Chroma subsampling

Chroma subsampling is a compression technique that stores colour (chroma) at lower resolution than brightness (luma). It exploits a well-documented quirk of human vision: we’re far more sensitive to brightness contrast than to colour contrast at small scales.

The notation looks like 4:2:0, 4:2:2, or 4:4:4. The three numbers describe a 4-pixel-wide reference block: how many luma samples (always 4), how many chroma samples in the first row, how many in the second row.

• 4:4:4 — no subsampling. Full colour resolution. Used for graphics, screen captures, anywhere edges and text matter.
• 4:2:2 — chroma at half horizontal resolution. Used in professional video editing.
• 4:2:0 — chroma at quarter resolution (half horizontal, half vertical). Used by JPG, MPEG, H.264, H.265, most consumer video. Saves ~50% of the chroma data with almost no perceived quality loss for photographs.

Where 4:2:0 fails: sharp colour edges, especially text. Saturated red text on a saturated blue background gets visibly fuzzy. This is why screenshots should be PNG (no subsampling) and photographs can be JPG (4:2:0 invisible).