How to align the absolute position to center?

问题:

I am trying to stack two canvas together and make it a double layers canvas.

I've saw an example here:

<div style="position: relative;">
 <canvas id="layer1" width="100" height="100" 
   style="position: absolute; left: 0; top: 0; z-index: 0;"></canvas>
 <canvas id="layer2" width="100" height="100" 
   style="position: absolute; left: 0; top: 0; z-index: 1;"></canvas>
</div>

But i would like to set both of the canvas align at the center of the screen. If i set the value of 'left' as a constant, while I change the orientation of the screen (as I'm doing aps on iPad) the canvas won't remain at the middle of the screen like how it act in

<div align="center">

Can anyone please help?

回答1:

If you set both left and right to zero, and left and right margins to auto you can center an absolutely positioned element.

position:absolute;
left:0;
right:0;
margin-left:auto;
margin-right:auto;

回答2:

If you want to center align an element without knowing it's width and height do:

position: absolute;
top: 50%;
left: 50%;
transform: translate(-50%, -50%);

Example:

*{
  margin:0;
  padding:0;
}
section{
  background:red;
  height: 100vh;
  width: 100vw;
}
div{  
  width: 80vw;
  height: 80vh;
  background: white;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

<section>
  <div>
    <h1>Popup</h1>
  </div>
</section>

回答3:

Have you tried using?:

left:50%;
top:50%;
margin-left:-[half the width] /* As pointed out on the comments by Chetan Sastry */

Not sure if it'll work, but it's worth a try...

Minor edit: Added the margin-left part, as pointed out on the comments by Chetan...

回答4:

position: absolute;
top: 0;
left: 0;
bottom: 0;
right: 0;
margin: auto;

回答5:

All you have to do is,

make sure your parent DIV has position:relative

and the element you want center, set it a height and width. use the following CSS

.layer {
    width: 600px; height: 500px;
    display: block;
    position:absolute;
    top:0;
    left: 0;
    right:0;
    bottom: 0;
    margin:auto;
  }
http://jsbin.com/aXEZUgEJ/1/

回答6:

Move the parent div to the middle with

left: 50%;
top: 50%;
margin-left: -50px;

Move the second layer over the other with

position: relative;
left: -100px;