The Link Between HDR, SDR and HLG Explained

November 28th, 2017 icon / message-square 10
The Link Between HDR, SDR and HLG Explained

Display technology has suppressed camera technology over the years, preventing filmmakers from unleashing their visual creativity. Nowadays, the boundaries are significantly reduced thanks to HDR TVs and HLG.

HLG

HLG connected to HDR

HDR (11 stops) vs SDR (6 stops)

First, a word about HDR. HDR (which stands for High Dynamic Range) means being able to display a bigger brightness and contrast range. An HDR TV display contains and is able to present up to ten times as much brightness compared to a regular TV with SDR (Standard Dynamic Range) in Rec. 709. 

It must be noted that not everything will be presented brighter in the HDR display. For example, a white piece of paper will be presented as pure white, but only direct sources of light will be presented brighter on a proper HDR display.

A Rec.709 TV (Standard Dynamic Range) displays ONLY 5-6 stops – enough to present people, skin tones and other objects that fall between black and white, and delivers a perfectly usable image.

Cameras are far more advanced than TVs in term of HDR

Cameras have been capable of capturing much more than 6 stops for a long time now. For example, when shooting with a Log profile, a camera is capable of capturing 14 stops, but a TV will squeeze those into only 6 by the SDR (Rec.709) display, and we will see a flat and washed-out image.

It is important to emphasize that there is no such thing “Flat” picture profile. Those milky images are caused by the limitations of the standard Rec. 709 TV dynamic range.

In other words, when we shoot Log, we shoot HDR!

HDR TVs as a filmmakers’s lifeline

HDR TVs can display 11 stops of dynamic range, and thus have the ability to reproduce the full amount of stops in the Log/flat picture profile, which leads to a vivid and contrasty image.

The video below by Alister Chapman presents a well-explained comparison between a Rec. 709 display and an HDR display receiving input from a camera shooting in a flat picture profile, and you can see the significant difference when switching between the two.

Conclusion: When you are shooting Log, use an HDR monitor in order to see the image correctly.

HLG – Hybrid Log Gamma as a great delivery option 

HLG stands for Hybrid Log Gamma, which means a hybrid between Rec 709 and Log. It is available as a picture profile on cameras such as the Sony FS5, Sony a7RIII, Sony AX700 just to name a few.

It must be noted that this is a delivery standard that has the ability to send HDR images to the display! Although this curve is optimized for delivery, it’s also a very capable gamma for acquisition and grading

When you view HLG on a regular Rec 709 TV (SDR), you will get a perfectly usable picture. It does look a little bit darker due to the 75% white recording, but when you take this profile and view it on an HDR monitor, you will see a much more vivid picture with much greater and improved highlights!

HLG delivers a signal that’s great for HDR but that’s usable for SDR as well. 

Watch the below video by Alister Chapman to see the impact of HLG.

Stops & NITS

For further info regarding Stops & NITS watch that great video below

Display technology is finally catching up to the cameras, so cinematographers and directors will be able to see what they are actually shooting in the most accurate way. Would you use HLG and HDR in your productions? Tell us in the comments below! 

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 Gavin Greenwalt
Member
December 4th, 2017

“Those milky images are caused by the limitations of the standard Rec. 709 TV dynamic range.”

That’s not exactly true. All displays are linear displays because that’s what the real world is: linear. Rec709 monitors take the Rec709 gamma and apply an inverse Rec709 gamma to re-linearize the output. Log is just another gamma curve. The reason your rec709 display looks “Milky” isn’t because of limitations but because the monitor is trying to ‘undo’ the rec709 color correction and doesn’t know it needs to ‘undo’ the log footage.

Log footage properly de-logged by a Standard Dynamic Range display wouldn’t look milky it would just look dark. Looking dark is the limitation of the display not milky.

 Bernard Shaw
Bernard Shaw
Member
December 1st, 2017

Perhaps it would be helpful to share the information of what the natural eye sees and how our brain interprets it.

Then readers can see the advantages of HDR over SDR. It is similar but not equivalent in the differences in vividness vs saturation in images. Creating images that fully show what we can see on a view where it’s a wide dynamic scene can be pleasing and emotionally involving.

 Jospeh Slomka
Jospeh Slomka
Member
December 1st, 2017
Reply to  Bernard Shaw

Bernard,
I have a post currently in moderation with link to the munsellcolor science laboratory which had dedicated significant time to how the human visual system reacts to hdr

http://rit-mcsl.org/fairchild/HDR.html

As well here are some smpte presentations on

Her

https://www.smpte.org/sites/default/files/23-1615-TS7-2-IProc02-Miller.pdf

https://www.smpte.org/sites/default/files/2015-10-15-ED-HDR-Slomka-V7-2-Handout.pdf

 roger nelson
roger nelson
Member
November 29th, 2017

HDR? STOPS?!? Contrast RATIOS? Please….There is no mention of the actual measurements of brightness levels in lumens of nits ….and what are your darkest values? No measurements of colors in RGB or LAB values…Whats the purpose of HDR? To make things surrealistic? Is there a book or manual explaining this?Ill let you guys keep talking all this gobble dee gook..Its like the trend of you movie guys is ether to make things look desaturated at 20 fps or to make things look hyper realistically contrastically vivid ….

