= Centered chroma
= Centred chroma
{synonym}
= Interstitially sited chroma
{synonym}
\b[Centered chroma], also called <interstitially sited chroma>, places colour-difference samples between the corresponding <luma (video)> sample positions along an axis being subsampled. The term <centred chroma> is the alternative spelling. A simple two-sample average illustrates how this location arises.<YUV Formats in the Linux Kernel documentation>[§2.7.1, centered/interstitial chroma]{ref}
Let the luma-grid coordinate be an integer $n$, and consider an original colour-difference ramp $c(n)=a+bn$. Averaging consecutive samples before keeping one result per pair gives
$$
d_k=\frac{c(2k)+c(2k+1)}2
=a+b\left(2k+\frac12\right).
$$
The output exactly equals the ramp at $x=2k+1/2$. Assigning that average to $x=2k$ would shift its interpretation by half a luma-sample spacing. For $c(n)=40+20n$, the first averages are50,90 and130 at positions0.5,2.5 and4.5.
The location describes the sampling geometry, not an obligation to use only this averaging filter. A more elaborate filter can have the same effective alignment. Nor does horizontal centering force vertical centering: the Linux documentation includes a <4:2:0 chroma subsampling> example with horizontal centering and vertical co-siting.<YUV Formats in the Linux Kernel documentation>[4:2:0 example]{ref}
A decoder performing <chroma upsampling> must interpret those offsets consistently. Treating centered samples as <co-sited chroma> can displace reconstructed colour relative to luma while preserving every stored byte. A <pixel format>'s plane arrangement and <framebuffer stride> describe memory addressing; they do not, by themselves, establish the sample coordinates.
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