Hybrid log–gamma explained

The hybrid log–gamma (HLG) transfer function is a transfer function jointly developed by the BBC and NHK for high dynamic range (HDR) display.[1] It is backward compatible with the transfer function of SDR (the gamma curve).[2] It was approved as ARIB STD-B67 by the Association of Radio Industries and Businesses (ARIB).[3] [4] It is also defined in ATSC 3.0, Digital Video Broadcasting (DVB) UHD-1 Phase 2, and International Telecommunication Union (ITU) Rec. 2100.[5] [6] [7]

HLG is an HDR format that uses the HLG transfer function, BT.2020 color primaries and a bitdepth of 10-bit.[8] HLG was designed to be backward compatible with SDR UHDTV. However, HLG is not intended to be fully backward compatible with traditional SDR displays that cannot interpret BT.2020 colorimetry.[9]

Both HLG transfer function and the HLG format are royalty-free. 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.[10] They are supported by HDMI 2.0b, HEVC, VP9, and H.264/MPEG-4 AVC,[11] [12] [13] [14] and are used by video services such as BBC iPlayer, DirecTV, Freeview Play, and YouTube.[15] [16] [17] [18] [19]

Description

HLG is designed to be better-suited for television broadcasting, where the metadata required for other HDR formats is not backward compatible with non-HDR displays, consumes additional bandwidth, 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.[20] [21] 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.[22] [23] [24] In contrast with the other HDR formats it does not use metadata.[25]

The HLG transfer function 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.[26] 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 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.[4] [27]

HLG reference OETF is as follows (as defined in ARIB STD-B67):

E'=\beginr \, \sqrt\ & 0 \le E \le 1,\!\\a \ln(E - b) + c & 1 < E\end

or as follows (as defined in Rec. 2100):

E'=\begin\sqrt\ & 0 \le E \le \frac,\!\\a \ln(12E - b) + c & \frac < E \le 1\end

where

\left[0,12\right]

in ARIB STD-B67 and in the range

\left[0,1\right]

in Rec. 2100.

The signal value is 0.5 for the reference white level while the signal value for 1 has a relative luminance that is 12 times higher than the reference white level. ARIB STD-B67 has a nominal range of 0 to 12.[28] HLG uses a logarithmic curve for the upper half of the signal values due to Weber's law.

HLG reference OOTF is as follows:

\beginF_D = OOTF[E]= \alpha Y_S^ E \!\\R_D = \alpha Y_S^ R_S \!\\G_D = \alpha Y_S^ G_S \!\\B_D = \alpha Y_S^ B_S \!\\Y_S = 0.2627 R_S + 0.6780 G_S + 0.0593 B_S\endwhere

FD

is the luminance of a displayed linear component

RD,GD,BD

in cd/m2.

E

is a signal for each colour component proportional to scene linear light normalized to the range

\left[0,1\right]

.

YS

is the normalized linear scene luminance.

\alpha

is the variable for user gain in cd/m2. It represents LW, the nominal peak luminance of a display for achromatic pixels.

\gamma

is the system gamma.

\gamma

= 1.2 at the nominal display peak luminance of 1000 cd/m2.

HLG reference EOTF is as follows:

FD=OOTF[OETF-1[max(0,(1-\beta)E'+\beta)]]

where

FD

is the luminance of a displayed linear component

RD,GD,BD

in cd/m2.

E'

is the non-linear electrical signal

R',G',B'

in the range

\left[0,1\right]

.

\beta=\sqrt{3(LB/

1/\gamma
L
W)

}

is the variable for user black level lift.

LW

is nominal peak luminance of the display in cd/m2 for achromatic pixels.

LB

is the display luminance for black in cd/m2.

HLG does not need to use metadata since it is compatible with both SDR displays and HDR displays. HLG can be used with displays of different brightness in a wide range of viewing environments.

