CSS : Background image Trick

CSS background image Trick

Emulating background image crop, background image opacity, background transforms, and improved background positioning. A few hacks relying on CSS pseudo-elements to emulate features unavailable or not yet widely supported by modern browsers.

Demos: Example CSS background image hacks

Pseudo-element hacks can fill some gaps in existing browser support for CSS features, without resorting to presentational HTML. In some cases, they even make it possible to emulate things that are not currently part of any W3C working draft, like background transforms and background image opacity.

Most of the hacks in this article tie in with the pseudo-element hack described in an earlier article – Multiple Backgrounds and Borders with CSS 2.1. That article already describes how to emulate multiple background support and its demo page shows several other uses of the basic principle. This article presents a few of those effects and applications in greater detail.

Emulating background-crop

Known support: Firefox 3.5+, Opera 10+, Safari 4+, Chrome 4+, IE 8+

Demo: Pseudo background-crop

Background image cropping can be emulated in modern browsers using only CSS 2.1. The principle behind a pseudo background-crop is to apply a background-image to a pseudo-element rather than the element itself. One example would be to crop an image to display in the background. Another would be to crop an image sprite to display icons alongside text in links.

In several cases, using pseudo-elements may have advantages over existing, alternative techniques because it combines their strengths and avoids some of their weaknesses.

Google, Facebook, and Twitter all make use of empty DOM elements to crop dense sprites and display icons next to certain links in their interfaces. The alternative is not to use empty elements but be forced into using multiple images and/or to design sub-optimal image sprites that have their component images spaced out.

Pseudo-elements can be used in much the same way as empty DOM elements. This simultaneously eliminates the need for presentational HTML and doesn’t depend so heavily on an image sprite’s design. Using pseudo-elements for this purpose does have its own drawback – a lack of support in legacy browsers like IE6 and IE7. However, the technique will progressively enhance capable browsers while leaving a perfectly usable experience for incapable browsers.

Example code: cropping a sprite

This example shows how to crop icons that are part of a dense image sprite that uses a 16px × 16px grid. It uses a simple list and specifies a class for each type of action.

<ul class="actions">
   <li class="save"><a href="#">Save</a></li>
   <li class="delete"><a href="#">Delete</a></li>
   <li class="share"><a href="#">Share</a></li>
   <li class="comment"><a href="#">Comment</a></li>
</ul>

Styling can be applied to present this list in whatever way is needed. From that base, a pseudo-element can be created and then treated as you would an empty, inline DOM element (e.g. <span>).

In this case, the :before pseudo-element is used and sized to match the sprite’s grid unit. It could be sized to whatever dimensions are required to match a section of the sprite that needs to be cropped.

.actions a:before {
   content:"";
   float:left;
   width:16px;
   height:16px;
   margin:0 5px 0 0;
   background:url(sprite.png);
}

.save a:before {background-position:0 -16px;}
.delete a:before {background-position:0 -32px;}
.share a:before {background-position:0 -48px;}
.comment a:before {background-position:0 -64px;}

Providing hover, focus, active, and “saved” states is just a case of declaring the correct background position in each case.

.save a:hover:before,
.save a:focus:before,
.save a:active:before {background-position:-16px -16px;}

.saved a:before {background-position:-32px -16px;}

Future alternatives

In the future, there will be other alternatives.  Firefox 3.6 added -moz-image-rect  to allow background images to be cropped. But this is not supported by other browsers and looks likely to be replaced by an alternative proposal (to use fragment identifiers) that is part of the CSS Image Values Module Level 3 specification. As far as I know, no stable release of any modern browser supports the use of fragment identifiers with bitmap images.

Emulating background-transform

Known support: Firefox 3.6+, Opera 10.5+, Safari 4+, Chrome 4+, IE 9+

Demo: Pseudo background-transform

Combining pseudo-elements and transforms makes it possible to emulate background transforms. A pseudo background-transform can be used to rotate, scale, and skew background images and sprites. There is no proposal for background-image transforms, so a pseudo-element hack is one way to emulate it.

Example: rotating a background image

The example of cropping sprites can be further developed by reducing the number of different images used in the sprite. Rather than applying transforms to images in a graphics package, they can be applied in the CSS.

The code to do this is relatively simple and might look something like:

.accordion a:before {
   content:"";
   float:left;
   width:16px;
   height:16px;
   margin:0 5px 0 0;
   background:url(sprite.png) no-repeat 0 0;
}

.accordion.open a:before {
   -webkit-transform:rotate(90deg);
   -moz-transform:rotate(90deg);
   -ms-transform:rotate(90deg);
   -o-transform:rotate(90deg);
   transform:rotate(90deg);
}

To apply a transform to a more conventional background image (e.g., a large graphic sitting behind some content that doesn’t affect the positioning of other components) requires use of the positioning technique detailed in the article Multiple Backgrounds and Borders with CSS 2.1.

