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Bootstrap 图像

目标

在本教程中，我们将讨论三个 Bootstrap 3 的 class 来为图像添加样式。同时，我们也会讨论有关 Bootstrap 3 为响应式图像所提供的内容。

一般的样式

在我们讨论 Bootstrap 3 提供的定义图像样式的特殊的 class 之前，我们将看到 Bootstrap 3 提供的定义图像的一般的样式。

img {
  border: 0;
}

这是在 Bootstrap 3 的 CSS 中找到的第一个为图像定义的 CSS 规则，当图像呈现时用来移除边框。

然后，在打印的媒体查询内，规定了这些规则。

 img {
    page-break-inside: avoid;
  }
  img {
    max-width: 100% !important;
  }

page-break-inside: avoid; 避免图像内的分页符。

max-width: 100% !important; 为图像定义的样式是显而易见的。这里它用于确定即使图像的宽度超出了容器，它也能被限制在容器内显示。它与 !important 一起使用，来覆盖其他任何破坏这种样式的样式。有时候，开发人员想让样式更好地支持移动设备上图像的友好呈现，会特别使用这两个规则。

这里还有另一个用于图像的规则

img {
  vertical-align: middle;
}

由于这条规则，一个图像会在 div 或者其他元素内垂直居中。如下面实例所示。

<!DOCTYPE html>
<html>
  <head>
    <title>example of rendering images in Bootstrap 3</title>
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <!-- Bootstrap -->
    <link href="dist/css/bootstrap.min.css" rel="stylesheet" media="screen">
    <style>
    body {
    padding: 50px
    }
     .mdl {
     background-color: silver;
     padding: 7px
    }
    </style>
    <!-- HTML5 Shim and Respond.js IE8 support of HTML5 elements and media queries -->
    <!-- WARNING: Respond.js doesn't work if you view the page via file:// -->
    <!--[if lt IE 9]>       <script src="https://oss.maxcdn.com/libs/html5shiv/3.7.0/html5shiv.js" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" ></script>       <script src="https://oss.maxcdn.com/libs/respond.js/1.3.0/respond.min.js" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" ></script>     <![endif]-->
  </head>
  <body>
<p class="mdl"><img src="icon-default-screenshot.png">Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh euismod tincidunt ut laoreet dolore magna aliquam erat volutpat.</p>
<!-- jQuery (necessary for Bootstrap's JavaScript plugins) -->
    <script src="https://code.jquery.com/jquery.js" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" ></script>
    <!-- Include all compiled plugins (below), or include individual files as needed -->
    <script src="dist/js/bootstrap.min.js"></script>
  </body>
</html>

点击这里，在线查看演示。

请注意，如果需要根据上下文垂直居中图像，则需要额外的样式。

特殊的样式

Bootstrap 3 提供了三个 class 用于呈现带有明确样式的图像。

img-rounded

您可以使用这个 class 来呈现带有圆角的图像。为了实现这点，Bootstrap 提供了 img-rounded class。该 class 的样式如下定义

.img-rounded {
  border-radius: 6px;
}

所以，它把 border-radius 属性设置为带有 6 像素值，用来定义相关图像的圆角。下面的实例演示了两个相同的图像，第一个图像不带 img-rounded class，第二个图像带有 img-rounded class。请注意，第二个图像是圆角的。您可以 点击这里，在线查看实例。

images with and without rounded corners 
<!DOCTYPE html>
<html>
  <head>
    <title>example of rendering images with rounded corners with Bootstrap 3</title>
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <!-- Bootstrap -->
    <link href="dist/css/bootstrap.min.css" rel="stylesheet" media="screen">
    <style>
    body {
    padding: 50px
    }
    </style>
    <!-- HTML5 Shim and Respond.js IE8 support of HTML5 elements and media queries -->
    <!-- WARNING: Respond.js doesn't work if you view the page via file:// -->
    <!--[if lt IE 9]>       <script src="https://oss.maxcdn.com/libs/html5shiv/3.7.0/html5shiv.js" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" ></script>       <script src="https://oss.maxcdn.com/libs/respond.js/1.3.0/respond.min.js" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" ></script>     <![endif]-->
  </head>
  <body>
<img src="rounded-corner-images.png" alt="image with rounded corners">
<img src="rounded-corner-images.png" alt="image with rounded corners" class="img-rounded">
<!-- jQuery (necessary for Bootstrap's JavaScript plugins) -->
    <script src="https://code.jquery.com/jquery.js" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" ></script>
    <!-- Include all compiled plugins (below), or include individual files as needed -->
    <script src="dist/js/bootstrap.min.js"></script>
  </body>
</html>

