Two fundamentally different ideas
Lossless compression is packing: it finds repetition and stores it more cleverly, like writing "50 blue pixels" instead of listing blue fifty times. Unpack it and you get every original pixel back, bit-for-bit. PNG works this way. So does ZIP.
Lossy compression is triage: it ranks image information by how visible it is to human eyes and permanently discards the least visible part. JPG, lossy WebP, AVIF and HEIC all do this. The discarded data is unrecoverable — and that's fine, because it was chosen precisely for being imperceptible.
| Lossless (PNG, WebP-lossless) | Lossy (JPG, WebP, AVIF, HEIC) | |
|---|---|---|
| Pixels after decode | Identical to original | Approximation (tuned to look identical) |
| Typical photo savings | 10–30% | 60–95% |
| Quality dial | None — always perfect | Yes — you choose the trade-off |
| Repeated re-saving | Safe forever | Degrades each generation |
| Best for | Masters, graphics, text, archives | Photos being viewed or shared |
Why throwing data away is usually right
Human vision has well-mapped blind spots: we resolve brightness far better than color, notice smooth areas more than busy ones, and can't perceive high-frequency texture beyond a threshold. Lossy encoders are built directly on those blind spots — JPG, for example, stores color at half resolution before doing anything else, and you have never noticed.
That is why a 6 MB photo can become 500 KB with no visible change: the discarded 92% was mostly imperceptible detail and sensor noise. Keeping it has a real cost (bandwidth, storage, load time) and zero visible benefit.
The trap: generation loss
The one real danger of lossy formats is re-encoding the same image repeatedly. Each save discards a little more; ten rounds of edit-save-edit-save produce visible mush. The workflow that avoids it:
- Keep one master — the camera original, RAW, or a PNG export.
- Edit from the master, never from a previous lossy export.
- Export lossy once, at the end, sized for the destination.
Which should you use?
Photos being shared or displayed → lossy, quality 80–85. Logos, screenshots, text → lossless (or high-quality WebP). Archival masters → lossless, always. When a hard size limit rules (forms, portals), lossy is the only tool that can hit an exact number — that's what our compress-to-size presets automate.