= Co-sited chroma
\b[Co-sited chroma] places a retained colour-difference sample at the same spatial coordinate as a selected <luma (video)> sample along the relevant axis. With horizontal reduction by two, an illustrative co-sited grid occupies $x=0,2,4,\ldots$ when luma samples occupy all integer positions. The Linux video documentation distinguishes this from <centered chroma>.<YUV Formats in the Linux Kernel documentation>[§2.7.1, co-sited chroma and odd-length filters]{ref}
Co-siting does not require simply discarding alternate original samples. An original symmetric three-tap filter can produce
$$
d_k=\frac{c(2k-1)+2c(2k)+c(2k+1)}4.
$$
For a linear ramp $c(n)=a+bn$, the terms simplify to $d_k=a+2bk=c(2k)$. This filter's output is therefore aligned with the middle luma position for the ramp, even though it combines neighbouring samples.
An alternating input shows why filtering matters. Let $c(n)=128+20(-1)^n$. Keeping only even-indexed samples without filtering gives the constant148, losing the alternating pattern through <aliasing>. The stated three-tap filter instead gives $(108+2(148)+108)/4=128$ at every retained position. This demonstrates suppression of this particular pattern; it does not prove rejection of every frequency that could alias.
Horizontal and vertical alignment are separate choices. A <4:2:0 chroma subsampling> grid can be co-sited vertically while centered horizontally. During <chroma reconstruction>, using the correct coordinates and an appropriate <low-pass filter> are related but distinct requirements: correct placement cannot undo detail already discarded during sampling.
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