JPGs use a complex compression algorithm, which can be applied on a sliding scale. It is capable of displaying millions of colours at once, without the need for dithering, allowing for the complex blend of hues that occur in photographic images. The 16-bit JPEG format (usually written without the E), was designed with photographs in mind. This single GIF file simply contains many frames of animation, which your browser shuffles through. As you can see, the background of the page shows through, but the edges are jagged, as this image was not designed for this background colour.įinally we have a rather rubbish animated GIF that I just knocked up in ten minutes. This is the same image again, but with the red background colour set to transparent.

The loss of colours becomes less obvious, and the filesize overall is about half as big. To remedy this, we can add some dithering (which does add a bit to the filesize), to produce the image on the right. The filesize is improved, but it doesn't look quite as smooth. This is the same graphic, but the number of available colours has been reduced. This first image is indicative of the optimum properties of a GIF image - small, low on colours, and containing large areas of continuous colour. GIFs (variously pronounced “gif” or “jif”) are a very good format for most of the graphics you’ll be using on your site, though PNGs below are better in many regards. This usually means a larger overall filesize, but it means that a version of the image gets placed onto the viewer’s screen much quicker, and so is beneficial in many situations, as it gives the impression of a speedier download. GIFs can also be interlaced, which is a way of saving a graphic so that it loads progressively - first a blurry, low-detail version is loaded, and then successive layers of detail are added. This allows the background colours of the web page to show through the image. GIFs allow single-bit transparency, which means when you are creating your image, you can specify one colour to be transparent. You will need an animation program to make your own animations. GIFs can be animated, which is another reason they became so successful. Because this can sometimes be tight, GIFs have the option to dither, and will mix pixels of two different available colours to create a suggestion of another colour. This makes it a poor format for photographic images. GIF files can be saved with a maximum of 256 colours. GIFs use a simple technique called LZW compression to reduce the filesizes of images by finding repeated patterns of pixels, but this compression never degrades the image quality. The format is loss-less, meaning it does not get blurry or messy.

It is ideal for small navigational icons and simple diagrams and illustrations where accuracy is required, or graphics with large blocks of a single colour. Compuserve’s 8-bit GIF format has long been the most popular on the Internet, mainly because of its small size.

File formats for pictures