Chroma Subsampling (4:4:4 / 4:2:2 / 4:2:0)

The notation that says how much colour (chroma) is sampled relative to brightness (luma). 4:4:4: full colour, no subsampling — essential for computer text and fine graphics. 4:2:2: chroma halved horizontally, the broadcast standard, invisible on a moving picture. 4:2:0: chroma halved in both directions, used by streaming and by HDMI when bandwidth runs short, and it softens coloured edges. The eye is far less sensitive to colour resolution than to luma — which is exactly what the trick exploits.