What the codec is actually doing
The accepted answer set it out directly: video is not stored as red, green and blue at full resolution. Brightness is kept at full resolution and colour is reduced — commonly to a half or a quarter — so that a block of pixels may share a single colour value while each keeps its own brightness. It is a large saving in data for a small loss in apparent quality, and it works because human vision is much more discriminating about light and dark than about hue.
Why red in particular
- Brightness is built mostly from green, with red contributing less and blue least. One answer gave the analogue-era weightings as roughly 59 per cent green, 30 per cent red and 11 per cent blue.
- That leaves a saturated red area carrying most of its information in the channel that was thrown away, and very little in the one that was kept at full resolution.
- The same answer noted red degrades first as a signal deteriorates, showing up as noise and smearing before other colours do.
Where the thread disagreed
- One answer disputed the premise, saying they had never observed red breaking up more than any other colour, and suggested the asker was seeing general compression artefacts.
- Another put it down to bright areas against a dark background, and to the encoder using fixed block sizes without subdividing them at edges — either because the bit rate did not allow it or the option was not enabled.
- A fourth said the problem is specifically highly saturated red, and treated it as a known constraint rather than a fault.
What to do about it
- Bring saturated reds down. The broadcast practice described was reducing the luma and chroma values of strong reds — graphics especially — until they sit perceptually alongside everything else, rather than designing to a pure red in the first place.
- Where the delivery format allows it, less aggressive colour subsampling keeps more of the information that reds depend on.
- Bit rate matters. Two answers pointed at insufficient bandwidth as the reason blocking appears where it otherwise would not.
- The thread is from 2012 and its examples are of their time, but subsampling is still how consumer delivery formats work, which is why the effect has not gone away.