在一周内建立一个响应式网站:图像和视频(第3部分)

  • 知识需要:中级CSS和HTML
  • 需要:文本编辑器,现代浏览器,图形软件
  • 项目时间:1小时(共5小时)下载源文件

+$Yesterday I described how we can craft fluid layouts, and suggested that to do this, we need to move away from pixels and instead embrace proportional units like ems and percentages.

+$This is all well and good when dealing with passages of text; if a column becomes too narrow then text can easily wrap on to a new line. But media assets like images and video have prescribed dimensions set in pixels. We can scale these down of course, but that means downloading larger files than necessary, and bandwidth can be expensive.

+$High definition (or Retina) displays pose another challenge. These displays pack a considerable number of pixels into a small area, allowing more detail to be shown. This can be a problem for high contrast imagery and iconography, which if not designed to match these native resolutions, can appear fuzzy and lack definition.

+$Today, I’ll look at these issues and provide insight into how we might tackle them. But it’s worth noting that this is a rapidly developing area, with new approaches emerging almost daily. This can be challenging – but exciting too!

+$Scaling images within a fluid layout

+$Yesterday we created a fluid layout for our thumbnail pattern. Unfortunately, images within each item were not adapting to the changing layout, breaking outside their containers or leaving large areas of white space:

Thanks to some stubbornly unresponsive images, our gallery layout breaks

+$Thanks to some stubbornly unresponsive images, our gallery layout breaks

+$Fortunately, there’s an easy way to fix this. First+$discovered by Richard Rutter+$(an early pioneer of fluid layouts), it involves adding the following rule to images you wish to scale with the layout:

  1. Img {
  2. 最大宽度:100%;
  3. }

+$This rule tells browsers that an image should fill the width of its containing element, up until the point where it reaches 100% of its size. Simple!

+$There is one small caveat, however. For this CSS declaration to work, you can’t add a height attribute to your images. From a performance point of view, that’s not ideal (the sooner the browser knows what size to render an image, the less time it spends redrawing the page). Continue to include width and height attributes for images that don’t need to scale however.

响应式图像

+$You’ll note that for these thumbnails, I’m using images that reflect the design of the desktop-orientated layout, and scaling them down for narrower viewports. This seems counter to everything we have described previously – this isn’t mobile first!

+$As with everything, it’s a question of judgement. We need to balance performance against maintainability – and our own sanity! For these thumbnails, the size differential is small enough (138px > 240px) that we can compensate for the file size in other ways. We will use the correct file format (JPEG can typically generate the smallest file size for photos) and apply an appropriate amount of compression. We will cover image optimisation in more detail during the final part of this tutorial.

Large images can appear on the media item page

+$Large images can appear on the media item page

+$However, on the media item page, a photo can be anywhere between 288 and 800 pixels wide. Regardless of file format and other optimisations, our largest image could still be anywhere up to 300kb in size, taking several seconds (if not longer) to download over a 3G connection. This is a problem.

+$Ideally the browser would report the size of image it needs to display and the average amount of bandwidth available and we could serve an appropriately sized image best matching these constraints. Sadly, no browser does this, and there is nothing that hints at how this might work either (well,+$not Yet+$anyway).

+$Such is the enormity of this problem, a few smart and talented developers have proposed some solutions. Many involve JavaScript, others use server-side detection; all have their advantages and trade-offs:

