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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.

A chroma sample location specifies where a colour-difference sample is positioned relative to the luma sampling grid. The counts in 4:2:2 chroma subsampling or 4:2:0 chroma subsampling do not fully specify those positions. Co-sited chroma shares selected luma positions; centered chroma lies between them.^[ref]

Use an original one-dimensional coordinate system with luma samples at integers. A colour-difference grid at $x=0,2,4,\dots$ is co-sited with every second luma sample. The equally dense grid at $x=0.5,2.5,4.5,\dots$ is centered between successive pairs. Both contain half as many horizontal samples, but they have different offsets.

Suppose the first two stored component codes are 20 and 100. At the luma position $x=1$, linear interpolation on the first grid givesOn the second grid it givesThe same stored array and interpolation rule yield different reconstructed values because the sample coordinates differ. An incorrect location convention can therefore shift colour relative to luma without corrupting the byte buffer.

Horizontal and vertical choices also need not match. The Linux documentation illustrates 4:2:0 with horizontally centered and vertically co-sited colour-difference samples.^[ref] By contrast, the original grid diagram in 4:2:0 chroma subsampling deliberately chooses centering in both directions. That is an illustration, not a claim that every format follows it.

A framebuffer stride locates rows in computer memory; it does not identify the physical sampling offset. A decoder performing chroma reconstruction therefore needs both the storage layout and the sample-location convention.

Co-sited chroma places a retained colour-difference sample at the same spatial coordinate as a selected luma sample along the relevant axis. With horizontal reduction by two, an illustrative co-sited grid occupies $x=0,2,4,\dots$ when luma samples occupy all integer positions. The Linux video documentation distinguishes this from centered chroma.^[ref]

Co-siting does not require simply discarding alternate original samples. An original symmetric three-tap filter can produceFor 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.