Re: Provenance of "sRGB for ICC profiles" on w3.org

Srgb is an old crt display specification.
000 is no darker than the display being turned off.
A turned off crt display was not infinitely black in a room with some light such as the specified luminance level.

A colorist has to take this glare into account when grading for shadow details.



-------- Original message --------
From: Peter Occil <poccil14@gmail.com>
Date: 9/3/17 10:13 AM (GMT-10:00)
To: public-colorweb@w3.org
Subject: Re: Provenance of "sRGB for ICC profiles" on w3.org

Thank you for your response.  For the record, as Phil further clarified
to me: "Basically 0 cd/m2 is the sRGB black point as defined by IEC".

To further clarify:  Is it true that sRGB (0, 0, 0) ideally has a
luminance of 0 cd/m^2, but that luminance is not practically achievable
in (most) real media, so that the actual luminance of sRGB (0, 0, 0) is
slightly higher when viewed in such media?

--Peter


On 09/03/2017 02:56 PM, Phil Green wrote:
> Hi Pierre, Peter
>
> Tristimulus normalization is a convention used in colour management
> where Y=1 for the white point. If you omit this step you will get some
> strange numbers when applying the component transfer function.
>
> The sRGB encoding is defined by IEC as having a black point of Y=0,
> but this is not expected to be achieved on any real display viewed in
> the sRGB viewing enviroment of 64 lux illuminance. The purpose of the
> 0.2 cd/m2 black point luminance of the reference display is to provide
> a practical recommendation for exchanging colorimetry between
> different real media, where if you assume a black point of zero cd/m2
> you can end up with effects such as clipping. However, the ICC v2 sRGB
> profile sRGB2014.icc (available from
> https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.color.org%2Fsrgbprofiles.xalter%23v2&data=02%7C01%7C%7C93155f5e0ccb44aa123008d4f3082def%7Cfa7b1b5a7b34438794aed2c178decee1%7C0%7C0%7C636400663862283413&sdata=o6TTV84dEJ5owRyDxZzyrWIPjaAGKmIj9tv5eN4LFc8%3D&reserved=0) is fully compatible with
> the IEC specification, as amended in 2014.
>
> Phil Green
>
> ICC Technical Secretary
>
>
> On 03/09/2017 20:26, Pierre-Anthony Lemieux wrote:
>> Hi Peter,
>>
>> As mentioned in Ref. 1, "These equations are not provided in IEC
>> 61966-2-1, and are an additional interpretation provided in this
>> document."
>>
>> There is no justification for these equations, and I would therefore
>> disregard them until someone provides one :)
>>
>> Best,
>>
>> -- Pierre
>>
>> On Sun, Sep 3, 2017 at 11:18 AM, Peter Occil <poccil14@gmail.com> wrote:
>>> That is actually only the case once XYZ is "normalized", so that Y =
>>> 0 is
>>> the (sRGB) black point and Y = 1 is the white point.
>>>
>>> However, the document I previously linked to (which is actually from
>>> the ICC
>>> and at ref. 1), at section A.6, suggests that XYZ values are normalized
>>> taking into account the veiling glare luminance ("black point"
>>> luminance)
>>> (in addition to the white point luminance), rather than taking into
>>> account
>>> just the white point luminance, which I presumed was the case until
>>> now.
>>> (Note that each of the two normalizations will result in a different
