How to achieve fully compatible frosted glass effect using CSS? (detailed code explanation)

This article will take you to understand the CSS backdrop-filter attribute, see the compatibility of this attribute, and introduce how to achieve a fully compatible frosted glass effect.

Through this article, you can learn the

• most basic use of CSS backdrop-filter to achieve frosting The effect of glass (frosted glass)

• Firefox browser that is not compatible with backdrop-filter so far, how to use some technical operations to cleverly achieve the frosted glass effect , so that this effect can really be applied in business

What isbackdrop-filter

backdrop- The filter CSS property allows you to add graphical effects (such as blur or color shift) to the area behind an element. Because it applies to all elements behind the element, in order to see the effect, the element or its background must be made at least partially transparent.

backdrop-filter is very similar to filter. The values ​​that can be taken are the same, but one is applied to the entire element and the other is only applied to the back of the element. Area.

backdrop-filter vs. filter

We use backdrop-filter vs. filter At the same time, a frosted glass effect is implemented for comparison. The pseudo code is as follows:

<div class="bg">
    <div>Normal</div>
    <div class="g-filter">filter</div>
    <div class="g-backdrop-filter">backdrop-filter</div>
</div>

.bg {
    background: url(image.png);
    
    & > div {
        width: 300px;
        height: 200px;
        background: rgba(255, 255, 255, .7);
    }
    .g-filter {
        filter: blur(6px);
    }
    .g-backdrop-filter {
        backdrop-filter: blur(6px);
    }
}

CodePen Demo -- Comparison between filter and backup-filter

Address: https://codepen.io/Chokcoco/pen/WNjebrr

Before backdrop-filter, I wanted to achieve the above-mentioned filter effect of only adding elements to the background It's still very difficult, and, fine for static images, if the background is a dynamic background that can be scrolled, CSS is usually powerless.

backdrop-filter was born to add filters to the content behind the element without affecting the element itself. It can be used to achieve frosted glass effect (frosted glass) very conveniently!

backdrop-filter Compatibility

backdrop-filter In fact, it has been around for a long time. However, firefox has not yet None are compatible with it!

For some PC-side businesses that have given up on IE, firefox still needs to be compatible. If you want to use backdrop-filter to achieve the frosted glass effect To implement the application, the Firefox compatibility issue must be resolved.

Achieve frosted glass effect in firefox

OK, the focus of this article is how to use backdrop-filter in firefox as much as possible Restore the effect of frosted glass.

First of all, let’s take a look. If backdrop-filter is used normally, or the above example has the following effect, there is no frosted glass effect:

Use background-attachment: fixed compatible with static background images

If you want to use the frosted glass effect on Firefox. The background using frosted glass elements is just a static background image. In fact, there are many methods.

We only need to superimpose the same picture behind the element, use background-attachment: fixed to position the picture superimposed under the element to the same coordinates as the background, and then use filter: blur() Just blur it.

The pseudo code is as follows:

<div class="g-glossy">frosted glass effect </div>

$img: 'https://static.pexels.com/photos/373934/pexels-photo-373934.jpeg';

body {
    height: 100vh;
    display: flex;
    background-image: url($img);
    background-repeat: no-repeat;
    background-attachment: fixed;
    background-size: cover;
}

.g-glossy {
    position: relative;
    width: 600px;
    height: 300px;
    background-color: rgba(255, 255, 255, 0.5);
    overflow: hidden;
    z-index: 10;
    
    &::before {
        content: "";
        position: absolute;
        top: 0;
        left: 0;
        right: 0;
        bottom: 0;
        background-image: url($img);
        background-repeat: no-repeat;
        background-attachment: fixed;
        background-size: cover;
        filter: blur(10px);
        z-index: -1;
    }
}

The effect is as follows:

This method is also used before there is no backdrop-filter , one of the most commonly used methods to achieve a simple frosted glass effect in various browsers.

