This blog post is a part of Mobilizing websites with responsive design and HTML5 tutorial. For all posts please see the Introduction post.
When you change your layout dynamically you need to re-fit media elements to match the new layout dimensions. There exist three basic techniques dealing with images in responsive designs
- Resizing images using CSS only – in this case the clients download the full image payload regardless of the image size and image visible sizes are adjusted using CSS styles
- Co-operative image resizer on the front end web server: when HTML is written from a template the image tags are written in such a way that <img src> attribute points to a resized image. The resizer rewrites image to match the vrowser screen dimensions. This is done e.g. in Web and Mobile for Plone CMS.
- Independent image resizer on the front end web server: Javascript + server-side stub component is used to resize the images. The server-side component does not depend on the rest of the front-end server system, thus making the front end code less complex. A client-side Javascript scans HTML DOM tree for <img> tags and rewrites <img src> to go through the resizer. For example Adaptive Images (below) is this kind of a solution. Alternatively, an independent image resizer can be a part of the middleware stack (e.g. WSGI in Python)
In this blog post we mostly discuss about the simplest approach: resizing images using CSS only.
1. Resizing images using pure CSS
If your responsive layout includes images, as contentish or style elements, they might not work well with small screens as is. You can fix the situation by resizing the images dynamically with CSS.
In our example case the page has only one image: the site logo. If viewed in a mobile browser using desktop CSS the logo might be too small or too large. Thus
- We remove any hard-coded width and height values for the <img> in HTML
- We apply CSS style for the image width where the image takes width from the screen width as percents, but still has max-width set in pixels so that tablet devices with more screen estate don’t scale the image too much
Related CSS:
/* Logo must not appear too large on mobile */
#portal-logo img {
width: 33%;
max-width: 180px;
}
Before fix:
After fix:
2. Server-side image resizing solutions
You can also resize images dynamically on the server-side, so that mobile browsers load the reduced size version, saving the precious mobile bandwidth. However, this adds additional server-side component to the mix, increasing the complexity of the solution and especially when dealing with legacy sites consider whether the improved user experience is enough to balance this trade-off.
Also, the dynamic image resizer component is easily subject to denial of service attack because the HTTP responses it serve are usually slow and computationally intensive. If you decide to use one of these components be sure you are aware of these implications.
You don’t need to change the server-side generated HTML to achieve the dynamic images sizes. There exist a technology neutral solutions relying on the client-side Javascript and additional serve-side proxy component.
Adaptive Images is one of server-side image resizing solutions based on Javascript and minimal server-side PHP script. It operates in independent manner and adheres the contemporary best practices of HTML5 and responsive web designs.
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