Scroll effect of changing images

<main>
  <section class="mast">
     <figure class="mast__bg" style="background-image:url(https://unsplash.it/1800/800?image=1033)"></figure>
    <header class="mast__header  js-clippy">
        <h5 class="mast__pretitle">Scroll Fool</h5>
        <h2 class="mast__title">Let's Get Clippy</h2>
        <hr class="sep">
    </header>
  </section>
  
  <section class="mast">
     <figure class="mast__bg" style="background-image:url(https://unsplash.it/1800/800?image=1008)"></figure>
    <header class="mast__header  js-clippy">
      <h5 class="mast__pretitle">Go On Mon</h5>
      <h2 class="mast__title">Clip Rect In Effect</h2>
      <hr class="sep">
    </header>
  </section>
    
  <section class="mast">
     <figure class="mast__bg" style="background-image:url(https://unsplash.it/1800/800?image=945)"></figure>
  <header class="mast__header  js-clippy">
    <h5 class="mast__pretitle">We Done Son</h5>
    <h2 class="mast__title">Getting All Recty</h2>
    <hr class="sep">
  </header>
  </section>
    
  </main>
  
  <div class="border-top"></div>
  <div class="border-left"></div>

<script src="//cdnjs.cloudflare.com/ajax/libs/jquery/2.1.3/jquery.min.js"></script>
//Скрипт js надо подключать здесь
//Автор: Stephen Scaff. @StephenScaff

*, *:before, *:after {
  box-sizing: border-box;
  min-width: 0;
  min-height: 0;
  margin: 0;
  padding: 0;
}

body {
  margin: 0;
  width: 100%;
  font-family: "Source Sans Pro";
  font-weight: 400;
  font-style: normal;
  -ms-text-size-adjust: 100%;
  -webkit-text-size-adjust: 100%;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  text-rendering: optimizeLegibility;
}

.border-top {
  width: 100%;
  height: 0.75em;
  top: 0;
  width: 0;
  -webkit-animation: ani-width ease-in-out 0.5s 0s both;
          animation: ani-width ease-in-out 0.5s 0s both;
}
.border-top:after {
  content: "";
  bottom: 0;
  width: 100%;
  height: 0.75em;
  right: 0;
  -webkit-animation: ani-width ease-in-out 0.5s 1s both;
          animation: ani-width ease-in-out 0.5s 1s both;
}

.border-left {
  height: 100%;
  width: 0.75em;
  left: 0;
  bottom: 0;
  -webkit-animation: ani-height ease-in-out 0.5s 1.5s both;
          animation: ani-height ease-in-out 0.5s 1.5s both;
}
.border-left:after {
  content: "";
  right: 0;
  top: 0;
  height: 100%;
  width: 0.75em;
  -webkit-animation: ani-height ease-in-out 0.5s 0.5s both;
          animation: ani-height ease-in-out 0.5s 0.5s both;
}

.border-top,
.border-top:after,
.border-left,
.border-left:after {
  position: fixed;
  z-index: 9;
  background-color: #fff;
}

h2 {
  font-family: "Playfair Display";
  font-weight: 700;
  font-size: 1.75em;
  line-height: 1.2;
}
@media (min-width: 32em) {
  h2 {
    font-size: 4em;
  }
}

hr.sep {
  border: 0px;
  height: 1px;
  background: #fff;
  margin: 1.4em auto;
  width: 3em;
}

main > section:nth-child(1):not(.site-menu), main > article:nth-child(1):not(.site-menu) {
  z-index: 2;
}
main > section:nth-child(2):not(.site-menu), main > article:nth-child(2):not(.site-menu) {
  z-index: 3;
}
main > section:nth-child(3):not(.site-menu), main > article:nth-child(3):not(.site-menu) {
  z-index: 4;
}

.mast {
  position: relative;
  display: flex;
  align-items: center;
  height: 100vh;
  width: 100%;
  color: #fff;
  margin-bottom: 1em;
}
.mast:after {
  content: "";
  z-index: 0;
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background: rgba(0, 0, 0, 0.7);
}
.mast__bg {
  position: absolute;
  top: 0;
  left: 0;
  height: 100%;
  width: 100%;
  margin: 0 auto;
  padding: 0;
  background-size: cover;
  background-color: #111;
  -webkit-animation: fade-in 1s ease both;
          animation: fade-in 1s ease both;
}
.mast__header {
  z-index: 5;
  position: fixed;
  width: 100%;
  top: 45%;
  text-align: center;
  -webkit-animation: fade-in 2s 1.5s ease both;
          animation: fade-in 2s 1.5s ease both;
}
.mast__pretitle {
  margin-bottom: 0.5em;
  font-size: 1em;
  font-weight: 400;
  text-transform: uppercase;
  letter-spacing: 0.15em;
}

@-webkit-keyframes fade-in {
  0% {
    opacity: 0;
  }
  100% {
    opacity: 1;
  }
}

@keyframes fade-in {
  0% {
    opacity: 0;
  }
  100% {
    opacity: 1;
  }
}
@-webkit-keyframes ani-width {
  0% {
    width: 0;
  }
  100% {
    width: 100%;
  }
}
@keyframes ani-width {
  0% {
    width: 0;
  }
  100% {
    width: 100%;
  }
}
@-webkit-keyframes ani-height {
  0% {
    height: 0;
  }
  100% {
    height: 100%;
  }
}
@keyframes ani-height {
  0% {
    height: 0;
  }
  100% {
    height: 100%;
  }
}

/*---------------------------------------------
Clippy Headings
- Creates a scrolling cliprect effect on fixed headings
----------------------------------------------*/

(function ($) {
  var s,
    clippy = {
      settings: {
        heading: $(".js-clippy")
      },
      init: function () {
        s = this.settings;
        this.bindEvents();
      },
      bindEvents: function () {
        $(window).on("load resize scroll", $.proxy(this.getClippy, this));
      },

      getClippy: function () {
        s.heading.each(function () {
          var layerOffset = $(this).closest("article, section").offset(),
            containerOffset = layerOffset.top - $(window).scrollTop(),
            clippy =
              containerOffset -
              $(this)
                .css("top")
                .replace(/[^-\d\.]/g, "") -
              $(this)
                .css("margin-top")
                .replace(/[^-\d\.]/g, "");
          $(this).css("clip", "rect(" + clippy + "px, auto, auto, auto)");
        });
      }
    };
  clippy.init();
})(jQuery);