Re: [EXTERNAL] Re: Proposal for HTMLCanvasElement HDR compositing modes

BT.2100 is a more complete specificaton as it includes specs on the reference display and reference environment.
BT.709 display was de-facto CRT. When new display technologies became available it was necessary to explicitly define the HDTV display. This was done in two separate Recs, BT.1886 spec'd gamma=2.4 and BT.2035 specs white at 100 nits and a viewing environment with a 10-nit background. I consider HDTV a display referred system, i.e. you create a pixel to produce known color/luminance on the reference display in the reference environment.

I can't emphasize enough the importance of rendering for display on a fully specified reference display in a reference environment. This is the stake in the ground that anchors everything together (sort of like Greewich defines the prime meridian for mapping and navigation, see below). This stake in the ground is essential for content to be produced in a consistent way, so that when an actual display deviates from reference for reasons of environment, technology, preference, etc. it will consistently produce an altered image. If the display differs and content differs you get a mess (like with maps before 1884).


https://www.nationalgeographic.org/encyclopedia/prime-meridian/
[https://media.nationalgeographic.org/assets/photos/000/279/27984.jpg]<https://www.nationalgeographic.org/encyclopedia/prime-meridian/>
prime meridian | National Geographic Society<https://www.nationalgeographic.org/encyclopedia/prime-meridian/>
The prime meridian is the line of 0 longitude, the starting point for measuring distance both east and west around the Earth. The prime meridian is arbitrary, meaning it could be chosen to be anywhere.Any line of longitude (a meridian) can serve as the 0 longitude line. However, there is an international agreement that the meridian that runs through Greenwich, England, is considered the ...
www.nationalgeographic.org

Governments did not always agree that the Greenwich meridian was the prime meridian, making navigation over long distances very difficult. Different countries published maps and charts with longitude based on the meridian passing through their capital city. France would publish maps with 0 longitude running through Paris. Cartographers in China would publish maps with 0 longitude running through Beijing. Even different parts of the same country published materials based on local meridians.

Finally, at an international convention called by U.S. President Chester Arthur in 1884, representatives from 25 countries agreed to pick a single, standard meridian. They chose the meridian passing through the Royal Observatory in Greenwich, England. The Greenwich Meridian became the international standard for the prime meridian.
________________________________
From: Simon Thompson-NM <Simon.Thompson2@bbc.co.uk>
Sent: Tuesday, March 16, 2021 8:03 AM
To: Seeger, Chris (NBCUniversal) <Chris.Seeger@nbcuni.com>; Jim Helman <jhelman@movielabs.com>; Lars Borg <borg@a1830.dscd.akamai.net>; Christopher Cameron <ccameron@google.com>; public-colorweb@w3.org <public-colorweb@w3.org>; Pierre-Anthony Lemieux <pal@sandflow.com>
Subject: Re: [EXTERNAL] Re: Proposal for HTMLCanvasElement HDR compositing modes


Hi



Two of the three solutions in the document under discussion are scene-referred:

  1.  ITU-R BT.2100 scene-referred linear floating point signal (Table 10) and
  2.  ITU-R BT.2100 hybrid log-gamma (Table 5)



The definitions of compositing levels for these two systems are based on %age signal level of these scene-referred signals and not the luminance output of an unknown display that the system is connected to.


If we're going to exclude scene-referred video systems then that will also exclude ITU-R BT.709, which is most definitely scene-referred.  Even the reference EOTF for SDR TV, specified 21 years later in ITU-R BT.1886, does not specify a nominal peak luminance (Lw) for the display.  It is often thought that BT.1886 specifies a nominal peak luminance of 100 cd/m^2 for an SDR display, however, this is only in an informative annex as an example that could be used to emulate a CRT display.  For television reference monitors, the EBU specified a range of luminances dependent on usage and viewing environment.  SDR TV is therefore clearly a scene-referred system.


Furthermore, even though the sRGB specification mentions 80cd/m^2, most modern computers do not use this nominal peak luminance as it's too dim for most viewing environments.