 Jospeh Slomka
Jospeh Slomka
Member
November 29th, 2017
Reply to  roger nelson

Roger. In this case the answer really doesn’t apply to you. Filmmakers are quite comfortable thinking in stops for lighting and acquisition. It’s not gobbledygook but the language that professionals use to describe images, lighting an displays.

My last release worked on was was a very natural looking digital presentation. It was neither desaturated or 20fps in hdr or sdr.

Hdr is a tool for a film maker to achieve their vision. In this case it allows for a more cinematic presentation in homes.

 roger nelson
roger nelson
Member
November 30th, 2017
Reply to  Jospeh Slomka

You didn’t leave a link to the HDR manual…and you do not have answers…have a nice day.

 Jospeh Slomka
Jospeh Slomka
Member
December 1st, 2017
Reply to  roger nelson

Roger,
I may not have all of the answers, but I have a great deal of experience in producing cinematic and home theater HDR, the underlying physiological process, signal processing and camera systems.

HDR has been in the world of academics and physics for a number of decades. It has had a longer than 10 year exposure to still photography and computer graphics. Only recently has the technology been mainstreamed in motion pictures. Since it is topic with both wide breadth and depth there are many ‘manuals’ to HDR.

A really great place to start would be the work of Dr. Mark Fairchild from the Munsell Color Science Laboratory. There is a wealth of information on HDR research over the last decade.

http://rit-mcsl.org/fairchild/HDR.html

I have given a few presentations at SMPTE on HDR technology and mastering.

If you have a SMPTE membership you can see the presentation
“Clarifying High Dynamic Range” on the smpte.org webiste

The printed materials are available online here

https://www.smpte.org/sites/default/files/2015-10-15-ED-HDR-Slomka-V7-2-Handout.pdf

I expect to have an updated presentation based on the work participated in the last two years.

In terms of specifics numbers I can go over a few.

For SDR the processional mastering standard is 100 nit (or Cd/m^2) for a maximum white of RGB max(1,1,1)
For a contrast ratio of 700:1 means diving 100 nits by 700
So the black level( rgb 0,0,0) measures .143 nits

IN terms of stops you have the maximum number to travel a stop.
So stops from white are
1 stop = 50 nits
2 stops= 25 nits
3 stops = 12 nits
4 stops = 6 nits
6 stops = 3 nits
7 stops = 1.5 nits
8 stops =.75 nits
9 stops = .375
10 stops would be .187 if it reached a full 10 stops

When you factor in HDR the most popular mastering Nit level is 1000
if you compare it to the 100 nit normal TV

100 = 0 stops brighter
200= 1 stop brighter
400 = 2 stops brighter
800 = 3 stops brighter
1600 would be 4 stops brighter

The HDR system maxes out at 10,000 nits.
Although the system max is 10x brighter it is still only 3 1/3 stops different
1000=0 stops brighter
2000= 1 stop brighter
4000= 2 stops brighter
8000= 3 stops brighter
16000 would be 4 stops brighter but the system maxes out at 10,000

The visual appearance of brightness is predicted by the Barten curve. You can find a discussion of that here

https://www.smpte.org/sites/default/files/23-1615-TS7-2-IProc02-Miller.pdf

So those are links to 3 HDR manuals. One to explain the photographic capture, one the explain the artistic process and one to explain the underlying physiological responses.

Lab in this context is a less then useful or intuitive model for color prediction. Lab is based on the perception of reflective objects with a defined relationship to a maximum white. There are now Lab models that content with that, but they are a bit different in terms of what you are referring to with a traditional Lab code-value.

Crimson Son
Crimson Son
Member
December 7th, 2017
Reply to  roger nelson

Why not just research it yourself?

 Jospeh Slomka
Jospeh Slomka
Member
November 29th, 2017

There is a little bit of clarification that can make this easier to understand for film makers. A lot of this talk is about bringing cinema style acquisition into a TV world in the terms of TV engineers and not the terms of cinema.

Stops vs contrast ratio.
Contrast ratio is the absolute amount of bright light divided by the dimmest shadow. A stop represents a doubling of light. For various reasons it is very useful in film and digital photography.

Contrast ratio can be converted to stops.
A DCP has a contrast ratio of 2000:1 = log 2000 base 2 =~ 11 stops

Digital cinema without any HDR is already almost 11 stops.

Modern TV’s are already showing a larger dynamic range than 6-7 which is based on a really outdated TV spec. 6 stops converted to a contrast ratio is 64:1!

A modern normal Non-HDR tv shows a contrast ratio of about 700:1 or 9.4 stops

Going from the current tv standard of 100 brightness to 1000 is about 3.3 stops, for about 12.5 stops. That brings you very close to what the audience sees in a theater.

HDR is clearly superior to standard dynamic range at home. For film makers it is finally providing the cinematic levels of dynamic range to the home audience, rivaling the best theatrical experience.

Used correctly HDR can provide experience beyond the the traditional theater. It’s just the broadcast TV centered numbers being talked about paint a confusing picture for filmmakers.

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