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              <main class="">
  <div></div>
  <div></div>
  <div></div>
  <div></div>
  <div></div>
  <div class="primary"></div>
</main>

<form>
  <button type="button" class="active" data-step="0">1</button>
  <button type="button" data-step="1">2</button>
  <button type="button" data-step="2">3</button>
</form>
<p class="step1">Add six frames. For each frame overlay a repeating gradient of 1px transparent and 5px white. Offset this gradient by 1px for each frame.</p>
<p class="step2">Apply a mix-blend-mode of multiply to each frame, letting the underlying frames to show instead of the white blocks from the gradient.</p>
<p class="step3">Add a cover on top that is 1px transparent and 5px black. Translate it 5px infinitely. The underlying frames will show through one at a time.</p>
            
          
!
            
              :root {
  --pixel: 1px;
  --cover: #212123;
  --duration: 1000ms;
  --covered: calc(var(--pixel) * 5); /*6 frames, 5 covered at a time */
  --space: white; /* will cause transparency when mix blend mode multiply */
  --w: 100vw;
  --h: 100vh;
  
  --depth-base: 0px;
  --direction: alternate;
  
  --blend: none;
}
main *, main *::after {
  opacity: 0;
}
main div {
  --offset: calc(var(--pixel) * 1);
  --background-image: radial-gradient(circle, cyan 0%, cyan);
  --background-cover-yes: linear-gradient(to right, transparent 0, transparent calc(var(--pixel)), var(--space) var(--pixel), var(--space) calc(var(--covered)));
  --background-cover-no: linear-gradient(to right, transparent, transparent);
  --background-cover: var(--background-cover-no);
  
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  overflow: hidden;
  background-color: #16161c;
  background-image: var(--background-cover),
    var(--background-image);
  mix-blend-mode: var(--blend);
  
  background-position: calc(var(--offset) - calc(var(--pixel) * 1)) 0, center center;
  background-size: calc(var(--pixel) * 6) 100%, cover;
  background-repeat: repeat, no-repeat;
  transition: background 750ms ease-in-out;
  
  filter: saturate(200%);
}

main div:nth-of-type(2) {
  --offset: calc(var(--pixel) * 2);
  --background-image: radial-gradient(circle, hotpink 0%, cyan 20%);
}
main div:nth-of-type(3) {
  --offset: calc(var(--pixel) * 3);
  --background-image: radial-gradient(circle, hotpink 20%, cyan 40%);
}
main div:nth-of-type(4) {
  --offset: calc(var(--pixel) * 4);
  --background-image: radial-gradient(circle, hotpink 40%, cyan 60%);
}
main div:nth-of-type(5) {
  --offset: calc(var(--pixel) * 5);
  --background-image: radial-gradient(circle, hotpink 60%, cyan 75%);
}
main div:nth-of-type(6) {
  --offset: calc(var(--pixel) * 6);
  --background-image: radial-gradient(circle, hotpink 80%, cyan);
}

/*want to try using your own frames? insert them here, with either a gradient or an image with url() */
/**/
main div:nth-of-type(1) {
  --background-image: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/61811/h1.jpg);
}
main div:nth-of-type(2) {
  --background-image: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/61811/h2.jpg);
}
main div:nth-of-type(3) {
  --background-image: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/61811/h3.jpg);
}
main div:nth-of-type(4) {
  --background-image: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/61811/h4.jpg);
}
main div:nth-of-type(5) {
  --background-image: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/61811/h5.jpg);
}
main div:nth-of-type(6) {
  --background-image: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/61811/h6.jpg);
}
/**/



/*the sliding cover that triggers the motion illusion */
.primary::after {
  content: '';
  position: absolute;
  top: 0;
  right: calc(var(--pixel) * -12);
  bottom: 0;
  left: calc(var(--pixel) * -12);
  background: linear-gradient(to right, transparent 0px, transparent calc(var(--pixel) * 1), var(--cover) calc(var(--pixel) * 1), var(--cover) calc(var(--pixel) * 6));
  will-change: transform, opacity;
  transition: opacity var(--duration) linear;
  mix-blend-mode: normal;
  opacity: var(--show, 0);
  transform: translateX(var(--slide-amount, 0));
  background-size: calc(var(--pixel) * 6) 100%;
  
  animation: var(--animation, none) var(--duration) infinite var(--direction) linear;
}

