Most of the things we see on a computer screen – words, numbers and pictures - are presented to us by software as images. These are presented to us digitally using either a raster model – which divides space into a rectangular grid – or a vector model, which represents areas of space as co-ordinates on an axis.
Both techniques can represent pictures via a browser. I can use html tags such as <img> to make my web page appealing [URL: http://www.student.city.ac.uk/~abhd820/index.html] or use the <a href> tags to link to an image held elsewhere, as below:
Fig 3. World map. Image source: Web Resources Depot (free vector maps page).
[URL: http://www.webresourcesdepot.com/free-vector-world-maps-collection/]
Most software formats used for images on web pages - Graphics Interchange Format (.gif), JPEG (.jpg) and Portable Network Graphics (.png) - use raster techniques, and employ some compression to reduce the size of images.
GIF [URL: http://en.wikipedia.org/wiki/Graphics_Interchange_Format] is an 8-bit format, enabling up to 256 colours to be used. Each image keeps colour information in a separate ‘lookup table’ which helps keep file size down.
JPEG [URL: http://en.wikipedia.org/wiki/JPEG] is a 24-bit format, which enables it to display many colours. JPEGs maximize compression which make them great for using on the web. However, the ‘lossy’ compression techniques used by the format often leave ‘artefacts’, which can be noticeable in images with large areas of a single colour (the GIF format is better for this type of image). The degradation is often less noticeable in photographs, which contain many different colours.
PNG [URL: http://en.wikipedia.org/wiki/Portable_Network_Graphics] is a 24-bit format that uses ‘lossless’ compression to avoid this degradation. The compression is not as great as with JPEG files, so file sizes are often bigger.
These image formats give me flexibility when working as a corporate information officer. I can use jpg files for our website [URL: http://www.mouchel.com/services_atoz/default.aspx] taking advantage of the better compression ratios. In contrast, for technical charts, GIF or PNG files may be a better option.
Word count (excluding figures and captions): 299 words.
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