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Old 13th April 2020, 05:44   #1  |  Link
wswartzendruber
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I Made a PQ to HLG Converter

I wrote a small utility for converting raw video streams from PQ to HLG. It's available on GitHub.

Code:
USAGE: pq2hlg [format] [nits] [width] [height] [input] [output]
  format - The raw pixel format to use. Supported options are:
           rgb48
  nits   - Luminance value of the brightest pixel in the input.
  width  - Width of the video stream in pixels.
  height - Height of the video stream in pixels.
  input  - Raw RGB48 input file; use - for STDIN.
  output - Raw RGB48 output file; use - for STDOUT.
My motivation was that I wanted to rip my 4K UltraHD library, but wanted to do so in a viewing format that is as universal as possible, even if that entails some amount of compromise. I quickly settled on HLG after reading about it, but couldn't find a conversion utility to do this. I learned that ffmpeg supports the use of lookup tables, but couldn't find any such thing for cheaper than a couple hundred dollars. Others were available from the BBC under license.

All in all, I got quite fed up with the situation and started researching PQ, HLG, metadata, gamma curves, etc. until I got nauseated and started work. Then I studied some more and then went back to coding. Wash, rinse, and repeat a few times. I'm at the point where I'm comfortable with releasing my work to the public. Principally, it uses, to the best of my understanding, the algorithm published by BBC Research & Development.

Future improvements may include passing dynamic metadata in for variable PQ->HLG tone mapping. I definitely want to add YUV10 support as soon as I can figure out how the U and V components are encoded.

This is also the first thing I've written in Rust, so it's probably not going to be as idiomatic or as proper as it could be.

EDIT: Attached a screenshot from Alita: Battle Angel using this utility, taken from a nightly build of VLC 3.0.9 on a SDR monitor. The video codec in use is VP9 Profile 2.

EDIT: I also forgot to mention: This isn't really suitable for any brightness value aside from 1,000 nits. If a PQ video has a peak brightness of less than 1,000 nits, pass 1,000 nits in as the second argument anyway. If any pixel ever exceeds 1,000 nits, then external tone mapping really needs to be applied to the PQ source before passing it into this utility. A peak brightness of other than 1,000 nits can be specified, but this will cause reference white to be pushed up or down. This addendum is in accordance with ITU-R BT.2390-8 Section 7.2.
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Last edited by wswartzendruber; 13th April 2020 at 07:05. Reason: Clarifying brightness setting.
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Old 13th April 2020, 20:17   #2  |  Link
Blue_MiSfit
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Fun project!

Here's one thing I've been trying to wrap my mind around regarding HLG: I get that one of the big appeals is "backwards compatibility", and I totally see how, from an EOTF standpoint it's somewhat backwards compatible with SDR. In other words, you could feed an HLG signal to a dumb SDR display and the luminance would look mostly correct.

However, I don't understand how (without some other component) it can be backwards compatible from a color gamut standpoint. If you're doing HLG with Rec2020 or P3, from what I understand that will NOT look correct on a non wide-gamut display. Gamut conversion is not trivial.

So... how does that work?

Your screenshot looks at least mostly correct to me, so I'm wondering how you / VLC converted the gamut from Rec2020 to Rec709.
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Old 13th April 2020, 21:36   #3  |  Link
wswartzendruber
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Quote:
Originally Posted by Blue_MiSfit View Post
Fun project!

Here's one thing I've been trying to wrap my mind around regarding HLG: I get that one of the big appeals is "backwards compatibility", and I totally see how, from an EOTF standpoint it's somewhat backwards compatible with SDR. In other words, you could feed an HLG signal to a dumb SDR display and the luminance would look mostly correct.

However, I don't understand how (without some other component) it can be backwards compatible from a color gamut standpoint. If you're doing HLG with Rec2020 or P3, from what I understand that will NOT look correct on a non wide-gamut display. Gamut conversion is not trivial.

