$imgWidth: 600;
$imgHeight: 300;
$photos: 3; // the total photos in the slideshow (must match the html)
$pieces: 50; // 50 divs for each "photo"
$segmentWidth: $imgWidth/$pieces; // the width of each piece
$time: 6s; // the time each photo will take to fade In and split
$totalTime: $photos * $time; // the full length of the animation for all photos
// this is needed because there's no such thing as animation-iteration-delay :(

// a helper function.. randomly returns -1 or 1
@function posOrNeg(){
  @return random() * 2 - 1;
}

html, body {
  height: 100%;
}

body {
  background: black;
  overflow: hidden;
}

.wrap {
  height: $imgHeight * 1px;
  width: $imgWidth * 1px;
  position: relative;
  // center the wrap
  top: 50%;
  left: 50%;
  margin-left: $imgWidth/2 * -1px;
  margin-top: $imgHeight/2 * -1px;
}

.photo {
  width: inherit;
  height: inherit;
  display: block;
  position: absolute;
  &:after { // clearfix
    content: "";
    display: table;
    clear: both;
  }
}

.p {
  height: $imgHeight * 1px; // set the height to the imageHeight 300 in this case
  width: $segmentWidth * 1px; // set the width to the segment width 600/50
  background-repeat: no-repeat; // stop background from repeating
  float: left;
  opacity: 0; // set the opacity to 0, so we can fade them in 
}

// we need to loop over the photo divs, set their z-index and 
// give the .p divs inside them separate background images
@for $i from 1 through $photos {
  .photo:nth-child(#{$i}){ // grab the photo div
    z-index: $photos - $i; // stack them on top of each other 3, 2, 1
    .p {
      // set a background image from lorempixel, used ?v= $i to break cache and randomize each image
      background-image: url(http://lorempixel.com/#{$imgWidth}/#{$imgHeight}/technics/?v=#{$i});
    }
  }

  @for $x from 1 through $pieces { // now we need to loop over each piece of this specific photo div
    $delay: ($i - 1) * $time; // this is part of the magic. $time is 6s in this case, we need each photo animation to wait + 6s before animating, so we add this delay on top of the .1s * $x animation delay. In the first iteration $delay will be 6s, next 12s, then 18s
    .photo:nth-child(#{$i}) .p:nth-child(#{$x}){ 
      animation-delay: ($x * .01s) + $delay; // set the animation delay
    }
  }
}

@for $i from 1 through $pieces { // we can loop over the pieces and set their keyframe animations/background positions
  .p:nth-child(#{$i}){ // grab this piece
    // this technique is the same as using CSS sprite sheets, each segment is 12px wide, and there's 50 of them. The first time around we position the background at 0px 0px, next -12px 0, next -24px 0 etc.. When they are slammed next to each other it gives the effect of an entire image. Here's a guide on CSS-Tricks about css sprites if you're unfamiliar with how they work https://css-tricks.com/css-sprites/
    // Here's some other pens where I used a similar technique 
    // CSS 3d RedBull Can -- https://codepen.io/natewiley/pen/IwDCq
    // 300 piece image animation -- https://codepen.io/natewiley/pen/pFABJ
    background-position: (($segmentWidth * ($i - 1)) * -1px) 0px; 
    animation: split#{$i} $totalTime infinite; // set each piece to its respective keyframe animation. Notice we use the $totalTime, so each animation is 18s in this case before it repeats itself. With the other 6s delays it gives the effect of an animation-iteration-delay
  }
  @keyframes split#{$i} {
    33% { // from 0%-33% we are fading in from 0, and not transforming
      transform: translate3d(0,0,0);
      opacity: 1;
    }
    50% { // from 33%-50% we are fading out, while each piece is randomly translated along the Y axis
      transform: translate3d(0, (((random($imgHeight) + ($imgHeight * 2)) * posOrNeg()) * 1px), 0);
      opacity: 0;
    }
  }
}