<!--
Sections animate in and out on scroll. Scroll up or down and the sections will wrap around after hitting the start or end. Uses GSAP for the animations.
-->

<header>
  <div>Animated Sections</div>
  <div><a href="https://codepen.io/BrianCross/pen/PoWapLP">Original By Brian</a></div>
</header>
<section class="first">
  <div class="outer">
    <div class="inner">
      <div class="bg one">
        <h2 class="section-heading">Scroll down</h2>
      </div>
    </div>
  </div>

</section>
<section class="second">
  <div class="outer">
    <div class="inner">
      <div class="bg">
        <h2 class="section-heading">Animated with GSAP</h2>
      </div>
    </div>
  </div>
</section>
<section class="third">
  <div class="outer">
    <div class="inner">
      <div class="bg">
        <h2 class="section-heading">GreenSock</h2>
      </div>
    </div>
  </div>
</section>
<section class="fourth">
  <div class="outer">
    <div class="inner">
      <div class="bg">
        <h2 class="section-heading">Animation platform</h2>
      </div>
    </div>
  </div>
</section>
<section class="fifth">
  <div class="outer">
    <div class="inner">
      <div class="bg">
        <h2 class="section-heading">Keep scrolling</h2>
      </div>
    </div>
  </div>
</section>

@import url("https://fonts.googleapis.com/css2?family=Bebas+Neue&display=swap");
@import url("https://fonts.googleapis.com/css2?family=Cormorant+Garamond&display=swap");

$bg-gradient: linear-gradient(
  180deg,
  rgba(0, 0, 0, 0.6) 0%,
  rgba(0, 0, 0, 0.3) 100%
);

* {
  box-sizing: border-box;
  user-select: none;
}

a {
  color: #fff;
  text-decoration: none;
}

body {
  margin: 0;
  padding: 0;
  height: 100vh;
  color: white;
  background: black;
  font-family: "Cormorant Garamond", serif;
  text-transform: uppercase;
}

h2 {
  font-size: clamp(1rem, 5vw, 5rem);
  font-weight: 400;
  text-align: center;
  letter-spacing: 0.5em;
  margin-right: -0.5em;
  color: hsl(0, 0, 80%);
  width: 90vw;
  max-width: 1200px;
}

header {
  position: fixed;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0 5%;
  width: 100%;
  z-index: 3;
  height: 7em;
  font-family: "Bebas Neue", sans-serif;
  font-size: clamp(0.66rem, 2vw, 1rem);
  letter-spacing: 0.5em;
}

section {
  height: 100%;
  width: 100%;
  top: 0;
  position: fixed;
  visibility: hidden;

  .outer,
  .inner {
    width: 100%;
    height: 100%;
    overflow-y: hidden;
  }

  .bg {
    display: flex;
    align-items: center;
    justify-content: center;
    position: absolute;
    height: 100%;
    width: 100%;
    top: 0;
    background-size: cover;
    background-position: center;

    h2 {
      z-index: 2;
    }

    .clip-text {
      overflow: hidden;
    }
  }
}

.first {
  .bg {
    background-image: $bg-gradient,
      url(https://images.unsplash.com/photo-1617478755490-e21232a5eeaf?crop=entropy&cs=srgb&fm=jpg&ixid=MnwxNDU4OXwwfDF8cmFuZG9tfHx8fHx8fHx8MTYxNzU1NjM5NA&ixlib=rb-1.2.1&q=75&w=1920);
  }
}

.second {
  .bg {
    background-image: $bg-gradient,
      url("https://images.unsplash.com/photo-1617128734662-66da6c1d3505?crop=entropy&cs=tinysrgb&fit=max&fm=jpg&ixid=MnwxNDU4OXwwfDF8cmFuZG9tfHx8fHx8fHx8MTYxNzc3NTM3MA&ixlib=rb-1.2.1&q=75&w=1920");
  }
}

.third {
  .bg {
    background-image: $bg-gradient,
      url(https://images.unsplash.com/photo-1617438817509-70e91ad264a5?crop=entropy&cs=srgb&fm=jpg&ixid=MnwxNDU4OXwwfDF8cmFuZG9tfHx8fHx8fHx8MTYxNzU2MDk4Mg&ixlib=rb-1.2.1&q=75&w=1920);
  }
}

.fourth {
  .bg {
    background-image: $bg-gradient,
      url(https://images.unsplash.com/photo-1617412327653-c29093585207?crop=entropy&cs=srgb&fm=jpg&ixid=MnwxNDU4OXwwfDF8cmFuZG9tfHx8fHx8fHx8MTYxNzU2MDgzMQ&ixlib=rb-1.2.1&q=75&w=1920);
  }
}

.fifth {
  .bg {
    background-image: $bg-gradient,
      url("https://images.unsplash.com/photo-1617141636403-f511e2d5dc17?crop=entropy&cs=srgb&fm=jpg&ixid=MnwxNDU4OXwwfDF8cmFuZG9tfHx8fHx8fHx8MTYxODAzMjc4Mw&ixlib=rb-1.2.1&q=75w=1920");
    background-position: 50% 45%;
  }
}

h2 * {
  will-change: transform;
}

gsap.registerPlugin(Observer);

let sections = document.querySelectorAll("section"),
  images = document.querySelectorAll(".bg"),
  headings = gsap.utils.toArray(".section-heading"),
  outerWrappers = gsap.utils.toArray(".outer"),
  innerWrappers = gsap.utils.toArray(".inner"),
  splitHeadings = headings.map(heading => new SplitText(heading, { type: "chars,words,lines", linesClass: "clip-text" })),
  currentIndex = -1,
  wrap = gsap.utils.wrap(0, sections.length),
  animating;

gsap.set(outerWrappers, { yPercent: 100 });
gsap.set(innerWrappers, { yPercent: -100 });

function gotoSection(index, direction) {
  index = wrap(index); // make sure it's valid
  animating = true;
  let fromTop = direction === -1,
      dFactor = fromTop ? -1 : 1,
      tl = gsap.timeline({
        defaults: { duration: 1.25, ease: "power1.inOut" },
        onComplete: () => animating = false
      });
  if (currentIndex >= 0) {
    // The first time this function runs, current is -1
    gsap.set(sections[currentIndex], { zIndex: 0 });
    tl.to(images[currentIndex], { yPercent: -15 * dFactor })
      .set(sections[currentIndex], { autoAlpha: 0 });
  }
  gsap.set(sections[index], { autoAlpha: 1, zIndex: 1 });
  tl.fromTo([outerWrappers[index], innerWrappers[index]], { 
      yPercent: i => i ? -100 * dFactor : 100 * dFactor
    }, { 
      yPercent: 0 
    }, 0)
    .fromTo(images[index], { yPercent: 15 * dFactor }, { yPercent: 0 }, 0)
    .fromTo(splitHeadings[index].chars, { 
        autoAlpha: 0, 
        yPercent: 150 * dFactor
    }, {
        autoAlpha: 1,
        yPercent: 0,
        duration: 1,
        ease: "power2",
        stagger: {
          each: 0.02,
          from: "random"
        }
      }, 0.2);

  currentIndex = index;
}

Observer.create({
  type: "wheel,touch,pointer",
  wheelSpeed: -1,
  onDown: () => !animating && gotoSection(currentIndex - 1, -1),
  onUp: () => !animating && gotoSection(currentIndex + 1, 1),
  tolerance: 10,
  preventDefault: true
});

gotoSection(0, 1);

// original: https://codepen.io/BrianCross/pen/PoWapLP
// horizontal version: https://codepen.io/GreenSock/pen/xxWdeMK