How to transition z-index? - html

Question

https://jsfiddle.net/vaf6nv36/1/
Can the balloons image slowly transition over the apple image?
I think that I need more transition parameters, or I should use opacity?
Can someone help me?
HTML:
<div class="img1">
</div>
<div class="img2">
</div>
CSS:
.img1, .img2{
border: 1px solid black;
transition: 1s;
position: absolute;
}
.img1{
left: 25%;
height: 500px;
width: 500px;
z-index: 1;
background-image: url(http://cdn.pcwallart.com/images/balloons-photography-vintage-wallpaper-1.jpg);
}
.img2{
right: 25%;
width: 500px;
height: 500px;
bottom: 0;
z-index: 2;
background-image: url(https://i.pinimg.com/736x/c1/7b/15/c17b150e93c4e9c50d963b076484bee7--apple-wallpaper-iphone-wallpaper.jpg);
}
.img1:hover{
z-index: 999;
}

Answer 1

Although theoretically you can transition z-index, it wouldn't make much sense, i.e. would not result in the crossfade effect which you obviously are looking for: z-index values are integers, which - when you change them in the smallest possible steps (integers, no commas) - results in states either before the other one OR behind the other one - no transitional "half states" in between. If you want to do a kind of continuous crossfade between two elements, you should use a transition on opacity.
In your particular case, since your DIVs are not directly above each other, but only overlap each other, you can solve that by having a second DIV identical to img2 (I called its class .img3), but with z-index: 0 and this CSS rule:
.img1:hover + .img2 {
opacity: 0;
}
This will fade out img2, but still show img3, which however is behind img1, creating the impression of a transition between img1 and img2.
https://jsfiddle.net/2a2epLfv/1/
.img1,
.img2,
.img3 {
border: 1px solid black;
transition: 1s;
position: absolute;
}
.img1 {
left: 20%;
height: 300px;
width: 300px;
z-index: 1;
background-image: url(http://cdn.pcwallart.com/images/balloons-photography-vintage-wallpaper-1.jpg);
}
.img2,
.img3 {
right: 20%;
width: 300px;
height: 300px;
top: 100px;
background-image: url(https://i.pinimg.com/736x/c1/7b/15/c17b150e93c4e9c50d963b076484bee7--apple-wallpaper-iphone-wallpaper.jpg);
}
.img2 {
z-index: 2;
}
.img3 {
z-index: 0;
}
.img1:hover+.img2 {
opacity: 0;
}
<div class="img1"></div>
<div class="img2"></div>
<div class="img3"></div>

Answer 2

I fear z-index transition only makes the element pass step by step through every layer. To make a nice effect you need to combine it with opacity transition and scale / position transition. The fiddle to show you the idea:
.img1, .img2{
border: 1px solid black;
transition: 1s;
position: absolute;
}
.img1{
left: 25%;
height: 500px;
width: 500px;
z-index: 1;
transform: scale(0.9);
opacity: 0.5;
background-image: url(http://cdn.pcwallart.com/images/balloons-photography-vintage-wallpaper-1.jpg);
}
.img2{
right: 25%;
width: 500px;
height: 500px;
bottom: 0;
z-index: 2;
background-image: url(https://i.pinimg.com/736x/c1/7b/15/c17b150e93c4e9c50d963b076484bee7--apple-wallpaper-iphone-wallpaper.jpg);
}
.img1:hover{
animation: fronte 1s linear forwards;
}
#keyframes fronte {
from { z-index: 0; transform: scale(0.9); opacity: 0.5; }
to { z-index: 4; transform: scale(1.1); opacity: 1; }
}
<div class="img1">
</div>
<div class="img2">
</div>

Answer 3

This is the trick I used.
.minus{animation:move 2s;animation-fill-mode:forwards;animation-delay:2s;}
#-webkit-keyframes move {
0 {z-index:1;opacity:1}
50% {opacity:0}
100% { z-index:-1;opacity:1}
}
#keyframes move {
0 {z-index:1;opacity:1}
50% {opacity:0}
100% {z-index:-1;opacity:1}
}
#main{background:red;width:100vw;height:100vh;position:fixed;top:0;left:0;opacity:.9}
.minus{position:fixed;top:10px;left:10px;z-index:1;color:#000}
<div id="main"></div>
<div class="minus">FADE</div>