The dynamic range that can be perceived by the human eye in a single image is around 14 stops.[27] An SDR video display with a 2.4 gamma curve and a bit depth of 8-bits per sample can display a range of about 6 stops without visible banding.[27] Professional SDR video displays with a bit depth of 10-bits per sample extend that range to about 10 stops.[27] When HLG is displayed on a 2,000 cd/m2 display with a bit depth of 10-bits per sample it can display a range of 200,000:1 or 17.6 stops without visible banding.[27]

HLG increases the dynamic range of the video compared to a conventional gamma curve by using a logarithmic curve for the upper half of the signal values.[27] HLG also increases the dynamic range by not including the linear part of the conventional gamma curve used by Rec. 601 and Rec. 709.[29] The linear part of the conventional gamma curve was used to limit camera noise in low light video but is no longer needed with HDR cameras.[29]

HLG is supported in Rec. 2100 with a nominal peak luminance of 1,000 cd/m2 and a system gamma value that can be adjusted depending on background luminance.[5] [30]

HLG is supported in HEVC with a formula that is mathematically equivalent to ARIB STD-B67 but has a nominal range of 0 to 1 instead of 0 to 12:[28]

V=\begin\sqrt \cdot L_c^ & \frac \ge L_c \ge 0,\!\\a \cdot \ln(12L_c - b) + c & 1 \ge L_c > \frac\end

where

History

Inception

On May 15, 2015, the BBC announced that they had begun work with the NHK to develop a joint HDR proposal that would be proposed to the International Telecommunication Union (ITU).[31] On June 9, 2015, HLG was proposed to the JCT-VC for High Efficiency Video Coding (HEVC) and added to the June 2015 draft of the screen content coding extensions.[32]

Later that year, Sony showed HLG video on a modified HDR display at the SMPTE 2015 conference.[33] [34] Colorfront announced that their Transkoder 2016 software would support HDR output using HLG.[35] LG announced that their 2015 4K OLED TVs would support HDR from HLG and perceptual quantizer (PQ).[36] Blackmagic Design released an update for DaVinci Resolve that added support for HLG.

SKY PerfecTV! announced that they will use HLG to transmit 4K UHDTV HDR programming to their satellite subscribers in Japan.[37] [38] Harmonic Inc. and NASA announced the HDR capture of an Atlas V launch which was broadcast the next day on NASA TV using HLG.[39] Vatican Television Center broadcast the ceremony of the Holy Door using HLG and the Rec. 2020 color space.[40]

2016

Industry bodies:

Hardware:

Software:

Broadcasters:

2017

Industry bodies:

Hardware:

Software:

Broadcasters:

