AQA · GCSE · Computer Science
Course version: 8525 · Version from Sept 2020
Describe how a bitmap represents an image using pixels and colour depth.
Full specification wording
Representing images: Describe how a bitmap represents an image using pixels and colour depth.
A bitmap is a grid of picture elements, or pixels, and each pixel stores a binary code for one colour.
Checked against the AQA 8525 specification (opens in a new tab) on .
- Point 8 of 18
Study notes
From the checked teaching notes for this specification point.
The idea
A bitmap is a grid of picture elements, or pixels, and each pixel stores a binary code for one colour. The image's width and height in pixels give its resolution, so a 4 × 3 grid holds 12 pixels. Colour depth is the number of bits used per pixel: with a depth of 1 bit each pixel is black or white, while 3 bits allow 2³ = 8 colours and 8 bits allow 2⁸ = 256 colours. The total storage is pixels × colour depth, so 12 pixels at 8 bits need 96 bits, which is 96 ÷ 8 = 12 bytes. A larger colour depth gives more realistic colour but increases file size; more pixels also increase file size. The bitmap does not store shapes or lines, only the colour code of every pixel, which is why enlarging it can make individual pixels visible.
What earns the marks
- A bitmap is a grid of pixels, each pixel being one small square of the image.
- Each pixel stores a binary value representing its colour.
- Resolution is the number of pixels across and down the image.
- Colour depth is the number of bits used to represent each pixel's colour.
- Number of available colours equals 2 raised to the power of the colour depth, e.g. 3 bits gives 2³ = 8 colours.
- File size in bits equals number of pixels multiplied by colour depth.
- Increasing colour depth or pixel count increases file size and improves detail or colour range.
Common mistakes and how to put them right
Confusing colour depth with resolution.
Put it right: Is that resolution counts pixels while colour depth counts bits per pixel.
Thinking more colour depth reduces file size.
Put it right: Is that more bits per pixel increases file size.
Calculating colours as colour depth × 2 rather than 2 raised to the power of the colour depth.
Forgetting to divide bits by 8 when converting a bit total to bytes.
Exam tips
- Underline the two required ideas in the question: pixels and colour depth, and address both.
- Show the calculation, e.g. 2³ = 8 colours or 12 × 8 = 96 bits, rather than only a final number.
- Check units: state whether your answer is in bits or bytes and convert by dividing or multiplying by 8 as needed.
Try the lesson
The start of the Representing images: Describe how a bitmap represents an image using pixels and colour depth. lesson, made from checked teaching notes for this specification point.
Have a go first
In a bitmap image, what is a pixel?
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