SRT Subtitles in HDR

[Etroxamin]

Hey,

trying to find a useful color for SRT subtitles to show a relative compareable white in HDR videos. Normal white doesn't work and is too bright. I've tried to change the color to Grey around 60%, seems to work.

However, I've noticed, in some videos, it may change to more grey, when the scene is really bright, or has very bright light somewhere. And the overall look may change from TV to TV and the setting as well, I would guess.

Just wondering if someone has tested this a bit more and could recommand a useful grey color?

[Blue_MiSfit]

I'm surprised the player doesn't adapt the subtitle presentation for PQ.

When you stream HDR content on Apple TV for example (typically with WebVTT subtitles) the subtitle rendering looks perfectly normal (meaning they did some work to render the subs at the appropriate light level)

[Etroxamin]

Originally Posted by Blue_MiSfit

I'm surprised the player doesn't adapt the subtitle presentation for PQ.

When you stream HDR content on Apple TV for example (typically with WebVTT subtitles) the subtitle rendering looks perfectly normal (meaning they did some work to render the subs at the appropriate light level)

Hey, thanks for your reply.

I converted SRT to VTT, using this web-tool here:

https://subtitletools.com/convert-to-vtt-online

the VTT subs are showing up when i put them into the same folder with the video file, just like other external subs would do. Seems to be quite similar to SRT (simple text based file).

I would guess the VTT Subs you watch then, may got a similar color correction applied to the VTT sub, meaning they eventually have applied a color shift from white to grey to show up "normal" by adding a color font to the subs. (which is the same thing i would want to do with SRT btw.)

Unless the VTT subtitle format has a HDR/SDR color function, but i would be suprised how it should work.

But maybe the player has a switch to do that, in practice the player should know if playing HDR or SDR, and could apply a color fix automatically.

Unfortunately MPC-HC doesnt seem to have a function for that and maybe other players as well i would guess (or i just haven't found it). HDR isn't quite new actually, MPC-HC last update was a month ago, so maybe its not as easy as i would guess, or nobody really cares.

Have you played HDR on PC too, and eventually know a player that can fix such problems, or maybe even a fix for MPC-HC?

And did you try to extract the vtt subtitles you were talking about, and see if they have a color added or if that is done by the player? Guess both would be possible, so it would be interesting to know.

[nevcairiel]

It should definitely be up to the player to do that. SRT subs don't even contain color information, so its entirely up to the player. Subtitles in HDR are a long-standing complaint though, and while some combinations of player and settings can get it to work, HDR in general is a bit plagued with growing pains even today still.

[Etroxamin]

Originally Posted by nevcairiel

It should definitely be up to the player to do that. SRT subs don't even contain color information, so its entirely up to the player. Subtitles in HDR are a long-standing complaint though, and while some combinations of player and settings can get it to work, HDR in general is a bit plagued with growing pains even today still.

Yes it should be better, if the player handles this. Only tried it with mpc and vlc, both show different colors (or brightness) for subtitles in SDR/HDR and both seem to have no HDR extra color settings.

Usually SRT has no color set, which defaults to pure white, or the color which is set in the player (most likely pure white), but it can be changed to any color, e.g. grey to get a similar "white" as SDR would show up, it can be changed that way, but the color cant be changed in the player again then, while the font can still be changed and of course it would be grey if the video gets tonemapped to SDR, so best to have HDR and SDR subs in one file or at least external for watching tonemapped.

So, my initial question wasn't to ask how SRT is handled by players when played in HDR (even if i would be happy to see a solution that way), but what color brightness value would seem to be useful for tonemapping subs for HDR content, considering that the SDR color would be pure white.

This question seems to be not easy to respond. Usually HDR is a little bit darker, there is a HDR/SDR brightness switch in Win10, if you open:

Settings > System > Display > Windows HD Color settings > HDR/SDR brightness balance

So the first question would be to ask what actually would be the best balance here, i think windows suggested it to be on 40 %, i have set it a bit lower.

So I was curious what brightness corrections some people here may would suggest to correct subtiles for HDR, this problem is of course not SRT only.

There obviously isn't a perfect value, but perhaps a very average value at least, depending on the TV color/brightness/gamme etc and those for windows/players/gpu etc it perhaps will change a little at least in comparison to what you see on screen.

Maybe there is also a clear value for brightness in HDR > SDR tonemapping, which would show always the same change for pure white to grey in HDR, but as far as i know different tonemapping algorithms will have different results and will be depending on calibration as well.

