Web Images: Bitmap Graphics
The most widely used formats on the Web at present are GIF (Graphic Interchange Format) and JPG/JPEG (Joint Photographic Experts Group). These are known as bitmap or raster graphics and are composed of tiny square picture elements (pixels) which when viewed at a distance look like a smooth image. (See figure 9.1a)
A small piece of computer memory is allocated to store each individual pixel. Therefore an image scaled to 50 per cent will consume less memory than the same image at 100 per cent in size.
Website designers can maintain relatively quick download times even at present if they utilize the capabilities of the image formats. For example GIFs can be interlaced, animated and made transparent.
Interlaced GIFs are those images which when downloaded from the Web, start out blocky and vague and gradually come into focus as their resolution sharpens. They are a good technique to use from a usability point of view because they provide the user with something to focus on before the image has fully downloaded. However, Weinman, (1999) stated: "although interlaced GIFs serve their purpose on non-essential graphics, they only frustrate end users when used on essential graphics" pointing out that "an imagemap or navigation icon, for example must be seen in order to fulfill its function."
Personally I believe interlaced GIFs serve a useful purpose, especially on sites that I view regularly where I know what to expect. ALT tags are valuable in such situations too. A combination of interlaced GIFs and annotations give a good indication of what an image is even before it has fully downloaded.
All images are created as square or rectangular files. By making pixels in the image transparent, one can create the illusion of irregularly shaped images. More interesting effects can be created as opposed to every graphic looking rectangular.
Animated GIFs allow multiple images to be stored as a single GIF. When the images are displayed one at a time very fast, they give the illusion of animation. One advantage of animated GIFs is that they require no additional plug-ins to play and they are relatively simple to create as well has having browser support.
JPEGs compress files in 24-bit colour. Specifically designed for photographic images, they are able to handle colour gradations subtly and with ease but handle solid blocks of colour poorly.