2018

2019

2020

See also

External links

Notes and References

  1. News: High Dynamic Range . . 2015-11-01.
  2. News: High Dynamic Range with Hybrid Log–Gamma. BBC. live. 8 November 2015. https://web.archive.org/web/20181109141347/https://custom.cvent.com/63DDE0970BB04F3EA62737749B39B60C/files/event/841C5F30AA5948D4B4D985BC90AE0B2C/426e265101984437b7f9bde81acf8364.pdf. 9 November 2018.
  3. News: High Dynamic Range with Hybrid Log–Gamma. BBC. 2015-11-08.
  4. News: 2015-07-03. ARIB STD-B67. Association of Radio Industries and Businesses. 2015-11-01.
  5. News: 2016-07-04. BT.2100 : Image parameter values for high dynamic range television for use in production and international programme exchange. International Telecommunication Union. 2016-07-04.
  6. News: 2016-11-17. DVB SB Approves UHD HDR Specification. Digital Video Broadcasting. 2017-01-07.
  7. News: 2017-05-19. ATSC Standard: Video. Advanced Television Systems Committee. 2017-05-19.
  8. Web site: 19 October 2020. Ultra HD Forum Guidelines v2.4. Ultra HD Forum.
  9. Web site: ITU-R Report BT.2390 - High dynamic range television for production and international programme exchange. 2021-09-11. ITU. en-US.
  10. Web site: 2019-12-23. All you wanted to know about Hybrid Log Gamma (HLG). 2020-05-27. TM Broadcast International. en-US.
  11. News: Introducing HDMI 2.0b. HDMI.org. 2017-01-07. 2016-12-03. https://web.archive.org/web/20161203212800/http://www.hdmi.org/manufacturer/hdmi_2_0/index.aspx. dead.
  12. News: Rajan Joshi. Shan Liu. Gary Sullivan. Gerhard Tech. Jizheng Xu. Yan Ye. 2015-09-05. HEVC Screen Content Coding Draft Text 4. JCT-VC. 2015-11-01.
  13. News: HDR Video Playback. Android. 2016-09-23.
  14. News: 2017-04-13. ITU-T Recommendation H.264 (04/2017). ITU. 2017-06-14.
  15. News: Upload High Dynamic Range (HDR) videos. Google Inc.. 2017-01-11.
  16. News: Julian Clover. 2016-09-23. Freeview updates for HEVC and Ultra HD. Broadband TV News. 2016-09-23.
  17. News: Leo Kelion. 2017-12-05. Blue Planet II comes to iPlayer in 4K HDR. BBC. 2017-12-05.
  18. News: Jason Dachman. 2017-12-14. AT&T DirecTV To Deliver First Live 4K HDR Telecast in U.S. With Tonight's Penguins-Golden Knights Game. Sportsvideo. 2018-01-10.
  19. News: Greg Tarr. 2017-12-15. DirecTV Presents NHL 4K Game With Live HDR, But Was Anyone Watching?. HDGuru. 2018-01-10.
  20. News: T. Borer. A. Cotton. A "Display Independent" High Dynamic Range Television System. BBC. live. 1 November 2015. https://web.archive.org/web/20160207193353/https://downloads.bbc.co.uk/rd/pubs/whp/whp-pdf-files/WHP309.pdf. 7 February 2016.
  21. Web site: 3 July 2015 . ARIB STD-B67 . live . https://web.archive.org/web/20170329122104/http://www.arib.or.jp/english/html/overview/doc/2-STD-B67v1_0.pdf . 29 March 2017 . 8 August 2017.
  22. Web site: Morrison. Geoffrey. What is HLG? Hybrid log gamma. Say what?. live. https://web.archive.org/web/20191007204408/https://www.cnet.com/news/all-about-hlg-what-hybrid-log-gamma-means-for-your-tv/. 7 October 2019. 7 October 2019. CNET. en.
  23. Web site: St. Leger. Henry. Hybrid Log Gamma: everything you need to know about HLG HDR. live. https://web.archive.org/web/20191007184226/https://www.techradar.com/news/hybrid-log-gamma-what-you-need-to-know-about-hlg. 7 October 2019. 7 October 2019. TechRadar. en.
  24. Web site: High Dynamic Range Television and Hybrid Log–Gamma. live. https://web.archive.org/web/20191007185732/https://www.bbc.co.uk/rd/projects/high-dynamic-range. 7 October 2019. 7 October 2019. BBC R&D.
