Peng Chen
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CSS揭秘之半透明边框

原创 CSS pengchen    2017, April 10

1.半透明边框

问题:

如果我们要为一个容器设置红色背景和一道黑色半透明边框,我们可能会这样写:

border: 20px solid rgba(0,0,0,0.5);
background: red;

但是效果却是这样的(图1-1.png);我们的半透明颜色怎么没有实现半透明边框?

图1-1.png

解决方案:

我们可以通过background-clip属性来调整上面的默认行为,把它是值设为padding-box,然后就出现了我们想要的效果(图1-2.png);

border: 20px solid rgba(0,0,0,0.5);
background: red;
background-clip: padding-box;

图1-2.png

2.background-clip

既然用到了background-clip属性,那我们就来看看这个属性吧;

background-clip: 设置元素的背景(背景图片或颜色)是否延伸到边框下面。

值(values ) 说明
border-box 默认初始值,背景延伸到边框外沿(但是在边框之下)
padding-box 边框下面没有背景,即背景延伸到内边距外沿
content-box 背景裁剪到内容区 (content-box) 外沿
text 实验API,背景裁剪到前景文本( foreground text)内。

示例

CSS content

span {
   border: 10px blue;
   border-style: dotted double;
   margin: 1em;
   padding: 2em;
   background: #F8D575;
}
.border-box { background-clip: border-box; }
.padding-box { background-clip: padding-box; }
.content-box { background-clip: content-box; }
.text { background-clip: text; }

HTML content

<span class="border-box">border-box</span>
<span class="padding-box">padding-box</span>
<span class="content-box">content-box</span>
<span class="text">text</span>

效果:(图2-1.png) 图2-1.png

3.border-style

4.border-image

初始值:

  • border-image-source: none
  • border-image-slice: 100%
  • border-image-width: 1
  • border-image-outset: 0s
  • border-image-repeat: stretch

4.1 border-image-source: none | <image>

where <image> = <url> | <image()> | <image-set()> | <element()> | <cross-fade()> | <gradient>

where <image()> = image([ [ <image> | <string> ]? , <color>? ]!)

<image-set()> = image-set(<image-set-option>#)

<element()> = element( <id-selecter> )

<cross-fade()> = cross-fade(<cf-mixing-image>,<cf-final-image>?)

<gradient> = <linear-gradient()> | <repeating-linear-gradient()> | <radial-gradient()> | <repeating-radial-gradient()>

点击这里查看文档.

Gradient示例:

<linear-gradient()> = linear-gradient( [<angle> | to <side-or-corner>]?, <color-stop-list>)

CSS content

.gradient { 
	border: 30px solid;
	border-image-source: linear-gradient(to right, red, green, blue);
    /*border-image-source: linear-gradient(90deg, red, green, blue);*/
	border-image-slice: 10;
	padding: 20px;
}

效果:(图4-1.png)

图4-1.png

4.2 border-image-slice:[ <number> | <percentage> ]{1,4}&& fill?

这个border-imge-slice属性传入1~4个参数(number没有单位专指像素或百分比值)将图片分割成9个部分,1,2,3,4四个区块是不会拉伸,不会平铺,称之为盲区,5,6,7,8四个区块可以通过border-image-repeat来控制拉伸平铺和重复(stretch:默认值,拉伸; repeat:平铺; round:整数次平铺;),第9区块不显示,传入参数fill则显示第9区块,分割情况如下图(图4-2.png && 图4-3.png):

图4-2.png

图4-3.png

我们通过上面这张图片(81px^81px)来看传入不同个数的参数是如何分割这张图片的;

1个参数

/* border-image-slice: slice */
border-image-slice: 27; 
border: 30px solid transparent;
padding: 20px;
border-image-source:url([https://mdn.mozillademos.org/files/4127/border.png](https://mdn.mozillademos.org/files/4127/border.png));

图4-4.png 图4-5.png(效果图)

2个参数(参考图4-3.png)

/* border-image-slice: vertical horizontal */
border-image-slice: 40 40.5; 
border: 30px solid transparent;
padding: 20px;
border-image-source:url([https://mdn.mozillademos.org/files/4127/border.png](https://mdn.mozillademos.org/files/4127/border.png));

图4-6.png 图4-7.png(效果图)

3个参数

/* border-image-slice: top horizontal bottom */
border-image-slice: 27 40 27; 
border: 30px solid transparent;
padding: 20px;
border-image-source:url([https://mdn.mozillademos.org/files/4127/border.png](https://mdn.mozillademos.org/files/4127/border.png));

图4-8.png(效果图)

4个参数(参考图4-2.png)

/* border-image-slice: top right bottom left */
border-image-slice: 27 40 27 27;
border: 30px solid transparent;
padding: 20px;
border-image-source:url([https://mdn.mozillademos.org/files/4127/border.png](https://mdn.mozillademos.org/files/4127/border.png));

图4-9.png(效果图)

4.3 border-image-width:[ <length> | <percentage> | <number> | auto ]{1,4}

语法:

border-image-width: all                        /* One-value syntax */       
E.g. border-image-width: 3;
border-image-width: vertical horizontal        /* Two-value syntax */  
E.g. border-image-width: 2em 3em;
border-image-width: top horizontal bottom      /* Three-value syntax */    
E.g. border-image-width: 5% 15% 10%;
border-image-width: top right bottom left      /* Four-value syntax */  
E.g. border-image-width: 5% 2em 10% auto;

设置边框图片的width,如果超出了设置的border-width,会向内扩展;查看下方示例,比较(图4-10.png && 图4-11.png);

示例:

border: 30px solid transparent;
padding: 20px;
border-image-source: url("https://mdn.mozillademos.org/files/4127/border.png");
border-image-slice: 27;

图4-10.png

border: 30px solid transparent;
padding: 20px;
border-image-source:url([https://mdn.mozillademos.org/files/4127/border.png](https://mdn.mozillademos.org/files/4127/border.png));
border-image-slice: 27;
border-image-width: 1 2 1 1;

图4-11.png

4.4 border-image-outset:[ <length> | <number> ]{1,4}

语法:

/* border-image-outset: sides */
border-image-outset: 30%;
/* border-image-outset:vertical horizontal */
border-image-outset: 10% 30%;
/* border-image-outset: top horizontal bottom */
border-image-outset: 30px 30% 45px;
/* border-image-outset:top right bottom left  */
border-image-outset: 7px 12px 14px 5px;

效果是将边框图片延伸到盒子外面,查看下放示例,比较(图4-12.png && 图4-13.png); 示例:

 border: 30px solid transparent;
 padding: 20px;
 border-image-source:url([https://mdn.mozillademos.org/files/4127/border.png](https://mdn.mozillademos.org/files/4127/border.png));
 border-image-slice: 27;
 margin: 60px;

图4-12.png

border: 30px solid transparent;
padding: 20px;
border-image-source:url([https://mdn.mozillademos.org/files/4127/border.png](https://mdn.mozillademos.org/files/4127/border.png));
border-image-slice: 27;
margin: 60px;
border-image-outset: 2 1 1 1;

图4-13.png

4.5 border-image-repeat:[ stretch | repeat | round ]{1,2}

值(value) 说明
stretch 默认初始值 ,;拉伸
repeat 平铺
round 整数次平铺

语法:

border-image-repeat: type                      /* One-value syntax */       
E.g. border-image-value: stretch;
border-image-repeat: horizontal vertical       /* Two-value syntax */       
E.g. border-image-width: round space;

本内容根据《css揭秘》, MDN 和自己的理解进行整理; 感谢您的阅读。