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SDR/HDR standard/high dynamic range

SDR/HDR

High Dynamic Range (HDR) is a technology that allows you to take and show more realistic pictures.

Bright view with dark areas of shadows is physically hard to be taken in one shot of the camera. The same is true for display. The human eye perceives much more information than any technology can show. However, HDR technologies are trying to challenge human perception at a new level. The HDR display can demonstrate more detail in dark and light scenes, so it comes to replace the most used Standard Dynamic Range (SDR). So, HDR technology can be understood as three components working together: how to take pictures (camera with HDR and post-processing), code formats (HDR standards and validation), and how to display pictures (displays with HDR).

Characteristics

HDR implementation deals with characteristics such as brightness, contrast, and color rendition. Let's explain these definitions in terms of HDR applications.

Brightness is the maximum level of white that the device can display. The unit of measurement is cd / m² (nit). Humans perceive brightness better than color - this known fact underlies the majority of image compression algorithms. This fact is also meaningful for dynamic range expansion. 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/s RGB gamut, while common HDR formats use Rec. 2100, which is a wide color gamut (WCG).

Contrast is the difference between the lightest and darkest portions of the screen. For example, a contrast ratio of 1000:1 means that white areas are 1000 times brighter than dark areas. The contrast level is determined by how saturated a dark color looks and how bright white is displayed. Generally, the higher the contrast, the better the detail. The ratio between the maximum and the minimum of the tonal value in an image is known as the dynamic range.

Color rendering is the number of colors and shades reproduced by the TV. Depends on the bitness of the matrix. SDR color depth is 8 bits. SDR does not prohibit the use of 10 bits, but in practice, the vast majority of video content is compressed at 8 bits. This means that each of the basic colors - red, green, and blue - can have 28 values, equal to 256 or the total number of 256x256x256 = 16,777,216 colors. This is a lot, but the human eye sees much more, therefore, in practice, it distinguishes between step transitions in SDR video. This is especially noticeable in gradient background scenes, such as the sky. HDR has a minimum color depth of 10 bits: 1,024 possible values ​​for each base color, or a total of 1,073,741,824 colors, which is 64 times more than SDR. Such an image is much closer to reality, however, under certain circumstances, the human eye is still able to notice the gradation of color transitions.

Characteristics

HDR implementation deals with characteristics such as brightness, contrast, and color rendition. Let's explain these definitions in terms of HDR applications.

Brightness is the maximum level of white that the device can display. The unit of measurement is cd / m² (nit). Humans perceive brightness better than color - this known fact underlies the majority of image compression algorithms. This fact is also meaningful for dynamic range expansion. 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/s RGB gamut, while common HDR formats use Rec. 2100, which is a wide color gamut (WCG).

Contrast is the difference between the lightest and darkest portions of the screen. For example, a contrast ratio of 1000:1 means that white areas are 1000 times brighter than dark areas. The contrast level is determined by how saturated a dark color looks and how bright white is displayed. Generally, the higher the contrast, the better the detail. The ratio between the maximum and the minimum of the tonal value in an image is known as the dynamic range.

Color rendering is the number of colors and shades reproduced by the TV. Depends on the bitness of the matrix. SDR color depth is 8 bits. SDR does not prohibit the use of 10 bits, but in practice, the vast majority of video content is compressed at 8 bits. This means that each of the basic colors - red, green, and blue - can have 28 values, equal to 256 or the total number of 256x256x256 = 16,777,216 colors. This is a lot, but the human eye sees much more, therefore, in practice, it distinguishes between step transitions in SDR video. This is especially noticeable in gradient background scenes, such as the sky. HDR has a minimum color depth of 10 bits: 1,024 possible values ​​for each base color, or a total of 1,073,741,824 colors, which is 64 times more than SDR. Such an image is much closer to reality, however, under certain circumstances, the human eye is still able to notice the gradation of color transitions.

Standards and 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 depending 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 to 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 backward 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 to 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 that aims to be backward compatible with SDR. As of 19 December 2020 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 standardized as ETSI TS 103 433 in August 2016. SL-HDR1 provides direct backward compatibility by using static (SMPTE ST 2086) and dynamic metadata (using SMPTE ST 2094-20 Philips and 2094-30 Technicolor formats) to reconstruct an 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