  25. Web site: Michael Bizzaco. Ryan Waniata. Simon Cohen. 19 December 2020. HDR TV: What it is and why your next TV should have it. live. https://web.archive.org/web/20201221081838/https://www.digitaltrends.com/home-theater/what-is-hdr-tv/. 21 December 2020. 2 January 2021. Digital Trends. Designtechnica Corporation.
  26. Web site: Ultra HD Forum. 19 October 2020. Ultra HD Forum Guidelines v2.4.
  27. News: A "Display Independent" High Dynamic Range Television System . T. Borer . A. Cotton . . 2015-11-01.
  28. News: HEVC Screen Content Coding Draft Text 6 . Rajan Joshi . Shan Liu . Gary Sullivan . Gerhard Tech . Ye-Kui Wang . Jizheng Xu . Yan Ye . JCT-VC . 2016-03-24 . 2016-03-26.
  29. News: Study Group Report High-Dynamic-Range (HDR) Imaging Ecosystem . Society of Motion Picture and Television Engineers . 2015-11-30.
  30. News: ITU announces BT.2100 HDR TV standard . Rasmus Larsen . 2016-07-05 . 2016-07-26.
  31. News: High Dynamic Range at the SES Industry Days . Simon Thompson . BBC . 2015-05-15 . 2015-11-01.
  32. News: Proposed addition of transfer characteristics in VUI . Y. Nishida . T. Yamashita . A. Ichigaya . T. Shimizu . JCT-VC . 2015-06-09 . 2015-11-01.
  33. News: Sony Describes Hybrid Log Gamma (HLG) at SMPTE 2015 Event . DisplayDaily . 2015-11-03.
  34. News: SMPTE 2015 Annual Technical Conference & Exhibition . . 2015-11-03.
  35. News: Colorfront spotlights amazing UHD HDR performance at SMPTE 2015 . American City Business Journals . 2015-10-27 . 2015-11-01.
  36. News: Vincent Teoh. 2015-11-06. LG Talk Up 4K OLED TV & HDR at UK Briefing. HDTVtest. 2015-11-08.
  37. News: SkyPerfect offers UHD-HDR by DTH . Chris Forrester . Advanced Television . 2015-11-04 . 2015-11-04.
  38. News: Brian Hallett. 2015-12-23. DaVinci Resolve 12.2 Update & Price Drop. Pro Video Coalition. 2015-12-23.
  39. News: NASA and Harmonic Explore the Boundaries of UHD at ISS Resupply Launch at Kennedy Space Center . . 2015-12-03 . 2015-12-05 . dead . https://web.archive.org/web/20151208170158/http://money.cnn.com/news/newsfeeds/articles/marketwire/11G074496-001.htm . 2015-12-08 .
  40. News: "Huge success" for UHD-HLG test in Rome . Advanced Television . 2015-12-14 . 2015-12-18.
  41. News: 2016-04-18. Ultra HD Forum Releases First Industry Guidelines for Deploying End-to-End Live & Pre-Recorded UHD Services in 2016. Business Wire. 2016-04-18.
  42. News: 2016-04-18. End-to-end guidelines for phase A implementation. Ultra HD Forum. 2016-04-18.
  43. News: James Grover. 2016-11-17. UHD-1 Phase 2 approved. TVBEurope. 2017-01-07. 2017-01-13. https://web.archive.org/web/20170113023123/http://www.tvbeurope.com/uhd-1-phase-2-approved/. dead.
  44. News: Ramus Larsen. 2016-12-12. HDMI 2.0b standard gets support for HLG HDR. flatpanelshd. 2017-01-07.
  45. News: Andrew Cotton. 2016-12-31. 2016 in Review - High Dynamic Range. BBC. 2017-01-07.
  46. News: LEADER Announces 12G SDI, HDR and Video over IP Options for its LV 5490 and LV 5480 4K-capable Waveform Monitors . CreativeCOW.net . 2016-01-12 . 2016-01-13.
  47. News: Harmonic Showcases Innovation for Agile Video Delivery at 2016 NAB Show . . 2016-03-29 . 2016-04-11.
  48. News: Canon reveals new camera and lens in advance of NAB . RedShark . 2016-04-13 . 2016-04-13.
  49. News: Sony Goes Beyond Definition at NAB 2016 . . 2016-04-18 . 2016-04-18.
  50. News: Sony to Update BVM-X300 4K Monitor in October . Bryant Frazer . studiodaily . 2016-08-03 . 2016-08-07.
  51. News: Sony Electronics Announces the Latest Addition to its 4K HDR Home Theater Projector Line-Up at CEDIA 2016 . PR Newswire . 2016-09-14 . 2016-09-23.
  52. News: Hybrid Log–Gamma: Why it's the next big thing in 4K TV . Steve May . Trusted Reviews . 2016-11-04 . 2017-01-10.
