How Image Compression Affects Quality
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Lossy vs lossless compression, what the quality slider really does, the artifacts to look for, and why re-saving JPEGs repeatedly hurts.
Lossless vs lossy
Lossless compression (PNG, lossless WebP) stores data more efficiently without changing any pixel — decompress it and you get the original back exactly. Lossy compression (JPEG, lossy WebP, AVIF) permanently discards detail to make files much smaller.
What the quality slider does
In JPEG and WebP, the quality number (roughly 1–100) controls how aggressively fine detail is discarded. It is not a percentage of the original quality, and the same number doesn’t look the same across formats or encoders. File size usually drops quickly from 100 to around 80 with little visible change, then the savings shrink while artifacts grow.
Artifacts to look for
- Blocking: visible 8 × 8 pixel squares, especially in smooth areas.
- Ringing: halos and speckles around sharp edges and text.
- Banding: smooth gradients such as skies turning into visible steps.
- Color smearing: fine colored details bleeding, because color is stored at lower resolution than brightness.
Generation loss
Every time a lossy image is decoded and saved again, more detail is lost. Keep an original and always compress from it, rather than compressing an already-compressed copy.
Resizing is often better than lower quality
Halving an image’s width and height removes three-quarters of its pixels. An image at the right display size and quality 80 usually looks better — and is smaller — than a full-size image at quality 40.
The Image Compressor includes a before/after comparison slider so you can check for these artifacts before downloading.
