Display profiles did not always include data about Luminance and I think it was
optional in the v2 profiles.
But the Luminance tag has to be populated now and contains the measured monitor
white point luminance.
The Black point is another story, its actual and useful measurement does vary.
It takes a good instrument to measure “no light”.
But I think there is a Black Point Luminance Tag in v4 profiles as well now.
Whether display profilers actually bother to populate these two tags with
useful data is another story.
The “better profilers” do it.
AFAIK, the CMM never uses the Luminance in any way since the viewer is supposed
to be adapted to its Luminance.
So a 350 cd/m2 or a 80 cd/m2 monitor is the same thing to a CMM.
Your assumptions are right, IMO, in that 100% “Brightness” or the monitor white
point Luminance (350 or 80) becomes L* = 100 a* = 0 and b* = 0 IN A RELATIVE
Same thing with Lab to RGB, Lab 100 0 0 becomes RGB 255, 255, 255 (8 bits) and
Lab 0 0 0 becomes RGB 0 0 0, depending on the “reference profile” but as a
simple explanation I believe this is what’s going on.
Graeme could better explains the ABSOLUTE COLORIMETRIC transform…
Black point mapping depends. In sRGB, “absolute black point” is 0,0,0 but
“effective black point” is like 12, 12, 12 to take into account various effects
you described below like flare.
Now the way Photoshop operates, during Simulate Paper color / Black ink is
shifting gears from a RelCol transform to the screen to an AbsCol transform.
If you ask for Simulate Paper Color, from memory, I think it automatically
kicks Black Ink too, it’s “automatic”, to be able to do a proper simulation.
And for those who want to have their cake and eat it too, there is the Black
ink option alone but to me it’s not a good idea.
I’m sure others more knowledgeable than me will kick in…
I have a web site that digs into some of these question but it’s in French…
Best / Roger
From: argyllcms-bounce@xxxxxxxxxxxxx <argyllcms-bounce@xxxxxxxxxxxxx> On Behalf
Of Jeremy Stephenson
Sent: Monday, July 23, 2018 9:53 PM
Subject: [argyllcms] display profile and soft-proofing behaviour
I have been digging into some of the icc white papers on www.color.org
<http://www.color.org> trying to understand a bit deeper what’s happening
under the hood with display profile behaviour and soft-proofing. I would
appreciate a bit of help to confirm if I’m understanding correctly or not.
As far as I understand (at least in v2 icc profiles), with display profiles and
how they’re used in CMM’s (I’m using apple, so colorsync) — display profiles
don’t include data on the actual brightness/intensity of the white point, or
the darkness of the black point?
If I understand that correctly then, the CMM will assign 100% brightness (100%
L* in Lab, or equivalent Y in XYZ) to RGB 100%,100%,100%, and 0% L* (or
equivalent Y in XYZ) to RGB 0,0,0?
I’m guessing that happens regardless of how bright the black point actually is
(ie. due to backlight leakage and flare, black on the monitor will be brighter
than absolute black)? So essentially it’s functioning similarly to Black Point
Compensation with Relative Intent on a print, by mapping absolute 0 to the
darkest black available?
My next query is regarding what’s happening under the hood with soft-proofing
in Photoshop and similar applications when selecting “simulate paper color /
I’ve read somewhere that Photoshop is effectively (for display purposes only)
converting the image to the printer profile (using whichever rendering intent
was selected), then converting back using absolute rendering intent.
The idea of doing this would be to map the white and black points to their
actual reflectivity in the PCS, and then to display that through the screen
using the display profile.
Is my understanding of both of these points correct?
I appreciate your input!