Color, Chroma Subsampling and HDR
Definition
The way a pixel's colour is encoded determines both the colour range of the image and the bandwidth it needs. The colour space defines the range of colours (gamut) a signal can represent, through its primaries and white point. HD video uses ITU-R BT.709; UHD and wide-gamut production use ITU-R BT.2020, whose gamut is much larger. A BT.2020 signal shown without conversion on a display that expects BT.709 looks desaturated and flat, which is the usual symptom of HDR material passed through an SDR chain. The reverse error, BT.709 content interpreted as BT.2020, looks oversaturated.
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The guide
Chroma subsampling relies on the lower sensitivity of human vision to colour detail than to brightness detail. The signal keeps the luma (Y') of every pixel and shares the colour-difference components (Cb and Cr) between neighbouring pixels. 4:4:4 keeps full colour resolution, 4:2:2 halves it horizontally, and 4:2:0 halves it both horizontally and vertically. The loss appears in fine coloured detail, so text, graphics and chroma keying are usually handled in 4:4:4 or 4:2:2 despite the extra bandwidth.
Bit depth sets the number of levels per component: 256 in 8-bit and 1024 in 10-bit. The additional levels keep smooth gradients, such as a clear sky, from breaking into visible bands. HDR (high dynamic range), defined for television in ITU-R BT.2100, uses the BT.2020 primaries with 10 or 12 bits and one of two transfer functions, PQ (standardised as SMPTE ST 2084) or HLG (hybrid log-gamma). Every device in the chain has to agree on colour space, subsampling, bit depth and transfer function, and colour errors usually appear at an uncontrolled conversion between them.
The rule
Colour space, subsampling, bit depth and transfer function stay the same from source to display.
Glossary terms
Sources
- ITU-R BT.709 : paramètres de la TVHD, primaires BT.709 · itu.int
- ITU-R BT.2020 : paramètres de l'UHDTV, gamut étendu, 10 et 12 bits · itu.int
- ITU-R BT.2100, HDR : fonctions de transfert PQ et HLG, primaires BT.2020, 10 ou 12 bits · itu.int
- Rec. 2100 (Wikipédia, pointeur secondaire) : PQ normalisée en SMPTE ST 2084, HDR10 et HLG10 · en.wikipedia.org
- Chroma subsampling (Wikipédia, pointeur secondaire) : schémas 4:4:4, 4:2:2 et 4:2:0 · en.wikipedia.org