Term: Chroma sampling
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Term: Chroma sampling
- Definition:
Because the human eye is less sensitive to color than to brightness, video codecs default to encoding the luma plane (Y') at full resolution, while using half or even quarter resolution for the chroma planes (Cb', Cr'). While slightly sacrificing video quality, this will result in data reduction and contributes to smaller file sizes.
The subsampling scheme is commonly expressed as a three-part ratio J:a:b (e.g. 4:2:2) or four parts, if alpha channel is present (e.g. 4:2:2:4). The parts are (in their respective order):
- J: horizontal sampling reference (width of the conceptual region). Usually, value = 4.
- a: number of chrominance samples (Cr, Cb) in the first row of J pixels. Common values = 4, 2, 1
- b: number of changes of chrominance samples (Cr, Cb) between first and second row of J pixels. b is usually either zero or equal to a (except in rare irregular cases like 4:4:1 and 4:2:1, which do not follow this convention). Common values = 4, 2, 1, 0
- Alpha: horizontal factor (relative to first digit). May be omitted if alpha component is not present, and is equal to J when present.
Higher chroma sampling values can contribute to reduced file sizes but can also can lead to visible artifacts, such as color bleeding and loss of detail in the color channels.
- Category:
- Image (still) and Moving Image (video and film)
- Resource:
- FADGI\'s Significant Properties for Digital Video
https://www.digitizationguidelines.gov/guidelines/sigpropvideo.html - FFmpeg definition of chroma subsampling
https://en.wikipedia.org/wiki/Chroma_subsampling - Wikipedia for explanation of values
https://en.wikipedia.org/wiki/Chroma_subsampling - See also:
- Chromaticity