How can I make a CSS glass/blur effect work for an overlay?

I am having trouble applying a blur effect on a semi-transparent overlay div. I'd like everything behind the div the be blurred, like this:

Here is a jsfiddle which doesn't work: http://jsfiddle.net/u2y2091z/

Any ideas how to make this work? I'd like to keep this as uncomplicated as possible and have it be cross-browser. Here is the CSS I'm using:

#overlay {
    position: absolute;
    left: 0;
    top: 0;
    right: 0;
    bottom: 0;

    background:black;
    background:rgba(0,0,0,0.8);

    filter:blur(4px);
    -o-filter:blur(4px);
    -ms-filter:blur(4px);
    -moz-filter:blur(4px);
    -webkit-filter:blur(4px);
}

Here is an example that uses svg filter.

The idea is to use an svg element with height same as the #overlay and apply the feGaussianblur filter on it. This filter is applied on an svg image element. To give it an extruded effect, you could use a box-shadow at the bottom of the overlay.

Browser Support for svg filters.

`Demo on Codepen`

body {
  background: #222222;
}
#container {
  position: relative;
  width: 450px;
  margin: 0 auto;
}
img {
  height: 300px;
}
#overlay {
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  z-index: 1;
  color: rgba(130, 130, 130, 0.5);
  font-size: 50px;
  text-align: center;
  line-height: 100px;
  box-shadow: 0 3px 5px rgba(0, 0, 0, 0.3);
}

<div id="container">
  <img src="http://lorempixel.com/450/300/sports" />
  <div id="overlay">WET</div>
  <svg width="450" height="100" viewBox="0 0 450 100" style="position: absolute; top: 0;">
    <defs>
      <filter id="blur">
        <feGaussianBlur in="SourceGraphic" stdDeviation="3" />
      </filter>
    </defs>
    <image filter="url(#blur)" xlink:href="http://lorempixel.com/450/300/sports" x="0" y="0" height="300px" width="450px" />
  </svg>
</div>

Thank you for the help everyone! I was able to piece together information from everyone here and from further Googling, and I came up with the following which works in Chrome and Firefox: http://jsfiddle.net/u2y2091z/12/. I'm still working on making this work for IE and Opera.

The key is putting the content inside of the div to which the filter is applied:

<div id="mask">
    <p>Lorem ipsum ...</p>
    <img src="http://www.byui.edu/images/agriculture-life-sciences/flower.jpg" />
</div>

And then the CSS:

#mask {
    position: absolute;
    left: 0;
    top: 0;
    right: 0;
    bottom: 0;
    background-color: black;
    opacity: 0.5;

    filter: blur(10px);
    -webkit-filter: blur(10px);
    -moz-filter: blur(10px);
    -o-filter: blur(10px);
    -ms-filter: blur(10px);
    filter: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg'><filter id='svgMask'><feGaussianBlur stdDeviation='10' /></filter></svg>#svgMask");
}

So mask has the filters applied. Also note the use of url() for a filter with an <svg> tag for the value -- that idea came from http://codepen.io/AmeliaBR/pen/xGuBr. If you happen to minify your CSS, you might need to replace any spaces in the SVG filter markup with "%20".

So now, everything inside the mask div is blurred.

If you're looking for a reliable cross-browser approach today, you won't find a great one. The best option you have is to create two images (this could be automated in some environments), and arrange them such that one overlays the other. I've created a simple example below:

<figure class="js">
    <img src="http://i.imgur.com/3oenmve.png" />
    <img src="http://i.imgur.com/3oenmve.png?1" class="blur" />
</figure>

figure.js {
    position: relative;
    width: 250px; height: 250px;
}

figure.js .blur {
    top: 0; left: 0;
    position: absolute;
    clip: rect( 0, 250px, 125px, 0 );
}

Though effective, even this approach isn't necessarily ideal. That being said, it does yield the desired result.

Here's a possible solution.