So... how does that work?

Your screenshot looks at least mostly correct to me, so I'm wondering how you / VLC converted the gamut from Rec2020 to Rec709.
It's entirely on the player to do that, as you have surmised. VLC does an outstanding job of it while Kodi doesn't even seem to try (everything looks washed out).

My assumption with VLC is that it is using some kind of logarithmic roll-off to compress the color dynamics. However they do it, the results are outstanding.

EDIT: I have to take that back. I put Alita through Kodi and Rec.2020->Rec.709 mapping was indeed happening. That other clip I played must not have had the right colorspace signaled.

Last edited by wswartzendruber; 14th April 2020 at 07:34.
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Old 15th April 2020, 20:40   #4  |  Link
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Quote:
Originally Posted by Blue_MiSfit View Post
Fun project!

Here's one thing I've been trying to wrap my mind around regarding HLG: I get that one of the big appeals is "backwards compatibility", and I totally see how, from an EOTF standpoint it's somewhat backwards compatible with SDR. In other words, you could feed an HLG signal to a dumb SDR display and the luminance would look mostly correct.

Interestingly enough, I do it the other way around:
Watching most SDR content with HLG.
This works quite well for most movie-like content
and b0rks most of the time with video-like content.

It seems that gradual luminance saturation (and chrominance desaturation) of the SDR film-curve in bright areas of the image fits the HLG-curve quite well.

On the other hand SDR with video-content peaking colors will look terrible with HLG applied.

My Sony OLED-TV allows to force the use of HLG on any type of input signal.
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Old 17th April 2020, 05:33   #5  |  Link
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I've attached some more screenshots for demonstration. All in all, the HLG-on-SDR picture is somewhat dim, but I find that this leads to a more pleasant viewing experience, especially with the lights turned out. Reference white comes in on my monitor somewhere around 100 nits and bright whites come in around 200. So the specular highlights are noticeable as the director intends. In contrast, the SDR grading of the film looks washed out and loud.
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Old 17th April 2020, 13:23   #6  |  Link
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Quote:
Originally Posted by wswartzendruber View Post
My motivation was that I wanted to rip my 4K UltraHD library, but wanted to do so in a viewing format that is as universal as possible, even if that entails some amount of compromise. I quickly settled on HLG after reading about it, but couldn't find a conversion utility to do this. I learned that ffmpeg supports the use of lookup tables, but couldn't find any such thing for cheaper than a couple hundred dollars. Others were available from the BBC under license.
Oh, really...? *coff coff* Link *coff coff*

By the way, the fact that people were charging a lot of money for those matrices of linear transformation is exactly what made me want to share them for free...
Of course, as I always said, the ones made by AVID, BlackMagic, BBC and so on have had a lot of engineers working on them, while I made mine together with my boss in a studio room and they have been tested against a single professional monitor by Sony and not a wide range of them, but still...

Quote:
Originally Posted by wswartzendruber View Post
All in all, I got quite fed up with the situation and started researching PQ, HLG, metadata, gamma curves, etc. until I got nauseated and started work. Then I studied some more and then went back to coding. Wash, rinse, and repeat a few times. I'm at the point where I'm comfortable with releasing my work to the public. Principally, it uses, to the best of my understanding, the algorithm published by BBC Research & Development.
Yep, starting with the BBC Whitepaper is always a good starting point as it basically explains how the conversion should be done.


Quote:
Originally Posted by wswartzendruber View Post
This is also the first thing I've written in Rust, so it's probably not going to be as idiomatic or as proper as it could be.
I don't actually know rust as programming language as I never studied it, but still hats off for researching and implementing your own converter.

Quote:
Originally Posted by Blue_MiSfit View Post
Fun project!

Here's one thing I've been trying to wrap my mind around regarding HLG: I get that one of the big appeals is "backwards compatibility", and I totally see how, from an EOTF standpoint it's somewhat backwards compatible with SDR. In other words, you could feed an HLG signal to a dumb SDR display and the luminance would look mostly correct.
Correct.

