How an image is stored

In this lesson

Explain how a bitmap image is stored as a grid of pixels in binary, define resolution and colour depth, relate colour depth to how many colours a pixel can hold in both directions, describe the effect of each on image quality and on file size, and say what metadata is and which board requires it.

What it covers

• What a PIXEL is - AQA's own words: the term pixel is short for picture element, and a pixel is a single point in an image
• An image is stored as a grid of pixels, and each pixel's colour is held as a binary code
• IMAGE SIZE / RESOLUTION: width in pixels by height in pixels, written width x height - and the UNITS ARE PIXELS, which Edexcel pays a separate mark for
• COLOUR DEPTH: the number of bits used to represent each pixel
• The relationship, BOTH WAYS: a colour depth of n bits gives 2 to the n different colours; and given a number of colours, the smallest colour depth that will hold them
• The effect of resolution and colour depth on the QUALITY of the image and on the SIZE of the image file - qualitatively, direction of change only
• That the two are INDEPENDENT: more bits per pixel does not add pixels, and more pixels does not add colours
• METADATA - additional information stored with the image such as height, width, colour depth, file size, or when and where it was taken. OCR REQUIRES IT; AQA and Edexcel print no metadata requirement at all
• The naming: all three boards say colour depth; OCR says resolution where AQA says image size

Key words

On the specification