CSS3 3D Rotation: How perspective work?

Question

I'm working on 3D bloc following mouse move on it. I'm updating CSS values to create a 3D effect. The Base look like:

document.addEventListener('DOMContentLoaded', function() {
  let mouseMove = function (e) {
    let el = e.currentTarget;
    let delta_x = parseFloat(e.offsetX / el.offsetWidth).toFixed(3)
    let delta_y = parseFloat(e.offsetY / el.offsetHeight).toFixed(3)

    var transform = "rotateY(" + ((delta_x - 0.5) * 50) + "deg) " +
        "rotateX(" + (-(delta_y - 0.5) * 50) + "deg)"
    var boxShadow = parseInt(-(delta_x - 0.5) * 8) +"px " + 
        parseInt(-((delta_y - 0.5) * 8) + 2) + 
        "px 4px rgba(34, 25, 25, 0.4);"

    el.setAttribute('style', 
                    "transform: " + transform + "; " + 
                    "box-shadow: " + boxShadow);
  }

  let els = document.getElementsByClassName("el")
  let len = els.length
  for(let i=0; i<len; i++) {
    let el = els[i]
    el.addEventListener("mousemove", mouseMove)
  }
}, false);

html, body, #wrapper { 
  height: 100%; 
  margin: 0; 
  padding: 0; 
}
#wrapper {
  background: #a4d24b;
  /*font-family: 'Open Sans', sans-serif; */
  font-weight: 400;
  font-size: 14px;
  display: flex;
  flex-wrap: wrap;
  perspective: 500px;
}
.container {
  position: relative;
  margin: auto;
  width: 20%;
  height: 40%;
  transition: all .25s;
  transform-origin: 50% 50%;
}
.container:hover {
  transform: translateZ(25px);
}
.el {
  height: 100%;
  background: #FFF; color: #000;
  box-shadow: 0 2px 4px rgba(34, 25, 25, 0.4);
}
.el:not(:hover) {
  transform: rotateY(0deg) rotateX(0deg) !important;
  box-shadow: 0 2px 4px rgba(34, 25, 25, 0.4) !important;
}

<div id="wrapper">
  <div class="container">
    <div class="el"></div>
  </div>
</div>

The JS concept is simple: When the mouse move on the bloc, I got the position (in %: position / bloc size) of the mouse on x and y.

• For x (0 -> 1) I move the transform property from rotate: rotateY(-12deg) to rotate: rotateY(12deg)
• Same thing for y (0 -> 1) from rotate: rotateX(-12deg) to rotate: rotateX(12deg)
• I also move the box-shadow (with x and y) to help visualization.

The result look good, but if I add a background to this bloc the result look weird.

(Same code with background)

document.addEventListener('DOMContentLoaded', function() {
  let mouseMove = function (e) {
    let el = e.currentTarget;
    let delta_x = parseFloat(e.offsetX / el.offsetWidth).toFixed(3)
    let delta_y = parseFloat(e.offsetY / el.offsetHeight).toFixed(3)

    var transform = "rotateY(" + ((delta_x - 0.5) * 50) + "deg) " +
        "rotateX(" + (-(delta_y - 0.5) * 50) + "deg)"
    var boxShadow = parseInt(-(delta_x - 0.5) * 8) +"px " + 
        parseInt(-((delta_y - 0.5) * 8) + 2) + 
        "px 4px rgba(34, 25, 25, 0.4);"

    el.setAttribute('style', 
                    "transform: " + transform + "; " + 
                    "box-shadow: " + boxShadow);
  }

  let els = document.getElementsByClassName("el")
  let len = els.length
  for(let i=0; i<len; i++) {
    let el = els[i]
    el.addEventListener("mousemove", mouseMove)
  }
}, false);

html, body, #wrapper { 
  height: 100%; 
  margin: 0; 
  padding: 0; 
}
#wrapper {
  background: #a4d24b;
  /*font-family: 'Open Sans', sans-serif; */
  font-weight: 400;
  font-size: 14px;
  display: flex;
  flex-wrap: wrap;
  perspective: 500px;
}
.container {
  position: relative;
  margin: auto;
  width: 20%;
  height: 40%;
  transition: all .25s;
  transform-origin: 50% 50%;
}
.container:hover {
  transform: translateZ(25px);
}
.el {
  height: 100%;
  background: #FFF url(https://images-na.ssl-images-amazon.com/images/M/MV5BMjQyODg5Njc4N15BMl5BanBnXkFtZTgwMzExMjE3NzE@._V1_SY1000_SX686_AL_.jpg);
  background-size: cover;
  background-position: 50% 50%; 
  color: #000;
  box-shadow: 0 2px 4px rgba(34, 25, 25, 0.4);
}
.el:not(:hover) {
  transform: rotateY(0deg) rotateX(0deg) !important;
  box-shadow: 0 2px 4px rgba(34, 25, 25, 0.4) !important;
}

<div id="wrapper">
  <div class="container">
    <div class="el"></div>
  </div>
</div>

Now, it's look like only the top part working, and the bottom isn't moving in the good direction. (Best reproduction when the mouse is moving from top-left to bottom-right).

Did I made a mistake ? Or it's a perspective problem ?

---

Update Some browser didn't like the CSS bellow, if the white box didn't move when you move the mouse on it, just remove this.

.el:not(:hover) { transform: rotateY(0deg) rotateX(0deg) !important; box-shadow: 0 2px 4px rgba(34, 25, 25, 0.4) !important; }

Answer 1

You need to have perspective applied on the immediate parent of the transformed element.

I solved using perspective: inherit

document.addEventListener('DOMContentLoaded', function() {
  let mouseMove = function (e) {
    let el = e.currentTarget;
    let delta_x = parseFloat(e.offsetX / el.offsetWidth).toFixed(3)
    let delta_y = parseFloat(e.offsetY / el.offsetHeight).toFixed(3)

    var transform = "rotateY(" + ((delta_x - 0.5) * 50) + "deg) " +
        "rotateX(" + (-(delta_y - 0.5) * 50) + "deg)"
    var boxShadow = parseInt(-(delta_x - 0.5) * 8) +"px " + 
        parseInt(-((delta_y - 0.5) * 8) + 2) + 
        "px 4px rgba(34, 25, 25, 0.4);"

    el.setAttribute('style', 
                    "transform: " + transform + "; " + 
                    "box-shadow: " + boxShadow);
  }

  let els = document.getElementsByClassName("el")
  let len = els.length
  for(let i=0; i<len; i++) {
    let el = els[i]
    el.addEventListener("mousemove", mouseMove)
  }
}, false);

html, body, #wrapper { 
  height: 100%; 
  margin: 0; 
  padding: 0; 
}
#wrapper {
  background: #a4d24b;
  /*font-family: 'Open Sans', sans-serif; */
  font-weight: 400;
  font-size: 14px;
  display: flex;
  flex-wrap: wrap;
  perspective: 500px;
}
.container {
  position: relative;
  margin: auto;
  width: 20%;
  height: 40%;
  transition: all .25s;
  transform-origin: 50% 50%;
  perspective: inherit;
}
.container:hover {
  transform: translateZ(25px);
}
.el {
  height: 100%;
  background: #FFF url(https://images-na.ssl-images-amazon.com/images/M/MV5BMjQyODg5Njc4N15BMl5BanBnXkFtZTgwMzExMjE3NzE@._V1_SY1000_SX686_AL_.jpg);
  background-size: cover;
  background-position: 50% 50%; 
  color: #000;
  box-shadow: 0 2px 4px rgba(34, 25, 25, 0.4);
}
.el:not(:hover) {
  transform: rotateY(0deg) rotateX(0deg) !important;
  box-shadow: 0 2px 4px rgba(34, 25, 25, 0.4) !important;
}

<div id="wrapper">
  <div class="container">
    <div class="el"></div>
  </div>
</div>