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Representing Images

FoundationHigherAQA

Learn Representing Images for GCSE Computer Science with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA GCSE Computer Science (8525). Bitmap images are made of pixels, each stored as a binary colour value.

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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA specification. Every worksheet comes with a full mark scheme.

Foundation

WorksheetFoundation · PDFMark SchemeFoundation · PDF

Higher

WorksheetHigher · PDFMark SchemeHigher · PDF

Topic overview

A bitmap image is made of a grid of pixels, and each pixel is stored as a binary code representing its colour.

Colour depth is the number of bits used per pixel. With n bits, 2ⁿ colours are available: 1 bit gives 2 colours, 8 bits gives 256, and 24 bits gives over 16 million. Resolution is the number of pixels in the image, usually given as width by height.

Both affect quality and file size together. Doubling the colour depth doubles the file size; doubling both dimensions of the resolution quadruples the number of pixels. Metadata is also stored — the dimensions, colour depth, and often the date and camera settings — so that software knows how to interpret the pixel data.

Revision notes

Pixels and colour depth

An image is a grid of pixels, each stored as a binary code for its colour.

Colour depth is the bits per pixel. With n bits there are 2ⁿ colours: 1 bit gives 2, 8 bits gives 256, 24 bits gives over 16 million.

Resolution

Resolution is the number of pixels, given as width × height.

Higher resolution means more detail but more pixels to store. Doubling both dimensions quadruples the pixel count, because both width and height double.

Metadata

Metadata is data about the image, stored alongside the pixel data.

It includes the width, height and colour depth, without which software could not interpret the file. It often also includes the date, time and camera settings.

Key points

  • A bitmap image is a grid of pixels.
  • Colour depth is the bits per pixel.
  • n bits give 2ⁿ colours.
  • Resolution is width × height in pixels.
  • Higher resolution and depth increase file size.
  • Metadata describes how to interpret the file.

Worked examples

Example 1

An image uses 8 bits per pixel. Calculate how many colours it can display. [2 marks]

Working

Each of the 8 bits can be 0 or 1, giving 2⁸ combinationsstate the calculation
2⁸ = 256 colourswork out the number

Example 2

State two items of metadata stored with an image and explain why they are needed. [2 marks]

Working

The width and height in pixels, and the colour depthname the metadata
Without them the software would not know how to arrange the pixel data or how many bits each pixel usesexplain why they are needed

Example 3

An image is 100 by 50 pixels. Its dimensions are both doubled. Work out the new pixel count. [3 marks]

Working

Original: 100 × 50 = 5000 pixelsfind the original count
Doubled: 200 × 100double both dimensions
= 20 000 pixels, four times the originalwork out the new count

Common mistakes

  • Confusing colour depth with resolution.

    Depth is bits per pixel; resolution is the number of pixels.

  • Calculating n × 2 instead of 2ⁿ.

    The number of colours is 2 to the power of the bit depth.

  • Forgetting metadata is stored.

    The file needs it to be interpreted correctly.

  • Saying doubling resolution doubles the pixels.

    Doubling both dimensions quadruples them.

Exam tips

  • Use 2 to the power of the colour depth.
  • Keep colour depth and resolution distinct.
  • Remember doubling both dimensions quadruples pixels.
  • Name specific metadata items when asked.

Key terms

Pixel
One dot of colour in a bitmap image.
Colour depth
The number of bits used per pixel.
Resolution
The number of pixels, as width × height.
Metadata
Data about the image, needed to interpret it.

Related topics

Written and reviewed against the current AQA specification. Spotted an error? Let us know.