Why JPEG produces artifacts
JPEG splits an image into small blocks and compresses each one using frequency data. When bits are dropped, the reconstruction of a block is approximate, and the seams between blocks can become visible.
Edges make the problem obvious. Rapid changes in brightness create ringing, the faint ripples that outline a hard edge, and smooth gradients pick up mottled patches where the encoder ran out of detail.
How to spot artifacts
Zoom to full size and inspect areas that should be clean: blue skies, white walls, lettering, and the borders of objects. Blocky squares and colored blotches are the usual giveaways.
Repeated saving makes them worse. Each pass recompresses the previous result, so artifacts compound even when the quality setting never changes.
- Visible square blocks in flat areas
- Halos or ripples around sharp edges
- Color fringes in high-contrast detail
- Soft, smudged text after several saves
Preventing and reducing artifacts
Compress once from the original and stop when the image still looks clean at its final size. Avoid round-tripping a JPEG through more editors than necessary.
Filekind compresses from a JPEG or PNG original to a JPEG at or below your target, and shows the result before download so you can raise the limit if artifacts appear. Everything runs in your browser. Starting from the cleanest available original keeps the visible damage to a single pass of compression.