How are bitmap images stored in binary, and how do resolution and colour depth affect file size?
Representing bitmap images as pixels, the meaning of resolution and colour depth, calculating the file size of an image, and the role of metadata.
A focused answer to the WJEC GCSE Computer Science Unit 1 content on representing images, covering bitmap images and pixels, resolution, colour depth and the number of colours, calculating image file size from dimensions and colour depth, and the role of metadata.
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What this topic is asking
WJEC wants you to know how a bitmap image is stored as pixels, what resolution and colour depth mean, how to calculate an image's file size, and what metadata is. This is part of the Data representation and data types content in Unit 1 of WJEC GCSE Computer Science (3500).
Bitmap images and pixels
Resolution
Colour depth
Calculating file size
The uncompressed file size of a bitmap, in bits, is the number of pixels multiplied by the colour depth.
Metadata
Try this
Q1. An image uses a colour depth of bits per pixel. How many different colours can it show? [1 mark]
- Cue. colours.
Q2. A black-and-white image is pixels by pixels with a colour depth of bit. Calculate its size in bytes. [2 marks]
- Cue. bits bytes.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC-style Unit 13 marksAn image is pixels wide and pixels high and uses a colour depth of bits per pixel. Calculate the file size in bytes, ignoring metadata. Show your working.Show worked answer →
A Unit 1 file-size calculation. Total pixels (1 mark). Bits bits (1 mark). Bytes bytes (1 mark). Markers reward multiplying width by height by colour depth and converting bits to bytes. A common error is to forget to divide by , leaving the answer in bits, or to leave out the colour depth.
WJEC-style Unit 14 marksExplain how a bitmap image is stored, and describe how increasing the colour depth affects both the number of colours available and the file size.Show worked answer →
A Unit 1 explain question. A bitmap image is made up of a grid of small dots called pixels, and the colour of each pixel is stored as a binary number (1 mark for grid of pixels, 1 mark for each pixel stored as binary). The colour depth is the number of bits used per pixel, and the number of available colours is raised to the colour depth, so increasing the colour depth increases the number of colours that can be shown (1 mark). However, more bits per pixel means each pixel needs more storage, so a higher colour depth increases the file size (1 mark). Markers reward the pixel grid, the link between colour depth and number of colours () and the effect on file size. A common error is to confuse colour depth with resolution.
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