PS Uses Relative Colorimetric Intent with BPC for Soft Proofing? - Adobe Color Management

Kwanyue,

several possible configurations are shown here
on pages ix, x and xi:
<http://www.color.org/ICC1V42.pdf>

The profile can contain up to three sets of
one-dimensional curves, a matrix and a multi-
dimensional LUT.

Did you ever encounter a monitor profile with
black point not a zero ?

Best regards --Gernot Hoffmann

Yes, monitor profiles do have black point tag.

For softproofing using monitor profile this will limit to the few possibilities in page ix of <http://www.color.org/ICC1V42.pdf>

However the available non-proprietary tags here in my monitor profile are:

lumi, rTRC, gTRC, bTRC, rXYZ, gXYZ, bXYZ, bkpt, wkpt, chad, A2B1, B2A1, A2B0, B2A0

and I expect the conversion possibilities there can only either be: a or d in page ix but which one would PS choose?

Kwanyue,

why should PhS not use the complete profile information,
mainly rXYZ,..., A2B1,... ? Any evidence ?

The doc says on p.24 top: matrix based display profiles
can contain LUTs. That's here the case for profiles
by ProfileMaker for 'large' profiles (probably case e
on p. ix), but 'default=small' profiles don't contain LUTs.

The media blackpoint tag is not required (Annex G).
If bkpt is missing (here), then the blackpoint is at 0/0/0,
which can be shown by ProfileEditor / GamutView / 2D(ab)
for L*=0..1 .

Again my question: do you have in your monitor profiles
the blackpoint NOT at 0/0/0 ?

Best regards --Gernot Hoffmann

But the monitor display profile contains also rTRC, gTRC, bTRC tags and only figure (a) in page ix needed these tags and also in figure (e) the matrix is 3x4 and not 3x3 required by RGB monitors.

Yes, the blackpoint tag in my monitor profile has values: X=0.002853, Y=0.002121 and Z=0.000702

My question again: Which tags does PhS uses for softproofing?

Kwanyue,

Fig.(e) on p.ix shows the general case: four outputs or
even more. This doesn't exclude three for the monitor.

If there are more than three channels then ONLY (d) or (e)
can be used.

The TRCs are probably always used, because the (inverse)
gamma encoding, as introduced for CRTs, is still the
expected style even for TFTs.
The matrix contains the essential color conversion XYZ--RGB
for linear RGB. The LUT contains small deviations.
The TRCs apply the global gamma functions.
If everything would be packed let's say into one 3x3 LUT,
then the accuracy would be very bad.

Which program delivers nonzero blackpoint and what's the
reference unit (1 or 100) ? Assumed it's 1:
Y=0.002 for 100cd/m2 delivers for black 0.2cd/m2.
IMO, there is no need to apply a correction. Please refer
to p.85/86 in Annex D.7.8 : monitor black is mapped to
PCS black.

I'm taking your questions as a chance for interpreting
the ICC specs once again. Corrections are appreciated.
Of course I don't know what PhS is really doing.

Best regards --Gernot Hoffmann

Fig.(e) on p.ix shows the general case: four outputs or even more. This
doesn't exclude three for the monitor.




agreed, that should include also scanner input profile.

If there are more than three channels then ONLY (d) or (e) can be used.




agreed, because PCS has only three channels.

The TRCs are probably always used, because the (inverse) gamma encoding,
as introduced for CRTs, is still the expected style even for TFTs.




I forget to mention the vcgt tag. This tag is used by the video card LUT to calibrate the monitor to the gamma values described in the TRCs tags.

The matrix contains the essential color conversion XYZ--RGB for linear
RGB. The LUT contains small deviations. The TRCs apply the global gamma
functions.




agreed

If everything would be packed let's say into one 3x3 LUT, then the accuracy
would be very bad.




could you elaborate?