Quote:
Originally Posted by Blue_MiSfit View Post
However, I don't understand how (without some other component) it can be backwards compatible from a color gamut standpoint.
In fact, it isn't.

Quote:
Originally Posted by Blue_MiSfit View Post
If you're doing HLG with Rec2020 or P3, from what I understand that will NOT look correct on a non wide-gamut display. Gamut conversion is not trivial.

So... how does that work?
It's not converted. Let me clarify that.
The whole reason why we want to air with HLG, as broadcasters, is that we wanna have both HDR and SDR on the same channel, BUT (and here's the catch) most people tend to think that when we say "SDR" we're actually referring to Linear BT709 100 nits, but we are not.

When we air a 4K UHD stream via satellite it has two things mainly: the colormatrix and the color curve.
If you have a new 4K UHD TV, it's gonna read the stream, understand the BT2020nc, understand the HLG curve and display the color curve correctly, thus showing you the right amount of nits (up to 1000) that the content has. You're definitely watching an HDR content.
If you have an old 4K UHD TV, however, when it's gonna read the stream, it's gonna correctly interpret the BT2020nc, but it's just gonna ignore any information relative to the color curve, thus displaying the content as BT2020nc SDR 100 nits. The more a content is graded towards HDR, the worse it's gonna look on BT2020nc SDR panels, the more is graded towards SDR, the less you're gonna have dynamic range on HDR monitors, hence the compromise and the 1000 nits limit.
For quite some time I had to grade live sports events to 400 nits maximum 'cause many people didn't have HDR panels at the very beginning.

For the records, the reason why you can't do that with something like PQ is that if you ignore the color curve information on a PQ stream, then everything is gonna look very dull, as PQ is a proper logarithmic curve, while HLG is indeed Hybrid and compromises details in the shadows to make it SDR compatible but still has room for the peak brightness to be way higher than 100 nits.

Out of curiosity, this is how you're gonna see the very same HDR HLG signal on two different monitors: one BT2020nc SDR which ignores the color curve and the other that reads and interprets the color curve:




As you can clearly see, it's a compromise and it's never gonna look exactly as we intended it to look, which is something that kinda pisses us all off (broadcast encoders), but hey, we barely have the bandwidth to have 25 Mbit/s H.265 on all our 4K channels, so it would be madness to have two 4K channels for everything...

Last edited by FranceBB; 17th April 2020 at 13:30.
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Old 17th April 2020, 13:31   #7  |  Link
videoh
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Quote:
I quickly settled on HLG after reading about it, but couldn't find a conversion utility to do this.
I cough like FranceBB too (no, not the virus!):

http://rationalqm.us/hdr/DGPQtoHLG_1.0.rar

It's an Avisynth filter that supports both SW mode and CUDA mode.

Last edited by videoh; 17th April 2020 at 13:37.
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Old 17th April 2020, 21:44   #8  |  Link
wswartzendruber
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Quote:
Originally Posted by FranceBB View Post
Yep, starting with the BBC Whitepaper is always a good starting point as it basically explains how the conversion should be done.
Am I crazy for wanting to add scene-specific tone mapping to compress anything over 1,000 nits via dynamic PQ metadata? This was a goal from the beginning, otherwise, I would have written this thing to just output a static LUT.

And what's this color curve you speak of? I don't remember reading anything about that. I know that I have to weigh R, G, and B by differing factors to determine the overall luminance of a PQ pixel, but that's about it. Everything else seems to treat R, G, and B independently of each other.