Well, still curios to see some more posts about it, and thanks for the replys so far and those that might come up in time.

[varekai]

@Etroxamin
Try free PotPlayer, works great for me.
There are a plethora of settings in PotPlayer and some great features for subtitles.
I use RGB248 for color, and RGB0 for outline, 2nd ouline and shadow, outline blur and outline gaussian blur.
To easily switch between SDR, HDR or other resolutions I export each PotPlayer settings to a reg file.

Using Windows 10
PotPlayer + madVR

[Etroxamin]

Originally Posted by varekai

@Etroxamin
Try free PotPlayer, works great for me.
There are a plethora of settings in PotPlayer and some great features for subtitles.
I use RGB248 for color, and RGB0 for outline, 2nd ouline and shadow, outline blur and outline gaussian blur.
To easily switch between SDR, HDR or other resolutions I export each PotPlayer settings to a reg file.

Using Windows 10
PotPlayer + madVR

Hey thanks for your suggestion, guess i will try some more players, but for now and maybe finally i guess it's just the easiest way to go with a prefixed color/brightness shift added to SRT file, just like subs on 4k blurays have all its tonemapped color. Actually i just dont want to change subs setting in anyway, maybe not even if it comes down to being as easy as to just press a button for it.

Personally i tried to find a value that is not glowing, but also not greyish and also fits to the overall feeling of HDR. my personal choice is now "#939393"

if someone is interested on what studios decide to use, i just checked a few of course, but so far i saw them at 52% - 62% of SDRs pure white, guess if i check more the difference may even grow higher.

[FranceBB]

Originally Posted by Etroxamin

Hey thanks for your suggestion, guess i will try some more players, but for now and maybe finally i guess it's just the easiest way to go with a prefixed color/brightness shift added to SRT file, just like subs on 4k blurays have all its tonemapped color. Actually i just dont want to change subs setting in anyway, maybe not even if it comes down to being as easy as to just press a button for it.

Personally i tried to find a value that is not glowing, but also not greyish and also fits to the overall feeling of HDR. my personal choice is now "#939393"

if someone is interested on what studios decide to use, i just checked a few of course, but so far i saw them at 52% - 62% of SDRs pure white, guess if i check more the difference may even grow higher.

You can do this:

1) Index the HDR source in Avisynth
2) Use
TurnRight.Histogram.TurnLeft
to see luma as a waveform
3) Add subtitles as pure white SDR with TextSub() and see how they skyrocket to the top
4) At that point, lower them in brightness in whatever subtitles editor you use, export them again and check them again in Avisynth to see how much lower they're.
5) Repeat the above 'till the waveform looks ok and subs don't skyrocket way over the waveform.


Some useful insight from my post here: https://forum.doom9.org/showthread.php?t=182986


Bonus tip: if there are credits at the beginning or the end in the original language, like in English or whatever and they're white, use a color picker in the subtitle editor you're using and get the very same value they're made of.
This way, you're gonna be ok 'cause it doesn't matter if they're in PQ or HLG and you're seeing them dimmed down, they're mastered at the peak brightness of the show and once you get yours with the same level, they're gonna be displayed as white correctly in an HDR monitor.

[Etroxamin]

Originally Posted by FranceBB

You can do this:

1) Index the HDR source in Avisynth
2) Use
TurnRight.Histogram.TurnLeft
to see luma as a waveform
3) Add subtitles as pure white SDR with TextSub() and see how they skyrocket to the top
4) At that point, lower them in brightness in whatever subtitles editor you use, export them again and check them again in Avisynth to see how much lower they're.
5) Repeat the above 'till the waveform looks ok and subs don't skyrocket way over the waveform.

Hey, thanks for your interesting post

In relation to HDR, I was thinking about if it would be possible to have scene based brightness adjustments to subtiles, theoretically yes, and for HDR it would be the only way to have "perfect" brightness for subs.

But guess it still wont really be possible, subtitles may appear over a longer time with scene cuts, were the same subtitle would need to be split in, the very extrem form, into seperated frames. That theoretically might be possible, but i dont know if there are avisynth functions to meassure each frame brightness and find the perfect matching "grey" for a perfect SDR looking white.

The question is if that really makes sense, and would be worth the work, in the end, if you show subs in HDR, it will change the brightness of the area were the subtitles are shown, unless there are TVs which would have a backlight for each pixel.

