High-dynamic-range video
High-dynamic-range television (HDR or HDR-TV) is a technology improving the signal that displays receive. It is contrasted with the retroactively-named standard dynamic range (SDR). HDR changes the way the luminance and colors of videos and images are represented in the signal. It allows representing brighter and more detailed highlights, darker and more detailed shadows, and a wider array of more intense colors.

HDR allows compatible displays to receive a higher quality image source. It does not improve a display's intrinsic properties (brightness, contrast, and color capabilities). Not all HDR displays have the same capabilities, and HDR content will thus look different depending upon the display used.

HDR-TV first came about in 2014 for video. It is now also available for still pictures. HDR-TV is a part of HDR imaging, an end-to-end process of increasing the dynamic range of images and videos from their capture and creation, to their storage, distribution and display. Beside dynamic range, HDR-TV also improves color gamut as Rec. 2100 and all common HDR formats requires HDR to be delivered with wide color gamuts (WCG).

Description 
Before HDR, improvements in the fidelity of displays were typically achieved by increasing the pixel quantity and density (resolution) and the display's frame rate. By contrast, HDR improves the perceived fidelity of the existing individual pixels themselves. Standard dynamic range (SDR) is still based on, and limited by, the characteristics of older cathode ray tubes (CRT), despite the huge advances in screen and display technologies since CRT's obsolescence. HDR aims to overcome these limits.

SDR formats are able to represent a maximum luminance level of around 100 nits. For HDR, this number goes up to at least 1000 nits, and in some cases up to 10000 nits. HDR also enables the representation of lower (i.e. darker) black levels and more saturated (i.e. more colorful) colors. The most common SDR formats are limited to the Rec. 709/sRGB gamut, while common HDR formats use Rec. 2100, which is a wide color gamut (WCG).

In practice, HDR is not always used at its limits. HDR contents are often limited to a peak brightness of 1000 or 4000 nits and P3-D65 colors, even if they are stored in formats capable of more. Content creators can choose to what extent they make use of HDR capabilities. They can constrain themselves to the limits of SDR even if the content is delivered in an HDR format.

The benefits of HDR depend on the display capabilities, which vary. No current display is able to reproduce the maximal range of brightness and colors that can be represented in HDR formats.

Benefits 
The highlights (i.e. the brightest parts of an image) can be brighter, more colorful, and more detailed. The larger capacity for brightness can be used to increase the brightness of small areas without increasing the overall image's brightness, resulting in, for example, bright reflections from shiny objects, bright stars in a dark night scene, and bright and colorful light-emissive objects (e.g. fire, and sunset).

The shadows/lowlights (i.e. the darkest parts of an image) can be darker and more detailed.

The colorful parts of the image can be even more colorful (if WCG is used).

The color dynamism and wider range of colors frequently attributed to HDR video is actually a consequence of a Wide Color Gamut (WCG). This has become a point of significant confusion among consumers, whereby HDR and WCG are either confused for each other or treated as interchangeable. Whilst HDR displays typically have WCGs and displays with WCGs are usually capable of HDR, one does not imply the other. Crucially, there are SDR displays with WCGs. Some HDR standards specify WCG as a prerequisite of compliance. Regardless, when a WCG is available on an HDR display, as is typically the case, the image as a whole can be more colorful due to the wider range of colors.

More subjective, practical benefits of HDR video include:

 More realistic luminance variation between scenes (such as sunlit, indoor, and night scenes).
 Better surface material identification.
 Better in-depth perception, even with 2D imagery.

Preservation of content creator' intents 
Modern display's capabilities are often higher than the capability of SDR to represent brightness, contrast and colors. The SDR images need to be altered by the display to make use of all of its capabilities. Content creators have no control on the process and the resulting image doesn't always preserve their creative intents. HDR allow them to decide how the image will look on high capable displays.

When the display's capabilities are insufficient to reproduce all the brightness, contrast and colors that are represented in the HDR content, the image need to be adjusted to fit the display's capabilities. Some HDR formats (such as Dolby Vision and HDR10+) allows the content creator to choose how the adjustment will be done. Other HDR formats (such as HDR10 and HLG) don't offer this possibility and thus, the content creator's intents are not ensured to be preserved on lower capable displays.

