When people say a picture or song has been “compressed”, they usually mean one of two very different things. In one case, nothing is lost and the original can be rebuilt perfectly. In the other, some information is thrown away for good in return for a much smaller file. Understanding the difference helps you choose formats, avoid damage and stop worrying about the wrong things.
Lossless: like a zip file
Lossless compression works by finding clever shorthand. If a picture has a thousand identical blue pixels in a row, the file can say “blue, one thousand times” instead of listing each one. When you open the file, the original is rebuilt exactly, pixel for pixel. PNG images and WAV or FLAC audio work this way. You can save them again and again without any damage, but the files stay relatively large.
Lossy: like a summary
Lossy compression decides what the human eye or ear is unlikely to notice and removes it. JPG drops tiny colour variations and fine texture that most people overlook. MP3 removes quiet sounds that are masked by louder ones. The file becomes much smaller, often ten times or more, but the discarded information cannot be recovered. Think of it as a summary of a book: shorter and often good enough, but you cannot rebuild the full text from it.
Where the damage shows up
Lossy artefacts have recognisable looks. In JPG, you see blocky squares in smooth areas such as skies, fuzzy halos around text, and a muddy look in fine textures like hair or grass. In MP3, low bitrates cause a watery, swishy sound in cymbals and a dull top end. The artefacts appear when quality is set too low, and they grow with every re-save.
Generation loss: the quiet problem
Every time you open a lossy file, edit it and save it as lossy again, a new round of loss begins. A picture saved ten times as JPG looks noticeably worse than one saved once. People often meet this with images that have been shared and re-shared through messaging apps, because each app recompresses them. That is why a screenshot of a screenshot looks so poor. The fix is to keep an original and always work from it.
Which one should you pick?
- Photos for sharing and the web: lossy (JPG or WebP). The savings are large and the damage is invisible at sensible quality.
- Screenshots, logos and diagrams: lossless (PNG). Sharp edges and text stay clean.
- Editing and archiving: lossless. Convert to PNG while you work, and export the lossy version at the end.
- Music for a phone: MP3 at 192 kbps or more. For editing or archives, use WAV.
Is lossy always bad?
Not at all. Lossy compression is the reason the internet works as well as it does. Without it, a web page of photos would take minutes to load, and a music library would fill a phone several times over. The skill is to use enough quality that the loss is invisible, and no more. For a photo viewed on a screen, 80 to 85 percent JPG quality is usually indistinguishable from the original for nearly everyone.
Can you turn lossy into lossless?
You can change the file type, but not the history. Converting a JPG to PNG makes a lossless container around a picture that has already lost detail. It stops further damage, which is useful, and it does not restore anything. The same applies to audio: turning an MP3 into a WAV makes a larger file with the same sound.
A simple test you can try
Take one photo and save it as JPG at 95, 80, 50 and 20 percent. Zoom in to 200 percent on an area with text or a sharp edge. At 95 and 80 you will struggle to see a difference. At 50, small blocks start to show. At 20, the damage is obvious. Doing this once gives you a feel for quality settings that no article can match.
What about WebP?
WebP is unusual because it can work in either mode. A lossy WebP behaves like a JPG with a smaller file, and a lossless WebP behaves like a PNG, also smaller. That flexibility is why websites like it. Remember that the mode is chosen when the file is made, so two files that both end in .webp can behave very differently. If you cannot tell which kind you have, treat it as lossy and avoid editing it repeatedly.
Tools for each job
When you want to lock in a picture before editing, use the JPG to PNG converter. When you want a light copy for sharing, use the PNG to JPG converter and choose a quality that suits the use. Both work in your browser and leave the original untouched.
The takeaway
Lossless means exact, and lossy means smaller. Use lossless where you edit or where sharp detail matters, use lossy where size matters and the content is photographic, and keep an original so you can start fresh when you need to.