Spotting Compression Artifacts: Blockiness, Banding and Halos
How to recognise the visual signs of over-compression and choose a quality setting that avoids them.
Heavy compression leaves telltale marks. Learning to spot them helps you pick a setting that saves space without spoiling the picture.
The common artifacts
| Artifact | What it looks like | Where it shows |
|---|---|---|
| Blockiness | visible squares, often 8 × 8 pixels | smooth areas, low quality JPEGs |
| Ringing or halos | faint outlines or ripples around sharp edges | text, high-contrast edges |
| Banding | visible stripes in a smooth gradient | skies, sunsets, dark backgrounds |
| Colour bleeding | colour smearing past edges | red and blue edges |
| Mosquito noise | shimmering speckles around edges | detailed edges |
| Loss of fine detail | smooth, plastic-looking textures | hair, foliage, fabric |
Why they happen
JPEG compression divides an image into small blocks and simplifies each one. At low quality, adjacent blocks simplify differently, and the seams show. Fine detail is discarded to save space.
How to check
- View at 100%. Zoom in to actual pixels, not the fit-to-screen view.
- Look at smooth areas, such as skies and skin.
- Look at edges, especially text and lines.
- Compare with the original side by side.
- View on the device where it will be shown.
Choosing a quality level
- High quality (about 85–95): artifacts rarely visible.
- Medium (about 70–85): usually fine for sharing; small artifacts possible in detail.
- Low (below about 60): artifacts obvious, often unacceptable.
If you see blockiness, raise the quality slightly, or reduce dimensions instead of lowering quality further.
Reduce dimensions versus reduce quality
For a size limit, resizing down often gives a better result than slashing quality, because fewer pixels means fewer to compress. A smaller image with moderate quality can look better than a large one at low quality.
Avoid double compression
Compressing an already compressed JPEG makes artifacts worse. Always go back to the original for a new version.
Which formats have fewer artifacts?
- PNG has none, being lossless, but files are larger.
- WebP and modern formats often look better than JPEG at the same size.
Screenshots
Text in a JPEG can develop halos. Use PNG for screenshots of text where possible, or high-quality JPEG.
Balancing act
Aim for the smallest file that looks fine at the size you'll use. A compression tool with an adjustable slider lets you find that point.
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