@keyframes cover {
  100% {
    transform: translateX(calc(var(--pixel) * 5))
  }
}

main {
  width: var(--w);
  height: var(--h);
  position: relative;
  transition: transform var(--duration) ease-in-out;
  background: white;
  transform: scale(1);
}
*, *::after {
  box-sizing: border-box;
}










body {
  width: 100%;
  height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;
  background: hsl(203, 50%, 4%);
  margin: 0;
  overflow: hidden;
  touch-action: none;
  
  font-family: system-ui, -apple-system, 'Segoe UI', sans-serif
}
form {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  padding: .2rem 1rem;
  text-align: center;
  
  background: rgba(245,245,255,.86);
  display: flex;
  align-items: center;
  justify-content: center;
  flex-wrap: wrap;
  --form-color-alpha: rgba(150, 250, 255, .1);
  overflow: auto;
}
form div {
  padding: .5rem .5rem;
  font-size: 12px;
  transform: translateX(1rem);
}
form label {
  transform: translateX(-1.5rem)
}
p {
  position: absolute;
  line-height: 1.334;
  bottom: 0;
  left:0;
  right:0;
  
  padding: .5rem 1rem;
  text-align: center;
  
  background: rgba(245,245,255,1);
  display: flex;
  align-items: center;
  justify-content: center;
  
  opacity: 0;
}
code {
  display: inline;
  font-family: monospace;
}


button {
  appearance: none;
  border: 0;
  border-radius: 50%;
  font-size: 1rem;
  margin: .5rem 1rem;
  width: 2rem;
  height: 2rem;
  background-color: hsl(193, 50%, 90%);
  transition: background-color 350ms ease-in-out;
}
button.active {
  background-color: hsl(193, 50%, 50%);
  color: #fafaff;
}
            
          
!
            
              var frames = Array.from(document.querySelectorAll('main div'));
var explanations = Array.from(document.querySelectorAll('p[class*=step]'));
var steps = document.querySelectorAll('button');
var duration = 1000;

buildFrames();

function buildFrames(finish) {
  document.documentElement.style.setProperty('--show', 0);
  changeExplanation(0);
  var dur = finish === true ? 0 : duration;
  for (let i = 0, l = frames.length; i < l; ++i) {
    let frame = frames[i];
    var framer = frame.animate({opacity: [0,1]}, {
      duration: dur,
      fill: 'both',
      delay: dur * 2 * i
    });
    framer.onfinish = function() {
      var masker = frame.animate({backgroundImage: ['var(--background-cover-no),var(--background-image)','var(--background-cover-yes),var(--background-image)']}, {
        duration: dur,
        fill: 'both',
        delay: 0
      });

      if (i === l-1) {
        masker.onfinish = startBlendModes;
      }
    };
  }
}


function startBlendModes(finish) {
  document.documentElement.style.setProperty('--show', 0);
  changeExplanation(1);
  var dur = finish === true ? 0 : duration;
  for (let i = frames.length - 1, count = 0; i >= 0; --i) {
    let frame = frames[i];
    var framer = frame.animate({mixBlendMode: ['normal','multiply']}, {
      duration: dur * 1.5,
      fill: 'both',
      delay: dur * (count++) * 1.5
    });
    
    if (i === 0) {
      framer.onfinish = startCover;
    }
  }
}

function startCover() {
  document.documentElement.style.setProperty('--show', 1);
  changeExplanation(2);
  
  setTimeout(() => {
    document.documentElement.style.setProperty('--animation', 'cover');
  }, duration * 3)
}

function changeExplanation(index) {
  explanations[index].animate({opacity:[0,1]}, {
    duration: 250,
    fill: 'both'
  });
  document.querySelector('.active').classList.remove('active');
  steps[index].classList.add('active');
}

function clearAnimations() {
  if (document.getAnimations) {
    document.getAnimations().forEach(function(anim) {
      anim.cancel();
    });
  }
}

// for (let i = 0; i < steps.length; ++i) {
//   steps[i].addEventListener('click', changeStep);
// }
function changeStep(e) {
  var index = parseInt(e.currentTarget.getAttribute('data-step'), 10);
  clearAnimations() 
  
  if (index === 0) {
    buildFrames();
  } else if (index === 1) {
    buildFrames(true);
    startBlendModes();
  } else {
    buildFrames(true);
    startBlendModes(true);
    startCover();
  }
}
            
          
!
999px
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