Best Regards



Simon



--

Simon Thompson MEng CEng MIET
Senior R&D Engineer

BBC Research and Development South Laboratory

________________________________
From: Seeger, Chris (NBCUniversal) <Chris.Seeger@nbcuni.com>
Sent: Tuesday, March 16, 2021 13:21
To: Jim Helman <jhelman@movielabs.com>; Lars Borg <borg@a1830.dscd.akamai.net>; Christopher Cameron <ccameron@google.com>; public-colorweb@w3.org <public-colorweb@w3.org>; Pierre-Anthony Lemieux <pal@sandflow.com>
Subject: Re: [EXTERNAL] Re: Proposal for HTMLCanvasElement HDR compositing modes


We agree.  Please stay in Display-Light.



From: Jim Helman <jhelman@movielabs.com>
Date: Monday, March 15, 2021 at 11:54 PM
To: Lars Borg <borg@a1830.dscd.akamai.net>, Christopher Cameron <ccameron@google.com>, public-colorweb@w3.org <public-colorweb@w3.org>, Pierre-Anthony Lemieux <pal@sandflow.com>
Subject: [EXTERNAL] Re: Proposal for HTMLCanvasElement HDR compositing modes

Hi,

I agree. Scene light has a very specific definition in the context of capture and in the color grading/correction world. In our context here, we should avoid getting into mappings from scene-referred to display-referred systems, which are usually highly subjective and depend on the desired creative "look."

- Jim

On 3/15/21 8:26 PM, Lars Borg wrote:

Chris C,



One concern I have is the term scene light.

If this means scene-referred, then that’s a problem as sRGB is defined only for display light, not for scene light.

So then additional conversions would be needed to convert sRGB to canvas.

And if scene light doesn’t mean scene-referred then what does it mean? It causes confusion.

Please find a way to stay in display-referred color space.



Lars



From: Christopher Cameron <ccameron@google.com><mailto:ccameron@google.com>
Date: Sunday, March 14, 2021 at 8:30 PM
To: "public-colorweb@w3.org"<mailto:public-colorweb@w3.org> <public-colorweb@w3.org><mailto:public-colorweb@w3.org>, Pierre Lemieux <pal@sandflow.com><mailto:pal@sandflow.com>
Subject: Re: Proposal for HTMLCanvasElement HDR compositing modes
Resent-From: <public-colorweb@w3.org><mailto:public-colorweb@w3.org>
Resent-Date: Sunday, March 14, 2021 at 8:29 PM



FYI, I have updated the PR to rely more heavily on standard broadcast terminology, and included a (hopefully-clarifying) definitions section at the top.





On Wed, Mar 10, 2021 at 2:42 PM Christopher Cameron <ccameron@google.com<mailto:ccameron@google.com>> wrote:

I put up a PR with a writeup of what I think is the best scheme for HDR compositing of HTMLCanvasElements, given our various constraints. Please review and/or comment for our next meeting on March 16!

https://github.com/w3c/ColorWeb-CG/pull/15<https://urldefense.com/v3/__https:/nam04.safelinks.protection.outlook.com/?url=https*3A*2F*2Fgithub.com*2Fw3c*2FColorWeb-CG*2Fpull*2F15&data=04*7C01*7Cborg*40adobe.com*7Cf4f9872e7f6a408760f808d8e77bbfcf*7Cfa7b1b5a7b34438794aed2c178decee1*7C0*7C0*7C637513866045481738*7CUnknown*7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0*3D*7C1000&sdata=vAN*2BA6ZzmQ4r*2F3m7SlyZVOU4s8nn5nu50rjT0S2XpfU*3D&reserved=0__;JSUlJSUlJSUlJSUlJSUlJSUlJSUlJQ!!PIZeeW5wscynRQ!6mlLIp7Z_m5FoUU-qfpGA4Tn3i4RMsTXJuy0E55neWP9Vv8L36ITozr3pAZp6rofTA$>



--

Jim Helman | MovieLabs | o: +1.650.646.2277 m: +1.650.576.1755 | jhelman@movielabs.com<mailto:jhelman@movielabs.com> | 222 Kearney St, Suite 300, San Francisco, CA 94108