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    background-size

    Summary

    background-size 设置背景图片大小。

    Note: 如果在设置 background-size 之后 background 又没有设置此属性,那么此属性取默认值(译注:css同一属性后面的覆盖前面的)。

    • 初始值 auto auto
    • 适用元素 all elements
    • 可继承 no
    • Percentages relative to the background positioning area
    • 适用媒体 visual
    • 计算值 as specified, with lengths made absolute
    • 可动画 yes, as a repeatable list of a simple list of a length, percentage or calc(); when both values are lengths, they are interpolated as lengths; when both values are percentages, they are interpolated as percentages; otherwise, both values are converted into a calc() function that is the sum of a length and a percentage (each possibly zero), and these calc() functions have each half interpolated as real numbers. . This means keyword values are not animatable.
    • Canonical order the unique non-ambiguous order defined by the formal grammar

    Syntax

    Formal syntax: <bg-size>#
    
    /* 关键字 */
    background-size: cover
    background-size: contain
    
    /* 一个值: 这个值指定图片的宽度,图片的高度隐式的为auto */
    background-size: 50%
    background-size: 3em
    background-size: 12px
    background-size: auto
    
    /* 两个值: 第一个值指定图片的宽度,第二个值指定图片的高度 */
    background-size: 50% auto
    background-size: 3em 25%
    background-size: auto 6px
    background-size: auto auto
    
    /* 逗号分隔的多个值:设置多重背景 */
    background-size: auto, auto     /* 不同于background-size: auto auto */
    background-size: 50%, 25%, 25%
    background-size: 6px, auto, contain
    
    background-size: inherit
    

    Values

    <length>
    <length> 值,指定背景图片大小,不能为负值。
    <percentage>
    <percentage> 值,指定背景图片相对背景区(background positioning area)的百分比。背景区由background-origin设置,默认为盒模型的内容区与内边距,也可设置为只有内容区,或者还包括边框。如果attachmentfixed,背景区为浏览器可视区(即视口),不包括滚动条。不能为负值。
    auto
    以背景图片的比例缩放背景图片。
    cover
    缩放背景图片以完全覆盖背景区,可能背景图片部分看不见。
    contain
    缩放背景图片以完全装入背景区,可能背景区部分空白。

    位图一定有固有尺寸与固有比例,矢量图可能两者都有,也可能只有一个。渐变视为只有固有尺寸或者只有固有比例的图片。

    Gecko 8.0 note
    (Firefox 8.0 / Thunderbird 8.0 / SeaMonkey 2.5)

    This behavior changed in Gecko 8.0 (Firefox 8.0 / Thunderbird 8.0 / SeaMonkey 2.5). Before this, gradients were treated as images with no intrinsic dimensions, with an intrinsic proportion identical to that of the background positioning area.

    -moz-element 生成的背景图片,(which actually match an element) are currently treated as images with the dimensions of the element, or of the background positioning area if the element is SVG, with the corresponding intrinsic proportion.

    Note: This is not the currently-specified behavior, which is that the intrinsic dimensions and proportion should be those of the element in all cases.

    背景图片大小计算:

    如果指定了 background-size 的两个值并且不是auto
    背景图片按指定大小渲染。
    containcover:
    保留固有比例,最大的包含或覆盖背景区。如果图像没有固有比例,则按背景区大小。
    autoauto auto:
    图像如果有两个长度,则按这个尺寸。如果没有固有尺寸与固有比例,则按背景区的大小。如果没有固有尺寸但是有固有比例, 效果同 contain。如果有一个长度与比例,则由此长度与比例计算大小。如果有一个长度但是没有比例,则使用此长度与背景区相应的长度。
    一个为 auto 另一个不是auto:
    如果图像有固有比例,则指定的长度使用指定值,未指定的长度由指定值与固有比例计算。如果图像没有固有比例,则指定的长度使用指定值,未指定的长度使用图像相应的固有长度,若没有固有长度,则使用背景区相应的长度。

