How RLE Works:

Replace repeated values with a count + value. Instead of "AAAAAABBCC", store "6A2B2C".

Example 1: Simple Pattern

  Original: WWWWWWBBBWWW (12 characters)
  RLE:      6W3B3W (6 characters)
  
  Saved: 50% compression ratio

Example 2: Binary Data

  Original: 00000000111111110000000 (23 bits)
  RLE:      8×0, 8×1, 7×0 (3 pairs)
  
  Stored as: 8,0,8,1,7,0

Example 3: Image Data (Scanline)

  Image row (colors): RRRRRRGGGGBBBBBB (16 pixels)
  RLE:                6R4G6B (3 pairs)
  
  Original: 16 pixels × 3 bytes = 48 bytes
  RLE:      3 pairs × (count + color) = 6 bytes
  Saved:    87.5%!

When RLE Works Well:

Large blocks of same color: Simple graphics, logos, cartoons
Black and white images: Scanned documents, faxes
Video frames: Large unchanged areas between frames

When RLE Fails:

No repetition: ABCDEFGH → 1A1B1C1D1E1F1G1H (BIGGER!)
Random data: Photos, complex images - little to no repetition
Can make file larger if there's no pattern

This key facts covers Run-Length Encoding (RLE) - Simple Lossless within Compression for GCSE Computer Science. Revise Compression in 3.3 Data Representation for GCSE Computer Science with 16 exam-style questions and 16 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 4 of 12 in this topic. Use this key facts to connect the idea to the wider topic before moving on to questions and flashcards.

Practice questions for Compression

Which statement correctly describes lossy compression?

A. The original file can be perfectly restored after decompression.
B. Data is permanently removed and the original cannot be exactly recreated.
C. The compressed file is always the same size as the original.
D. No data is removed during the compression process.

1 markfoundation

Explain how run-length encoding (RLE) works to compress data.

3 marksstandard

Run-Length Encoding (RLE) - Simple Lossless

Practice questions for Compression

Quick recall flashcards

16 questions on Compression: practise free