For optimal quality, standards require video to be created and viewed in a relatively dark environment. Dolby Vision IQ and HDR10+ Adaptive adjust the content according to the available light.

Other HDR technologies 

HDR image capture techniques have been used for years in photography to increase the dynamic range of photographs. Older formats with HDR support, such as raw and logarithmic formats, were only intended to be used for storage. Now, the additional range can either be kept through export to a consumer HDR image format (e.g. HDR10) or reduced via tone mapping to SDR and exported to one of the many SDR image formats for legacy displays.

Formats
Since 2014, multiple HDR formats have emerged including HDR10, HDR10+, Dolby Vision, and HLG. Some formats are royalty-free and others require a license. The formats vary in their capabilities.

Dolby Vision and HDR10+ include dynamic metadata while HDR10 and HLG do not. The dynamic metadata are used to improve image quality on limited displays that are not capable of reproducing an HDR video to its fullest intended extent. Dynamic metadata allows content creators to control and choose the way the image is adjusted. When less capable displays are used and dynamic metadata is not available, the result will vary dependent upon the display, and the creator's intent may not be preserved.

HDR10

HDR10 Media Profile, more commonly known as HDR10, is an open HDR standard announced on 27 August 2015 by the Consumer Technology Association. It is the most widespread of the HDR formats. It is not backward compatible with SDR displays. It is technically limited to a maximum of 10,000 nits peak brightness; however, HDR10 content is commonly mastered with a peak brightness between 1000 and 4000 nits.

HDR10 lacks dynamic metadata. On HDR10 displays that have lower color volume than the HDR10 content (e.g. lower peak brightness capability), the HDR10 metadata provides information to help the display adjust to the video. The metadata, however, is static and constant with respect to each individual video and doesn't inform the display exactly how the content should be adjusted. The interaction between display capabilities, video metadata, and the ultimate output (i.e. the presentation of the video) is mediated by the display, with the result that the original producer's intent may not be preserved.

Dolby Vision

Dolby Vision is an end-to-end ecosystem for HDR video. It covers content creation, distribution, and playback. It is a proprietary solution from Dolby Laboratories that emerged in 2014. It does use dynamic metadata and is capable of representing luminance levels up to 10,000 nits. Dolby Vision certification requires displays for content creators to have a peak luminance of at least 1000 nits.

HDR10+

HDR10+, also known as HDR10 Plus, is an HDR video format, announced on 20 April 2017. It is the same as HDR10 but with the addition of a system of dynamic metadata developed by Samsung. It is free to use for content creators and has a maximum $10,000 annual license for some manufacturers. It has been positioned as an alternative to Dolby Vision without the same expenses.

HLG10 (HLG format) 

HLG10, commonly simply referred as the HLG format, is an HDR format that can be used for both video and still images. It uses the HLG transfer function, Rec. 2020 color primaries, and a bit depth of 10 bits. The format is backwards compatible with SDR UHDTV but not with older SDR displays that do not implement the Rec. 2020 color standards. It doesn't use metadata and is royalty free.

PQ10 (PQ format) 

PQ10, sometimes simply referred as the PQ format, is an HDR format that can be used for both video and still images. It is the same as the HDR10 format without any metadata. It uses the PQ transfer function, Rec. 2020 color primaries and a bit depth of 10-bits. It is not backward compatible with SDR.

Other formats 

 Technicolor Advanced HDR: An HDR format which aims to be backwards compatible with SDR.  there is no commercial content available in this format.
 SL-HDR1 (Single-Layer HDR system Part 1) is a HDR standard that was jointly developed by STMicroelectronics, Philips International B.V., and Technicolor R&D France. It was standardised as ETSI TS 103 433 in August 2016. SL-HDR1 provides direct backwards compatibility by using static (SMPTE ST 2086) and dynamic metadata (using SMPTE ST 2094-20 Philips and 2094-30 Technicolor formats) to reconstruct a HDR signal from an SDR video stream that can be delivered using existing SDR distribution networks and services. SL-HDR1 allows for HDR rendering on HDR devices and SDR rendering on SDR devices using a single-layer video stream. The HDR reconstruction metadata can be added either to HEVC or AVC using a supplemental enhancement information (SEI) message. Version 1.3.1 was published in March 2020.
SL-HDR2
SL-HDR3

Comparison of HDR formats

Notes

Displays 
Display devices capable of greater dynamic range have been researched for decades, primarily with flat panel technologies like plasma, SED/FED and OLED.

