Image Compressor

How lossy compression actually works

A lossy image codec does not store an image pixel-for-pixel. It transforms blocks of pixels into a frequency representation, keeps the parts of that representation the human eye is most sensitive to, and discards or coarsens the rest. The quality setting controls how aggressively that discarding happens: a high quality keeps most of the detail and produces a large file; a low quality throws more away and produces a small one, at the cost of visible artefacts once enough detail is gone. This is why the same slider produces very different results on different images — a busy, detailed photo has more high-frequency information to lose than a smooth gradient does.

Choosing a format

JPEG is the oldest and most universally supported lossy format, and remains a safe default when compatibility matters most. WebP generally reaches a smaller file than JPEG at equivalent visual quality, and adds transparency support that JPEG lacks. AVIF, based on a newer video codec, can compress further still at low-to-medium quality, though support for opening AVIF files is less universal than WebP's. PNG is lossless — it never discards detail — which is exactly why it is the right choice for flat-colour graphics and the wrong choice for photographs, where its files can run many times larger than a lossy equivalent.

Quality versus file size is not a straight line

Pushing the quality dial down does not save file size at a constant rate. Near the top of the range, small reductions in quality barely move the file size, because there is little redundant information left to remove. Further down, the same size reduction in the slider removes proportionally more, and visible artefacts appear more quickly. That is why a moderate setting around 75–85 tends to sit in the useful part of the curve for photographs: enough of the detail the codec would discard is genuinely redundant, so the size drops noticeably before quality loss becomes obvious.

When to resize instead of just compressing

Quality and resolution are two separate levers, and they are not interchangeable. Compressing harder keeps every pixel but represents each one less precisely; resizing removes pixels outright. When an image is going to be displayed smaller than its original size — a thumbnail, an inline web image, an email attachment — resizing down to roughly the size it will actually be shown at removes information that was never going to be visible anyway, before compression even gets involved. That combination, downscale first and then compress at a moderate quality, generally beats compressing the full-size original harder to hit the same target file size.