img-thumbnail

这是另一个 Bootstrap 3 的 CSS class，它给图像添加了一个缩略图。该 class 的代码显示如下

.img-thumbnail {
  display: inline-block;
  height: auto;
  max-width: 100%;
  padding: 4px;
  line-height: 1.428571429;
  background-color: #ffffff;
  border: 1px solid #dddddd;
  border-radius: 4px;
  -webkit-transition: all 0.2s ease-in-out;
          transition: all 0.2s ease-in-out;
}

下面是一个使用了该 class 的实例。您可以 点击这里，查看在线演示。

two images without and with img-thumbnail 
<!DOCTYPE html>
<html>
  <head>
    <title>Images thumbnail with Bootstrap 3</title>
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <!-- Bootstrap -->
    <link href="dist/css/bootstrap.min.css" rel="stylesheet" media="screen">
    <style>
    body {
    padding: 50px
    }
    </style>
    <!-- HTML5 Shim and Respond.js IE8 support of HTML5 elements and media queries -->
    <!-- WARNING: Respond.js doesn't work if you view the page via file:// -->
    <!--[if lt IE 9]>       <script src="https://oss.maxcdn.com/libs/html5shiv/3.7.0/html5shiv.js" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" ></script>       <script src="https://oss.maxcdn.com/libs/respond.js/1.3.0/respond.min.js" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" ></script>     <![endif]-->
  </head>
  <body>
<img src="image-with-thumbnail.png" alt="image without thumbnail corners">
<img src="image-with-thumbnail.png" alt="image with thumbnail corners" class="img-thumbnail">
<!-- jQuery (necessary for Bootstrap's JavaScript plugins) -->
    <script src="https://code.jquery.com/jquery.js" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" ></script>
    <!-- Include all compiled plugins (below), or include individual files as needed -->
    <script src="dist/js/bootstrap.min.js"></script>
  </body>
</html>

img-circle

通过使用 border-radius 属性，Bootstrap 3 创建了一个以圆形呈现的图像。img-circle class 的 CSS 代码如下

.img-circle {
  border-radius: 50%;
}

点击这里，在线查看实例。下面是截屏和代码。

image without and with img-circle 
<!DOCTYPE html>
<html>
  <head>
    <title>Images with circle with Bootstrap 3</title>
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <!-- Bootstrap -->
    <link href="dist/css/bootstrap.min.css" rel="stylesheet" media="screen">
    <style>
    body {
    padding: 50px
    }
    </style>
    <!-- HTML5 Shim and Respond.js IE8 support of HTML5 elements and media queries -->
    <!-- WARNING: Respond.js doesn't work if you view the page via file:// -->
    <!--[if lt IE 9]>       <script src="https://oss.maxcdn.com/libs/html5shiv/3.7.0/html5shiv.js" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" ></script>       <script src="https://oss.maxcdn.com/libs/respond.js/1.3.0/respond.min.js" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" ></script>     <![endif]-->
  </head>
  <body>
<img src="image-with-circle.png" alt="image without circle shape">
<img src="image-with-circle.png" alt="image with circle shape" class="img-circle">
<!-- jQuery (necessary for Bootstrap's JavaScript plugins) -->
    <script src="https://code.jquery.com/jquery.js" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" ></script>
    <!-- Include all compiled plugins (below), or include individual files as needed -->
    <script src="dist/js/bootstrap.min.js"></script>
  </body>
</html>

响应式图像

Bootstrap 3 没有提供立即可用的响应式图像。您必须添加 img-responsive class 到图像上让它具有响应性。该 class 的 CSS 代码如下。