TV sets with enhanced dynamic range and upscaling of existing SDR/LDR video/broadcast content with reverse tone mapping have been anticipated since early 2000s. In 2016, HDR conversion of SDR video was released to market as Samsung's HDR+ (in LCD TV sets) and Technicolor SA's HDR Intelligent Tone Management.

As of 2018, high-end consumer-grade HDR displays can achieve 1,000 cd/m2 of luminance, at least for a short duration or over a small portion of the screen, compared to 250-300 cd/m2 for a typical SDR display.

Video interfaces that support at least one HDR Format include HDMI 2.0a, which was released in April 2015 and DisplayPort 1.4, which was released in March 2016. On 12 December 2016, HDMI announced that Hybrid Log-Gamma (HLG) support had been added to the HDMI 2.0b standard. HDMI 2.1 was officially announced on 4 January 2017, and added support for Dynamic HDR, which is dynamic metadata that supports changes scene-by-scene or frame-by-frame.

Compatibility 
As of 2020, no display is capable of rendering the full range of brightness and color of HDR formats. A display is called an HDR display if it can accept HDR content and map them to its display characteristics. Thus, the HDR logo only provides information about content compatibility and not display capability.

Certifications 
Certifications have been made in order to give consumers information about the display rendering capability of a screen.

VESA DisplayHDR

The DisplayHDR standard from VESA is an attempt to make the differences in HDR specifications easier to understand for consumers, with standards mainly used in computer monitors and laptops. VESA defines a set of HDR levels; all of them must support HDR10, but not all are required to support 10-bit displays. DisplayHDR is not an HDR format, but a tool to verify HDR formats and their performance on a given monitor. The most recent standard is DisplayHDR 1400 which was introduced in September 2019, with monitors supporting it released in 2020. DisplayHDR 1000 and DisplayHDR 1400 are primarily used in professional work like video editing. Monitors with DisplayHDR 500 or DisplayHDR 600 certification provide a noticeable improvement over SDR displays, and are more often used for general computing and gaming.

*Wide Color Gamut, at least 90% of DCI-P3 in specified volume (peak luminance)

Other certifications 
UHD Alliance certifications:

 Ultra HD Premium
 Mobile HDR Premium: for mobile devices.

Technical details
HDR is mainly achieved by the use of PQ or HLG transfer function. Wide Color Gamut (WCG) is also commonly used along HDR up to Rec. 2020 color primaries. A bit-depth of 10 or 12 bits is used to not see banding across the extended brightness range. Some additional metadata are sometimes used to handle the variety in displays brightness, contrast and colors. HDR video is defined in Rec. 2100.

Color space

ITU-R Rec. 2100

Rec. 2100 is a technical recommendation by ITU-R for production and distribution of HDR content using 1080p or UHD resolution, 10-bit or 12-bit color, HLG or PQ transfer functions, full or limited range, the Rec. 2020 wide color gamut and YCBCR or ICTCP as color space.

Transfer function 

SDR uses a gamma curve transfer function that is based on CRT's characteristics and that is used to represent luminance levels up to around 100 nits. HDR uses newly developed PQ or HLG transfer functions instead of the traditional gamma curve. If the gamma curve would have been extended to 10,000 nits, it would have required a bit-depth of 15 bits to avoid banding. PQ and HLG are more efficient.

HDR transfer functions:

 Perceptual Quantizer (PQ), or SMPTE ST 2084, is a transfer function developed for HDR that is able to represent luminance level up to 10,000 cd/m2. It is the basis of HDR video formats (such as Dolby Vision, HDR10 and HDR10+) and is also used for HDR still picture formats. PQ is not backward compatible with SDR, because no tonemapping to SDR is perfect. PQ encoded in 12 bits does no longer allow for human vision to decipher any differences.
 Hybrid Log-Gamma (HLG) is a transfer function developed by the NHK and BBC. It is backward compatible with SDR's gamma curve. It is the basis of an HDR format that is also named HLG or sometimes referred to as HLG10. (The HLG10 format uses Rec. 2100 color primaries and is therefore not backward compatible with all SDR displays, but neither BT.2020 primaries, nor BT.2020-NCL matrix are as visible as PQ function, when incorrectly presented.) The HLG transfer function is also used by other video formats such Dolby Vision profile 8.4 and for HDR still picture formats.