  • 响应式图像由开发斯科特杰尔+$, this approach (a combination of server-side rewrites, cookies and JavaScript) aims to detect the width of the viewport before any images are downloaded. If the viewport is small, then the default small image is requested. Yet if the viewport is large enough, requests will be rewritten to a directory containing larger images, and these will be downloaded instead. Unfortunately, as browsers increasingly focus on performance – typically using techniques that involve prefetching of assets – this approach has become fragile. It also detects the display width to decide which images to load, yet it is not always true that a larger display warrants larger images.
  • 自适应图像+$A PHP solution developed by+$Matt Wilcox+$and inspired by the work of Scott, this approach is compatible with existing markup and allows for multiple image sizes to be used. Again, it loads images based on screen resolution, which is not always an accurate test, and requires PHP and the GD library for it to work.
  • 响应式增强+$While many approaches have focused on trying to work out which image to download before a page loads,乔希艾默生+$has taken a different approach. Like other solutions, it involves referencing the smallest image in the markup. But rather than detect whether a larger image is required before the page is loaded, here the smallest image is downloaded regardless. Only once this has loaded (and if a larger image is required) does it get replaced with a higher resolution version. In many ways this is a modern take on the+$lowsrc+$attribute that mysteriously disappeared from HTML. Oh, how we long for its return.

+$Jason Grisby has written more about this problem+$and goes into fantastic detail about the pros and cons of some of the above solutions. Meanwhile,+$Matt Marquis has suggested the need for a new HTML element+$, And Started+$a community group at the W3C+$to work out the details.

+$The long and short of it? This isn’t a solved problem. All we can do is decide which of the options mentioned above fits our needs the best. For this tutorial, we’ll choose the responsive enhance technique.

+$First, we need to link to the JavaScript file in the

  1. <>
  2. ...
  3. <脚本+$src="/_js/responsive-enhance.js">脚本>
  4. <标题+$>Arriving in Las Vegas标题>
  5. 头>

+$Then On Our+$element we need to add a new+$data-fullsrc+$attribute that references the source of the larger image:

  1. <IMG+$id="media-object" src="http://farm8.staticflickr.com/7131/6998741089_0d87917944_n.jpg" data-fullsrc="http://farm8.staticflickr.com/7131/6998741089_0d87917944_c.jpg" alt="Sunset over Arizona"/>

+$We also need to add the following line of JavaScript below the image:

  1. +$

+$This tells the browser which image to replace, and at what viewport width to replace it. When JavaScript is disabled (or unavailable), users will get the smallest image. Otherwise, when the viewport is larger than 320px, the larger image will be displayed after the smallest one has finished downloading.

+$Finally, we need to override the default styling of images. For the detection to work, we need the smaller image to scale larger than its native size. We can do this as follows:

  1. +$img.media-object {
  2. 保证金:0自动;
  3. 宽度:100%;
  4. +$max-width: 50em; /* 800px */
  5. }

+$Again, this is not a perfect solution (especially as we can only reference one additionally sized image), but it’s far better than serving larger images regardless.

+$Resolution Independent Images

+$Images don’t just exist within markup of course: we can also reference them from CSS. Things are a little easier here, as media queries mean we can choose which images to show in different layouts.

+$As many of our background images in CSS will be icons, now is a good time to touch on high resolution displays, and in particular, two methods which allow us to use resolution-independent icons:

  • +$SVG Image Sprites:+$On an increasingly pixel-less web, Scalable Vector Graphics (SVG) are enjoying something of a revival after years in the wilderness. With good support in most modern browsers – including IE9 and above – we can use this format in a number of ways. We can reference it from an element, we can embed SVG code directly in our markup, and we can reference it from within CSS. David Bushell has+$documented a technique that uses SVG-based image sprites+$, and it’s well worth a read.
  • +$Icon Fonts:+$Another means of including scalable icons is with specially designed web fonts that include icons rather than letters and numbers. This means we can style icons in the same way we do for text, and enjoy vector based iconography, too. Jon Hicks+$wrote about icon fonts in more detail on 24 ways

+$Ratio Ga Ga

+$Okay, enough about images – let’s talk about video. Thankfully, making videos responsive is less problematic. Such is their size, approaches to serving videos in a bandwidth-friendly manner are more commonplace, be that via streaming or the many different compression codecs available. And of course, videos should only start downloading once you hit play.

+$But, we want our videos to scale within the confines of our layout. If we are using native HTML5 video, we can again use the following CSS:

  1. 视频 {
  2. 最大宽度:100%;
  3. }

+$However, Embedded Objects (

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