YUV/YCbCr 420 means that there is one set of chroma samples (Cb+Cr) for each 2x2 block of luma samples (pixels). Often, the chroma samples fall on pixels in ev

YUV/YCbCr 420 means that there is one set of chroma samples (Cb+Cr) for each 2x2 block of luma samples (pixels). Often, the chroma samples fall on pixels in even rows and on even lines. So pixels in odd rows or on odd lines, have to borrow (interpolate) their chroma values from neighbouring pixels.