Representing images: colour depth.

The idea

Colour depth is the number of bits used to store the colour of each pixel in a bitmap image. With a colour depth of n bits, each pixel can represent 2ⁿ different colours. For example, a colour depth of 1 bit gives 2¹ = 2 colours, 3 bits gives 2³ = 8 colours, and 8 bits gives 2⁸ = 256 colours. A greater colour depth allows more colours and more realistic images but increases the file size, because each pixel needs more bits. Colour depth is therefore a trade-off between image quality and storage. It is distinct from resolution, which concerns the number of pixels rather than the bits per pixel.

What earns the marks

• Colour depth is the number of bits used to represent each pixel's colour.
• The number of available colours is 2ⁿ where n is the colour depth in bits.
• A colour depth of 8 bits allows 2⁸ = 256 colours.
• Increasing colour depth increases file size because more bits are stored per pixel.
• Colour depth is separate from resolution, which is the number of pixels.

Common mistakes and how to put them right

• Thinking colour depth is the number of colours directly.

  Put it right: It is the number of bits, and colours = 2ⁿ.

• Confusing colour depth with resolution.

  Put it right: Resolution concerns pixel count, while colour depth concerns bits per pixel.

• Assuming more colour depth reduces file size.

  Put it right: More bits per pixel increases file size.

Exam tips

• Use the relationship colours = 2ⁿ and show the power calculation.
• Link colour depth to file size when explaining trade-offs.
• Distinguish colour depth from resolution in your answer.