AQA · GCSE · Computer Science
Course version: 8525 · Version from Sept 2020
Represent data in RLE frequency/data pairs. Students could be given a…
Full specification wording
Data compression: Represent data in RLE frequency/data pairs. Students could be given a bitmap representation and they would be expected to show the frequency and value pairs for each row,
To encode a bitmap with RLE, work along each row from left to right.
Checked against the AQA 8525 specification (opens in a new tab) on .
- Point 12 of 13
Study notes
From the checked teaching notes for this specification point.
The idea
To encode a bitmap with RLE, work along each row from left to right. Count how many identical pixels occur in a run, write that count, then write the pixel value, and repeat for the next run until the row ends. For example, a row of five white then three black pixels becomes 5/white then 3/black. Do this separately for every row, because runs do not carry across a row boundary. The result is a list of frequency/data pairs per row that can be decoded back to the original bitmap exactly. Learners should practise reading a grid of pixels and producing the pairs, keeping the count and value in the correct order and starting a new count whenever the value changes.
What earns the marks
- Work along each row from left to right, grouping consecutive identical pixel values into runs.
- For each run, record the frequency (number of identical pixels) followed by the data value (the pixel colour).
- Start a new pair whenever the pixel value changes, so each pair covers one uninterrupted run.
- Treat each row separately; a run does not continue from the end of one row into the start of the next.
- The pairs must reconstruct the original bitmap exactly when decoded, confirming the encoding is correct.
Common mistakes and how to put them right
Continuing a run across a row boundary.
Put it right: Each row is encoded separately, so a run ends at the end of the row.
Reversing the order and writing the value before the count.
Put it right: Write the frequency first, then the value, for example 5/white.
Merging two different colours into one pair.
Put it right: A new pair begins whenever the pixel value changes.
Exam tips
- Count carefully along each row and double-check the total of all frequencies equals the number of pixels in that row.
- Keep the same order throughout (frequency then value) so the encoding is consistent and decodable.
- When checking your answer, decode your pairs back into pixels to confirm they match the original bitmap.
Try the lesson
The start of the Data compression: Represent data in RLE frequency/data pairs. Students could be given a bitmap representation and they would be expected to show the frequency and value pairs for each row, lesson, made from checked teaching notes for this specification point.
Have a go first
A single row of a black-and-white bitmap is 11110000. Using run length encoding, which frequency/data pair represents this row?
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