The hybrid log–gamma (HLG) transfer function is a
transfer function
In engineering, a transfer function (also known as system function or network function) of a system, sub-system, or component is a function (mathematics), mathematical function that mathematical model, models the system's output for each possible ...
jointly developed by the
BBC
The British Broadcasting Corporation (BBC) is a British public service broadcaster headquartered at Broadcasting House in London, England. Originally established in 1922 as the British Broadcasting Company, it evolved into its current sta ...
and
NHK
, also known by its Romanization of Japanese, romanized initialism NHK, is a Japanese public broadcasting, public broadcaster. It is a statutory corporation funded by viewers' payments of a television licence, television license fee.
NHK ope ...
for
high dynamic range
High dynamic range (HDR), also known as wide dynamic range, extended dynamic range, or expanded dynamic range, is a signal with a higher dynamic range than usual.
The term is often used in discussing the dynamic ranges of images, videos, audio o ...
(HDR) display.
It is backward compatible with the transfer function of SDR (the
gamma curve).
It was approved as ARIB STD-B67 by the
Association of Radio Industries and Businesses
The , commonly known as , is a standardization organization in Japan. ARIB is designated as the center for promotion of the efficient use of the radio spectrum and the designated frequency change support agency. Its activities include those previo ...
(ARIB).
It is also defined in
ATSC 3.0,
Digital Video Broadcasting
Digital Video Broadcasting (DVB) is a set of international open standards for digital television. DVB standards are maintained by the DVB Project, an international industry consortium, and are published by a Joint Technical Committee (JTC) o ...
(DVB)
UHD-1 Phase 2, and
International Telecommunication Union
The International Telecommunication Union (ITU)In the other common languages of the ITU:
*
* is a list of specialized agencies of the United Nations, specialized agency of the United Nations responsible for many matters related to information ...
(ITU)
Rec. 2100.

HLG is an
HDR format that uses the HLG transfer function,
BT.2020 color primaries and a
bitdepth of 10-bit.
HLG was designed to be
backward compatible
In telecommunications and computing, backward compatibility (or backwards compatibility) is a property of an operating system, software, real-world product, or technology that allows for interoperability with an older legacy system, or with inpu ...
with
SDR UHDTV. However, HLG is not intended to be fully backward compatible with traditional SDR displays that cannot interpret BT.2020 colorimetry.
Both HLG transfer function and the HLG format are
royalty-free
Royalty-free (RF) material subject to copyright or other intellectual property rights may be used without the need to pay royalties or license fees for each use, per each copy or volume sold or some time period of use or sales.
Computer standards ...
.
The backward compatibility allows them to be used with existing transmission standards when the receiver is compatible with the BT.2020 colour container, reducing complexity and cost for both equipment manufacturers and content distributors.
They are supported by
HDMI
High-Definition Multimedia Interface (HDMI) is a proprietary digital interface used to transmit high-quality video and audio signals between devices. It is commonly used to connect devices such as televisions, computer monitors, projectors, gam ...
2.0b,
HEVC
High Efficiency Video Coding (HEVC), also known as H.265 and MPEG-H Part 2, is a video compression standard designed as part of the MPEG-H project as a successor to the widely used Advanced Video Coding (AVC, H.264, or MPEG-4 Part 10). In co ...
,
VP9, and
H.264/MPEG-4 AVC,
and are used by video services such as
BBC iPlayer
BBC iPlayer (stylised as iPLAYER or BBC iPLAYER) is a video on demand service from the BBC. The service is available Over-the-top media service, over-the-top on a wide range of devices, including Mobile phone, mobile phones and Tablet computer ...
,
DirecTV
DirecTV, LLC is an American Multichannel television in the United States, multichannel video programming distributor based in El Segundo, California. Originally launched on June 17, 1994, its primary service is a digital Satellite television, s ...
,
Freeview Play, and
YouTube
YouTube is an American social media and online video sharing platform owned by Google. YouTube was founded on February 14, 2005, by Steve Chen, Chad Hurley, and Jawed Karim who were three former employees of PayPal. Headquartered in ...
.
Description
HLG is designed to be better-suited for
television broadcasting
A television broadcaster or television network is a telecommunications network for the distribution of television content, where a central operation provides programming to many television stations, pay television providers or, in the United ...
, where the metadata required for other HDR formats is not
backward compatible
In telecommunications and computing, backward compatibility (or backwards compatibility) is a property of an operating system, software, real-world product, or technology that allows for interoperability with an older legacy system, or with inpu ...
with non-HDR displays, consumes additional
bandwidth
Bandwidth commonly refers to:
* Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range
* Bandwidth (computing), the rate of data transfer, bit rate or thr ...
, and may also become out-of-sync or damaged in transmission. HLG defines a non-linear
optical-electro transfer function, in which the lower half of the signal values use a
gamma curve and the upper half of the signal values use a
logarithmic curve
In mathematics, logarithmic growth describes a phenomenon whose size or cost can be described as a logarithm function of some input. e.g. ''y'' = ''C'' log (''x''). Any logarithm base can be used, since one can be converted to anot ...
.
In practice, the signal is interpreted as normal by
standard-dynamic-range displays (albeit capable of displaying more detail in highlights), but HLG-compatible displays can correctly interpret the logarithmic portion of the signal curve to provide a wider dynamic range.
In contrast with the other HDR formats it does not use metadata.
The HLG
transfer function
In engineering, a transfer function (also known as system function or network function) of a system, sub-system, or component is a function (mathematics), mathematical function that mathematical model, models the system's output for each possible ...
is backward compatible with
SDR's
gamma curve. However, HLG is commonly used with
Rec. 2020 color primaries which produce a de-
saturated image with visible
hue shifts on non-compatible devices.
HLG is therefore backward compatible with SDR-
UHDTV and will show color distortion on common SDR devices that only support
Rec. 709 color primaries.
Technical details
HLG defines a
nonlinear
In mathematics and science, a nonlinear system (or a non-linear system) is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathe ...
transfer function
In engineering, a transfer function (also known as system function or network function) of a system, sub-system, or component is a function (mathematics), mathematical function that mathematical model, models the system's output for each possible ...
in which the lower half of the signal values use a
gamma curve and the upper half of the signal values use a
logarithmic curve
In mathematics, logarithmic growth describes a phenomenon whose size or cost can be described as a logarithm function of some input. e.g. ''y'' = ''C'' log (''x''). Any logarithm base can be used, since one can be converted to anot ...
.
HLG reference OETF is as follows (as defined in ARIB STD-B67):
or as follows (as defined in Rec. 2100):
where
*''E'' is the linear light signal normalized by the reference white level in the range