Image Compression Explained (for Non-Engineers)
Lossy throws away pixels you won't miss. Lossless keeps everything but compresses redundancy. Here's the rest.
Lossless vs lossy
Lossless compression rebuilds the exact original bit-for-bit. PNG and WebP-lossless work this way; they just store the same image more efficiently.
Lossy compression throws away data that the human eye is bad at perceiving. JPEG is the classic example — it drops high-frequency detail that we don't notice. The result is dramatically smaller files.
Why JPEG is so good for photos
Photos have lots of high-frequency noise (texture, grain) that's expensive to store and that we don't consciously perceive. JPEG identifies it and dumps it. PNG can't dump anything, so PNG photos are huge.
Why PNG is so good for screenshots
Screenshots have lots of flat color regions (UI panels, text backgrounds). PNG compresses runs of identical pixels into tiny encoded entries. JPEG would smear those flat regions with its frequency tricks; PNG keeps them crisp.
Where WebP fits in
WebP has both modes. Lossy WebP beats JPEG by 20-30% at the same visual quality. Lossless WebP beats PNG by 20-30% on most images. It's the modern default.
Frequently asked questions
Does compression reduce image quality?
Lossy compression reduces it slightly; well-tuned settings make the loss invisible. Lossless compression doesn't reduce quality at all.
Why does my compressed file look the same?
Because the compressor threw away only what your eyes are bad at seeing. That's the whole game.
Is it safe to share compressed photos?
Yes. Compression is universal and standard. Every viewer decodes it transparently.