HTML

<img id="source" src="http://www.byui.edu/images/agriculture-life-sciences/flower.jpg" />

<div id="crop">
    <img id="overlay" src="http://www.byui.edu/images/agriculture-life-sciences/flower.jpg" />
</div>

CSS

#crop {
    overflow: hidden;

    position: absolute;
    left: 100px;
    top: 100px;

    width: 450px;
    height: 150px;
}

#overlay {
    -webkit-filter:blur(4px);
    filter:blur(4px);

    width: 450px;
}

#source {
    height: 300px;
    width: auto;
    position: absolute;
    left: 100px;
    top: 100px;
}

I know the CSS can be simplified and you probably should get rid of the ids. The idea here is to use a div as a cropping container and then apply blur on duplicate of the image. Fiddle

To make this work in Firefox, you would have to use SVG hack.

#bg, #search-bg {
  background-image: url('https://images.pexels.com/photos/719609/pexels-photo-719609.jpeg?w=940&h=650&auto=compress&cs=tinysrgb');
  background-repeat: no-repeat;
  background-size: 1080px auto;
}

#bg {
  background-position: center top;
  padding: 70px 90px 120px 90px;
}

#search-container {
  position: relative;
}

#search-bg {
  /* Absolutely position it, but stretch it to all four corners, then put it just behind #search's z-index */
  position: absolute;
  top: 0px;
  right: 0px;
  bottom: 0px;
  left: 0px;
  z-index: 99;

  /* Pull the background 70px higher to the same place as #bg's */
  background-position: center -70px;

  -webkit-filter: blur(10px);
  filter: url('/media/blur.svg#blur');
  filter: blur(10px);
}

#search {
  /* Put this on top of the blurred layer */
  position: relative;
  z-index: 100;
  padding: 20px;
  background: rgb(34,34,34); /* for IE */
  background: rgba(34,34,34,0.75);
}

@media (max-width: 600px ) {
  #bg { padding: 10px; }
  #search-bg { background-position: center -10px; }
}

#search h2, #search h5, #search h5 a { text-align: center; color: #fefefe; font-weight: normal; }
#search h2 { margin-bottom: 50px }
#search h5 { margin-top: 70px }

<div id="bg">
  <div id="search-container">
    <div id="search-bg"></div>
    <div id="search">
      <h2>Awesome</h2>
      <h5><a href="#">How it works »</a></h5>
    </div>
  </div>
</div>

I came up with this solution.

Click to view image of blurry effect

It is kind of a trick which uses an absolutely positioned child div, sets its background image same as the parent div and then uses the background-attachment:fixed CSS property together with the same background properties set on the parent element.

Then you apply filter:blur(10px) (or any value) on the child div.

*{
    margin:0;
    padding:0;
    box-sizing: border-box;
}
.background{
    position: relative;
    width:100%;
    height:100vh;
    background-image:url('https://images.unsplash.com/photo-1547937414-009abc449011?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&auto=format&fit=crop&w=1350&q=80');
    background-size:cover;
    background-position: center;
    background-repeat:no-repeat;
}

.blur{
    position: absolute;
    top:0;
    left:0;
    width:50%;
    height:100%;
    background-image:url('https://images.unsplash.com/photo-1547937414-009abc449011?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&auto=format&fit=crop&w=1350&q=80');
    background-position: center;
    background-repeat: no-repeat;
    background-attachment: fixed;
    background-size:cover;
    filter:blur(10px);
    transition:filter .5s ease;
    backface-visibility: hidden;
}

.background:hover .blur{
    filter:blur(0);
}
.text{
    display: inline-block;
    font-family: sans-serif;
    color:white;
    font-weight: 600;
    text-align: center;
    position: relative;
    left:25%;
    top:50%;
    transform:translate(-50%,-50%);
}

<head>
    <title>Blurry Effect</title>
</head>
<body>
    <div class="background">
        <div class="blur"></div>
        <h1 class="text">This is the <br>blurry side</h1>
    </div>
</body>
</html>

view on codepen

来源：https://stackoverflow.com/questions/27583937/how-can-i-make-a-css-glass-blur-effect-work-for-an-overlay

标签

html

css

css3

blur

css-filters