And subtitles (text) shown on display also usually start a bit more greyish, so splitting them into different colors, could make it even worse, causing some flickering, so it maybe would be needed to have the average brightness of a single subtitle range (e.g. frame 400-450).
and find best color for that frame range.

For the future, it may be possible to see TVs that can overlay an extra SDR layer just for subtitles, this might be the best way to have a real flawless workaround for subs in HDR, but perhaps this isn't really a priority for any TV manufactory and i also think most people wouldn't really care for such features anyway, if it is software or hardware based. The difference is perhaps too small.

Or maybe there will be a HDR subtitle format, that do the adjusments automatically.

Selecting the brightness level of already included text (like credits) is of course an idea, but i am not sure if it is really useful for subs in general. I mean just normal subs, not advanced .ass like some fansubbers do for anime - in HDR videos, these subbers perhaps will do just that for the forced subs, for common subs, one value throught the entire feature is maybe the better option.

Having only an eye cancer causing pure white in HDR makes it quite hard to define what white actually is.

But for now, i would suggest to simply use a color that looks like white, without being too grey. Unless there comes a way to have automatic adjustments with a clearly improvement and without a lot of trouble to get these created and working.

And about your hardcode thread, these will be of course only a few scenes, and it might be an idea to to do some extra work to have different tonemapping for each scene, but I still would doubt most studios would do that at all and absolutely not for non hardcoded subs over a whole movie.

Still would be interesting to see a way to have easy to do auto-adjusments to subs for hdr ...

[FranceBB]

Well I don't think having scene-adaptive subs is gonna be easy to achieve.
The best idea would be to just work with the waveform and set the subs to a gray-ish color that matches the highest peak in the image so that you're sure they're gonna be displayed as pure white in an HDR monitor.
My best bet for you would be to pick the color of the credits, apply it to all subs in every scene and get away with it.
I mean, sure, it's not idea and it could have been done better and in a sort of adaptive way, but I guess that this is the best chance you have to get something good out of it without going through a considerable effort.
In my case, you're right, I had to hardsub some signs, so I was good as I literally copied the color and levels of the official English ones using a color picker and cross checking with a waveform monitor.

Oh and just FYI that wasn't fansubbing, that's the Italian version of an official movie we had to localize which went on air on TV


Side note: professionally speaking almost no one does soft sub in HDR (and definitely not in broadcasting), but the only guys I've seen doing those things were from Netflix and they were using Colorfront Transkoder which was allowing them to pick sRGB or other color curves like HLG and PQ when they loaded the subs from an IMF Package and those were in... uhm... I think .ttml but my memory might be wrong.

[Etroxamin]

Originally Posted by FranceBB

My best bet for you would be to pick the color of the credits, apply it to all subs in every scene and get away with it.

In certain scenes, some subs may be shown slightly more white or grey, depending on the brightness of a scene, that's why i was thinking about scene based/adaptive coloring, but as you said it may would be hard to archive and maybe quite time consuming as well.

This also depends on the "quality" of HDR brightness range applied to a movie, it can happen a scene has too much brightness, turning the subs color looking like SDR pure white throughtout 95% of the movie into something more greyish in the other 5% of the movie.

Not every movie has white credits, I am actually not even sure if soft subs, like srt would be effected by the brightness settings for the video, but maybe just for HDR range overall, but there sure can be scenes that effect the subs and vice versa.

Originally Posted by FranceBB

In my case, you're right, I had to hardsub some signs, so I was good as I literally copied the color and levels of the official English ones using a color picker and cross checking with a waveform monitor.

Well, that's why i also was looking at the colors used by professional studios, but even for the same movie, looking the same (at least for brightness / colors) these values can be different - perhaps, there might be different opinions about what white should be in HDR and also a matter of subjective preferences.

I mean, picking up the colors from an official source, is perhaps never wrong, but you may end up with really ugly colors, e.g. something yellow-white-like, in such a case, most people would agree its not the best idea to do that. While in other cases, it may be the opposite, and you feel like the color of a subtitle is fitting so perfectly for the content, that it has to be used for either different languages or even picture based to text based transition, but that isn't limited to HDR of course, can happen in SDR too.