Oh, and here are some more screenshots.
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Old 30th July 2020, 07:04   #9  |  Link
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I've Been Busy

I've made some modifications. Here's the new help screen:

Code:
PQ2HLG 0.1.0
Converts from HDR-PQ (HDR10) to HDR-HLG

USAGE:
    pq2hlg [OPTIONS] <INPUT-FILE> <OUTPUT-FILE> --height <PIXELS> --width <PIXELS>

FLAGS:
        --help       Prints help information
    -V, --version    Prints version information

OPTIONS:
    -h, --height <PIXELS>     Height of the input video stream
    -r, --ref-white <NITS>    Brightness of the input video stream's reference white level [default: 203]
    -w, --width <PIXELS>      Width of the input video stream

ARGS:
    <INPUT-FILE>     Raw input video stream (PQ, Rec.2020, RGB48)
    <OUTPUT-FILE>    Raw output video stream (HLG, Rec.2020, RGB48)

This utility follows the BBC R&D method for converting PQ to HLG. Any pixel brighter than 1,000 nits will be clipped and
no tone mapping will be applied. If a reference white value is supplied, then the input signal will first be linearly
scaled so that the reference white level becomes 203 nits.
I would appreciate someone letting me know if linear scaling for reference white adjustment is the way to go. When it comes to Alita: Battle Angel, I've noticed that the darker areas of the picture are marginally darker than with the native Rec.709 grading from the 1080p Blu-ray. Again, this is when comparing in VLC on a SDR monitor without WCG.

As is the usual case, there are a few more screenshots. These were taken from an HLG encode that uses the new reference white scaling. I sampled the credits and determined that this movie's PQ grading uses 157 nits. So that was linearly scaled up to 203 across all three color channels.

In the future, I intend to add support for Kim-Kautz tone mapping.

EDIT: Imgur Album of Latest Screenshots

Last edited by wswartzendruber; 31st July 2020 at 05:45.
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Old 30th July 2020, 11:52   #10  |  Link
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I'd suggest you to use an image host like imgur 'cause attached screenshot have to be approved first and that can take time (and it also uses space from the Doom9 server).
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Old 31st July 2020, 05:45   #11  |  Link
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Done. :-)
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Old 31st July 2020, 12:58   #12  |  Link
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wswartzendruber
Once you're strong in HLG may be you can help us (SAT-amateurs) in writing good working (giving near real colors) HLGtoSDR shader https://forum.doom9.org/showthread.php?t=176909 ?
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Old 31st July 2020, 20:40   #13  |  Link
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Following up on some posts from above, it sounds like the plan is to have a single 4K BT. 2020 HLG stream that will serve two groups of customers:

1) Folks with modern 4K TVs with HLG and 2020 support who will see the extended dynamic range that HLG brings in addition to the wide 2020 color gamut

2) Folks with first generation 4K TVs that are SDR, but still support 2020/WCG. They get the wider gamut, but and will still be able to watch the content acceptably even being totally naive to HLG. I imagine folks with modern 4K TVs that have HDR10 and DoVi, but still lack HLG for some reason (like my older LG B6) would fall into this category as well.

This leaves everyone else, but you'll still uplink a standard HD SDR 709 feed for them?

So... the industry is going through the hassle and compromises of HLG just to help out that small group of 4K early adopters with SDR panels that support BT. 2020? Seems a little silly to me, honestly! How big is that user-base, and how much of even the P3-D65 gamut will they even see (forget about 2020)?

There must be some other reason for using HLG. Is it just the momentum around HLG in production workflows (cameras, etc)?
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Old 31st July 2020, 23:06   #14  |  Link
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Quote:
Originally Posted by Blue_MiSfit View Post
Following up on some posts from above, it sounds like the plan is to have a single 4K BT. 2020 HLG stream that will serve two groups of customers:

1) Folks with modern 4K TVs with HLG and 2020 support who will see the extended dynamic range that HLG brings in addition to the wide 2020 color gamut

2) Folks with first generation 4K TVs that are SDR, but still support 2020/WCG. They get the wider gamut, but and will still be able to watch the content acceptably even being totally naive to HLG. I imagine folks with modern 4K TVs that have HDR10 and DoVi, but still lack HLG for some reason (like my older LG B6) would fall into this category as well.

