Semiplanar pixel format

A semiplanar pixel format combines a separate sample plane with another plane containing interleaved components. A common YCbCr arrangement stores luma samples in one plane and pairs of colour-difference samples in another. It therefore has two planes while representing three components. Linux's video API distinguishes this from the three separate planes of a fully planar pixel format.^[ref]

In NV12, one byte represents each sample. Chroma subsampling halves the colour-difference sampling count in both directions, and the second plane stores Cb followed by Cr in each pair. NV21 reverses that pair order. For even width $W$ and height $H$, tightly stored active samples occupyThus a stipulated $4\times 2$ image contains eight Y samples and two Cb/Cr pairs: 12 active bytes. The eight Y bytes can be followed by Cb00 Cr00 Cb01 Cr01. The component names identify stored codes; their colour interpretation also requires the declared YCbCr conventions.^[ref]

Active sample count differs from allocated storage. With an eight-byte framebuffer stride for both planes in this example, two Y rows and one interleaved row occupy $2(8)+1(8)=24$ bytes including padding. A consumer needs dimensions, strides, component order and whether planes are adjacent or separately allocated. Reinterpreting NV21 as NV12 swaps the two colour-difference components; it does not recover the intended colours merely because the buffer size matches.