Wirebook › Glossary › Compression artefact
What are compression artefacts (why videos go blocky)?
In one sentence: The blocks, smudges and fuzzy halos you see when a picture or video has been squished too hard and detail was thrown away.
A little more
A compression artefact is a mistake you can see because a picture or video was squished hard. The common ones are blocking (the 8x8 squares the squisher works in start to show), mosquito noise (fuzzy, buzzing halos around sharp edges), banding (smooth skies turning into stripes of colour) and smearing in fast movement.
They appear when there aren't enough bits for the picture: a low bitrate, a slow connection, a dark or busy scene, or a video that has been re-squished many times as it was forwarded. Making the file bigger again doesn't remove them, because the detail was thrown away; the fix is to squish the original more gently, or lower the resolution so each pixel gets more bits.
See it in action
- BonusWhat happens when you squish a video too much?Squish a real (tiny) picture the way phones squish video: watch thin whiskers crumble first, then 8x8 blocks appear, then a splat.10 min · interactive
- BonusWhy does a video call go blurry when the connection is slow?Dial a pretend video call's bandwidth thinner, spend a tiny 10-bit budget on eyes vs mouth vs wallpaper to still read a facial expression, then reveal the clear face.7 min · interactive
- BonusCan a blocky picture be made sharp again?Guess a mystery blob made of just 16 squares, buy back real patches of detail with five coins, then see the original.7 min · interactive
Do it for real
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