It involves setting the background image on a pseudo-element which is then positioned behind the content layer of an element using absolute positioning and z-index.

Example: mirroring a background image

There are instances when mirroring a background image might be desired. The approach is similar to that for rotating an image, but this time usestransform:scale().

Producing an exact mirror of an element or pseudo-element can be done using transform:scaleX(-1), transform:scaleY(-1), andtransform:scale(-1,-1) to mirror along the x-axis, y-axis, and both axes, respectively.

The following code is an example of how a pseudo background-transform might be used for pagination links. A pseudo-element displays a single image (or region of a sprite) and is then mirrored. The image’s appearance is such that a rotation cannot produce the desired counterpart. Only a scale operation can do it.

.prev a:before,
.next a:before {
   content:"";
   float:left;
   width:16px;
   height:16px;
   margin:0 5px 0 0;
   background:url(sprite.png) no-repeat 0 0;
}

.next a:before {
   float:right;
   margin:0 0 0 5px;
   -webkit-transform:scaleX(-1);
   -moz-transform:scaleX(-1);
   -ms-transform:scaleX(-1);
   -o-transform:scaleX(-1);
   transform:scaleX(-1);
}

There is no support for this in IE 8. Even if you’re a fan of using IE filters to work around some missing CSS support, they won’t work on pseudo-elements.

Future alternatives

There don’t seem to be any future alternatives in any CSS working draft. For the moment, it looks like pseudo-element hacks will be needed to emulate effects like background transforms and background perspective without resorting to presentational HTML.

Emulating background-position

Known support: Firefox 3.5+, Opera 10+, Safari 4+, Chrome 4+, IE 8+

Demo: Pseudo background-position

The CSS 2.1 specification limits the values of background-position to offsets from the left and top sides. It’s possible to emulate positioning a background image from the right and bottom sides by applying the background image to a pseudo-element and using it as an additional background layer.

This hack is easily combined with the other hacks in this article. More details on the pseudo background-position hack can be found in the article on Multiple Backgrounds and Borders with CSS 2.1.

Example code

In this example, a pseudo-element is created and placed behind the element’s content layer. The background image is 500px × 300px and declared for the pseudo-element, which is also given dimensions that match those of the image. Since the pseudo-element is absolutely positioned, it can be positioned from the bottom and right of the element using offsets.

#content {
   position:relative;
   z-index:1;
}

#content:before {
   content:"";
   position:absolute;
   z-index:-1;

  bottom:10px;

right:10px;

   width:500px;
   height:300px;
   background:url(image.jpg);
}

Future alternatives

There is a part of the CSS Backgrounds and Borders module working draft that describes an improvement to the background-position property to allow positions to be set from any side. At the moment, Opera 11 is the only stable release of a browser that has implemented it.

Emulating background-opacity

Known support: Firefox 3.5+, Opera 10+, Safari 4+, Chrome 4+, IE 9+

Demo: Pseudo background-opacity

Changing the opacity of a pseudo-background is as simple as modifying the value of the opacity property. There is no IE 8 support for opacity and IE filters will not work on pseudo-elements.

Example code

This example code shows a pseudo-element being created and positioned behind the rest of the element’s content so as not to interfere with it. The pseudo-element is then sized to fit the element using offsets (but could be offset by other values or given an explicit size), given a background image, and has its opacity changed.

#content {
   position:relative;
   z-index:1;
}

#content:before {
   content:"";
   position:absolute;
   z-index:-1;
   top:0;
   bottom:0;
   left:0;
   right:0;
   background:url(image.jpg);
   opacity:0.7;
}

Notes

For now, and as far as I am aware, using CSS 2.1 pseudo-elements is the only widely supported (and backwards compatible) way to emulate background image crop, background transform, background opacity, and improved background positioning with semantic HTML.

Even when alternatives in CSS working drafts (e.g., the improved background-position and use of fragment identifiers) are widely implemented, pseudo-element background-image hacks will still have the advantage of letting you use other CSS properties like opacity, border-radius, border-image, box-shadow, transforms, etc., which may prove useful in certain situations. It can’t hurt to be aware of these options.

It’s worth mentioning that although you can only generate 2 pseudo-elements from a DOM element, in many cases you can easily use descendant elements to provide more pseudo-elements to play with. This idea was used to help create the rotated example on the CSS drop-shadows demo page and several of the CSS3 examples at the bottom of the pure CSS speech bubbles demo page.

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