Lossless
/ˈlɔːsləs/ · adjective · compression
Compression that preserves every pixel exactly. PNG is lossless.
In more detail
Lossless compression shrinks a file while allowing it to be reconstructed bit-for-bit. Nothing is discarded, so quality is untouched.
PNG is the common lossless image format. The trade-off is that files are usually larger than a lossy equivalent.
How lossless compression works
Lossless encoders find repetition and predictable patterns, then describe them more efficiently without removing information. Decompression reverses those instructions to recover the original data exactly, much like extracting a ZIP archive.
PNG uses filters and general-purpose compression for pixel rows. FLAC predicts audio samples and stores the prediction error. The techniques differ, but the defining test is the same: decoded content can match the source exactly.
When lossless is the right choice
Choose lossless compression for logos, screenshots, diagrams, text-heavy graphics, intermediate editing files, source data, and anything that will be processed repeatedly. Sharp edges and repeated flat colours often compress well without introducing visible artefacts.
Lossless does not automatically mean small. Camera photos, noisy scans, and video contain complex variation that is difficult to describe compactly, so a lossless version may be many times larger than a good lossy copy. Keep a lossless master and create smaller delivery versions when appropriate.
Key points
- Lossless compression can reconstruct the encoded content exactly.
- It is ideal for masters, repeated editing, text, diagrams, and flat-colour graphics.
- A lossless file can still differ from an earlier source if it was created from a lossy copy.