 NV21M

NV21M is a two-memory-plane semiplanar pixel format for eight-bit YCbCr with 4:2:0 chroma subsampling. Like NV21, its colour-difference plane stores Cr before Cb in each pair. Unlike ordinary contiguous NV21, the two image memory planes can have independent allocations.^[ref]

For an original example, stipulate an accepted width-four, height-four layout whose luma plane begins at 1000 with an eight-byte framebuffer stride, while its Cr/Cb plane begins at 5000 with a four-byte stride. Luma position $(3,3)$ is at $1000+3(8)+3=1027$. The corresponding subsampled cell $(1,1)$ has Cr at $5000+1(4)+2(1)=5006$ and Cb at 5007. This ordering is the reverse of NV12M.

The active sample count is $4(4)+2(2)(2)=24$ bytes. The declared allocation sizes are $4(8)+2(4)=40$ bytes because luma rows have padding. Neither 40 nor 24 is an offset that can replace the explicitly supplied second-plane base.

A layout conversion to NV12M can exchange the two bytes in each active colour-difference pair while respecting each plane's stride. It does not require changing luma values or reducing the colour sample count. By contrast, choosing a different YCbCr matrix would be a colour conversion.

The distinction between a multi-planar format and the V4L2 multi-planar API matters here: the interface can carry formats with different memory organizations, while the format identifies which organization the supplied descriptors represent.^[ref]