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 mathematical function that theoretically models the system's output for each possible input. They are widely used ...
jointly developed by the
BBC #REDIRECT BBC
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and
NHK
, also known as NHK, is a Japanese public broadcaster. NHK, which has always been known by this romanized initialism in Japanese, is a statutory corporation funded by viewers' payments of a television license fee.
NHK operates two terrestria ...
for
high dynamic range
High dynamic range (HDR) is a dynamic range higher than usual, synonyms are wide dynamic range, extended dynamic range, expanded dynamic range.
The term is often used in discussing the dynamic range of various signals such as images, videos, a ...
(HDR) display.
It's backward compatible with the transfer function of SDR (the
gamma curve
Gamma correction or gamma is a nonlinear operation used to encode and decode luminance or tristimulus values in video or still image systems. Gamma correction is, in the simplest cases, defined by the following power-law expression:
: V_\text = ...
).
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 of promotion of the efficient use of the radio spectrum and designated frequency change support agency. Its activities include those previously ...
(ARIB).
It is also defined in
ATSC 3.0
ATSC 3.0 is a major version of the ATSC standards for television broadcasting created by the Advanced Television Systems Committee (ATSC).
The standards are designed to offer support for newer technologies, including HEVC for video channels of u ...
,
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 is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Union ...
(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
Backward compatibility (sometimes known as backwards compatibility) is a property of an operating system, product, or technology that allows for interoperability with an older legacy system, or with input designed for such a system, especially ...
with
SDR UHDTV
Ultra-high-definition television (also known as Ultra HD television, Ultra HD, UHDTV, UHD and Super Hi-Vision) today includes 4K UHD and 8K UHD, which are two digital video formats with an aspect ratio of 16:9. These were first proposed by ...
. 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 standard ...
.
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 audio/video interface for transmitting uncompressed video data and compressed or uncompressed digital audio data from an HDMI-compliant source device, such as a display controller, ...
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 compar ...
,
VP9
VP9 is an open and royalty-free video coding format developed by Google.
VP9 is the successor to VP8 and competes mainly with MPEG's High Efficiency Video Coding (HEVC/H.265).
At first, VP9 was mainly used on Google's video platform YouTube. ...
, 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 on a wide range of devices, including mobile phones and tablets, personal computers and smart televisions. iPlayer services de ...
,
DirecTV
DirecTV (trademarked as DIRECTV) is an American multichannel video programming distributor based in El Segundo, California. Originally launched on June 17, 1994, its primary service is a digital satellite service serving the United States. ...
,
Freeview Play
Freeview is the United Kingdom's sole digital terrestrial television platform. It is operated by Digital UK Ltd and DTV Services Ltd, a joint venture between the BBC, ITV, Channel 4, Channel 5 and Sky. It was launched on 30 October 2002, t ...
, and
YouTube
YouTube is a global online video sharing and social media platform headquartered in San Bruno, California. It was launched on February 14, 2005, by Steve Chen, Chad Hurley, and Jawed Karim. It is owned by Google, and is the second most vis ...
.
Description
HLG is designed to be better-suited for
television broadcasting
A television network or television broadcaster is a telecommunications network for distribution of television program content, where a central operation provides programming to many television stations or pay television providers. Until the mid-1 ...
, where the metadata required for other HDR formats is not
backward compatible
Backward compatibility (sometimes known as backwards compatibility) is a property of an operating system, product, or technology that allows for interoperability with an older legacy system, or with input designed for such a system, especially ...
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
Gamma correction or gamma is a nonlinear operation used to encode and decode luminance or tristimulus values in video or still image systems. Gamma correction is, in the simplest cases, defined by the following power-law expression:
: V_\text = ...
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''). Note that any logarithm base can be used, since one can be convert ...
.
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 mathematical function that theoretically models the system's output for each possible input. They are widely used ...
is backward compatible with
SDR's
gamma curve
Gamma correction or gamma is a nonlinear operation used to encode and decode luminance or tristimulus values in video or still image systems. Gamma correction is, in the simplest cases, defined by the following power-law expression:
: V_\text = ...
. However, HLG is commonly used with
Rec. 2020
ITU-R Recommendation BT.2020, more commonly known by the abbreviations Rec. 2020 or BT.2020, defines various aspects of ultra-high-definition television (UHDTV) with standard dynamic range (SDR) and wide color gamut (WCG), including pictur ...
color primaries which produce a de-
saturated
Saturation, saturated, unsaturation or unsaturated may refer to:
Chemistry
* Saturation, a property of organic compounds referring to carbon-carbon bonds
**Saturated and unsaturated compounds
**Degree of unsaturation
**Saturated fat or fatty acid ...
image with visible
hue
In color theory, hue is one of the main properties (called color appearance parameters) of a color, defined technically in the CIECAM02 model as "the degree to which a stimulus can be described as similar to or different from stimuli that ...
shifts on non-compatible devices.
HLG is therefore backward compatible with SDR-
UHDTV
Ultra-high-definition television (also known as Ultra HD television, Ultra HD, UHDTV, UHD and Super Hi-Vision) today includes 4K UHD and 8K UHD, which are two digital video formats with an aspect ratio of 16:9. These were first proposed by ...
and will show color distortion on common SDR devices that only support
Rec. 709
Rec. 709, also known as Rec.709, BT.709, and ITU 709, is a standard developed by ITU-R for image encoding and signal characteristics of high-definition television.
The most recent version is BT.709-6 released in 2015. BT.709-6 defines the P ...
color primaries.
Technical details
HLG defines a
nonlinear
In mathematics and science, a nonlinear 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, mathematicians, and many other ...
transfer function
In engineering, a transfer function (also known as system function or network function) of a system, sub-system, or component is a mathematical function that theoretically models the system's output for each possible input. They are widely used ...
in which the lower half of the signal values use a
gamma curve
Gamma correction or gamma is a nonlinear operation used to encode and decode luminance or tristimulus values in video or still image systems. Gamma correction is, in the simplest cases, defined by the following power-law expression:
: V_\text = ...
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''). Note that any logarithm base can be used, since one can be convert ...
.
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