DITA 04 - Images & Graphics

Graphics and images can be of many types e.g. letter character fonts, maps, photographs or satellite images and weather charts.

The most basic storage method for images is the bitmap (windows .bmp) where a grid of squares is populated by data words that describe intensity and colour of the picture elements (pixels). Bitmaps are prone to create blurred images when enlarged by zooming into an image of this type (i.e. pixellated) and these files are large memory users. Alternative file types can store a compressed version of this data e.g JPEG (windows .JPG) - this format sacrifices detail (information) to compress the image further. In lab tests an image was 3841 Kb as a .BMP and compressed to just 127 Kb as a lossy .JPG. An example of a lossless bitmap compression file format is GIF.

An alternative is using a vector image containing information about shapes (e.g. lines) and their position and length. Vector images can be scaled with shape and detail being preserved - an example is the Scaleable Vector Graphics (SVG) file format.

The way that colour in images is stored and presented on the WWW uses a binary representation of a colour per display pixel using the additive Red Green Blue (RGB) colour system - all 3 colours produces white, as opposed to subtractive systems e.g. painting RYB and printing CMY systems where adding all gives black.

Due to human perceptions of colours and particularly the occurance of colour blindness in the population so primary colour schemes are best avoided for websites.

A format manipulation test image file is here: Session 3 mini website.