The Mystery Of CSS Sprites: Techniques, Tools And Tutorials
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CSS Sprites are not new. In fact, they are a rather well-established technique and have managed to become common practice in Web development. Of course, CSS sprites are not always necessary, but in some situation they can bring significant advantages and improvements – particularly if you want to reduce your server load. And if you haven’t heard of CSS sprites before, now is probably a good time to learn what they are, how they work and what tools can help you create and use the technique in your projects.
What Are CSS Sprites?
The term “sprite” (similar to “spirit,” “goblin,” or “elf”) has its origins in computer graphics, in which it described a graphic object blended with a 2-D or 3-D scene through graphics hardware. Because the complexity of video games has continually increased, there was a need for smart techniques that could deal with detailed graphic objects while keeping game-play flowing. One of the techniques developed saw sprites being plugged into a master grid (see the image below), then later pulled out as needed by code that mapped the position of each individual graphic and selectively painted it on the screen.
Sprites were displayed over a static or dynamic background image, and the positioПing of the sprite was controlled simply by the hardware controllers. The term was coined because the sprites seemed to “haunt” the display and didn’t really exist in the graphic memory.
The Pokemon Sprite Sheet, consisting of over 1000 graphic objects. Found here 2. You can click on the image for the larger version (thanks, Ryan! ).
Time passed, and at the beginПing of the 2000s, when progressive Web designers started to seek alternatives to JavaScript-based rollover menus (with onMouseOver and onMouseOut effects), sprites saw a renaissance in Web development. With CSS, the simple implementation of sprites was possible, and it was much easier and clearer than its JavaScript-based predecessor.
In 2004, Dave Shea suggested 3 a simple CSS-based approach to CSS sprites based on the practice established by those legendary video games. In this case, multiple images used throughout a website would be combined into the so-called “master image.” To display a single image from the master image, one would use the background-position property in CSS, defiПing the exact position of the image to be displayed. Any hover, active or фокус effects would be implemented using the simple definition of the background-position property for the displayed element.
When the page is loaded, it would not load single images one by one (nor hover-state images per request), but would rather load the whole master image at once. It may not sound like a significant improvement, but it actually was: the main disadvantage of the onMouse effects is that JavaScript-based hover effects require two HTTP requests for each image, which takes time and creates that unpleasant “flickering” of images. Because the master image is loaded with the whole page only once with CSS sprites, no additional HTTP requests are needed for hover, active or фокус effects (because the image is already loaded), and no “flickering” effect occurs.
Consequence: CSS sprites reduce HTTP requests and the loading time of pages. This is the main reason why CSS sprites are often used on websites with heavy traffic, where millions of page impressions would need “only” a tiny fraction of what could otherwise be 30,000,000. Hence, CSS sprites are commonly used, particularly for navigation (such as for hover effects), icons and buttons.
Where Are CSS Sprites Used?
CSS sprites can be used in various settings. Large websites can combine multiple single images in a meaПingful manner, creating clearly separated “chunks” of the master images – the purpose being to keep the design maintainable and easy to update. The large empty space between the images is often used to make sure that the text resizing in browser doesn’t cause side effects such the display of multiple images in the background. In fact, sprites usually work well in a pixel-based design, but they are hard to use in elastic (em-based) designs due to the restricted background-position-property. Essentially, the structure that sprites take depends on the trade-off between maintainability and reduced server load; thus, it varies depending on the project you are working on.
Here are some inspiring (and not so inspiring) examples:
Xing uses various icons and buttons, as well as its logo, in the sprite.
