Week 4 - Graphics & Images

This session introduced two ways to capture graphical information, Raster and Vector.

Rasters work well for complex images. They consist of discrete units, cells, that divide space and so information recorded can be very blocky in appearance. A sequence of cells, usually in a grid is allocated a numeric value that defines the shape of what it's representing. This value may also represent colour and shade. Raster cells are often referred to as 'pixels' picture elements.

Vector divides information into discrete elements, records position and shape and they are useful for images that have defined boundaries. Geometric elements are recorded as to their location using a co-ordinated system.

The Graphics Interchange Format (GIF) and the Joint Photographic Experts Group (JPEG) are the two most commonly used raster formats. GIF is an 8 bit format using 8 bits for every pixel, 256 distinguishable colours. It is best used at a 2:1 or 3:1 compression ratio. GIF is generally more suitable for less complex images such as graphs or diagrams.

JPEG is defined by ISO standard 10918 1991. As a 24 bit format is can contain 16 million colours. It can sacrifice some of the original data using complex computer compression techniques. For this reason it is often recommended to save originals before loading on www. It can compress to 20:1 and often alteration of colour cannot be seen to the human eye. They generally work well with photography and detailed imagery.

The use of Portable Network Graphics (PNG) is not as prolific but is on the increase and a useful alternative. Designed to use the best of JPG and GIF, it is wise again to store a master copy.

GIF, JPEG and more often now PNG can be included in web pages. These can be embedded within the document or included as links to a separate source. Appropriate img tags are used to do this.