Lossless vs lossy: keep everything, or keep enough

4 minAS Level1.3.2

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end you can define lossless and lossy compression, explain the difference, and say which type common file formats use.

Two ways to pack a suitcase

There are only two ways to make a file smaller. Pack it more cleverly, so that everything still comes out at the other end. Or leave some things behind that you've decided nobody will miss.

The first is lossless compression. The second is lossy.

Definition

Lossless compression

Compression that reduces the file size without losing any data. When the file is decompressed, the original is reconstructed exactly, bit for bit.

Definition

Lossy compression

Compression that reduces the file size by permanently removing some data, usually detail that is unnecessary or unlikely to be noticed. The original file cannot be reconstructed exactly.

Lossless gives back exactly what went in. Lossy gives back something close, and a much smaller file.

Quick check

Multiple choiceQuestion 1 · Recall

Which statement describes lossless compression?

AThe file is made smaller, and decompressing it reconstructs the original exactly.BData that people are unlikely to notice is permanently removed.CThe file stays the same size but is stored more securely.DOnly the less important data is lost, so the original can still be rebuilt.

One-line answerQuestion 2 · Recall

Name the type of compression in which some data is permanently removed, so the original file cannot be reconstructed exactly.

Answer the 2 questions above to continue.

Previous lessonWhy compress? Smaller files, faster everything Next lessonLossy or lossless? Justify your choice