[Cin] EOTF, PQ, HLG ....

[Andrew Randrianasulu]

It seems that at least for "PQ" HDR systems their EOTF can be used to give
some reading into waveform scopes etc? As they set to "Nits" and not
-10-110% as usual for SDR (and may be Hybrid Log Gamma?) systems.

https://github.com/colour-science/colour/discussions/712

=====

A true PQ curve is an absolute cv-nit scale. The normalized CVs of a few
different nit levels are:

100 nit = 0.5081
600 nit = 0.6963
1000 nit = 0.7518
4000 = 0.9026

Here's Steve Shaw's write-up on the matter which further backs up my
concerns: https://www.lightillusion.com/uhdtv.html
[image: hdr_luminance_levels]
<https://user-images.githubusercontent.com/30381480/38954716-4812c0ba-4307-11e8-9137-9c0b9fd829a3.png>

This behavior is how reference displays should work (notwithstanding HDR
metadata and consumer-grade secret-sauce), and I can confirm is the way the
Dolby Pulsar, Sony x300, and x550 all work. When fed RGB(.5081, .5081,
.5081) on all three displays set to D65 ST2084, you should measure 100
nits. If you feed a linear ramp to all the displays, you will see a hard
clip of all values above the associated code value for that display's peak
luminance.

===== quote end ====


Apple's document

HDR and Wide Color Gamut in Final Cut Pro

White Paper November 2020

says:

====

Wide Color Gamut Displays that support wider color gamuts with a
traditional luminance range are sometimes referred to as wide-gamut,
standard-dynamicrange (SDR) displays. Current models of Mac, iPhone, and
iPad support a wider-gamut SDR color space called Display P3. Display P3
uses the same color gamut as DCI P3, the standard for movie theater digital
projectors, combined with a white point and gamma appropriate for typical
viewing environments. The figure below shows that the P3 color gamut
contains more colors than the Rec. 709 gamut, with notable improvements in
the range of red and green areas of the color spectrum.


[..]

High Dynamic Range Displays that support both wider color gamuts and higher
luminance ranges are referred to as high-dynamic-range (HDR) displays. HDR
displays can produce not only high peak luminance values but also very low
black levels, revealing more detail in both shadows and highlights. HDR
displays typically reach peak brightness levels above 1000 nits while
maintaining very high contrast ratios, much higher than the brightness
levels and contrast ratios of SDR displays. These levels of brightness mean
that images appear more true to life, with a dynamic range that more
closely reflects what the eye sees in the real world. The figure below
shows how increased brightness levels (measured in nits) combined with a
wider color gamut greatly expand the range of reproducible color.



HDR displays typically process video at 10 bits per color component rather
than 8 bits. The additional color data lets HDR displays render more
discrete steps from the minimum to maximum brightness value in each color.
In order to encode and transmit HDR images more efficiently, new
mathematical transfer functions are used instead of gamma. Two popular
standards are the Perceptual Quantizer (PQ) function, defined in both SMPTE
ST 2084 and ITU-R Recommendation BT.2100, and Hybrid Log-Gamma (HLG), also
defined in Rec. 2100.


About Changing a Project’s Color Space After you set a library to
wide-gamut HDR, any project with HD resolution or higher created in that
library can be set to one of the following color spaces using the Modify
command in the Project Properties inspector (described in step 3 above):

• Standard - Rec. 709

• Wide Gamut - Rec. 2020

• Wide Gamut HDR - Rec. 2020 PQ

• Wide Gamut HDR - Rec. 2020 HLG


[..]


When performing these adjustments, it may be helpful to show the Waveform
video scope and set it to either RGB Parade or RGB Overlay view. In these
views, the vertical axis of the waveform is labeled in units of cd/m2
(nits), which is a measure of the display’s output brightness level
corresponding to the PQ signal level. Note that for the Luma and Chroma
views, and for any non-PQ content, the vertical axis reverts to IRE (where
100 IRE represents 100% signal level). This is because the waveform signal
in these cases does not correspond directly to  absolute cd/m2 values.

=== quote ends here finally =====


So, it seems that we currently lack easy user choice between PQ/HLG
variants of Wide Gamut bt 2020 color space, I think in ffmpeg/swscale they
hide as "trc" (transfer curve) ... but I can be wrong.


Also tonemapper on cpu, at least on my potato,  tend to be slow, unless you
downscale to like 1/4 out of 4k source?

So it will be helpful to look into vf_libplacebo, opencl_tonemap etc on
machines where they work.


Sorry if it feels like too much of confusing info, I think while
technically cingg currently  capable of *encoding* at least some variants
of HDR -  more accurate displaying, easy user settings, waveform scope
display "morphing" to HDR mode, HDR10+ dynamic metadata calculations all
missed, relative to FCP/Resolve.
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