CodePen Demo -- Use background-attachment: fixed | filter: bulr() to achieve frosted glass effect

Address: https://codepen.io/Chokcoco/pen/XWRrVma

Using background-attachment: fixed is compatible with the disadvantages of static background images

However, this method also has two disadvantages:

1. Pseudo elements are used to superimpose a layer of background. Due to the hierarchical relationship, the background of the parent element is at the bottom, so the background color of the element itself is not fully reflected. You can compare the actual renderings of the two methods:

The solution is to overlay a layer of background color through another pseudo-element. This background color should be originally assigned to the parent element itself.

The effect after superposition is as follows:

CodePen Demo -- 使用 background-attachment: fixed | filter: bulr() 实现毛玻璃效果优化

地址：https://codepen.io/Chokcoco/pen/abWbzKG

2、上述效果已经非常接近了，硬要挑刺的话，就是应用了模糊滤镜的伪元素的边缘有白边瑕疵，这一点其实是滤镜本身的问题，也非常好解决，我们只需要将伪元素的范围扩大一点即可：

.g-glossy {
    overflow: hidden;
    ....
    &::before {
        content: "";
        position: absolute;
        top: -100px;
        left: -100px;
        right: -100px;
        bottom: -100px;
    }
}

定位的代码由 top: 0px; 改为 top: -100px，四个方位都是如此即可。如此一来，就能做到基本上是百分百的模拟。

使用 moz-element() 配合 filter: blur() 实现复杂背景毛玻璃效果

下面这种方法就非常巧妙了，正常而言，运用毛玻璃效果的背景元素，都不是一张图片那么简单！背后通常都是整个页面复杂的结构，多层 DOM 的嵌套。

那么通过叠加一张简单的图片，就无法奏效了，我们得想办法模拟整个 DOM 元素。

而恰好，在 Firefox 中，有这么一个属性 -- -moz-element()。

何为 -moz-element()？MDN-element) 的解释是，CSS 函数 element() 定义了一个从任意的 HTML 元素中生成的图像 <image> 值。该图像值是实时的，这意味着如果被指定的 HTML 元素被更改，应用了该属性的元素的背景也会相应更改。

它其实是个草案规范，但是一直以来，只有 Firefox 支持它 -- CAN I USE -- CSS element()：

它有什么作用呢？

-moz-element() 如何使用

那么 -moz-element() 如何使用呢？简而言之，它能够复制一个元素内部渲染出来的 UI，并且能够实时同步变化。

假设我们有这样一个简单的结构，元素背景和内容都在运动：

<div id="bg" class="g-normal">
    <p>Content</p>
</div>

.g-normal {
    margin: auto;
    width: 200px;
    height: 200px;
    animation: change 5s infinite;
    background: linear-gradient(deeppink, yellowgreen);
}

p {
    animation: move 5s infinite;
}

@keyframes change {
    0% {
        filter: hue-rotate(0);
    }
    100% {
        filter: hue-rotate(360deg);
    }
}

@keyframes move {
    0% {
        transform: translate(0, 0);
    }
    100% {
        transform: translate(150px, 150px);
    }
}

它的效果大概是这样：

我们就假设这个结构就是我们页面某一块的内容，然后，我们就可以使用 background: -moz-element(#id) 这种方式，将这个元素内绘制的 UI 内容完全拷贝至另外一个元素，看看效果。

我们添加一个元素 <div class="g-element-copy"></div>，在这个元素内模拟 #bg 内的内容：

<div id="bg" class="g-normal">
    <p>Content</p>
</div>
<div class="g-element-copy"></div>

.g-element-copy {
    margin: auto;
    width: 200px;
    height: 200px;
    // 核心代码
    background: -moz-element(#bg);
}

它可以完全复制另外一个元素内绘制出来的 UI，并且能追踪实时变化：

CodePen Demo -- -moz-element Demo(Firefox Only)

地址：https://codepen.io/Chokcoco/pen/jOmOPPL

在 firefox 中使用 element 复制 UI，用作毛玻璃元素背景

这样，有了上面的铺垫，下面的内容就比较好理解了。

和上述的 background-attachment: fixed 方案对比，我们还是通过伪元素叠加一层背景，只不过背景内的内容由单纯一张图片，变成了由 -moz-element() 复制的整段 UI 内容。