Which program delivers nonzero blackpoint and what's the reference unit
(1 or 100) ? Assumed it's 1: Y=0.002 for 100cd/m2 delivers for black 0.2cd/m2.
IMO, there is no need to apply a correction. Please refer to p.85/86 in
Annex D.7.8 : monitor black is mapped to PCS black.




baICColor Display 4
I believe it is normalized to 1. Thanks for the info. Is Annex D.7.8 trying to say that monitor black should be equal to zero which is also PCS black?

I'm taking your questions as a chance for interpreting the ICC specs once
again. Corrections are appreciated. Of course I don't know what PhS is
really doing.




My guess is PhS uses figure (a) for softproofing and figure (d) for color transform with another ICC profile which I like it to be confirmed..

Kwanyue,

about the accuracy:
A LUT has for each input axis a number N input grid points.
E.g. N=15. Input values between the grid points require
an interpolation for the output values. This is normally
a LINEAR interpolation, using nearest neighbours (either
cube, prism, pyramid or tetrahedron).
If the data in the LUT are linear, then the interpolation
is exact, otherwise an approximation.
One-dimensional: y=x^2. This is a smooth parabola, but
the interpolation represents a polyline which connects
the grid points.
Therefore the essential nonlinearities (here the TRC)
should not be contained in a LUT.
In order to improve this further, the LUTs can have input
curves as well, thus optimizing the LUT content for linear
interpolation.

Reading Annex D.7.8 again, I'm not sure what it means.

But HOW can we take the blackpoint of the monitor into
account, if this should matter ?
Softproofing a printer simulates the printer on the monitor.
Using the blackpoint of the monitor would result in the
hardly understandable task 'simulate the monitor on the
monitor'.

Best regards --Gernot Hoffmann

about the accuracy: A LUT has for each input axis a number N input grid
points. E.g. N=15. Input values between the grid points require an interpolation
for the output values. This is normally a LINEAR interpolation, using
nearest neighbours (either cube, prism, pyramid or tetrahedron). If the
data in the LUT are linear, then the interpolation is exact, otherwise
an approximation. One-dimensional: y=x^2. This is a smooth parabola, but
the interpolation represents a polyline which connects the grid points.
Therefore the essential nonlinearities (here the TRC) should not be contained
in a LUT. In order to improve this further, the LUTs can have input curves
as well, thus optimizing the LUT content for linear interpolation.




agreed, all these the 1D LUTs, multi-dimensional LUTs matrix coefficient values are stored in the AtoB or BtoA tags for the CMM to reference.

Reading Annex D.7.8 again, I'm not sure what it means.





But HOW can we take the blackpoint of the monitor into account, if this
should matter ? Softproofing a printer simulates the printer on the monitor.
Using the blackpoint of the monitor would result in the hardly understandable
task 'simulate the monitor on the monitor'.






Reading again Annex D.7.8 I now think what it tries to say here is to calibrate the monitor black and white point to the white and black point of the paper?

Kwanyue & Gernot,

Thanks for revisiting the topic of monitor blackpoints.

I think that D.7.8 just says that, since most users expect R=G=B=0 to produce max black on the monitor, most users would also want BPC in the conversion from PCS to monitor space (so that shadows are never clipped).

Whether or not one could see the effect of BPC would, I should think, depend on the the level of black that your monitor can achieve: the higher the blackpoint, the more noticeable. Of course, the monitor profile would have to have a blackpoint tag or equivalent, and Photoshop would need to honor it.

For Y=0.002121, I’m not sure you’ll notice any outright clipping even if playing with a pure gradient. You might notice a slight fogging up of the scene when switching on BPC (with RelCol), due to the linear rescaling of all scene luminances upward to fit inside the monitor bp/wp range.

Suppose Photoshop does honor version 4 profile bp tags, but you don’t see much change, then I would try to bump the bp value up (maybe manually edit the tag with a binary editor) to see if this does anything. That’s what I would do anyway if I have the time (and a profile with a nonzero bp).

Regards,

Larry