Chroma sample location - what should I do?

[bokeron2020]

Hi!

I'm transcoding a 4K HDR10 bluray to H.265 1080 HDR. Supposedly chroma sample location should be top-left but ffprobe shows it's center-left.
So... what should I do?

1. Keep it center-left?
2. zscale it to top-left and flag it in x265 to get a within-the-specs output?

Does this "trans-locating" degrades the image further considering I am downscaling at the same time?

[pandy]

firstly ffprobe need to double verified with other tool - it may incorrectly/wrongly report this parameter,
secondly if possible best is to simply flag correct sample location (if possible),

and last action you may correct chroma sample location if first step is true and second step is false:
you can upscale chroma planes (i recommend point sampling as it is perfect from path perspective), upsampling factor depends on desired accuracy but i think 4 times upsampling at least is required, then you can shift upsampled chroma planes - 1 pixel shift with 4 times oversampling will give you 0.25 pixel accuracy.
Last step is downsampling by same factor as upsampling, you should use best resizer algorithm available - finally you should get chroma samples in desired position.

[bokeron2020]

Thanks for your answer, pandy!

It didn't cross my mind to upsample first. I'll need to re-read your answer a few times and do more research/tests. This needs far more technical knowledge than what I have at the moment but your answer gives me the coordinates I need to find my way - hopefully.

[pandy]

Thanks for your answer, pandy!

It didn't cross my mind to upsample first. I'll need to re-read your answer a few times and do more research/tests. This needs far more technical knowledge than what I have at the moment but your answer gives me the coordinates I need to find my way - hopefully.

Upsampling is crude workaround if signal processing is performed with help of the regular video editing tools - in math domain such operation is valid same as any other.
I frequently use oversampling (upsampling) to achieve subpixel accuracy.
But firstly i would do everything to be sure that such radical operation is required anyway (especially that even 4 times oversampling for UHD will give 16k+ size of video data to process - even in modern HW a lot of data to process...)

[jagabo]

It's my opinion that it's a waste of time if you're downscaling. You'll won't see any difference except in special test signals.

[bokeron2020]

It's my opinion that it's a waste of time if you're downscaling. You'll won't see any difference except in special test signals.

I've started to wonder about this, too. Diminishing returns. For a home user like me, what difference would it make?

I mean, I want to do this properly within-specs but it seems publishers of content aren't all following current standards. I'm finding some UHD blurays with left instead of topleft, and downladable media is also all over the place when it comes to this.
I'm spending time and brains trying to repair what professionals did wrong only to keep the files for myself. I'm not a publisher nor a professional, though I still want to understand the whole thing, but I'm not so sure right now that, as you pointed, it's worth the effort to fix it.

So, repeating what I just said above, for a home user like me who just wants to do 4K-HDR-Bluray/4K-HDR-download >> 1080-HDR-mkv, new questions:

What are the problems I might face when playing media now or in the future if:

1. I just leave it as is, chroma location left, and transcode keeping it left.
2. I don't fix it but just flag it topleft [I can imagine this is a bad idea, but I'm curious about the consequences]