Component video
From nonlinear R′G′B′ to Y′CbCr: luma versus luminance, BT.601/709/2020 matrices, Cb/Cr normalisation, narrow/full range, subsampling, chroma siting and common matrix errors.
Y′CbCr is not a colour space like Rec.709. It is a way to represent an already-defined nonlinear RGB signal as a luma-like Y′ plus two colour-difference components, which is useful for video coding and chroma subsampling.
Quick reference
For narrow-range digital representation.
Y′CbCr
The matrix is normally applied to nonlinear R′G′B′ values after OETF/transfer encoding, so the result is written Y′. CIE luminance Y is derived from linear tristimulus/light relationships. These quantities are related but not interchangeable. Saying 'the Y channel is luminance' is technically too crude.
First Y′ is formed as a weighted sum of R′G′B′. Cb stores a blue-difference component and Cr a red-difference component. In normalized form, it is useful to think of Cb=(B′−Y′)/(2−2Kb) and Cr=(R′−Y′)/(2−2Kr), where Kr/Kb depend on the standard. Neutral grey has Cb=Cr=0 before integer offsets are applied.
For HDTV BT.709: Y′=0.2126R′+0.7152G′+0.0722B′. Correspondingly Cb≈(B′−Y′)/1.8556 and Cr≈(R′−Y′)/1.5748. These coefficients are tied to BT.709 primaries/colorimetry and should not be applied automatically to an SD BT.601 signal.
Legacy SD BT.601 uses Kr=0.299, Kg=0.587 and Kb=0.114. Old SD video interpreted with a BT.709 matrix therefore acquires small but systematic hue/luma changes. This is one classic source of 'why did the old clip change after transcode?'
For BT.2020 NCL: Y′=0.2627R′+0.6780G′+0.0593B′, Cb=(B′−Y′)/1.8814 and Cr=(R′−Y′)/1.4746. The standard also defines a BT.2020 constant-luminance variant; it is a different representation with separate matrix_coefficients signalling and should not be confused with normal BT.2020 NCL.
In narrow-range video, Y′ normally occupies 16–235 in 8-bit or 64–940 in 10-bit. Chroma has a neutral offset: 128 in 8-bit / 512 in 10-bit, with nominal extremes of 16–240 / 64–960. This is why neutral chroma is not encoded as zero and why range conversion uses different nominal spans for Y′ and Cb/Cr.
Y′ carries much of the spatial detail while Cb/Cr carry colour-difference detail. Human vision is generally less sensitive to fine spatial chroma, so 4:2:2/4:2:0 reduce bandwidth with a smaller perceptual cost than an equivalent loss of luma resolution. The advantage becomes much less forgiving on graphics and keying.
In 4:2:0, chroma samples can be co-sited or centred relative to the luma grid depending on system/codec. H.26x/VUI metadata can signal location types. If an upsampler assumes a different phase, coloured edges shift and false colour/softness can appear on vertical or horizontal transitions.
YUV historically refers to analogue composite/component concepts, YPbPr to analogue component representation, and YCbCr to digital component coding. Everyday language often mixes them and pixel-format names use 'yuv' as an umbrella label, but technical descriptions should name the actual representation.
Wrong BT.601/709/2020 decoding rarely looks like a simple uniform tint. Channel relationships, saturation and luma contribution change, so skin and saturated colours shift differently. If the problem depends on object chroma while black/white range is normal, matrix mismatch is a more plausible hypothesis than a levels error.
Resolve normally decodes YCbCr through the codec/media pipeline into internal RGB processing. The user therefore depends on correct media tags, Data Levels and input colour space/transfer; a YCbCr matrix mismatch can happen before the nodes. With a suspicious transcode, compare source and decode and inspect stream matrix_coefficients instead of trying to 'restore the colour' manually with wheels.
To stay oriented, first name the source R′G′B′ colorimetry/transfer, then matrix_coefficients, then range. Only after those three questions do Y′CbCr numbers become unambiguous.
Technical definitions are anchored to standards bodies and primary specifications wherever that distinction matters.
HDTV luma/colour-difference matrix and quantisation.
Legacy SD luma weights 0.299/0.587/0.114 and digital component representation.
BT.2020 NCL/CL matrices and digital representation.
matrix_coefficients, video_full_range and colour-description code points.
