Lossy vs Lossless Image Compression

Compression makes a file smaller by storing the same picture with fewer bits. There are two families, and the difference decides which formats and settings make sense.

Lossless

The original pixels can be reconstructed exactly. It works by finding redundancy — runs of identical colour, repeated patterns — and encoding it compactly. PNG, lossless WebP and GIF are lossless. Great for graphics; limited savings on photos, where little is truly repeated.

Lossy

Some detail is permanently thrown away, chosen to be what the eye is least sensitive to: fine high-frequency texture and subtle colour shifts. JPEG and lossy WebP work this way. The "quality" slider controls how aggressive the discarding is.

The re-save trap

Because lossy compression is destructive, every time you open a JPEG and save it again, a little more is lost — a generation-loss effect. Always compress from the highest-quality original you have, once. Tools like the JPG Compressor show the before and after size so you can find the point of diminishing returns.

Which to choose

Lossless for anything with text, sharp edges or that will be edited further. Lossy for final-delivery photographs, where a 5× smaller file for an invisible quality cost is an easy trade.