Cambridge O Level Computer Science · Syllabus 2210 · Data Representation
Lossless Compression
What is Lossless Compression?
Lossless compression reduces the file size without permanently losing any data, so the original file can be reconstructed exactly; run-length encoding is an example.
This definition is part of the Data Representation chapter in Cambridge O Level Computer Science.
Lossless Compression in context
Lossy compression reduces the file size by permanently removing data. The original cannot be reconstructed exactly, and quality may fall — the more the file is compressed, the more noticeable that loss becomes. Lossless compression reduces the file size without permanently losing any data, so the original can be reconstructed exactly. Run-length encoding is the syllabus example of a lossless method.
Common mistakes with Lossless Compression
- M21. "Lossless compression always produces a smaller file, and RLE works equally well on all data." Why it fails Lossless methods exploit patterns. Where there is no pattern there is nothing to exploit, and RLE in particular stores two items for every run — so data with no repetition gets bigger. Correct model RLE is most effective on data containing long runs of identical values, such as a two-colour logo with large flat areas. On a detailed photograph the runs are one pixel long and the encoded form can exceed the original. Exam-safe "Run-length encoding reduces file size only when the data contains long runs of repeated values; with little repetition each single value is stored as a pair, so the file may not shrink and could grow." Test yourselfHideQ. Apply RLE to ABCDEF and count the items.A. 1A 1B 1C 1D 1E 1F — 6 pairs = 12 items, against 6 originally. It has doubled.
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