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Starting from a RGB image (or from a video streaming) I know how it is possible to obtain other image/video formats (for example YCrCb 4:2:2 or 4:2:0). I know the relation between the RGB pixels and the YCrCb and I know how to subsample in order to obtain a 4:2:2 or 4:2:0. The question is: why this notation? Where does this notation come from? What do the numbers mean?

Leos313
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    See https://en.wikipedia.org/wiki/Chroma_subsampling, "Sampling systems and ratios" section - it's explanatory enough. To put it short, it's J:a:b:A, where J means the length of a 2-row region to examine, a means the number of chroma samples in the first row of the region, b indicates vertical alteration, A means alpha presence. – kikobyte Jan 12 '17 at 15:58
  • Nice! The wiki page has a great graphical explanation as well! Thank for the suggestion – Leos313 Jan 13 '17 at 09:25

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These numbers mean the ratio of luminance (Y) to chrominance (Cr,Cb) values used in representation. Often number of bytes for chrominance are reduce in order to reduce the size of the image.

4:4:4 means you transmit a Y, a Cr, and a Cb value for each pixel. 4:2:2 means that you transmit a Y value for each pixel, but you transmit Cr and Cb values once for every to rows of the image. 4:2:0 means you transmit a Y value for each pixel but you downsample the Cr and Cb by 2 (i.e. you send one Cr and one Cb for every 2x2 block).

Alper Kucukkomurler
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