 Chroma reconstruction

Chroma reconstruction estimates colour-difference values on a desired spatial grid from the samples retained by chroma subsampling. In YCbCr video, chroma upsampling can bring the reduced Cb and Cr arrays onto the luma grid before a full-resolution conversion to RGB. The sampling ratio, chroma sample location and chosen interpolation rule all affect the result.^[ref]

For an original one-dimensional example, let a component have codes $c_0=40$ at $x_0=0$ and $c_1=120$ at $x_1=2$. Linear interpolation givesAt $x=0.5$, this gives 60, while nearest-neighbor interpolation selects 40. Both methods use the same retained samples; they make different assumptions about the values between them.

No reconstruction rule can infer every discarded variation from these two numbers alone. The functionsagree at zero and two, yet at $x=1$ they give 80 and 90. Thus the retained endpoints do not uniquely identify the interior signal. Recovery guarantees require additional restrictions on the signal and sampling process.

A practical reconstruction also specifies how to handle image edges, where a filter may ask for samples beyond the available array. The Microsoft example states its progressive-scan assumption and supplies separate edge equations; those details are part of the method rather than consequences of the ratio 4:2:0.^[ref] Increasing the number of output samples therefore does not, by itself, restore the original colour detail.