Chroma Subsampling

Chroma subsampling is a bandwidth-saving technique that reduces the resolution of color (chroma) information while keeping the brightness (luma) at full resolution, exploiting the human visual system's lower sensitivity to color detail than to luminance detail. The convention is written as J:a:b, where J is the number of luma samples per row in a reference block (always 4), a is the number of chroma samples in the top row, and b is the number of chroma samples in the bottom row. Common subsampling ratios are 4:4:4 (no subsampling, full color resolution, used in VFX and high-end finishing), 4:2:2 (half-horizontal chroma resolution, used in broadcast cameras, ProRes 422, DNxHR HQ and most mastering pipelines), and 4:2:0 (half-resolution chroma in both axes, used in H.264, H.265, AV1 distribution codecs and in consumer cameras). Subsampling matters for compositing and keying: 4:2:0 footage may chroma-key poorly due to soft color edges, and chroma-subsampled material can show color bleed on saturated edges (red text on green background, for example). Mastering and grading should always be done at 4:2:2 or 4:4:4 to preserve grading flexibility, with subsampling applied only at the final delivery encode. Reference: ITU-R BT.601 (where 4:2:0 originated) and the Wikipedia article on chroma subsampling for the geometric sampling diagrams.