其次，上面的方案我们使用 background-attachment: fixed 使背景图和伪元素内叠加的图片的位置对齐，在这里，我们需要借助 Javascript 进行简单的运算，确定背景内容元素的相关位置，计算对齐量。

来看这样一个 DEMO：

    
模拟真实 DOM
    
模拟真实 DOM
    
模拟真实 DOM
    
模拟真实 DOM
    
模拟真实 DOM
    
模拟真实 DOM
    
模拟真实 DOM
    
模拟真实 DOM
    
模拟真实 DOM

<div class="g-glossy">frosted glass effect </div>

其中，.g-glossy 是在正常情况下 backdrop-filter 兼容时，我们的毛玻璃元素，而 .g-glossy-firefox 则是不兼容 backdrop-filter 时，我们需要模拟整个 DOM 背景 UI时候的元素，可以通过 CSS 特性检测 CSS @support 进行控制：

核心 CSS 代码：

.bg {
    // 整个页面的 DOM 结构
}

.g-glossy {
    position: fixed;
    width: 600px;
    height: 300px;
    background-color: rgba(255, 255, 255, 0.5);
    backdrop-filter: blur(10px);
}

.g-glossy-firefox {
    display: none;
}

@supports (background: -moz-element(#bg)) {
    .g-glossy-firefox {
        display: block;
        position: fixed;
        width: 600px;
        height: 300px;
        background: -moz-element(#bg) no-repeat;
        filter: blur(10px);
    }
}

简单解读一下：

• 对于兼容 backdrop-filter 的，.g-glossy 内的代码将直接生效，并且 .g-glossy-firefox 不会展示

• 对于 Firefox 浏览器，因为 backdrop-filter 必然不兼容，所以 .g-glossy 内的 backdrop-filter: blur(10px) 不会生效，而 @supports (background: -moz-element(#bg)) 内的样式会生效，此时 .g-glossy-firefox 将会利用 background: -moz-element(#bg) no-repeat; 模拟 id 为 bg 的元素

当然，这里我们需要借助一定的 JavaScript 代码，计算我们的模拟页面 UI 的元素 .g-glossy-firefox 相对它模拟的 #bg 元素，也就是页面布局的一个定位偏差：

$(function() {
        let blur = $('.g-glossy-firefox')[0].style;
        let offset = $('.g-glossy').eq(0).offset();

        function updateBlur() {
            blur.backgroundPosition = 
                `${-window.scrollX - offset.left}px ` + 
                `${-window.scrollY - offset.top}px`;
        }
        document.addEventListener('scroll', updateBlur, false), updateBlur();
});

OK，至此，我们就能完美的在 Firefox 上也实现毛玻璃的效果了：

它相对于上面的第一种方案而言，最大的不同之处在于，它可以模拟各式各样的背景元素，背景元素可以不仅仅只是一张图片！它可以是各种复杂的结构！

这种方案是我的 CSS 群中，风海流 同学提供的一种思路，非常的巧妙，并且，他自己也对这种方案进行了完整的阐述，你可以戳这里看看：在网页中实现标题栏「毛玻璃」效果，本文也是经过他的同意，重新整理发出。

上述效果的完整代码，你可以戳这里：

CodePen Demo -- 兼容 Firefox 的复杂背景毛玻璃（磨砂玻璃）效果

https://codepen.io/Chokcoco/pen/ExWqaQG

总结一下

简单对上述内容进行一个总结：

• 你可以使用 backdrop-filter 对兼容它的浏览器非常简单的实现毛玻璃（磨砂玻璃）效果
• 对于不兼容 backdrop-filter 的浏览器，如果它只是简单背景，可以使用 background-attachment: fixed 配合 filter: blur() 进行模拟
• 对于 firefox 浏览器，你还可以使用 moz-element() 配合 filter: blur() 实现复杂背景毛玻璃效果
• 对于不兼容的上述 3 种效果的其他浏览器，设置了毛玻璃效果的元素，可以通过设置类似 background: rgba(255, 255, 255, 0.5) 的样式，使之回退到半透明效果，也算一种非常合理的降级效果，不会引起 Bug

原文地址：https://segmentfault.com/a/1190000040263452

作者：ChokCoco

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