What is the difference between lossy and lossless compression?

Lossy compression permanently discards some data to achieve much smaller files (JPEG, AVIF, WebP lossy). Lossless compression discards nothing — the original is reconstructed exactly — so files are larger but perfect (PNG, WebP lossless, OxiPNG optimization).

Yes, PNG is always lossless. Any tool that visibly degrades a PNG is converting it, not optimizing it. PNG optimization works by improving filters and DEFLATE compression, not by removing pixels.

Should I use lossless or lossy compression for web images?

Lossy for photographs (WebP or AVIF at quality 80 — visually identical, far smaller). Lossless for flat-color graphics, logos, screenshots and anything with text, where compression artifacts are obvious.

Lossless compression

Compression that shrinks a file without discarding any data — the original can be reconstructed bit for bit.

Definition

Lossless compression reduces file size by encoding data more efficiently, while discarding nothing. The decompressed image is mathematically identical to the original — pixel for pixel. PNG and WebP's lossless mode are lossless formats; OxiPNG and zlib re-compression are lossless operations.

Lossless vs lossy

Lossy compression (JPEG, AVIF, WebP lossy) permanently discards data the eye is unlikely to notice — typically achieving far smaller files. Lossless compression never discards anything, so its savings are smaller but its fidelity is perfect. Use lossless for flat graphics, logos, screenshots and medical or technical imagery; lossy for photographs.

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