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 
> http://www.color.org/srgbprofiles.xalter#v2) 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. http://www.color.org/chardata/rgb/sRGB.pdf
>>>
>>>
>>>
>>> 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://webstore.iec.ch/publication/6169
>>>>
>>>> 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://www.w3.org/Graphics/Color/srgb
>>>>>>
>>>>>> 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://lists.w3.org/Archives/Public/www-style/2016Sep/0061.html
>>>>>>
>> .
>>
>
>
>

Received on Sunday, 3 September 2017 20:12:51 UTC