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2.2.2Bitmap images

Edexcel GCSE Computer Science (1CP2) · Data › Data representation

Practise Bitmap images. 12 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.

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Quick recall

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Define the term 'pixel'.
The smallest single-coloured element (dot) of a bitmap image.
Give the number of colours that can be represented by a colour depth of 2 bits.
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Sample questions

Written for this site in the style of Edexcel exam questions. They are not taken from real past papers.

Question 1Easy3 marks
(a) Define the term 'pixel'.[1]
(b) Define the term 'colour depth'.[1]
(c) State the number of different colours that can be represented with a colour depth of 3 bits.[1]
Show the answer and mark scheme
(a) Answer: The smallest single-coloured element (dot) of a bitmap image.
  • the smallest element of a bitmap image / a single point (dot) of one colour in the image
(b) Answer: The number of bits used for each pixel's colour.
  • the number of bits used to store the colour of each pixel
(c) Answer: 8
  • 8 (23)
Question 2Medium4 marks
A photo-editing app allows a user to change the colour depth and the resolution of a bitmap image.
(a) Explain the effect of reducing the colour depth of a photo from 24 bits to 8 bits.[2]
(b) Explain the effect of increasing the resolution of the photo.[2]
Show the answer and mark scheme
(a) Answer: Only 256 colours instead of over 16 million, so the photo looks less realistic, but the file is a third of the size.
  • fewer colours can be represented (256 instead of 16,777,216), so the photo looks less realistic / colours may appear as bands
  • the file size is reduced (to a third of the original)
(b) Answer: More pixels give more detail and a sharper image, but the file size increases.
  • there are more pixels, so more detail can be shown / the image is sharper / less pixelated
  • more pixels must be stored, so the file size increases
Question 3Hard5 marks
A game uses a small sprite that is 4 pixels wide and 3 pixels high with a colour depth of 2 bits. The colour codes are: 00 = white, 01 = red, 10 = blue and 11 = black. The sprite is stored row by row, starting with the top row:
Row 1: 00 01 01 00
Row 2: 10 11 11 10
Row 3: 00 01 01 00
(a) State the colour of the pixel in row 2, column 1.[1]
(b) State the number of bits needed to store the sprite.[1]
(c) The designer wants to add green to the sprite's colours.
Explain why the colour depth must increase, and state the new colour depth needed.[2]
(d) State the increase in the size of the sprite, in bits, when the new colour depth is used.[1]
Show the answer and mark scheme
(a) Answer: blue
  • blue
(b) Answer: 24 bits
  • 24 (12 pixels × 2 bits)
(c) Answer: All 4 two-bit codes are in use, so a fifth colour needs 3 bits (up to 8 colours).
  • 2 bits give only 4 codes / colours, and all four are already used
  • a fifth colour needs a colour depth of 3 bits (23 = 8 colours)
(d) Answer: 12 bits
  • 12 (12 pixels × 1 extra bit)

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