    注意,对于没有固有尺寸或固有比例的矢量图不是所有的浏览器都支持。特别注意测试Firefox 7- 与Firefox 8+,以确定不同之处能否接受。

    Examples

    background-size: cover 演示 与  background-size: contain 演示 在新窗口打开,这样你可以看到当背景区大小变化时 containcover 是怎样的。 系列演示:background-size 及其与background-*属性的关联  很好的说明了单独使用 background-size 及与其它属性共同使用。

    Notes

    如果用渐变作为背景并且对它使用了background-size ,最好不要只用一个auto, 或者只指定一个宽度值 (例如 background-size: 50%)。对这两种情况 Firefox 8有所改变, 并且目前各浏览器表现不一致,不是全部浏览器都完全支持 CSS3 background-size 规范CSS3 Image Values gradient 规范

    .bar {
           width: 50px; height: 100px;
           background-image: gradient(...);
    
           /* 不推荐 */
           background-size: 25px;
           background-size: 50%;
           background-size: auto 50px;
           background-size: auto 50%;
    
           /* 可行 */
           background-size: 25px 50px;
           background-size: 50% 50%;
    }
    

    特别不推荐对渐变px与auto一起用, 因为Firefox 8之前不能重复渲染,并且对于没有实现Firefox 8渲染特性的浏览器,不知道指定了背景的元素的确切大小。

    Specifications

    Specification Status Comment
    CSS Backgrounds and Borders Module Level 3 Candidate Recommendation  

    Browser compatibility

    Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
    Basic support 1.0-webkit [2] 3.6 (1.9.2)-moz [4] 9.0 [5] 9.5-o
    with some bugs [1]
    3.0 (522)-webkit
    but from an older CSS3 draft [2]
    3.0 4.0 (2.0) 10.0 4.1 (532)
    Support for
    contain and cover
    3.0 3.6 (1.9.2) 9.0 [5] 10.0 4.1 (532)
    Support for SVG backgrounds ? 8.0 (8.0) ? ? ?
    Feature Android Firefox Mobile (Gecko) Windows Phone Opera Mobile Safari Mobile
    Basic support (Yes)-webkit
    2.3
    ? ? ? ?
    Support for SVG backgrounds ? 8.0 (8.0) ? ? ?

    [1] Opera 9.5's computation of the background positioning area is incorrect for fixed backgrounds.  Opera 9.5 also interprets the two-value form as a horizontal scaling factor and, from appearances, a vertical clipping dimension. This has been fixed in Opera 10.

    [2] WebKit-based browsers originally implemented an older draft of CSS3 background-size in which an omitted second value is treated as duplicating the first value; this draft does not include the contain or cover keywords.

    [3] Konqueror 3.5.4 supports -khtml-background-size.

    [4] While this property is new in Gecko 1.9.2 (Firefox 3.6), it is possible to stretch a image fully over the background in Firefox 3.5 by using -moz-border-image.

    .foo {
      background-image: url(bg-image.png);
    
      -webkit-background-size: 100% 100%;           /* Safari 3.0 */
         -moz-background-size: 100% 100%;           /* Gecko 1.9.2 (Firefox 3.6) */
           -o-background-size: 100% 100%;           /* Opera 9.5 */
              background-size: 100% 100%;           /* Gecko 2.0 (Firefox 4.0) and other CSS3-compliant browsers */
     
      -moz-border-image: url(bg-image.png) 0;    /* Gecko 1.9.1 (Firefox 3.5) */
    }

    [5] Though Internet Explorer 8 doesn't support the background-size property, it is possible to emulate some of its functionality using the non-standard -ms-filter function:

    -ms-filter: "progid:DXImageTransform.Microsoft.AlphaImageLoader(src='path_relative_to_the_HTML_file', sizingMethod='scale')";

    This simulates the value cover.

    See also

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