 Chroma subsampling

Chroma subsampling represents colour-difference components on a coarser spatial grid than luma. In YCbCr video, 4:4:4 chroma sampling keeps full sample counts for every component; 4:2:2 chroma subsampling halves the colour-difference count horizontally; 4:2:0 chroma subsampling halves it horizontally and vertically.^[ref]

For an original eight-bit $4\times 2$ image with no padding or alpha, full sampling uses eight samples in each of three components: 24 bytes. With horizontal halving, the count becomes $8+4+4=16$ bytes. Halving in both directions gives $8+2+2=12$ bytes. These count active samples, not the allocation overhead of a particular pixel format.

The reduction loses information unless the discarded variations are otherwise constrained. For a stipulated pair of neighbouring samples with constant luma, let one colour-difference channel have values $+0.1$ and $-0.1$. Averaging the pair and retaining only the average stores zero. Reconstructing both positions as zero cannot recover which one was originally positive. Fine colour variation can therefore disappear even though both luma samples remain intact.

A low-pass filter before reducing the sampling rate controls aliasing; merely dropping alternate samples need not. Later chroma reconstruction estimates the missing positions. It must respect chroma sample location: equal sample counts do not determine where those samples lie relative to the luma grid. The Microsoft primary guide explicitly distinguishes different 4:2:0 grids and discusses interpolation during format conversion.^[ref]

Storage organization is independent of this sampling ratio. NV12 interleaves the two reduced components in a second plane, while a fully planar pixel format can store them in separate planes. Rearranging the same samples changes layout, not sampling resolution.

Chroma subsampling is a technique used in video compression and image processing that reduces the amount of color information (chrominance) in an image while retaining the luminance information (brightness) relatively intact. This method exploits the human visual system's greater sensitivity to brightness (luminance) than to color (chrominance), allowing for a more efficient representation of images without a significant loss in perceived quality.