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

See proposed revised sRGB to HLG algorithm at:

https://github.com/w3c/ColorWeb-CG/pull/107

On Tue, Oct 3, 2023 at 7:44 AM Pekka Paalanen
<pekka.paalanen@collabora.com> wrote:
>
> On Fri, 29 Sep 2023 15:27:12 +0000
> Simon Thompson - NM <Simon.Thompson2@bbc.co.uk> wrote:
>
> > Hi
> >
> > Apologies for the brief reply, I’m on my phone.
> >
> > BT.709 and HLG are not display referred by the definition that ISO
> > uses, which is the one that almost all other standards bodies
> > reference.
>
> Hi Simon,
>
> thank you for the reply.
>
> Indeed, I have realised that my confusion comes from terminology.
>
> > For a signal to be display rereferred, the signal has to represent
> > the exact expected output of a display.  Neither HLG nor BT.709 do
> > that – they refer to the scene and the display is free to apply its
> > EOTF (complete with adaptations for screen and ambient luminance) and
> > in the case of a TV, the manufacturer’s “look”.
>
> I think my mistake was assuming that if you adjust the image look while
> looking at a grading/mastering display, you'd be adjusting the image and
> defining how it needs to be displayed, making it display-referred.
>
> After reading a section of
>
> https://library.imaging.org/admin/apis/public/api/ist/website/downloadArticle/cic/10/1/art00021
> Kevin Spaulding, Requirements for Unambiguous Specification of a Color Encoding: ISO 22028-1,
> IS&T/SID Tenth Color Imaging Conference
>
> I understood that you're not adjusting the image, but modifying the
> scene of a scene-referred signal. The displaying of the image
> remains... undefined.
>
>
> > So you can put the
> > same HLG signal in to 500, 1000 and 4000 nit monitors, they’ll look
> > perceptually similar, with similar levels of detail in the shadows
> > and mid-tones and saturation, but they will be different brightnesses.
>
> If I understand right, the key for display-referred is that the
> signal-to-radiance function must be fixed by the signal definition.
> Since HLG EOTF includes the HLG OOTF which is parameterised by the
> actual display and viewing conditions, it is variable and not fixed.
> BT.709 says to decode with BT.1886 which also is parameterised, and not
> fixed.
>
> > HLG is designed to be a very natural looking image – a bit like
> > looking out of the window.  This does not match what sports or drama
> > productions use, hence the in-camera artistic controls.  However, the
> > signal is still referred to the scene rather than a single monitor
> > and can be adapted for any display.  There are use cases where no
> > artistic controls are used and accurate colours are required – such
> > as medical imaging – so it’s important that any software
> > implementation can handle this.  (ISO split this into something like
> > “scene referred” and “scene referred with scene chromaticies”)
>
> Right. For Wayland, the desire to support ICC workflows and rendering
> intents should ensure we can also keep accurate colorimetry when wanted.
>
>
> Thanks,
> pq

Received on Tuesday, 3 October 2023 18:40:17 UTC