Re: Display on operating systems requiring tone-mapping

> Using the HLG equations and iterator or pre-determined look-up table find
the HDR peak luminance and system gamma for the given, mandated SDR White
level.

FWIW, this is how I approached this when attempting to convert HLG into a
PQ render target (for displaying on Windows), given a predefined/forced SDR
level (such as the Windows SDR slider). I basically did a binary search to
find which HLG peak luminance yielded the input SDR level at 0.75, and then
I'd convert the image into that destination peak luminance. I was honestly
surprised to not see anyone else doing this, as it has always felt to me
like the anchor point for HLG was ensuring that all incoming contents' 0.75
signal was a consistent luminance when compositing.

If anyone is curious to see what such a search looks like, I have it here:
https://github.com/joedrago/colorist/blob/master/lib/src/transform.c#L117





On Thu, Oct 21, 2021 at 11:30 AM Lars Borg <borg@adobe.com> wrote:

> I had the same thought.
>
> We only want the peak to move. Not the gray card.
>
>
>
> *From: *Chris Seeger <Chris.Seeger@nbcuni.com>
> *Date: *Thursday, October 21, 2021 at 8:27 AM
> *To: *Simon Thompson - NM <Simon.Thompson2@bbc.co.uk>, "
> public-colorweb@w3.org" <public-colorweb@w3.org>
> *Subject: *Re: Display on operating systems requiring tone-mapping
> *Resent-From: *<public-colorweb@w3.org>
> *Resent-Date: *Thursday, October 21, 2021 at 8:26 AM
>
>
>
> I have one concern here related to graphic white.  Using this method,
> won’t it move graphic white around by pushing down the entire raster
> average brightness?
>
>
>
> Cheers,
>
> Chris
>
>
>
> *From: *Simon Thompson - NM <Simon.Thompson2@bbc.co.uk>
> *Date: *Thursday, October 21, 2021 at 2:10 PM
> *To: *public-colorweb@w3.org <public-colorweb@w3.org>
> *Subject: *[EXTERNAL] Display on operating systems requiring tone-mapping
>
> Hi all,
>
>
>
> At the last meeting there was some discussion on the application of
> tone-mapping on operating systems, particularly looking at what should be
> done when the SDR white level was mandated by the operating system.
>
>
>
> I’ve investigated and I think one possible solution could be:
>
> 1.       Using the HLG equations and iterator or pre-determined look-up
> table find the HDR peak luminance and system gamma for the given, mandated
> SDR White level.
>
> 2.       If the calculated peak luminance is higher than that of the
> target monitor:
>
> a.       Convert the image from the extended sRGB format to display light
> by converting to HLG and then applying the HLG EOTF with the peak luminance
> and system gamma calculated in step 1
>
> b.       Apply a tone mapping curve on the Y signal – we’ve had good
> results calculating a bézier curve or a logarithmic curve
>
> c.       Convert the image back to extended sRGB
>
>
>
> To reduce processing time, it may be necessary  to pre-compute some of the
> curves and apply the nearest match.
>
>
>
> In the following examples I use a bézier curve.  The images are HLG but
> even when viewed on an sRGB monitor, differences can be seen in the
> highlights – the lights, clouds and the posters.  The shadows and mid-tones
> are untouched.
>
>
>
> [image: A picture containing text, outdoor, way, road Description
> automatically generated][image: Chart, line chart Description
> automatically generated][image: A picture containing text, outdoor, way,
> road Description automatically generated][image: Chart Description
> automatically generated]
>
>
>
> I hope to join on Monday night, but am traveling and may be delayed.
>
>
>
> Best Regards
>
>
>
> Simon
>
>
>
>
>
> --
>
> *Simon Thompson MEng CEng MIET*
> Senior R&D Engineer
>
> *BBC Research and Development South Laboratory*
>
>
>