[Help] adding black borders preserving HDR

[spoRv]

Let's say that I have a "natively cropped" HDR file, and I want to add back black borders, to get it 1.78:1 and subsequently burn it on a UHD-BD.
Is there any free tool that would let me do it, preserving the HDR? Davinci Resolve, maybe?

[deleted user]

I don't quite understand what you mean by "natively".

But in general, you can add black borders to a HDR file just fine in AviSynth+ while preserving the bit depth and YUV color space. Just add the borders like you would normally, not much else you have to do in theory. But of course you will have to reencode and that can take a while with x265. You can also encode it in x264, but that is of course pointless, as it does not support HDR metadata, hence no player will play it, and BD compatibility won't exist.

When reencoding with x265, just add the HDR parameters from the original file as encoding parameters (all those mastering display and matrix data and whatnot). I believe those are mainly for helping a TV map the values to its own dynamic range if it doesn't support the full 1000 nits or whatever, which is why you don't have to take them into account when adding the black bars or processing in AviSynth.

Now, I have heard of the existence of metadata that are actually in the stream itself and not constant, meaning they might change per scene. I don't really know anything about those, you would have to do some research yourself. But I don't think they are that much of a big deal probably, as again they would only affect the mapping I think.

If you hate the idea of AviSynth+, I would check and see whether x265 has some integrated filters to add black bars, it's not entirely implausible.

You will likely also have to research x265 parameters for UHD BD compatibility, I can't help with that unfortunately.

[spoRv]

"Natively" is just a fancy word that mean... well, the file had a not 1.78:1 aspect ratio - like, you know, 1.85:1, 2.35:1 and so on - that it makes not UHD-BD compliant.

Avisynth+: always wanted to start to use it, yet afraid about its compatibility with plain old Avisynth plugins/scripts... but I guess I shoud try it, sooner or later!

Where do I "get" the HDR metadata? Oooffh, this HDR "thing" don't want to enter in my brain!

[deleted user]

"Natively" is just a fancy word that mean... well, the file had a not 1.78:1 aspect ratio - like, you know, 1.85:1, 2.35:1 and so on - that it makes not UHD-BD compliant.

Avisynth+: always wanted to start to use it, yet afraid about its compatibility with plain old Avisynth plugins/scripts... but I guess I shoud try it, sooner or later!

Where do I "get" the HDR metadata? Oooffh, this HDR "thing" don't want to enter in my brain!

Ah I see! Gotcha.

Okay, so first off, check my AviSynth+ thread here, I posted to it recently. Out of all my dozens of AviSynth plugins, there was only 1 file that was not compatible with AviSynth+ and I don't even know what the hell it was. After removing it, I never had problems again. In other words: All you have to really do is install AviSynth+ and there's a 90% chance it will just work like it always has, as it will automatically detect the old plugin folders.

You will have to update your FFMS2 plugin though to a newer version, so that you get support of high bit depths for importing.

Worst case scenario, you will have to spend 10 minutes to sort out your plugins to see which one is causing problems. 

The HDR metadata you can see in MediaInfo of the original file in the "Video" part.