Chroma subsampling
Chroma subsampling is a video compression technique that stores color information at lower resolution than brightness, because human eyes notice brightness detail far more than color detail. Chroma subsampling is written as ratios such as 4:4:4 (full color), 4:2:2 or 4:2:0. Most streaming video uses 4:2:0, which keeps a quarter of the color samples.
Why it matters
Chroma subsampling removes a large share of the raw data before compression even starts, with little visible loss for normal viewing. That lowers delivery costs. It can hurt in specific cases, such as sharp colored text, screen recordings and green screen work, so production and delivery often use different settings.
How it works
- Each pixel's color is split into brightness (luma) and two color-difference values (chroma).
- Brightness is kept for every pixel, because eyes are most sensitive to it.
- Color values are shared across small groups of pixels, such as each 2 by 2 block for 4:2:0.
- On playback, the player stretches the shared color back across the group before showing the frame.
4:4:4 vs 4:2:2 vs 4:2:0: what is the difference?
The three ratios differ in how much color detail is kept. 4:4:4 keeps all color, 4:2:2 halves it horizontally and 4:2:0 halves it both horizontally and vertically. Streaming almost always uses 4:2:0, while editing and broadcast production often use 4:2:2 or 4:4:4.
Why does red text look blurry in my video?
Red or other saturated text often looks blurry because of 4:2:0 chroma subsampling. Color is stored at a quarter of the resolution, so thin colored edges get softened, while the brightness edges stay sharp. Using thicker fonts, adding outlines, or encoding screen recordings at 4:4:4 when the player supports it reduces the problem.
Where it's used
OTT services and short video apps deliver 4:2:0 video because it cuts data with little visible loss. Production teams keep fuller color while editing so green screen work stays clean.
By the numbers
- YouTube's recommended upload encoding settings specify 4:2:0 chroma subsampling. (YouTube Help: Recommended upload encoding settings)
- With 4:2:0 subsampling, each 2 by 2 block of pixels keeps one Cb and one Cr sample, halving total data compared with 4:4:4. (Wikipedia: Chroma subsampling)
