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FranceBB
15th February 2019, 13:46
Hi,
some studios are sending over XAVC-Intra Class 300 UHD 10bit BT2100 HLG 4:2:2 planar master files as well as some Apple ProRes UHD 10bit BT2100 PQ 4:4:4 planar master files. The problem is that neither of them have metadata; in other words, I have no clue about how many nits they have. I.E: no Master Display, no maxCll minCll, nothing.
I know that the new version of the Tektronix Waveform Monitor has a new function that makes you see the waveform for the nits rather than for the old standard video levels in Volt (0.0v - 0.7v -> 16-235 Limited range). Unfortunately, I can't get it for a reasonable price (of course) and I still have to check it and make sure everything is fine.
Let's put it this way: since we air in HLG, I could even potentially blindly trust the Studio that sent me the file, but I absolutely can't for PQ, 'cause whenever I have to convert it, I gotta make sure that it's converted to 1000 nits (maximum amount of nits allowed by HLG), but since PQ allows up to 10'000 nits (and some movies like Blade Runner were graded to 5000 nits) how can I check how many nits a content has and convert it accordingly? Is there a way I can do it using Avisynth? Are there any free 3rd party tools that allows it?
I'd like to know the peak amount of nits, the lowest amount of nits and the medium amount of nits.
An Avisynth plugin that does this and stores everything in a txt file (like the ssim plugin does for ssim) would be really appreciated.
Right now, I'm using HDRTools and my LUTs "blindly" -> I change the parameters for the PQ to HLG conversion trying to guess the amount of nits the PQ has based on the output result of the encoded HLG file (which I know is limited to 1000 nits).

Thank you in advance,
Frank.

ifb
19th February 2019, 02:17
VapourSynth's PlaneStats() would work for min/max/avg, but I can't help you with Avisynth.

You can't just cheat and look at a waveform? 1000 nit PQ peaks at 75% (full-range at 80% - should be able to determine limited/full range from the black level).

If you look at the steps (http://downloads.bbc.co.uk/rd/pubs/papers/HDR/BBC_HDRTV_PQ_HLG_Transcode_v2.pdf) to convert PQ->HLG, you only need to know the peak brightness so that you can map it to 100% HLG. I'm not sure how/if that works with the newest BT.2408-1 (https://www.itu.int/dms_pub/itu-r/opb/rep/R-REP-BT.2408-1-2018-PDF-E.pdf) recommendation though. Diffuse white should be 75% HLG and 58% PQ.

BTW, HLG is scene-referred. There's no inherent brightness limit. BBC claim HLG is good to 4000+ nit (probably why they have a LUT for both 1K and 4K PQ to HLG).