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Image Formats

As with video format it is also important to mention image format. Images shared internally though an organisation’s information management system can be large in file size, but images stored for the web would be much smaller. Images stored on the organisation’s information system would be used for publications and presentations, where image quality is the important factor. One important criteria for web pages is the speed with which they open, and smaller image sizes are more suitable.

An important format for the web is JPG. This is a compressed format, much smaller than bmp or tiff. JPG format is often the preferred format for common digital cameras. When reducing the file size in JPGs, there are two things which are specific to the format, compression and smoothing. Resolution and natural file dimensions can be changed in any format to reduce file size, but with JPG compression is also applied. During compression dividing lines between shades or colours can become visible. To reduce the impact of this, smoothing is applied, which further reduces the file size. Too much smoothing will cause blurring of the image so the two techniques have to be used together to reduce file size and maintain acceptable clarity. The combination of compression and smoothing creates a smaller file size which can still be of reasonable quality for the web, if it isn’t overdone. When storing for the web it is a good practice to size the image to the required dimensions on the web page. Placing an image on a webpage at its natural size means that its quality is not further reduced by stretching to a larger size and also the page is optimised for open speed with no image larger in file size than necessary.

Another useful image format is GIF. GIF supports transparency. A single colour in the image can be set as transparent, typically the background colour. When choosing a colour be aware that other parts of the images as well as the background may also become transparent. GIF is also a good format for storing scanned documents. When storing as GIF the number of colours in the image can be reduced. A black and white page doesn’t require 256 colours, and by choosing 4 colours the document will appear to have a bit more depth than black and white and will be much smaller in file size than using 256 colours. Of course resolution and natural image size can be adjusted to reduce the file size. GIF also supports animation. A series of pictures in a looped animation can be produced and placed on a web page examples of which could be compressed digital photos, cartoons or even a sequence of captured computer screen images.

EPS differs from JPG and GIF in so far as it supports vector based drawings, drawings made from lines and fills rather than pixels. This is particularly handy for exchanging images such as logos. EPS files can be opened in applications such as Adobe Illustrator and vector objects stretched without reduction in quality. Text stored in an EPS as vectors also maintains smooth lines when resized.

There are many image formats, but these three highlight examples of the most popular and typical uses.