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Old 8th December 2017, 08:36   #1  |  Link
varekai
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Ultra-HD downscale to 1080p?

Hello,

Is there a way to encode/recode/compress an Ultra-HD to 1080p BD or to 1080p mkv without getting washed out colors?
Done a few tests and I always get very pale colors.
I'm shure there is an explantions to this but I can't figure it out.
Is it even possible to downscale to 1080p and get "normal" full BD colors?
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Old 8th December 2017, 09:46   #2  |  Link
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You probably need to do a color space conversion to BT.709. Most UHD content is encoded as either BT.709 or BT.2020 and if it was BT.709 colors would look correct.

The reason colors look pale is because BT.2020 is a very wide color space, 50% red in BT.2020 is much redder than 50% red in BT.709. Without doing a conversion all the BT.2020 color values are simply displayed as if they were BT.709 color values.

Another option is to tag your encodes as BT.2020, for x264 and assuming an SDR source this might be:
--colorprim bt2020 --transfer bt2020-10 --colormatrix bt2020nc

I have no idea how to do a HDR -> SDR conversion.
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Old 8th December 2017, 10:37   #3  |  Link
Sparktank
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for HDR -> SDR you can use FFMPEG's internal tone-mapping.
https://ffmpeg.org/ffmpeg-filters.html#tonemap
You'll have test various parts to ensure that different scenes don't look too out of place (light vs dark vs natural light vs etc).

It also looks pale if it's HDR content, which will need to be converted to SDR (via FFMPEG).

You could encode to HEVC in 1080p and keep the HDR metadata to passthru and let MadVR convert to SDR (this limits to PC playback only).

There's a vapoursynth script somewhere here to convert HDR -> SDR. Not sure how well it works compared to FFMPEG conversions.
https://forum.doom9.org/showthread.php?t=174415

Those are the only automatic processes that are free and easily accessible.
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Old 8th December 2017, 11:28   #4  |  Link
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Ahh... that's a lot more complicated than I was hoping for... :/
I'll just leave that project, 'cause it's a bit over my head.

Thanks for the the input though, appreciate that!

Best regards,
varekai
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