  53. News: Atomos Shogun Inferno Firmware 8.1 Unlocks Sony FS 2K/Raw Super-Slow Motion . Ogy Stoilov. 4K Shooters . 2016-12-25 . 2017-01-10.
  54. News: How Avid is Advancing Key Industry Trends Highlighted at the HPA Tech Retreat 2016 . Wim Van den Broeck . . 2016-03-11 . 2016-04-04.
  55. News: Avid Media Composer 8.5 Adds HDR, Simplifies Menus, Offers Better Feedback on the Timeline . Bryant Frazer . studiodaily . 2016-01-29 . 2016-04-04.
  56. News: Android TV 7.0 supports Dolby Vision, HDR10 and HLG . Ramus Larsen . flatpanelshd . 2016-09-07 . 2016-09-23.
  57. News: Dome Productions to Begin Trialing of HLG for Delivering HDR . Yahoo! Finance . 2016-04-15 . 2016-04-15.
  58. News: 4K HDR from SKY Perfect JSAT . Colin Mann . Advanced Television . 2016-07-29 . 2016-07-30.
  59. News: IBC 2016: Eutelsat fast forwards in the video age . Eutelsat . 2016-09-07 . 2016-09-07.
  60. News: True colors: adding support for HDR videos on YouTube . Steven Robertson . 2016-11-07 . 2017-01-11.
  61. News: BBC announces arrival of HLG content on its iPlayer IPTV platform. Dave Bevan. 2016-12-08. 2016-12-08. BBC.
  62. News: LG Takes Aim At Samsung With New 'Quantum Dot-Beating' TV Technology . John Archer . Forbes . 2017-01-03 . 2017-01-07.
  63. News: Panasonic launches 'world's first' HDR, ultra-HD OLED TV . Matt Burgess . Wired . 2017-01-04 . 2017-01-07.
  64. News: Sony Unveils Debut OLED TV Range . John Archer . Forbes . 2017-01-04 . 2017-01-07.
  65. News: New JVC 4K e-Shift4 Projectors Deliver Higher Brightness and Contrast, Add Low Latency Mode . . 2017-01-05 . 2017-01-16.
  66. News: LG will bring HLG HDR to 2016 OLED & LCDs through firmware update . Ramus Larsen . flatpanelshd . 2017-01-04 . 2017-01-07.
  67. News: Sony: Android TV 7.0 coming to 2017, 2016 & 2015 TVs . Ramus Larsen . flatpanelshd . 2017-01-16 . 2017-01-16.
  68. News: Panasonic will add HLG HDR support to select 2016 TVs . Ramus Larsen . flatpanelshd . 2017-02-17 . 2017-03-04.
  69. News: Philips: HLG HDR update coming to 2016 TVs . Ramus Larsen . flatpanelshd . 2017-03-13 . 2017-03-13.
  70. News: Android 7.0 will roll out to Sony TVs starting May 31 . Ramus Larsen . flatpanelshd . 2017-05-31 . 2017-05-31.
  71. News: Panasonic Releases GH5 Firmware 2.0 with Anamorphic and More . Daron James . No Film School . 2017-09-28 . 2017-12-09.
  72. News: Panasonic releases HLG HDR update for select 2016 TVs . Ramus Larsen . flatpanelshd . 2017-11-27 . 2017-12-05.
  73. News: Qualcomm lifts the veil on the Snapdragon 845 SoC . Tech Report . Bruno Ferreira . 2017-12-06 . 2017-12-09.
  74. News: The Latest (and Greatest!) for Premiere Pro CC and Media Encoder . . 2017-04-19 . 2017-04-20.
  75. News: Final Cut Pro X 10.4 adds HDR support, VR video editing, and (finally!) curves . DPReview . Brittany Hillen . 2017-12-14 . 2017-12-17.
  76. News: Eutelsat scales up Ultra HD content with two new channels at the popular Hotbird neighbourhood . . 2017-01-04 . 2017-01-14.
  77. News: How can I watch Blue Planet II in Ultra HD on my connected TV? . BBC . 2017-12-05 . 2017-12-06.
  78. News: Vizio courts cord cutters, challenges OLED, and adds new HDR options to its 2018 smart TV lineup. TechHive. 2018-10-26. en.
  79. News: Pansonic S1, S1R to ship in march, HLG photo and high res mode added to feature list. DpReview news. 2019-01-10. en.
  80. News: Sky Q will finally get HDR on Wednesday 27 May 2020. Pocket Lint News. 2020-05-25. en.
  81. Web site: Patel. Nilay. 2020-10-20. Apple iPhone 12 Pro review: ahead of its time. 2020-10-20. The Verge. en.