Both PQ and HLG are royalty-free.

Color primaries 
SDR for HD video uses a system chromaticity (chromaticity of color primaries and white point) specified in Rec. 709 (same as sRGB). SDR for SD used many different primaries, as said in BT.601, SMPTE 170M and BT.470.

HDR is commonly associated to a Wide Color Gamut (a system chromaticity wider than BT.709). Rec. 2100 (HDR-TV) uses the same system chromaticity that is used in Rec. 2020 (UHDTV). HDR formats such as HDR10, HDR10+, Dolby Vision and HLG also use Rec. 2020 chromaticities.

HDR contents are commonly graded on a P3-D65 display and then contained in a HDR format that uses Rec. 2020 color primaries.

Bit depth 
Because of the increased dynamic range, HDR contents need to use more bit depth than SDR to avoid banding. While SDR uses a bit depth of 8 or 10 bits, HDR uses 10 or 12 bits. This, combined with the use of more efficient transfer function (i.e. PQ or HLG), is enough to avoid banding.

Matrix coefficients 
Rec. 2100 specifies the use of the RGB, the YCbCr or the ICTCP signal formats for HDR-TV.

ICTCP is a color representation designed by Dolby for HDR and wide color gamut (WCG) and standardized in Rec. 2100.

IPTPQc2 (or IPTPQc2) with reshaping is a proprietary format by Dolby and is similar to ICTCP. It is used by Dolby Vision profile 5.

Signaling color space 
Coding-independent code points (CICP) is used to signal the transfer function, color primaries and matrix coefficients. It is defined in both ITU-T H.273 and ISO/IEC 23091-2. It is used by multiple codecs including AVC, HEVC and AVIF. Common combinations of H.273 parameters are summarized in ITU-T Series H Supplement 19.

Metadata

Static metadata 
Static HDR metadatas give information about the whole video.

 SMPTE ST 2086 or MDCV (Mastering Display Color Volume): It describes the color volume of the mastering display (i.e. the color primaries, the white point and the maximum and minimum luminance). It has been defined by SMPTE and also in AVC and HEVC standards.
 MaxFALL (Maximum Frame Average Light Level)
 MaxCLL (Maximum Content Light Level)

Those metadatas do not describe how the HDR content should be adapted to an HDR consumer displays that have lower color volume (i.e. peak brightness, contrast and color gamut) than the content.

The values of MaxFALL and MaxCLL should be calculated from the video stream itself (not including black borders for MaxFALL) based on how the scenes appear on the mastering display. It is not recommended to set them arbitrarily.

Dynamic metadata 
Dynamic metadatas are specific for each frame or each scene of the video.

Dynamic metadatas of Dolby Vision, HDR10+ and SMPTE ST 2094 describe what color volume transform should be applied to contents that are shown on displays that have different color volume from the mastering display. It is optimized for each scene and each display. It allows for the creative intents to be preserved even on consumers displays that have limited color volume.

SMPTE ST 2094 or Dynamic Metadata for Color Volume Transform (DMCVT) is a standard for dynamic metadata published by SMPTE in 2016 as six parts. It is carried in HEVC SEI, ETSI TS 103 433, CTA 861-G. It includes four applications:

 ST 2094-10 (from Dolby Laboratories), used for Dolby Vision.
 ST 2094-20 (from Philips). Colour Volume Reconstruction Information (CVRI) is based on ST 2094–20.
 ST 2094-30 (by Technicolor). Colour Remapping Information (CRI) conforms to ST 2094-30 and is standardized in HEVC.
 ST 2094-40 (by Samsung), used for HDR10+.
ETSI TS 103 572: A technical specification published in October 2020 by ETSI for HDR signaling and carriage of ST 2094-10 (Dolby Vision) metadata.

