ITU-R Recommendation BT.2100, more commonly known by the abbreviations Rec. 2100 or BT.2100, introduced high-dynamic-range television (HDR-TV) by recommending the use of the perceptual quantizer (PQ [SMPTE ST 2084]) or hybrid log–gamma (HLG) transfer functions instead of the traditional "gamma" previously used for SDR-TV.[1] [2]
It defines various aspects of HDR-TV such as display resolution (HDTV and UHDTV), frame rate, chroma subsampling, bit depth, color space, color primaries, white point, and transfer function.[3] [4] It was posted on the International Telecommunication Union (ITU) website on July 4, 2016. Rec. 2100 uses a wide color gamut (WCG) which is the same as Rec. 2020's.
See also: Transfer functions in imaging. Rec. 2100 defines two sets of HDR transfer functions which are perceptual quantization (PQ) and hybrid log-gamma (HLG). HLG is supported in Rec. 2100 with a nominal peak luminance of 1,000 cd/m2 and a system gamma value that can be adjusted depending on background luminance. For a reference viewing environment the peak luminance of display should be 1000 cd/m2 or more for small area highlights and the black level should be 0.005 cd/m2 or less. The surround light should be 5 cd/m2 and be neutral grey at standard illuminant D65.
Within each set, the documented transfer functions include an:
Rec. 2100 uses the same color primaries as Rec. 2020 which is a Wide Color Gamut.
Rec. 2100 specifies three resolutions of 1920 × 1080 ("Full HD"), 3840 × 2160 ("4K UHD"), and 7680 × 4320 ("8K UHD"). These resolutions have an aspect ratio of and use square pixels.
Rec. 2100 specifies the following frame rates: 120p, 119.88p, 100p, 60p, 59.94p, 50p, 30p, 29.97p, 25p, 24p, 23.976p.[3] Only progressive scan frame rates are allowed.[3]
Rec. 2100 specifies a bit depth of either 10-bits per sample or 12-bits per sample, with either narrow range or full range color values. A future-use and intermediate linear RGB format using IEEE 16 bit floating point representation for each channel is also specified. For narrow range color, 10-bit per sample signals use video levels where the black level is defined as 64, achromatic gray level as 512 and the nominal peak as 940 in RGB, Y, and I encoding and 960 in Cb/Cr, and Ct/Cp component encoding. 12-bit per sample signals define 256 as the black level, 2048 as the gray level and the nominal peak is 3760 in RGB, Y, and I component encoding and 3840 in Cb/Cr, and Ct/Cp component encoding. Narrow range signals may extend below black or above peak white (super-black and super-white respectively), but must always be clipped to the signal range of 4-1019 for 10-bit signals or 16-4079 for 12-bit signals.[3]
Rec. 2100 specifies the use of RGB, YCbCr, and ICtCp. ICtCp provides an improved color representation that is designed for high dynamic range (HDR) and wide color gamut signals (WCG).[5]
Rec. 2100 allows for RGB, YCbCr, and ICtCp signal formats with 4:4:4, 4:2:2, and 4:2:0 chroma subsampling.[3] Rec. 2100 specifies that if a luma (Y') signal is made that it use the same matrix coefficients as Rec. 2020: 0.2627 for red, 0.678 for green, and 0.0593 for blue (derived from BT.2020 primaries and white point).[3]
Before Rec. BT.2020 the chroma sample location that was in use was center left. But in H.265 (2018-02) top-left chroma siting was mandated for BT.2020-2 and BT.2100-1, that must be described in VUI (video usability information) as such. First value of VUI should be 2 for top-left chroma and 0 for center left. Blu-ray also uses top-left chroma for HDR, including for Dolby Vision.