Originally Posted by FranceBB

Oh and just FYI that wasn't fansubbing, that's the Italian version of an official movie we had to localize which went on air on TV

Fansubbing can vary strongly i guess, i mentioned that, cause I believe some fansubbers turned the whole "Subs Thing" into pure art, i often was thinking their subs are part of the video tbh.

here is a guide for anime fansubbing bringing it to a really high level:

https://unanimated.github.io/ts/index.htm

It's really interesting and worth to have a look at. But looking through, you may wonder why all kind of professional releases still use "ancient" stuff like PGS subs

Originally Posted by FranceBB

Side note: professionally speaking almost no one does soft sub in HDR (and definitely not in broadcasting), but the only guys I've seen doing those things were from Netflix and they were using Colorfront Transkoder which was allowing them to pick sRGB or other color curves like HLG and PQ when they loaded the subs from an IMF Package and those were in... uhm... I think .ttml but my memory might be wrong.

found this link here:

https://partnerhelp.netflixstudios.c...ile-look-like-

seems like they use ttml.

also found this info thingy:

https://www.w3.org/AudioVideo/TT/doc...-Profiles.html

but nothing pops up when i search for "HDR", i would guess these formats may will have a color added to fit for HDR, but thats as already said in a prvious post, just guessing.

The question that now comes up is:

can one of these sub formats actually have a standard white, looking like "normal" white in HDR and are they compatible with mkv and common players?

The only disadvantage about adding a color to srt is that the color can't be changed later in the video player, as it will be fixed. On the other side, how much people will actually insist to have ... i dont know, red, green or whatever subs, guess most would just prefer white.

Still, it would be great to have a HDR subtitle format, automatically switching to a "perfect white" - whatever, and whoever may decided how it looks.

So, do you maybe know about such a sub format or to be more precise, is ttml able to be set to HDR content? So that subs will be shown natively as a "normal white" if no colors are added. WebVTT was mentioned, but on that sub info site link, its descibed as "simple", while ttml is described as complex

but to me it looks like, even it can natively show a useful white, it may be too advanced for most players to have other options like size and font changing. I would then, prefer a simple one like srt, i guess.

[FranceBB]

Originally Posted by Etroxamin

In certain scenes, some subs may be shown slightly more white or grey, depending on the brightness of a scene, that's why i was thinking about scene based/adaptive coloring, but as you said it may would be hard to archive and maybe quite time consuming as well.

Yeah, of course if a movie in PQ peaks at like 5000 nits but all the scenes are dark / low at around 400-800 nits except for explosions etc making subs peak at 5000 nits through the movie would be eye cancer. On top of that it would also make life hard for TVs that have like fewer nits than the ones at which the content peaks 'cause they would have to re-scale everything down, but the subs would get in the way...

Oh boy, this is complicated...

Originally Posted by Etroxamin

It's really interesting and worth to have a look at. But looking through, you may wonder why all kind of professional releases still use "ancient" stuff like PGS subs

Trust me, we don't, or at least we try not to.
I used to be a fansubber a long time ago, so I know about LUA, I know about AssDraw etc and I spent countless hours typesetting, which is why I tried to reflect this on my work.
This would open a far wider topic that is outside the scope of the original discussion, but if I had to briefly say why, I think it comes down to two things:

1) Hardsub
2) Softsub

Let's start with 1, Hardsub.

Whenever it comes to hardsub, we (and I'm talking on behalf of professional studios) generally receive a file which has a clock, bars note, the content like a tv series episode etc and then at the very end we get the textless so the bits of the original movie or tv series or whatever in which there were captions or signs or whatever in the original language (generally English). What we do is to trim out those parts and append the textless we found at the bottom there, trying to make those match perfectly per frame.
Of course "textless" are... well... without text so we gotta make sure that the localized version we create (like the Italian one etc) looks exactly the same as the original one and this is indeed done in Aegisub + Mocha or using other softwares like Virtz + AVID Media Composer for asset tracking etc. So... we do actually try to typeset.
If there's no typesetting in hardsubbed materials it comes down to 2 possibilities which I'm gonna call "A" and "B":

- A - We didn't get textless, so we can just put an {\an8} and insert the translated text at the top.

- B - We don't have the rights to modify the original video. Sometimes the company which produced the movie or tv series or whatever doesn't allow any kind of modification to their creation, of any kind, which includes typesetting, so we can't do anything at all. In this case, if there's a sign like a person is looking a mobile phone and gets a text message from someone we make the main character read the message in his/her own mind in the dubbed localized version.

Let's go to 2, Softsub.

