Lossy
/ˈlɔːsi/ · adjective · compression
Compression that discards detail you’re unlikely to notice. JPG is lossy.
In more detail
Lossy compression achieves much smaller files by permanently throwing away detail your eye is least likely to miss. JPG, WebP, HEIC, and AVIF are all lossy by default.
Because the discarded data can’t be recovered, repeated re-encoding compounds the loss — always keep an original.
What lossy compression removes
A lossy encoder transforms the source into components that can be prioritised, then quantises or discards information judged less noticeable. In a photo that may simplify fine texture and subtle colour changes; in audio it may remove frequencies masked by louder sounds.
At sensible settings the result can look or sound extremely close to the source. At aggressive settings, common artefacts include blocky image edges, colour banding, mosquito noise, smeared video detail, metallic audio, or watery high frequencies.
Using lossy formats safely
Lossy formats are appropriate for delivery: web photographs, streaming media, email attachments, and other cases where a smaller file improves speed or compatibility. Export from the best available original and choose the lowest size that still looks or sounds good in the real destination.
Do not repeatedly open and save a lossy file as another lossy file. Each generation makes new decisions using already simplified data, so damage accumulates. Preserve a lossless or highest-quality master for future edits and create fresh delivery copies from it.
Key points
- Lossy compression permanently removes selected source information.
- Quality settings trade file size against the visibility of compression artefacts.
- Repeated lossy encoding causes generation loss; export again from the master instead.