 Centered chroma

Centered chroma, also called interstitially sited chroma, places colour-difference samples between the corresponding luma 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.^[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 givesThe 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.^[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.