Whenever it comes to softsub, we have our hands tied 'cause by standard TX files of any broadcasters are .mxf and the only sub you can mux in an .mxf is a .stl for SD files and .890 for HD and FULL HD files. This is a big fat problem 'cause those subtitles are muxed as OP47 like so:

Text #1
ID: 8-777
Format: Teletext Subtitle
Muxing mode: Ancillary data / OP-47 / SDP
Language : Italian

Text #2
ID: 8-778
Format: Teletext Subtitle
Muxing mode: Ancillary data / OP-47 / SDP
Language : English

And this is a rather big problem as those subs don't support any kind of typesetting at all and on top of that they have a very limited set of characters that are supported in each line, so... you know... there's nothing we can do, we're (us all as broadcasters and professional studios around the world) hog-tied to old standards which have been there since forever...

Originally Posted by Etroxamin

The question that now comes up is:

can one of these sub formats actually have a standard white, looking like "normal" white in HDR and are they compatible with mkv and common players?

mkv as container? Standard consumer players like MPV, MPC-HC, VLC etc?

I would say: No...

Originally Posted by Etroxamin

So, do you maybe know about such a sub format or to be more precise, is ttml able to be set to HDR content? So that subs will be shown natively as a "normal white" if no colors are added.

WebVTT was mentioned, but on that sub info site link, its descibed as "simple", while ttml is described as complex

Yeah, well, from what I understand (and keep in mind that I might say something wrong as I've never worked with them) I think they don't have such a setting in the sense that they can be set to sRGB, HLG or PQ by whatever is rendering it and they will be rendered accordingly and if set on an HDR curve they also have a static brightness level that can be adjusted, but I have no idea how to do this. This is from what I remember about a discussion I heard.

I might pass the ball to someone else if anyone wants to join the conversation.

It's a shame though, we're on Doom9, there's me from Sky, Ben from Amazon, Derek from Disney, Alex from Hulu / Discovery... but no one from Netflix?

Anyone from Netflix in here by any chance? xD

[Etroxamin]

Originally Posted by FranceBB

Yeah, of course if a movie in PQ peaks at like 5000 nits but all the scenes are dark / low at around 400-800 nits except for explosions etc making subs peak at 5000 nits through the movie would be eye cancer. On top of that it would also make life hard for TVs that have like fewer nits than the ones at which the content peaks 'cause they would have to re-scale everything down, but the subs would get in the way...

Oh boy, this is complicated...

well, guess there is nothing more to say here

Originally Posted by FranceBB

- A - We didn't get textless, so we can just put an {\an8} and insert the translated text at the top.

That's one of the few cases where i use .ass - and just put it into a box, so it hides the hardsubbed text completely, but i would guess for your work it might not to be considered, cause:

Originally Posted by FranceBB

- B - We don't have the rights to modify the original video. Sometimes the company which produced the movie or tv series or whatever doesn't allow any kind of modification to their creation, of any kind, which includes typesetting, so we can't do anything at all. In this case, if there's a sign like a person is looking a mobile phone and gets a text message from someone we make the main character read the message in his/her own mind in the dubbed localized version.

Originally Posted by FranceBB

standards which have been there since forever...

Well, there are quite a lot of pointless standards, on the other side, these must have been useful in the past and whenever new technologies get into consumers life, it's often associated with new problems based on the old standards, guess that will never change, but over time, will get better, it just can take a long time to get there.

Originally Posted by FranceBB

mkv as container? Standard consumer players like MPV, MPC-HC, VLC etc?

I would say: No...

Well, what isn't there now, might be in future

Originally Posted by FranceBB

Yeah, well, from what I understand (and keep in mind that I might say something wrong as I've never worked with them) I think they don't have such a setting in the sense that they can be set to sRGB, HLG or PQ by whatever is rendering it and they will be rendered accordingly and if set on an HDR curve they also have a static brightness level that can be adjusted, but I have no idea how to do this. This is from what I remember about a discussion I heard.

I might pass the ball to someone else if anyone wants to join the conversation.

It's a shame though, we're on Doom9, there's me from Sky, Ben from Amazon, Derek from Disney, Alex from Hulu / Discovery... but no one from Netflix?

Anyone from Netflix in here by any chance? xD

Well, let's see who maybe is, or may isn't lurking in depths of doom9.

And thanks for the interesting discussion here (so far).

May this grant some people a glimpse of the interesting world of HDR subs

... or maybe we see more facts coming here over time as well.

[Atlantis]

For HDR viewing I simply set them to Dark Gray. Either in the player itself (KODI) or you can add color in SRT files. <font color="#808080">