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HDR to SDR conversion in component wrong?

Any issues that are tvOS specific
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Sunfox
Posts: 3
Joined: 30 Sep 2018, 17:18

HDR to SDR conversion in component wrong?

Post by Sunfox » 30 Sep 2018, 17:40

I’m attempting to get playback of 4K HDR files to a 4K SDR television (Sony X850B) working. So far, my problem is - when comparing the HDR playback to a SDR 1080p encode played on my trusty Chromebox on the same TV, the HDR file has crushed blacks and blown highlights.

First, the AppleTV 4K (TvOS 12.0.1) is set to component (which is inherrantly limited range) with 4:2:0 chroma, as RGB is not supported at 60hz on this TV. I can force the TV into full range, and doing this creates grey blacks. So there’s no range mismatch there.

If I tell MRMC to use videotoolbox for HDR, or disable hardware acceleration entirely, bingo there’s all my missing detail - but there’s too much as there seems to be no conversion from HDR at all, and the picture is flat and lifeless.

It kind of seems to me like the HDR-SDR conversion is going to Full Range, but then that’s being output as component / Limited range without further conversion. Thus crushed/blown. Note that the “use limited colour space” option seems to have no useful effect on the output video.

Ideas?

cosmoxl
Posts: 1408
Joined: 12 Dec 2015, 14:16

Re: HDR to SDR conversion in component wrong?

Post by cosmoxl » 30 Sep 2018, 21:45

HDR to SDR conversion isn't a feature of MrMC.

But, using the AVFoundation pathway will lead to tonemapping in tvOS 12 or 12.0.1. It looks quite good actually.

In MrMC Settings>system>video how is "user limited color range" set? Mine is off (so it'll use full).

In Settings>video>acceleration how is "disable limited to full video rescaling" set? Mine is off.

My understanding is that those settings are bypassed by AVF anyway, but I figure it's worth asking. I could be wrong. :)

Sunfox
Posts: 3
Joined: 30 Sep 2018, 17:18

Re: HDR to SDR conversion in component wrong?

Post by Sunfox » 30 Sep 2018, 23:13

Use limited color range is “off”. Turning it on has no effect to video.

I tried the limited to full option and it again didn’t do anything (I think it would be limited to SDR videos anyways?)

I tried the video through tvOS VLC and it looked less crushed colour wise, although playback was otherwise unusable.

Sunfox
Posts: 3
Joined: 30 Sep 2018, 17:18

Re: HDR to SDR conversion in component wrong?

Post by Sunfox » 01 Oct 2018, 01:37

Further testing:

* The 1080p SDR BD looks as expected through the ATV
* The 4k HDR file still looks poor at 1080p rgb high/low or component 4:4:4
* The 4k HDR file also looks poor at 4k 24hz RGB or component 4:4:4.

So, basically, it just seems like the HDR-SDR conversion is not quite appropriate for this particular HDR encode. Reading up, it seems like that's a problem for a lot of players at this time, never mind that UHD BD players provide so many conversion options just because there doesn't seem to be a "one size fits all" solution.

The file DOES look correct with MadVR on my PC (not shown on that TV though), so I hold out hope that some sort of tweaking options might become available (like gamma curve?).

Kind of doubting that I'll ever be able to see proper 4K on this 4K TV, though. Yeah, I was a very early 4k adopter, and I guess that's the pitfalls!

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