HLG: Hybrid Log-Gamma Transform

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Hybrid Log-Gamma Transform

Description

The Hybrid Log-Gamma OETF is a transfer function that allows for the display of high dynamic range (HDR) video. The version here is that supported by the ACES (Academy Color Encoding System) and HEVC (High Efficiency Video Coding) standards.
For use with ACES, a specialized HLG-based OOTF is provided that references the Perceptual Quality (PQ) EOTF. It converts the ST.2084 (PQ) output to HLG using the method specified in Section 7 of BT.2390-0.

Usage

HLG.OETF()
HLG.OOTF( gamma=1.2, Lb=0, Lw=1000 )

Arguments

gamma	the applied exponent, from scene linear to display linear
Lb	the luminance of black, in cd/m^2, or nit.
Lw	the luminance of white, in cd/m^2, or nit.

Details

HLG.OOTF() is 3D and does not operate on each channel independently. It uses a scaling factor based on these RGB weights - (0.2627,0.6780,0.0593) - from Section 7 of BT.2390.

Value

HLG.OETF() returns a univariate TransferFunction that maps linear scene RGB to signal display RGB. The interval [0,1] maps to [0,1] (as in the HVEC standard).
HLG.OOTF() returns a multivariate TransferFunction of dimension 3 that maps linear scene RGB to linear display RGB. It maps the cube [0,1]^3 to the cube [Lb,Lw]^3, but the image is only a proper subset of the cube.

References

ST-2084. SMPTE Standard - High Dynamic Range Electro-Optical Transfer Function of Mastering Reference Displays. 2014.

BT.2390. High dynamic range television for production and international programme exchange. April 2018.

H.265 : High Efficiency Video Coding. https://www.itu.int/rec/T-REC-H.265-201802-I/en. 2018-02-13.

See Also

TransferFunction, PQ.EOTF

spacesRGB documentation built on April 12, 2025, 1:55 a.m.