoaded", () => {
  initImageSequenceScroll();
});