2. Bitmaps

The idea behind a 'bitmap image' is to build up the image from a number of smaller rectangular blocks of single colour. If you are far away enough from the image, your eye / brain is fooled into seeing the image as one smooth entity.

Each square or rectangular block of colour in the image is called a 'pixel' (shortened from 'picture element').

Resolution

The more pixels used to produce an image the more detailed that image can be i.e. the higher its resolution. For instance a 10 Megapixel digital camera makes use of over 10 million pixels per image thus offering a very high photographic quality. On the other hand a mobile phone may only have a 2 Megapixel camera and so the image is 'ok' but hardly high quality.

If you look too closely or magnify the image too much, the illusion is spoilt as you begin to see the individual pixels. This is called 'pixellation'.

Addressable

A pixel can only be one colour at any given time but each pixel is addressable. This means its colour can be rapidly changed under computer control. This rapid update is why video is possible on a computer screen, where a new bitmap image replaces the previous one at least 25 times a second for smooth-looking video.

Colour

The colour within a pixel is normally represented by 3 bytes, one byte for each of the three primary colours of Red, Green and Blue. An eight bit scheme allows for 256 different levels of each primary colour and so 256 x 256 x 256 = 16 million+ colours are possible within each pixel.

Other schemes are possible such as Black and White but they are fairly rare.

File size and compression

Bitmap image files can be very large depending on the number of pixels used. For instance an uncompressed 1000px x 1000px image will be at least 3 Megabytes (for 3 bytes per pixel). This means more disk storage space and slower loading times.

An image often has areas of almost common colour and so this lends itself to some clever file compression methods. A good file compression method will be looking for such patterns within the image in order 'encode' the information more efficiently.

Another approach to reducing file size is to throw away some pixel information at the expense of lower quality. JPEG files use 'lossy' compression method.

Exam questions

In an exam question, if you are asked to describe a bitmap graphic, here is a summary of the points made above: