Representing images: D = colour depth in bits.

The idea

Colour depth, symbol D, is the number of bits used to store the colour of each individual pixel in a bitmap image. Because each pixel's colour is coded in binary, a depth of D bits gives 2^D distinct colour codes per pixel, so D = 1 allows 2 colours, D = 3 allows 8 colours and D = 8 allows 256 colours. The total storage needed for the pixel data is therefore width × height × D bits, since every pixel contributes D bits. Increasing D raises both the number of available colours and the file size in direct proportion.

What earns the marks

• States that D is the number of bits used to represent the colour of one pixel.
• Explains that D bits give 2^D possible colour codes per pixel.
• Applies the relationship that image storage in bits equals width × height × D.
• Distinguishes colour depth D (bits per pixel) from the number of colours (2^D).
• Recognises that increasing D increases both colour range and file size.

Common mistakes and how to put them right

• Confusing D with the number of colours.

  Put it right: D is bits per pixel and the colour count is 2^D.

• Treating D as bytes rather than bits.

  Put it right: D is measured in bits, so convert to bytes only at the end by dividing by 8.

• Assuming a larger D always means a larger physical image.

  Put it right: D affects colour range and file size, not the pixel dimensions.

Exam tips

• Underline whether the question asks for bits per pixel or number of colours before choosing an option.
• Write the formula storage = width × height × D and substitute values to test each option.
• Check units: keep the answer in bits unless the question specifies bytes or kilobytes.