Dual-layer video 
Some Dolby Vision profiles use a dual-layer video composed of a base layer (BL) and an enhancement layer (EL). Depending on the Dolby Vision profile (or compatibility level), the base layer can be backward compatible with SDR, HDR10, HLG, UHD Blu-ray or no other format in the most efficient IPTPQc2 color space (uses full range and reshaping).

ETSI GS CCM 001 describes a Compound Content Management functionality for a dual-layer HDR system, icluding MMR (multivariate multiple regression) and NLQ (non linear quantisation).

Adoption

Guidelines

Ultra HD Forum guidelines 

UHD Phase A are guidelines from the Ultra HD Forum for distribution of SDR and HDR content using Full HD 1080p and 4K UHD resolutions. It requires color depth of 10 bits per sample, a color gamut of Rec. 709 or Rec. 2020, a frame rate of up to 60 fps, a display resolution of 1080p or 2160p, and either standard dynamic range (SDR) or high dynamic range that uses Hybrid Log-Gamma (HLG) or Perceptual Quantizer (PQ) transfer functions. UHD Phase A defines HDR as having a dynamic range of at least 13 stops (213=8192:1) and WCG as a color gamut that is wider than Rec. 709. UHD Phase A consumer devices are compatible with HDR10 requirements and can process Rec. 2020 color space and HLG or PQ at 10 bits.

UHD Phase B will add support to 120 fps (and 120/1.001 fps), 12 bit PQ in HEVC Main12 (that will be enough for 0.0001 to 10000 nits), Dolby AC-4 and MPEG-H 3D Audio, IMAX sound in DTS:X (with 2 LFE). It will also add ITU's ICtCp and Color Remapping Information (CRI).

Video industry

Gaming industry

Still images

HDR image formats 
The following image formats are compatible with HDR (Rec.2100 color space, PQ and HLG transfer functions, Rec.2100/Rec.2020 color primaries):

 HEIC (HEVC codec in HEIF file format)
 AVIF (AV1 codec in HEIF file format), supports even ICTCP
 JPEG XL
HSP (a format used by Panasonic cameras for photo capture in HDR with the HLG transfer function)
Other image formats, such as JPEG, JPEG 2000, PNG, WebP, don't support HDR by default. They could in theory support it by the use of ICC profile. However, usually, existing applications do not take into account the absolute luminance value defined in ICC profiles. W3C is working to add HDR support to PNG.

Adoption of HDR in still images 
Panasonic: Panasonic's S-series cameras (including Lumix S1, S1R, S1H and S5) can capture photos in HDR using the HLG transfer function and output them in a HSP file format. The captured HDR pictures can be viewed in HDR by connecting the camera to an HLG-compliant display with an HDMI cable. A plug-in allowing to edit the HLG stills (HSP) in Photoshop CC has been released by Panasonic. The company also released a plug-in for displaying thumbnails of those HDR images on a PC (for Windows Explorer and macOS Finder).

Canon: EOS-1D X Mark III and EOS R5 are able to capture still images in the Rec.2100 color space by using the PQ transfer function, the HEIC format (HEVC codec in HEIF file format), the Rec. 2020 color primaries, a bit depth of 10 bit and a 4:2:2 YCbCr subsampling. The captured HDR pictures can be viewed in HDR by connecting the camera to an HDR display with an HDMI cable. Captured HDR pictures can also be converted to SDR JPEG (sRGB color space) and then viewed on any standard display. Canon refers to those SDR pictures as "HDR PQ-like JPEG". Canon's Digital Photo Professional software is able to show the captured HDR pictures in HDR on HDR displays or in SDR on SDR displays. It is also able to convert the HDR PQ to SDR sRGB JPEG.

Sony: Sony α7S III and α1 cameras can capture HDR photos in the Rec.2100 color space with the HLG transfer function, the HEIF format, Rec. 2020 color primaries, a bit depth of 10 bit and a 4:2:2 or 4:2:0 subsampling. The captured HDR pictures can be viewed in HDR by connecting the camera to an HLG-compliant display with an HDMI cable.

