Re: Incorrect Algorithm and mDCv for HLG (From last night's meeting)

On Thu, 7 Sep 2023 16:17:40 +0000
Simon Thompson - NM <Simon.Thompson2@bbc.co.uk> wrote:

> Hi all,
> 
> I just thought I’d make comment on the two agenda items that we
> didn’t get to last night, so that they can be discussed at next
> week’s face-to-face meetings.
> 

Hello Mr. Thompson,

I wonder if you could entertain a couple side questions below.

> 
> Incorrect sRGB to HDR conversions listed in Canvas document:
> 

...

> function convertSRGBtoREC2100HLG(r, g, b) {
>   const systemGamma = 1.0;  // Can be removed, included for completeness
>   const linearLightScaler = 0.26496256042100724;
> 
>   const r1 = srgb_eotf(r);
>   const g1 = srgb_eotf(g);
>   const b1 = srgb_eotf(b);

Speaking of srgb_eotf(), which one do you prefer here: the two-part
function with a linear segment near zero, or the 2.2 power function,
and why?

> 
>   const [r2, g2, b2] = matrixXYZtoBT2020(matrixSRGBtoXYZ(r1,g1,b1));
> 
>   const r3 = linearLightScaler * r2;
>   const g3 = linearLightScaler * g2;
>   const b3 = linearLightScaler * b2;
> 
>   const [r4, g4, b4] = hlg_inverse_ootf(r3, g3, b3, systemGamma);  // Can be removed, included for completeness
>   const [r5, g5, b5] = hlg_oetf(r4, g4, b4);
> 
>   return [r5, g5, b5]
> }
> 
> Chris Needham and I will endeavour to add this to the example webpage
> before the next Zoom call.
> 
> 
> Question on the use of mDCv Maximum and Minimum Luminance for HLG
> 
> HLG is a relative scene light signal, so the settings of the
> mastering monitor are not required by the target display.

What about the color gamut?

BT.2100 is based on monochromatic primaries, and consumer displays tend
to use less saturated primaries. How does the HLG system avoid
saturation clipping or how would one describe it? How do you make the
best use of the available color gamut in a monitor or TV without
exceeding it?

Is there some assumption of what the color gamut is in a HLG signal,
and what do displays do to map that to their hardware primaries and
white point?

I would imagine that knowing the primaries of the mastering display
would help with color gamut mapping. Is that not necessary?


Thanks,
pq

> 
> There are published formulas for how a display should adapt the OOTF
> (system gamma) for different display peak luminance and different
> ambient illumination, i.e. the distributed signal remains constant
> irrespective of display, the display adapts the incoming signal
> appropriately.  These adaptations have been tested over a wide range
> of available screen luminances from lower than 400 cd/m2 to 4000
> cd/m2 (we’ve no reason to believe that it won’t work above this
> luminance, just we haven’t seen any production screens above that
> level to test it on).
> 
> In professional monitors the black level is set using a PLUGE signal
> (i.e. set to the lowest perceptible black level for the given ambient
> conditions) and the white level and OOTF set to suit usage.  The
> incoming signal is then mapped according to these parameters.  (For
> example, in a dark OB truck environment, many operators are turning
> down the brightness of an HDR monitor to 600 cd/m2 to a) better match
> luminance of shadow and mid-tones to the SDR monitors and b) reduce
> eyestrain from prolonged use.  Because of the display OOTF
> adaptation, this signal is still valid when displayed at any other
> luminance)
> 
> In consumer television displays and monitors, this OOTF adaptation is
> also applied to the input signal in the device based on the screen’s
> peak luminance and ambient viewing environment (sometimes by ambient
> light sensor, sometimes by having different viewing condition modes).
>  Unlike in a professional monitor, the parameter values are not
> usually exposed to the user.
> 
> Adaptation for screen peak luminance is given in Note 5f (and
> associated footnote) in
> https://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.2100-2-201807-I!!PDF-E.pdf
> 
> Description of PLUGE signal for setting screen black level and
> adaptation for ambient illumination is given in
> https://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.814-4-201807-I!!PDF-E.pdf
> 
> --
> Simon Thompson
> Senior R&D Engineer
> BBC Research and Development
>