>>> meaning
>>> for Y = 0.) Hence my question on what luminance (either 0 or 0.2
>>> cd/m^2)
>>> sRGB's "black point" is.
>>>
>>> Ref. 1. https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.color.org%2Fchardata%2Frgb%2FsRGB.pdf&data=02%7C01%7C%7C93155f5e0ccb44aa123008d4f3082def%7Cfa7b1b5a7b34438794aed2c178decee1%7C0%7C1%7C636400663862283413&sdata=MkJMM7EnHLIfOZyZ%2FdtxM%2BHGbu%2BeCFqxFzP0ebyqAWs%3D&reserved=0
>>>
>>>
>>>
>>> On 09/03/2017 01:00 PM, Pierre-Anthony Lemieux wrote:
>>>>>    Is it true that the "sRGB black point" (what sRGB defines as
>>>>> black) has
>>>>> a luminance of 0.2 cd/m^2 (absolute Y = 0.2)
>>>>> rather than 0 cd/m^2 (absolute Y = 0, the start of the absolute XYZ
>>>>> scale)?
>>>> ISO 61966-2-1 [1] specifies that [X Y Z] = [0 0 0] yields [R G B] =
>>>> [0 0 0] (see equation 8).
>>>>
>>>> Furthermore, quantized 8-bit R8 = 255 R' , where R' is non-linear R,
>>>> (see equation 4)
>>>>
>>>> and R' = 12.92 R when R' < 0.04045. (see equation 5)
>>>>
>>>> ... so R8 = 0 when [X Y Z] = [0 0 0] , with the same reasoning
>>>> applying to G8 and B8.
>>>>
>>>> Let me know if I got this wrong.
>>>>
>>>> Best,
>>>>
>>>> -- Pierre
>>>>
>>>> [1] https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwebstore.iec.ch%2Fpublication%2F6169&data=02%7C01%7C%7C93155f5e0ccb44aa123008d4f3082def%7Cfa7b1b5a7b34438794aed2c178decee1%7C0%7C0%7C636400663862283413&sdata=28MAEa1GYxYO1d4zSrJKSRHcfIwg31XAAtKsNqp1MF8%3D&reserved=0
>>>>
>>>> On Sun, Sep 3, 2017 at 5:05 AM, Peter Occil <poccil14@gmail.com>
>>>> wrote:
>>>>> While I'm at it, that document contains a very questionable statement
>>>>> about
>>>>> the "black point" of sRGB, suggesting that the "black point" has a
>>>>> "veiling
>>>>> glare luminance" of 0.2 cd/m^2 (and indeed that suggestion appears
>>>>> further
>>>>> in some of the formulas in that document).  Is it true that the "sRGB
>>>>> black
>>>>> point" (what sRGB defines as black) has a luminance of 0.2 cd/m^2
>>>>> (absolute
>>>>> Y = 0.2) rather than 0 cd/m^2 (absolute Y = 0, the start of the
>>>>> absolute
>>>>> XYZ
>>>>> scale)?
>>>>>
>>>>>
>>>>>
>>>>> On 09/02/2017 03:28 PM, Peter Occil wrote:
>>>>>> I'm aware of the following document posted on the W3C Web site:
>>>>>>
>>>>>> https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.w3.org%2FGraphics%2FColor%2Fsrgb&data=02%7C01%7C%7C93155f5e0ccb44aa123008d4f3082def%7Cfa7b1b5a7b34438794aed2c178decee1%7C0%7C0%7C636400663862283413&sdata=ql4IWJ2z0GW1BoktV0%2Bo0UOV8VGN9nXmF88PDfsX1RQ%3D&reserved=0
>>>>>>
>>>>>> I find it very useful as a reference, but: Where did this
>>>>>> document come
>>>>>> from?  Who were its authors?  When was it posted?  I couldn't
>>>>>> find it
>>>>>> linked
>>>>>> anywhere on the W3C site except on a mailing list message (ref. 1).
>>>>>>
>>>>>> Ref. 1.
>>>>>> https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.w3.org%2FArchives%2FPublic%2Fwww-style%2F2016Sep%2F0061.html&data=02%7C01%7C%7C93155f5e0ccb44aa123008d4f3082def%7Cfa7b1b5a7b34438794aed2c178decee1%7C0%7C0%7C636400663862283413&sdata=%2FbCw2KrQb5QGTf2vj1djSgbzbkzhlF%2FV8Kb%2FVKIt544%3D&reserved=0
>>>>>>
>> .
>>
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