Maintaining a CSS translate3d position when swapping classes

Question

I have an infinite scrolling background image in a div as you can see in the below snippet.

The idea behind this is that its a timeline that I can change the speed of, ie. Normal speed, Fast, Slow and even Reverse. Using javascript I can successfully change the "speed class" that I have on the div and the image will successfully update to the speed and direction that I wish it to go.

However my issue is that when I'm changing class, I lose the relative translate3d position of the image, so as the class changes, the background image location gets reset and then the new animation starts.

I've tried to address this a few ways purely from a css point of view, by trying a complete swap of classes, as well as running a single class full time and applying the selected animation as an additional class, however I am unable to figure out how to maintain the relative background position when the new class is applied.

How can I maintain the background images location when I swap class so that it doesn't look like the image is jumping from one position to another?

.timeline_frame {
  width: 100%;
  min-width: 1500px;
  background-color: #444;
  height: 250px;
  left: 0;
  right: 0;
  margin: auto auto;
  position: relative;
}

.timeline {
  position: absolute;
  left: 0;
  right: 0;
  bottom: 0;
  top: 0;
  margin: auto auto;
  width: 1500px;
  height: 200px;
}

.timeline_container {
  width: 1500px;
  height: 200px;
  overflow: hidden;
  position: relative;
}

.timeline_background {
  background: url("https://www.aidanwardman.com/timeline-bg.png") repeat-x;
  background-color: rgba(0, 0, 0, 0.5);
  width: 3000px;
  height: 200px;
  vertical-align: bottom;
}

.timeline_speed_normal {
  animation: slide 20s linear infinite;
}

.timeline_speed_slow {
  animation: slide 40s linear infinite;
}

.timeline_speed_reverse {
  animation: slide-reverse 2s linear infinite;
}

@keyframes slide {
  0% {
    transform: translate3d(0, 0, 0);
  }
  100% {
    transform: translate3d(-1500px, 0, 0);
  }
}

@keyframes slide-reverse {
  0% {
    transform: translate3d(-1500px, 0, 0);
  }
  100% {
    transform: translate3d(0, 0, 0);
  }
}

.button_frame {
  left: 0;
  right: 0;
  margin: auto;
  width: 400px;
  padding: 20px;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="button_frame">
  <button id="forward-normal">Forward Normal</button>
  <button id="forward-slow">Forward Slow</button>
  <button id="reverse">Reverse</button>
</div>
<div class="timeline_frame">
  <div class="timeline">
    <div class="timeline_container">
      <div class="timeline_background timeline_speed_normal"></div>
    </div>
  </div>
</div>

<script>
  $("#forward-normal").click(function() {
    $(".timeline_background").removeClass("timeline_speed_fast timeline_speed_slow timeline_speed_reverse").addClass("timeline_speed_normal");
  });
  $("#forward-slow").click(function() {
    $(".timeline_background").removeClass("timeline_speed_fast timeline_speed_normal timeline_speed_reverse").addClass("timeline_speed_slow");
  });
  $("#reverse").click(function() {
    $(".timeline_background").removeClass("timeline_speed_fast timeline_speed_slow timeline_speed_normal").addClass("timeline_speed_reverse");
  });
</script>

Answer 1

I would simulate such thing using transition instead of animation:

.timeline_frame {
  width: 100%;
  min-width: 1500px;
  background-color: #444;
  height: 250px;
  left: 0;
  right: 0;
  margin: auto auto;
  position: relative;
}

.timeline {
  position: absolute;
  left: 0;
  right: 0;
  bottom: 0;
  top: 0;
  margin: auto auto;
  width: 1500px;
  height: 200px;
}

.timeline_container {
  width: 1500px;
  height: 200px;
  overflow: hidden;
  position: relative;
}

.timeline_background {
  background: url("https://www.aidanwardman.com/timeline-bg.png") repeat-x;
  background-color: rgba(0, 0, 0, 0.5);
  width: 3000px;
  height: 200px;
  vertical-align: bottom;
  transition:10s all;
}

.timeline_speed_normal {
  transform: translate3d(-1500px, 0, 0);
}

.timeline_speed_slow {
  transition:30s all;
  transform: translate3d(-1501px, 0, 0);
}

.timeline_speed_reverse {
  transform: translate3d(0, 0, 0);
}

.button_frame {
  left: 0;
  right: 0;
  margin: auto;
  width: 400px;
  padding: 20px;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="button_frame">
  <button id="forward-normal">Forward Normal</button>
  <button id="forward-slow">Forward Slow</button>
  <button id="reverse">Reverse</button>
</div>
<div class="timeline_frame">
  <div class="timeline">
    <div class="timeline_container">
      <div class="timeline_background"></div>
    </div>
  </div>
</div>

<script>
  $("#forward-normal").click(function() {
    $(".timeline_background").removeClass("timeline_speed_fast timeline_speed_slow timeline_speed_reverse").addClass("timeline_speed_normal");
  });
  $("#forward-slow").click(function() {
    $(".timeline_background").removeClass("timeline_speed_fast timeline_speed_normal timeline_speed_reverse").addClass("timeline_speed_slow");
  });
  $("#reverse").click(function() {
    $(".timeline_background").removeClass("timeline_speed_fast timeline_speed_slow timeline_speed_normal").addClass("timeline_speed_reverse");
  });
</script>