RE: Sanity check, ITU and IEC definitions of sRG are identical, right?

Chris – as far as I am aware, the IEC document is the only official (normative) reference for sRGB.  It is the one referred to by numerous other specifications and standards.  To use anything else, even if “identical” would only cause confusion for implementors.

Leonard

From: Todd, Craig <CT@dolby.com>
Sent: Wednesday, May 1, 2019 11:50 AM
To: Chris Lilley <chris@w3.org>; public-colorweb@w3.org
Subject: RE: Sanity check, ITU and IEC definitions of sRG are identical, right?

Note that the ITU document is a Report (informative) and not a Recommendation (which would  be a normative specification).

Craig Todd
Dolby Fellow
Dolby Laboratories
1275 Market St.
San Francisco, CA  94103
T +1 415-558-0221  M +1 415 672-0221
ct@dolby.com<mailto:ct@dolby.com> | http://www.dolby.com<https://nam04.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.dolby.com&data=02%7C01%7Clrosenth%40adobe.com%7Cf67a084d1fe24666f56a08d6ce4cb4a0%7Cfa7b1b5a7b34438794aed2c178decee1%7C0%7C1%7C636923226209318692&sdata=zyzTeli583DYW%2FOPaQh%2FEtl82o%2BYRRlmFrlIEa399Q8%3D&reserved=0>

From: Chris Lilley <chris@w3.org<mailto:chris@w3.org>>
Sent: Wednesday, May 1, 2019 8:18 AM
To: public-colorweb@w3.org<mailto:public-colorweb@w3.org>
Subject: Sanity check, ITU and IEC definitions of sRG are identical, right?


Learned people,

Currently, all W3C specifications (bar one, which is being fixed [1]) refer to the IEC definition of sRGB as authoritative. [2]

Multimedia systems and equipment - Colour measurement and management - Part 2-1: Colour management - Default RGB colour space - sRGB<https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Furldefense.proofpoint.com%2Fv2%2Furl%3Fu%3Dhttps-3A__webstore.iec.ch_publication_6169%26d%3DDwMDaQ%26c%3DlI8Zb6TzM3d1tX4iEu7bpg%26r%3D_F90w7eDD4MBjtHx1p0KJg%26m%3D86nhgkWcBMyAZtdf1CJ-FZR_ODNPVLcBs7NNCK2zIV8%26s%3DIYyuGYlT2vemcxSBHQWFTfhyHDZes9JGpETCZEBJ4sc%26e%3D&data=02%7C01%7Clrosenth%40adobe.com%7Cf67a084d1fe24666f56a08d6ce4cb4a0%7Cfa7b1b5a7b34438794aed2c178decee1%7C0%7C1%7C636923226209318692&sdata=U8JXImhqxtH9spZdJjqi9b4zh%2FEQQ7fhJ0YFY8i5I48%3D&reserved=0>.

However, the link goes to the IEC web store[3], where one can pay real money for a copy. In consequence, many people don't have a copy (including myself, hence this mail).

I recently learned [4] (and sorry, I didn't know this before) that sRGB is also defined by the ITU, on p.14 of:

Television colorimetry elements. Report  ITU-R  BT.2380-2 (10/2018)
https://www.itu.int/dms_pub/itu-r/opb/rep/R-REP-BT.2380-2-2018-PDF-E.pdf<https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Furldefense.proofpoint.com%2Fv2%2Furl%3Fu%3Dhttps-3A__www.itu.int_dms-5Fpub_itu-2Dr_opb_rep_R-2DREP-2DBT.2380-2D2-2D2018-2DPDF-2DE.pdf%26d%3DDwMDaQ%26c%3DlI8Zb6TzM3d1tX4iEu7bpg%26r%3D_F90w7eDD4MBjtHx1p0KJg%26m%3D86nhgkWcBMyAZtdf1CJ-FZR_ODNPVLcBs7NNCK2zIV8%26s%3DZxm1z8sjWiUl4oGKRc3fp_jLMVvPhs5DL-XT8Y2ctIM%26e%3D&data=02%7C01%7Clrosenth%40adobe.com%7Cf67a084d1fe24666f56a08d6ce4cb4a0%7Cfa7b1b5a7b34438794aed2c178decee1%7C0%7C1%7C636923226209328697&sdata=XmaxhM1RJtjGXYrdEtFiRxX57msaNt3lt4HNfeInkXY%3D&reserved=0>

and this specification has the merit of being freely available to download.

I see that the ITU report does reference IEC 61966-2-1 so I presume that the chromaticity coordinates, EOTF and OETF are copied correctly from there.

I notice that the EOTF breakpoint is specified to more significant figures, 0.40449936 rather than 0.04045 which I have seen elsewhere.

