- From: Peter Occil <poccil14@gmail.com>
- Date: Sun, 3 Sep 2017 16:12:16 -0400
- To: public-colorweb@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