Simulating additive animations using Web Animations API issue with getComputedStyle(element).transform

Question

I need to "simulate" additive animations using Web Animations API.

As described in the documentation additive animations are not supported yet and the use of getComputedStyle(element) is suggested.

The problem I am facing is that the matrix3d returned by getComputedStyle(element).transform probably does not contain enough information on perspective, so if I am using as start keyframe, animations are not the same (issue with Red box).

In this example, the blue box has the desired animation, red box animation is not correct:

I would like to know: - If a solution to this problem exists - If the implementation of additive animations would support transform with perspective.

Thanks for your time.

var elem = document.getElementById('target');
var style = window.getComputedStyle(elem);

elem.animate([{
  transform: style.transform // using matrix3d returned from getComputedStyle()
}, {
  transform: 'rotateY(179deg) scaleZ(2) translateZ(200px)'
}], {
  duration: 3000
});

var elem2 = document.getElementById('target2');
elem2.animate([{
  transform: 'rotateY(-179deg) scaleZ(2) translateZ(200px)'
}, {
  transform: 'rotateY(179deg) scaleZ(2) translateZ(200px)'
}], {
  duration: 3000
});

#wrapper,
#wrapper2 {
  perspective: 1000px;
}

#target,
#target2 {
  position: absolute;
  top: 100px;
  width: 200px;
  height: 300px;
}

#target {
  left: 100px;
  background-color: red;
  transform: rotateY(-179deg) scaleZ(2) translateZ(200px);
}

#target2 {
  left: 400px;
  background-color: blue;
  transform: rotateY(-179deg) scaleZ(2) translateZ(200px);
}

<div id="wrapper">
  <div id="target">wrong animation</div>
</div>

<div id="wrapper2">
  <div id="target2">correct animation</div>
</div>

Answer 1

You have some different problems in the snippet.

The main one is that the target elements are positioned different respect to the parent. (One has a left value higher than the other). Since the perspective value is in the wrapper, the perspective origin is also, and so it's very difficult to make them match.

I have redone the snippet that show that there is not a real issue with the perspective, when this problems are solved.

But there is still the problem with the missing information, as discussed in a previous question, that as I said is unsolvable.

var elem2 = document.getElementById('target2');
var style = window.getComputedStyle(elem2);


var elem = document.getElementById('target');
elem.style.transform = style;

#wrapper,
#wrapper2 {
  width: 200px;
  height: 300px;
  border: solid 1px blue;
  top: 100px;
  position: absolute;
}
#wrapper {
  left: 400px;
}
#wrapper2 {
  perspective: 1000px;
  left: 600px;
}
#target,
#target2 {
  width: 200px;
  height: 300px;
  position: absolute;
}
#target {
  background-color: red;
  transform: perspective(1000px) rotateY(-45deg) scaleZ(2) translateZ(200px);
}
#target2 {
  background-color: blue;
  transform: rotateY(-45deg) scaleZ(2) translateZ(200px);
}

<div id="wrapper">
  <div id="target">wrong animation</div>
</div>

<div id="wrapper2">
  <div id="target2">correct animation</div>
</div>