Qualcomm: Snapdragon 888 mobile SoC allows the capture of 10-bit HDR HEIF still photos.

Krita 5.0, released on 23 December 2021, added support for HDR HEIF and AVIF images with Rec.2100 PQ and HLG encoding.

Web 
Work is in progress at W3C to make Web compatible with HDR. This include:

 HDR capabilities detection
 HDR in CSS

History

2014 
In January 2014, Dolby Laboratories announced Dolby Vision.

In August 2014, the perceptual quantizer (PQ) was standardized in SMPTE ST 2084.

In October 2014, the HEVC specification incorporates code point for PQ. Previously, it also incorporates the Main 10 profile that supports 10 bits per sample on their first version.

In October 2014, SMPTE standardized the Mastering Display Color Volume (MDCV) static metadata in SMPTE ST 2086.

2015 
In March 2015, HLG was standardized in ARIB STD-B67.

On 8 April 2015, The HDMI Forum released version 2.0a of the HDMI Specification to enable transmission of HDR. The Specification references CEA-861.3, which in turn references SMPTE ST 2084 (the standard of PQ). The previous HDMI 2.0 version already supported the Rec. 2020 color space.

On 24 June 2015, Amazon Video was the first streaming service to offer HDR video using HDR10 format.

On 27 August 2015, Consumer Technology Association announced HDR10.

On 17 November 2015, Vudu announced that they had started offering titles in Dolby Vision.

2016 
On 1 March 2016, the Blu-ray Disc Association released Ultra HD Blu-ray with mandatory support for HDR10 and optional support for Dolby Vision.

On 9 April 2016, Netflix started offering both HDR10 and Dolby Vision.

On June to September 2016, SMPTE standardized multiple dynamic metadata for HDR in SMPTE ST 2094.

On 6 July 2016, the International Telecommunication Union (ITU) announced Rec. 2100 that defines the image parameters for HDR-TV and use two HDR transfer functions—HLG and PQ.

On 29 July 2016, SKY Perfect JSAT Group announced that on 4 October, they will start the world's first 4K HDR broadcasts using HLG.

On 9 September 2016, Google announced Android TV 7.0, which supports Dolby Vision, HDR10, and HLG.

On 26 September 2016, Roku announced that the Roku Premiere+ and Roku Ultra will support HDR using HDR10.

On 7 November 2016, Google announced that YouTube would stream HDR videos that can be encoded with HLG or PQ.

On 17 November 2016, the Digital Video Broadcasting (DVB) Steering Board approved UHD-1 Phase 2 with a HDR solution that supports Hybrid Log-Gamma (HLG) and Perceptual Quantizer (PQ). The specification has been published as DVB Bluebook A157 and will be published by the ETSI as TS 101 154 v2.3.1.

2017 
On 2 January 2017, LG Electronics USA announced that all of LG's SUPER UHD TV models now support a variety of HDR technologies, including Dolby Vision, HDR10, and HLG (Hybrid Log Gamma), and are ready to support Advanced HDR by Technicolor.

On 20 April 2017, Samsung and Amazon announced HDR10+.

On 12 September 2017, Apple announced the Apple TV 4K with support for HDR10 and Dolby Vision, and that the iTunes Store would sell and rent 4K HDR content.

2019 
On 26 December 2019, Canon announced the adoption of the PQ format (PQ10) for still photography.

2020 
On 13 October 2020, Apple announced the iPhone 12 and iPhone 12 Pro series, the first smartphone that can record and edit video in Dolby Vision right in the camera roll. iPhone uses HLG compatible profile 8 of Dolby Vision with only L1 trim and MMR reshaping.

2021 
In June 2021, Panasonic announced a plug-in for Photoshop CC allowing to edit HLG stills.

See also
 High dynamic range 
 Gamma correction
 Rec. 2100

Further reading
We need to talk about HDR by Yoeri Geutskens
ITU-R Rep. BT.2390 "High dynamic range television for production and international programme exchange", a report by ITU providing background information on HDR in general, and for the perceptual quantization (PQ) and hybrid log–gamma (HLG) HDR signal parameters specified in the Rec.2100.

References

Film and video technology
High dynamic range
ITU-R recommendations
 
Color
Display technology
Television technology