This leaves everyone else, but you'll still uplink a standard HD SDR 709 feed for them?

So... the industry is going through the hassle and compromises of HLG just to help out that small group of 4K early adopters with SDR panels that support BT. 2020? Seems a little silly to me, honestly! How big is that user-base, and how much of even the P3-D65 gamut will they even see (forget about 2020)?

There must be some other reason for using HLG. Is it just the momentum around HLG in production workflows (cameras, etc)?
If you are speaking to me, then you misunderstand. This isn't about the industry or what they want, this is about me and what I want. With that said, I am also a very big believer in open source software and in sharing work. At least a few other people will likely find this useful as well. Eventually, that is.

I want a process to rip a 4K disc such that I have a single encode that can play back on any device with a competent player. Right now, I have Alita on my desktop (Rec.709), my phone (Rec.709), and it will also be on my HTPC when I get that, which will be Rec.709 first and then Rec.2020 later on. I want a single encode for all of these devices.

And this is because I find PQ to be an absolute joke for content distribution, and a bad one at that. I am utterly convinced that the reason Dolby persuaded everyone to go along with it was because it created an artificial need for Dolby Vision, which they would in turn profit from.

My goal is to create a single workflow for PQ content that allows me to adjust and transcode anything I have into a consistent HLG stream. This means, for example, that each transcode will have a reference white level of 75% signal strength (or as best as I can manage this). VLC and Kodi already handle 2020->709 amazingly well as they are, so I don't have a colorspace issue. And I don't have a luminosity problem, either, because I've determined that VLC clips HLG at precisely the 75% mark, which is exactly what I want it to do.

Something I'm really interested in at the moment is scene-specific tone mapping via dynamic metadata, since I have to stay within 1,000 nits.

But regardless of how I get to HLG, I will never need metadata during playback. What a farce. I seriously cannot believe everyone went along with this.

EDIT: Well, I guess the BBC and NHK didn't go along with it.

Last edited by wswartzendruber; 31st July 2020 at 23:32.
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Old 1st August 2020, 01:32   #15  |  Link
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Oh no, I wasn't directing that at you I was directing it at FranceBB's description of their transmission process

To be honest though, as someone who's implemented PQ based workflows (HDR10 and Dolby Vision) for streaming services, it all works fine. For HDR10, Most of the displays just ignore the metadata and do whatever they want.

The Dolby Vision stuff is great, and works really well for preserving high luminance details and getting consistent color reproduction, especially on cheaper devices (TCL TVs etc). It also kicks in special rendering pipelines on the displays that do less bad things, and can result in sane output settings. People do wacky things to their TVs

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Old 1st August 2020, 11:52   #16  |  Link
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Derek was referring to this post

