Sampling Systems and Ratios
The subsampling scheme is commonly expressed as a three part ratio J:a:b (e.g. 4:2:2), although sometimes expressed as four parts (e.g. 4:2:2:4), that describe the number of luminance and chrominance samples in a conceptual region that is J pixels wide, and 2 pixels high. The parts are (in their respective order):
- J: horizontal sampling reference (width of the conceptual region). Usually, 4.
- a: number of chrominance samples (Cr, Cb) in the first row of J pixels.
- b: number of (additional) chrominance samples (Cr, Cb) in the second row of J pixels.
- Alpha: horizontal factor (relative to first digit). May be omitted if alpha component is not present, and is equal to J when present.
An explanatory image of different chroma subsampling schemes can be seen at the following link: http://lea.hamradio.si/~s51kq/subsample.gif (source: "Basics of Video": http://lea.hamradio.si/~s51kq/V-BAS.HTM) or in details in Chrominance Subsampling in Digital Images, by Douglas Kerr.
| 4:1:1 | 4:2:0 | 4:2:2 | 4:4:4 | ||||||||||||||||||||
| Y'CrCb | |||||||||||||||||||||||
| = | = | = | = | ||||||||||||||||||||
| Y' | |||||||||||||||||||||||
| + | + | + | + | ||||||||||||||||||||
| 1 | 2 | 3 | 4 | J = 4 | 1 | 2 | 3 | 4 | J = 4 | 1 | 2 | 3 | 4 | J = 4 | 1 | 2 | 3 | 4 | J = 4 | ||||
| (Cr, Cb) | 1 | a = 1 | 1 | 2 | a = 2 | 1 | 2 | a = 2 | 1 | 2 | 3 | 4 | a = 4 | ||||||||||
| 1 | b = 1 | b = 0 | 1 | 2 | b = 2 | 1 | 2 | 3 | 4 | b = 4 | |||||||||||||
| ¼ horizontal resolution, full vertical resolution |
½ horizontal resolution, ½ vertical resolution |
½ horizontal resolution, full vertical resolution |
full horizontal resolution, full vertical resolution |
||||||||||||||||||||
The mapping examples given are only theoretical and for illustration. Also note that the diagram does not indicate any chroma filtering, which should be applied to avoid aliasing.
To calculate required bandwidth factor relative to 4:4:4 (or 4:4:4:4), one needs to sum all the factors and divide the result by 12 (or 16, if alpha is present).
Read more about this topic: Chroma Subsampling
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