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

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 18:27:31 UTC