= 4:2:2 chroma subsampling
\b[4:2:2 chroma subsampling] keeps the full <luma (video)> sample count while sampling each colour-difference component at half the horizontal rate and the full vertical rate. Thus a horizontal group with four Y samples has two Cb and two Cr samples.<Recommended 8-Bit YUV Formats for Video Rendering by Gary Sullivan and Stephen Estrop>[YUV Sampling,4:2:2 definition]{ref}
For an original sample-count example, let the grid be six positions wide and three rows high. It contains 18 Y samples and nine samples of each colour-difference component. At eight bits per sample, its active payload is
$$
18+9+9=36\ \mathrm{bytes}.
$$
The corresponding <4:4:4 chroma sampling> payload is 54 bytes. The reduction is one third, or from 24 to 16 <bits> per original <pixel>. These counts omit padding and any <alpha channel>; they describe a sampling grid rather than every storage API's alignment constraints.
Horizontal fine colour detail can be lost even though the vertical sample count is unchanged. If a colour-difference row alternates between codes 40 and 160 at every luma position, pair averaging gives three samples of 100 on the stipulated six-position row. Interpolating from those averages cannot identify the original alternation. A <low-pass filter> and an explicit <chroma sample location> describe how the reduction is performed; the ratio alone does not specify either.
For a grid whose colour samples share the luma row coordinates, <chroma reconstruction> needs to estimate missing horizontal positions without inventing a missing vertical row. A later conversion to <4:2:0 chroma subsampling> additionally reduces the colour sampling vertically. Rearranging the same 4:2:2 samples between <packed pixel formats> and <planar pixel formats> changes layout without making that further sampling reduction.
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