[FFmpeg-user] [Cin] FranceBB's LUT collection

[Andrew Randrianasulu]

вс, 18 мая 2025 г., 04:36 Mark Filipak <markfilipak.imdb at gmail.com>:

> For people who are wondering what this is all about, I have a summary at
> the bottom.
>
> On 17/05/2025 16.46, Andrew Randrianasulu wrote:
> > On Sun, May 18, 2025 at 2:37 AM Mark Filipak <markfilipak.imdb at gmail.com>
> wrote:
> >> ...
> >> I tried a few 'cubes'. I was unsure which 'cubes' to use based solely
> on their names -- for example,
> >> what does 'linear' mean in this context
> >
> > As far as I understand "linear" here means linear light, without any
> > gamma curve applied, mostly for  further processing?
>
> For example, HLG_BT2020_to_Linear_BT709.cube. What does "Linear_BT709"
> mean? Linear... how? BT709 is
> not linear, so what's Linear_BT709?
>

Try to ask this question on github, where this project hosted? Or at doom9
forum ....

I suspect (from name patterns) linear is opposed to HLG/PQ *gamma* (EOTF?)
and BT2020/BT709 is colour primaries etc.




> To sum-up to this point...
>
> Let's call the next lines "tonemap-based".
>
> set _TWEAK_=eq=contrast=1.28:brightness=0.0725
> ffmpeg^
> ...
>   -filter_complex^
>   "[0:v]scale=w=iw/2:h=ih/2^
> ,zscale=transfer=linear:npl=100,format=gbrpf32le^
> ,tonemap=tonemap=hable:desat=0^
> ,zscale=primaries=bt709:transfer=bt709:matrix=bt709:range=tv^
> ,%_TWEAK_%^
> ,format=yuv420p[v]"^
>   -map "[v]"^
> ...
>
> as you see, that's not much tweaking.
>
> Now, let's call the next lines "LUT-based".
>
> ffmpeg^
> ...
>   -filter_complex^
>   "[0:v]scale=w=iw/2:h=ih/2^
> ,lut3d=file='BT2100_HDR_PQ_to_BT2020_SDR.cube'^
> ,lut3d=file='BT2020_to_BT709.cube'
> ,format=yuv420p[v]"^
>   -map "[v]"^
>   ...
>
> In contrast to tonemap-based, LUT-based conversions needed a lot more
> tweaking -- not shown because
> I didn't actually waste my time tweaking what obviously required much more
> tweaking.
>
> But LUT-based conversions are not supposed to need tweaking!
>
> My understanding is that your friends in France took various Hybrid
> Log-Gamma (HLG) curves for
> BT2100, BT2020, BT709, etc. and made LUTs of them for use doing
> conversions.
> My understanding is that the LUTs are formatted to work with FFmpeg's
> 'lut3d' filter.
> Something is wrong with my understandings, methinks.
>
> What do you think? Or do I need to go to France? :-)
>
>
> For people who are wondering what this is all about, I have a summary: HLG
> acts like ordinary gamma
> (almost 1:1 source-to-target transfer) for low-to-mid input luminance:
> 0.0..0.5, but acts like a
> logarithm (like compression) for mid-to-high input luminance: 0.5..1.0.
> Basically, it smoothly and
> progressively reduces the HDR values that are 'tending' to go whiter than
> SDR white, HDR values that
> blow out skies and sun-glints when in SDR, without losing HDR's deep
> blacks very much and without
> SDR's gray banding. The alternative to all this curve stuff is taking the
> HDR values that don't fit
> into SDR, and just clipping-to-white, blowing out skies and glints and
> highlights, etc.
>
> Of course, if you don't have an HDL source, then you're stuck with
> ordinary SDL and there's nothing
> you can do about it.
>
> As a bonus that's unrelated to color tones, HDR means UHD resolution (i.e.
> 2160x2040 or more), so
> you'll be able to transcode to HD (i.e. 1920x1080) and not only _not_ lose
> resolution, but actually
> get better resolution (better textures, cleaner edges) than most HD
> Blu-rays have.
>
> I have taken a 55 GiB, 2 hour 36 minute UHD movie -- HEVC encoded -- and
> made a 3.5 GiB HD copy --
> AVC encoded -- that looks better than the native HD Blu-ray of the same
> movie, better in every way.
>
>
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