4K UHD Blu-ray to DCP for playback on 2K commercial projector?

[Carsten]

okay

The resulting MKV is readable, VLC recognises it as h.265/HVEC 10 Bit 4:2:0, BT.2020. DCP-o-matic the same, as expected, the contrast (visible in preview) is reduced, looks dull, some scenes grainy and streaky. Many blown-out highlights in the rec.709 version have surprisingly low luminance values in the UHD version. I guess that is what they were trying to do when they created the HDR version of the film original - recover as much highlight detail as possible.
The video waveform shows very few X'Y'Z' values above 3600 or so.

So, I don't think that currently there is anything useful I can do in DCP-o-matic with it. I will try to play around with some gamma settings, but while I expect to lift the contrast a little bit that way, I don't think it will benefit much compared to the plain rec.709 version. I guess this disc will still look better when played through an UHD player directly into the projector with some tone mapping options. I am pretty sure some people have found ways to play back this kind of file in a better way on a computer, newer VLC versions have options for HDR tone mapping.

[maxwellwCT]

This is what I came here wondering about. Didn't want to wait overnight if it was going to come out pale and washed out like HDR screenshots look. That is what DOM gives me in the preview!

[Carsten]

I think currently the only useful way to marry a UHD disc with a commercial cinema projector is to play it live through a HDFury Vertex(2) or Arcana.


Barcos ICMP Server has HDMI 2.0 inputs and HDR color conversion, but it is an extra license, and as far as I know, it currently does not support dynamic metadata, only static.

- Carsten

[Carsten]

Maybe Carl has the time to have a look into this, as now FFMPEG seems to support it:

https://codecalamity.com/encoding-uhd-4 ... th-ffmpeg/

[carl]

I can build DoM with a newer FFmpeg and we could see what happens...

[Carsten]

Honestly I am not quite sure if I understand what kind of procedures this guy Chris is doing. But he seems to have/offer a free transcoding tool, is talking about 10/12 Bit processing pipelines, and seems to understand the whole issue. Maybe it would be worth sending him a mail and explaining the task.

As far as I understood, Handbrake now also supports HDR file formats.

Wondering WHY so many people seem to be talking about ENCODING HDR/UHD videos, though. Shouldn't they have players that can play UHD-RIPs in their original format - usually MKV? Why bother about transcoding then? Just to have a streamlined library?

It probably also seems to be relevant that UHDs use different methods, like HDR, HDR10, Dolby Vision, or two methods at the same time. So, maybe one should start with an easier one (which is probably HDR10).

From the screenshot in that other thread, it seems that DOM/FFMPEG already detects the UHD/HDR format properly.


https://dcpomatic.com/forum/download/file.php?id=924

I guess the main issue is to transform the BT2020 material in the best possible way into a 12Bit X'Y'Z'. What I understand from some of Chris' blog posts is that most UHDs, while technically being BT2020, internally only represent the P3-D65 color range. So it should be possible to encode that into an X'Y'Z' that should render nice enough on a P3 cinema projector. There are some unresolved issues, though, e.g. the fact that UHD/HDR usually is encoded at least towards a 1000nit display, and we are very far from that in a cinema at 50nits. Maybe some locations (e.g. ours), can up this to 100 or even 150nits, but even that is far away from a typical direct view display.


Do you have any HDR source material to play with?

[carl]

Yes I haven't really looked into this at all, so I have no real idea what's going on and have no test material!

I'm away for a couple of weeks so won't have much time - perhaps when I get back we can take a look.

[Kewl]

Just adding my 2 cents here...

In the last week, I have converted a certain number of HDR UHD Blu-ray remuxes in MKV files to DCPs. I don't pretend the method is perfect, but it seems pretty good. Before sending the MKV remuxes to DOM, I use ffmpeg and a Mac Studio GPU to convert them to BT709 SDR 10 bit.

Code: Select all

ffmpeg -i INFILE -map 0:0 -map 0:1 -vf zscale=t=linear,tonemap=hable,zscale=p=709:t=709:m=709 -c:v hevc_videotoolbox -b:v 70000k -profile:v main10 -c:a copy OUTFILE

It's been a while since I watched a movie projected in a theater (summer 2018?) and I have an OLED at home, so I'm biased in evaluating contrast. Anyway, I made 4K scope DCPs with DOM and ingested them in a Barco 4K ICMP server/projector. The result on screen is pretty good. A bit on the green side and lacking contrast (compared to an OLED). And our projection room is not in total darkness, so that's also a factor.

So, is DOM suppose to be able to do a HDR to SDR conversion?

[Carsten]

Guess it would be interesting to compare the result with a standard Bluray created from the same master (on many UHD discs, an additional standard Bluray is included).

I do not doubt that you can convert any UHD disc to a tone mapped rec.709 intermediate format. However, the original idea is not only to retain the 4k resolution, but to transfer the higher dynamic range to a higher dynamic range X'Y'Z' DCP. Using rec.709 (even 10Bit) SDR will lose the extra tone range of UHD.

As DCP-o-matic offers a BT.2020 conversion, it would make more sense to convert the UHD MKV to a 12 Bit BT.2020 intermediate for the DCP conversion.

- Carsten

[Kewl]

I dit not try to make DCPs directly from the BT.2020 HDR because the preview in DOM seemed off. This https://dcpomatic.com/mantis/view.php?id=2318 seems related to what I saw.

If DOM can/could directly tonemap HDR to X'Y'Z', so much the better.