high-dynamic-range video

High-dynamic-range television (HDR or HDR-TV) is a technology that improves the quality of display signals. 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, and allows brighter and more detailed highlight representation, 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 look different depending on the display used.

HDR-TV was first used in 2014 to enhance videos, and 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. HDR-TV also improves color gamut, as Rec. 2100 and all common HDR formats require HDR to be delivered with wide color gamuts (WCG).

Description

Before HDR, improvements in display fidelity were typically achieved by increasing the pixel quantity, density (resolution) and the display's frame rate. By contrast, HDR improves the perceived fidelity of the existing individual pixels. 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.

SDR formats are able to represent a maximum luminance level of around 100 nits. For HDR, this number increases to around 1,000–10,000 nits. HDR can represent darker black levels and more saturated 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 1,000 or 4,000 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—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 or lowlights—the darkest parts of an image—can be darker and more detailed.

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

The color dynamism and wider range of colors frequently attributed to HDR video is actually a consequence of a 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. While HDR displays typically have WCGs and displays with WCGs are usually capable of HDR, one does not imply the other; 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, 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, and better in-depth perception, even with 2D imagery.

Preservation of content creator intent

When a display’s capabilities are insufficient to reproduce all the brightness, contrast and colors that are represented in the HDR content, the image needs to be adjusted to fit the display’s capabilities. Some HDR formats (such as Dolby Vision and HDR10+) allow the content creator to choose how the adjustment will be done. Other HDR formats (such as HDR10 and hybrid log–gamma (HLG)) do not offer this possibility, so 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 ambient light.

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.

HDR10

The 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, and is not backward compatible with SDR displays. It is technically limited to a maximum peak brightness of 10,000 nits; 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 (such as lower peak brightness capability), the HDR10 metadata provides information to help the display adjust to the video. The metadata is static and constant with respect to each individual video, and does not 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, and covers content creation, distribution, and playback. It uses dynamic metadata and is capable of representing luminance levels of up to 10,000 nits. Dolby Vision certification requires displays for content creators to have a peak luminance of at least 1,000 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.

HLG

HLG format is an HDR format that can be used for 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 does not use metadata and is royalty-free.

PQ10 (PQ format)

PQ10, sometimes referred to as the PQ format, is an HDR format that can be used for video and still images. It is the same as the HDR10 format without any metadata. It uses the perceptual quantizer (PQ) transfer function, Rec. 2020 color primaries and a bit depth of 10-bits.

HDR Vivid

HDR Vivid is an HDR format developed by the China Ultra HD Video Alliance (CUVA) and released in March 2021. It uses dynamic metadata standardized in CUVA 005-2020.

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 an 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

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, so the HDR logo only provides information about content compatibility and not display capability.

Displays that uses global dimming, such as most edge-lit LED displays, cannot display the advanced contrast of HDR content. Some displays implement local dimming technologies, such as OLED and full-array LED-backlighting, to more properly display advanced contrast.

Certifications

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.

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. WCGs are 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. In some cases, additional metadata are 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 '' YC_BC_R'' or ''IC_TC_P'' as color space.

Transfer function

SDR uses a gamma curve transfer function that is based on CRT characteristics, and 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.

HDR transfer functions:

* 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. PQ encoded in 12 bits does not produce visible banding.
* HLG is a transfer function developed by the NHK and BBC
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