I would have greater peace of mind, before submitting a pull request on the spec database to point to the freely available specification, if someone who has read both specifications could confirm for me that the definitions are identical?

[1] Web Content Accessibility Guidelines refers to an obsolete 4th working draft of the IEC specification, which still has the erroneous rounded-off coefficient that led to a discontinuity in the transfer function.
Although the impact on 8bit per component systems is invisible, I now have agreement to fix that
https://github.com/w3c/wcag/issues/360#issuecomment-453741912<https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Furldefense.proofpoint.com%2Fv2%2Furl%3Fu%3Dhttps-3A__github.com_w3c_wcag_issues_360-23issuecomment-2D453741912%26d%3DDwMDaQ%26c%3DlI8Zb6TzM3d1tX4iEu7bpg%26r%3D_F90w7eDD4MBjtHx1p0KJg%26m%3D86nhgkWcBMyAZtdf1CJ-FZR_ODNPVLcBs7NNCK2zIV8%26s%3D0PPYZJiOTCA6FD5IEiZiOANkjJ4AbHbms3XXNv-wdA4%26e%3D&data=02%7C01%7Clrosenth%40adobe.com%7Cf67a084d1fe24666f56a08d6ce4cb4a0%7Cfa7b1b5a7b34438794aed2c178decee1%7C0%7C1%7C636923226209328697&sdata=hgomLrU9380yKatkYe28%2F0dOonki7vvvvsFeEpTaOpA%3D&reserved=0> and following
[2] https://www.specref.org/?q=srgb<https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Furldefense.proofpoint.com%2Fv2%2Furl%3Fu%3Dhttps-3A__www.specref.org_-3Fq-3Dsrgb%26d%3DDwMDaQ%26c%3DlI8Zb6TzM3d1tX4iEu7bpg%26r%3D_F90w7eDD4MBjtHx1p0KJg%26m%3D86nhgkWcBMyAZtdf1CJ-FZR_ODNPVLcBs7NNCK2zIV8%26s%3DblQS3hXydiwA2j9rzuJF4CKBHTAafd9RDIzg-2PhHq4%26e%3D&data=02%7C01%7Clrosenth%40adobe.com%7Cf67a084d1fe24666f56a08d6ce4cb4a0%7Cfa7b1b5a7b34438794aed2c178decee1%7C0%7C1%7C636923226209338702&sdata=8S15JGjl6NicfiMzaW7KbQMOLS9z2KE7BQud0yh2fF4%3D&reserved=0>
[3]  https://webstore.iec.ch/publication/6169<https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Furldefense.proofpoint.com%2Fv2%2Furl%3Fu%3Dhttps-3A__webstore.iec.ch_publication_6169%26d%3DDwMDaQ%26c%3DlI8Zb6TzM3d1tX4iEu7bpg%26r%3D_F90w7eDD4MBjtHx1p0KJg%26m%3D86nhgkWcBMyAZtdf1CJ-FZR_ODNPVLcBs7NNCK2zIV8%26s%3DIYyuGYlT2vemcxSBHQWFTfhyHDZes9JGpETCZEBJ4sc%26e%3D&data=02%7C01%7Clrosenth%40adobe.com%7Cf67a084d1fe24666f56a08d6ce4cb4a0%7Cfa7b1b5a7b34438794aed2c178decee1%7C0%7C1%7C636923226209348712&sdata=ARjDYn9p8ZmZyYNFVCjNPCc%2F0ZzCviNJVSo5f7N5tYs%3D&reserved=0>
[4] https://github.com/w3c/wcag/issues/360#issuecomment-487476773<https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Furldefense.proofpoint.com%2Fv2%2Furl%3Fu%3Dhttps-3A__github.com_w3c_wcag_issues_360-23issuecomment-2D487476773%26d%3DDwMDaQ%26c%3DlI8Zb6TzM3d1tX4iEu7bpg%26r%3D_F90w7eDD4MBjtHx1p0KJg%26m%3D86nhgkWcBMyAZtdf1CJ-FZR_ODNPVLcBs7NNCK2zIV8%26s%3Dk3PIiA0oDHLQLNvFImi8mxZ2rNokitQf97DxGIQcgz4%26e%3D&data=02%7C01%7Clrosenth%40adobe.com%7Cf67a084d1fe24666f56a08d6ce4cb4a0%7Cfa7b1b5a7b34438794aed2c178decee1%7C0%7C1%7C636923226209348712&sdata=Bx2vmnoftSC3oxRiLV3n62EDuLBoddpN8WVNN7oir9c%3D&reserved=0>

--

Chris Lilley

@svgeesus

Technical Director @ W3C

W3C Strategy Team, Core Web Design

W3C Architecture & Technology Team, Core Web & Media