Subtitle Brightness for Different Transfer Functions

[wswartzendruber]

I've noticed that the PGS subtitles on 4K Blu-rays are somewhat gray instead of white. This tells me that for HDR10 output, the luminosity of each PGS pixel is ultimately interpreted by the display using the PQ EOTF.

Is this true?

[FranceBB]

Well, I never muxed subtitles for UHD HDR-BD, however I've had hardsubbed UHD HDR HLG contents and I can tell you that subtitles MUST be with the HLG curve applied too!
If you try to apply a normal subtitle, like the one you would have in Linear BT709 SDR, what you get is a huge spike in the waveform monitor on the subtitle, which not only will screw up the subtitle, but the whole image as well!
If you're mastering a file and you put the subtitles without the HLG color curve applied, you would see the subtitles being incredibly bright, over contrasted and it will force the monitor to raise up the nits corresponding to those subtitles, which will cause a huge spike on OLED TV (as they'll push those pixels to the very maximum brightness possible) and make them annoying to read. On LCD TV, though, it will screw up things even more, 'cause to achieve the maximum brightness, it will have to raise up the brightness of the whole panel, so it will screw the background image, blacks will be pushed higher than they should and it will NOT look ok. Besides, it's even harmful as subtitles come and go all the time, so the TV will have to handle those intermittent spikes which is not good for the panel (LCD), not good for the individual pixels (OLED) and not good for the viewer (in general).
The same applies to signs, logos and other kind of captions in general.
When you're using a NLE like AVID or Davinci, you generally deal with those individually and then apply them to the video.
If the graphic department was kind enough to provide them with the right color curve + alpha channel, then fine, you just apply them on top of your HLG (or PQ) video, however if they made them in... I don't know like Linear BT2020 SDR without any kind of color curve applied, then you must perform a linear transformation by applying a LUT to bring them to HLG or PQ and then modify them according to your needs / tastes by constantly watching the waveform monitor of the whole file.
For instance, if your contents has been mastered at 400 nits, then a sign or a logo or subtitles can't be at HLG 800 nits for instance.
I mean, even if they're mathematically correct and with the right color matrix and color curve applied, they would look far too bright, so you should bring them down.
I generally like to keep them just slightly lower than the peak maximum of the nits of the scene if they're signs / logos, while if they're subtitles and you need to display them across the whole content, then I would get the value of the AVERAGE of the whole content and set them to that value, then see how they look like in a few scenes with the waveform monitor (HDR Scope) and eventually adjust them accordingly.
Remember that we're talking about color curves with a potentially huge set of values, so getting these things right is a MUST!

[Emulgator]

I can only concur to that.
Any subtitles no matter if I had watched them for reference or rendered them myself
came out more pleasing when they had NOT used maximum opacity, saturation, brightness.
As with classic music the max is reserved for only that singular occasion wich may not even be happening in that piece.
Steady 80% may well be too much already and steady 100% is definitaly a pain for the audience.
The better the equipment the more the power of ..anything, in that case: pain.

[wswartzendruber]

I'm tempted to go with 75% on the subtitles. If the player applies the HLG EOTF to them, then they'll be reference white (not that bright). If it applies the PQ EOTF, they'll be about 1,000 nits (which is kind of bright), but not fully bright (on future displays).