.img-responsive {
  display: block;
  height: auto;
  max-width: 100%;
}

它定义了图像必须显示为块级元素，高度将与图像高度相同，图像的最大宽度是 100%，来限制图像根据它所在的设备来呈现。

如需让图像默认具有响应特性，您可以添加这些 CSS 代码到 img{ }。

使用该 class 的实例如下。您可以 点击这里，在线查看演示。

<!DOCTYPE html>
<html>
  <head>
    <title>Responsive image example with Bootstrap 3</title>
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <!-- Bootstrap -->
    <link href="dist/css/bootstrap.min.css" rel="stylesheet" media="screen">
    <style>
    body {
    padding: 50px
    }
    img {
    margin-bottom:30px
    }
    </style>
    <!-- HTML5 Shim and Respond.js IE8 support of HTML5 elements and media queries -->
    <!-- WARNING: Respond.js doesn't work if you view the page via file:// -->
    <!--[if lt IE 9]>       <script src="https://oss.maxcdn.com/libs/html5shiv/3.7.0/html5shiv.js" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" ></script>       <script src="https://oss.maxcdn.com/libs/respond.js/1.3.0/respond.min.js" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" ></script>     <![endif]-->
  </head>
  <body>
<img src="image-with-circle.png" alt="without responsive image feature">
<img src="image-with-circle.png" alt="with responsive image feature" class="img-responsive">
<!-- jQuery (necessary for Bootstrap's JavaScript plugins) -->
    <script src="https://code.jquery.com/jquery.js" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" ></script>
    <!-- Include all compiled plugins (below), or include individual files as needed -->
    <script src="dist/js/bootstrap.min.js"></script>
  </body>
</html>

这个向图像添加响应特性的方法有局限性。

分别在大屏幕设备和移动设备（屏幕尺寸更小，可能会引发性能问题）上加载同样大的高分辨率的图像。由于浏览器会在 CSS 和 JS 加载之前预先加载图像，在一个低速的网络连接中，也会引发性能问题。试想一下，您有一个大图像和其内的一个特定对象，当您在移动设备上查看图像时，图像会被缩小到一个较小的版本，这样图像看起来就会很小，简称 艺术指导 问题。

开发人员已经想出克服这些限制的解决方案。我们将看到一个 Marc Grabanski 和 Christopher Schmitt 使用 HiSRC 的实例。这是一个会自动创建图像的低、中、高分辨率版本的 Jquery 插件，呈现的版本取决于显示图像设备的分辨率和带宽。

在我们后边的 响应式图像教程 中，我们将详细讨论所有这些方法。

点击这里，在线查看演示。代码如下：

<!DOCTYPE html>
<html>
  <head>
    <title>Responsive image with HiSRC example with Bootstrap 3</title>
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <!-- Bootstrap -->
    <link href="dist/css/bootstrap.min.css" rel="stylesheet" media="screen">
    <style>
    body {
    padding: 50px
    }
    img {
    margin-bottom:30px
    }
    </style>
    <!-- HTML5 Shim and Respond.js IE8 support of HTML5 elements and media queries -->
    <!-- WARNING: Respond.js doesn't work if you view the page via file:// -->
    <!--[if lt IE 9]>       <script src="https://oss.maxcdn.com/libs/html5shiv/3.7.0/html5shiv.js" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" ></script>       <script src="https://oss.maxcdn.com/libs/respond.js/1.3.0/respond.min.js" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" ></script>     <![endif]-->
  </head>
  <body>
<img src="5216852563_eca0af1b0d_m.jpg" data-1x="5216852563_eca0af1b0d.jpg" data-2x="5216852563_90c3f9c402_o.jpg" class="hisrc" />
<!-- jQuery (necessary for Bootstrap's JavaScript plugins) -->
    <script src="https://code.jquery.com/jquery.js" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" ></script>
    <!-- Include all compiled plugins (below), or include individual files as needed -->
    <script src="dist/js/bootstrap.min.js"></script>
<script>
$(document).ready(function(){
  $(".hisrc").hisrc();
});
</script>
<p>Photo courtesy: /http://www.flickr.com/photos/cubagallery/</p>
  </body>
</html>

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