Lossy
/ˈlɔːsi/ · adjective · compression
Lossy compression makes files much smaller by removing detail you are unlikely to notice. JPG is lossy.
In more detail
Lossy compression makes much smaller files by throwing away detail for good. It removes the detail your eyes or ears are least likely to miss. JPG, MP3, and most WebP, HEIC and AVIF files are lossy.
The removed data can't be recovered. Each new lossy save adds more loss, so always keep an original.
What lossy compression removes
A lossy encoder breaks the source into parts it can rank by importance. Then it rounds off or drops the parts people are less likely to notice. In a photo, that can mean simpler fine texture and color. In audio, it can mean sounds hidden behind louder ones.
At sensible settings, the result looks or sounds very close to the source. At aggressive settings you start to see and hear artifacts. Common ones are blocky edges, color banding, smeared video detail and a metallic or watery sound in audio.
Lossy vs lossless: when to use each
Lossy formats are for sharing and delivery. Think web photos, streaming, email attachments and other cases where a smaller file loads faster or fits a limit. Export from the best original you have. Pick the smallest size that still looks or sounds good where it will be used.
Don't keep opening a lossy file and saving it as another lossy file. Each round makes new cuts on data that was already simplified, so the damage adds up. Keep a lossless or top-quality master for future edits, and make fresh copies from it.
Key points
- Lossy compression removes part of the source data for good.
- The quality setting trades file size against visible or audible artifacts.
- Saving a lossy file again and again adds loss. Export from the master instead.