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

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 19:37:05 UTC