 NV21

NV21 is an eight-bit YCbCr semiplanar pixel format using 4:2:0 chroma subsampling. It has a full-resolution luma plane followed by interleaved colour-difference samples in Cr, Cb order. NV12 uses the reverse pair order, Cb, Cr. The separately allocated Linux variant is NV21M.^[ref]

An original $4\times 2$ tightly stored image has eight luma bytes followed by four colour-difference bytes. Suppose the latter are 200 40 150 90. Under NV21, the two pairs are $(C_r,C_b)=(200,40)$ and $(150,90)$. Reading the same bytes as NV12 instead assigns $(C_b,C_r)=(200,40)$ and $(150,90)$. The buffer length remains correct while its colour-difference channels are exchanged.

Converting this declared NV21 buffer to NV12 requires swapping adjacent bytes in the interleaved plane, giving 40 200 90 150, while retaining the luma data. This is a layout conversion, not a conversion to the RGB color model. The numerical colour interpretation still depends on the declared YCbCr encoding.

The active footprint is $3WH/2$ bytes for even width $W$ and height $H$ under eight-bit 4:2:0 sampling. Framebuffer stride can add padding, so the next row need not start immediately after the last active sample. A converter must apply the actual row and plane boundaries: swapping pairs across padding or across the luma plane would corrupt the image rather than exchange the intended channels.