As to the reason, believe it or not, that's one of the main reasons. What happened is that some companies reacted quicker than others in a period in which there was a huge technological switch and that made things complicated. The first thing is of course about manufacturers that released TV that were able to see BT2020 only without the HLG color curve as I mentioned above. In that period for instance (I believe 2015) we rolled out UHD in an H.265 stream with BT2020 SDR 100 nits. That worked like a charm. Many companies did the same but when we had to release it across the group as well like in Italy and other countries two years later, there were HLG and PQ already and we wanted to have a process as consistent as possible by launching HDR but also keeping old users with the old UHD BT2020 SDR option to be able to see contents. Being like "you can't watch UHD anymore unless you buy a new TV from today" after as little as two years wasn't really ideal for users... So we ended up with the HLG compromise as we didn't really want to air two separate 4K streams and waste twice the bandwidth, especially 'cause it's extremely costly on HotBird as it's really overcrowded. Sure, people may say that internet based decoders will be the future, but not everyone has a good connection and you're also moving the cost from a satellite to a CDN which isn't exactly cheap (although it's way cheaper than the satellite), which is why we're inviting customers to subscribe to our fiber internet offer.
Anyway, I'm digressing.
The thing is that this is our situation and many companies did the same. You also mentioned the cameras and that's true, for live sporting events they can shoot directly in BT2020nc HLG 10bit which is a big added bonus 'cause you can easily connect them all via SDI, route them to your beloved video server, adjust them and you're fine. You can also live downscale and tonemap the signal to FULL HD Linear BT709 SDR 100 nits and since it's a 50p stream it can be easily divided in fields to get a 25i all live with specialized hardware so that it can be aired in FULL HD as well (and in 720x576 25i Linear BT601 SD from the FULL HD stream for what matters). So it's a win win. If you wanna go to PQ instead, it becomes a bit trickier. Cameras don't generally shoot in PQ, they either use Linear BT709, HLG or Log (where Log can be SLog, CLog, LogC, FLog, VLog etc) depending on the manufacturers to get as many stops/nits as possible. Of course you can't air in Log (although I've had much fun laughing at other companies that did this wrong and ended up with things like highlights in Log on air by mistake xD) so you have to apply a matrix of Linear Transformation to go from Log to PQ and that has to be manually written every single time, for every event and possibly in the same even, adjusted for every camera. So you still have everything connected through SDI which goes to your hardware which performs the linear transformation and delivers it to the video server where you can adjust everything and go on air. Now, what about the ones with a TV that ignores the PQ colour curve? They're gonna have to watch the FULL HD Linear BT709 SDR 100 nits 25i stream... Right, speaking of which, how are you going to produce it? The same way as before? Oh no, that would be lame, you shouldn't start from an already linearly transformed signal (the PQ one) and tonemap it to Linear BT709 'cause your original signal is in Log, so you should start from the Log. Therefore, you're gonna need a second tech ops, encoding and ingest department that is gonna do exactly the same as the 1st one but instead of linearly transforming to PQ, they linearly transform to Linear BT709 and divide the 50p in fields to get the 25i. This is a mess, you have redundant people doing the same thing and you're gonna have to put extra stress on your departments and have two separate production teams... And for what, since many TV and cameras were not able to go past 780 nits leaving alone 1000 nits 'till not so long ago? If you take all these things into account, then HLG suddenly looks like a feasible option...


Side note: I think that BT Sport actually tried a similar workflow but I'm not sure 'cause I don't work there but apparently some former Sky employees are working there and these were the rumors, go figure...


Side note II: this is what I learned from my very personal experience, things might be different in other countries and other companies, but from what I saw I'm pretty confident that we're all in the same shoes more or less.

Last edited by FranceBB; 1st August 2020 at 11:58.
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Old 1st August 2020, 18:04   #17  |  Link
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Wow, and here I am liking HLG simply because it's relative and not absolute.
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Old 1st August 2020, 18:10   #18  |  Link
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Originally Posted by wswartzendruber View Post
Wow, and here I am liking HLG simply because it's relative and not absolute.
I'm with you, bro. PQ requires a reference background brightness that diverse environments cannot all satisfy.

Last edited by videoh; 1st August 2020 at 18:13.
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Old 2nd August 2020, 04:05   #19  |  Link
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Avengers is encoding now. I pegged reference white to the "MARVEL" text at the start. Hopefully it turns out good.

Quote:
Originally Posted by Blue_MiSfit View Post
To be honest though, as someone who's implemented PQ based workflows (HDR10 and Dolby Vision) for streaming services, it all works fine.
I genuinely want to know why streaming these days is not done in HLG.

Last edited by wswartzendruber; 2nd August 2020 at 06:20.
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Old 4th August 2020, 11:41   #20  |  Link
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I genuinely want to know why streaming these days is not done in HLG.
I believe due to not all players (devices like TVsets etc) support HLG as well as SDR but in nearest future HLG must becomes standard de facto because declared backward compatibility with SDR.
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