[argyllcms] Re: Question regarding White Point and spotread
- From: "Brad Funkhouser" <brad.funkhouser@xxxxxxxxxxx>
- To: <argyllcms@xxxxxxxxxxxxx>
- Date: Sun, 02 Nov 2014 14:54:58 -0600
Robert,
I think I see why I was confused.
I'm bringing art from a camera into BetaRGB as absolute colors in standard
D50. If all of the artwork's colors are in-gamut for a printer space, then
I can simply convert to that space using Absolute Colorimetric and I'll get
a match to the original art when viewed under D50 light. There's no
shifting of colors due to the difference between BetaRGB white and paper
white in this absolute in-gamut case. The D50 colors are converted into
printer space as accurately as they can be.
So my mind naturally thinks of Lab values as D50. I have Photoshop
conversion set to Absolute in color settings so the info palette Lab values
I see are D50, and my i1pro measurements of printer patches and camera
target patches are always D50 Lab.
When some of an artwork's D50 colors are outside the gamut of a printer
space, I have a variety of ways to bring them inside the space depending on
what colors are outside and to what extent.
So I think I see why I was confused about your wanting to compensate
spotread Lab values for paper white. We're looking at how to use Lab from
different perspectives. I'm always thinking in terms of absolute colors and
D50 Lab is my standard. I know what all my paper whites and blacks are in
terms of D50 Lab, but I don't ever change Lab in my mind to be those
different printer spaces. Doing that would be confusing to me.
- Brad
From: argyllcms-bounce@xxxxxxxxxxxxx [mailto:argyllcms-bounce@xxxxxxxxxxxxx]
On Behalf Of robert@xxxxxxxxxxxxxxxxxx
Sent: Sunday, November 02, 2014 12:33 PM
To: argyllcms@xxxxxxxxxxxxx
Subject: [argyllcms] Re: Question regarding White Point and spotread
Hi, yes, if I use Absolute rather than Relative the Lab value changes (but
stays the same in Relative . providing, of course, that the color is
in-gamut).
The reason for this, I think, is that since the white point is not shifted
in Absolute, all the colors will get shifted, even those that are in-gamut.
For example, if I take a color with Lab value of 50, -8, 48 and convert it
with AbsCol the values change to 51, -8, 51 so the print will be more
yellow. As the spotread on the paper white gave 97.74, 0.2, -0.59 this
makes sense (L is increased, a is presumably decreased but we're not seeing
it because the figure is rounded, and b is increased).
Actually, I think the way that AbsCol is described (that is that the white
point is not shifted) is very confusing to common mortals like me. What
really happens is that the destination white is matched to the source white.
If the conversion is from working space to print, all colors will be shifted
towards yellow if the working space white is yellower than the print white,
as in this case.
I have to say it surprises me a bit to find this out for the Canson Baryta
because I would have said that it is a yellowish paper. But it does have
some OBAs, and perhaps that's throwing out the readings as I didn't profile
with OBA compensation.
>> but when that setting is changed to "Absolute Colorimetric", the reported
Lab value in the info box changes to 87 -12 78. If I printed that color
and measured with spotread, I would expect to get a close match to the
Absolute Colorimetric value.
I think if you print the AbsCol values and compare the spotread to the
RelCol value . well, you will to some extent be compensating for the paper
white, but you're likely to screw up the colors, so I can't see it working.
I still think that it is probably necessary to compensate the spotread value
for the paper white. After all, the profile does compensate for the paper
white . but spotread has no way of doing this, so it's reading is almost
bound to be wrong (unless the working space white happened to be identical
to the paper white, which is very unlikely).
With colverify you can normalise the file whites to XYZ, so presumably if
you print a test target, use fakeread and then compare the fakeread values
to the chartread values the whites will 'align' and the comparisons should
be valid.
This is the colverify test that (I understand was suggested by Graham):
1. targen -v -d2 -G -f100 iPFTest
2. copy iPFTest.ti1 iPFRef.ti1
3. printtarg -v -r -ii1 -a1.0 -T300 -M6 -pA4 iPFTest
4. cctiff -v -ir -e iPF6400_Canson_Baryta iPFTest.tif iPFTestO.tif
5. move /Y iPFTestO.tif iPFTest.tif
Pause Print iPFTest.tif using no color management.
6. chartread iPFTest
7. fakeread -v -Ir iPF6400_Canson_Baryta iPFRef
Pause The test results will be in iPFValidate.txt
8. colverify -v2 -N -k -s -w -x -L iPF6400_Canson_Baryta iPFRef.ti3
iPFTest.ti3 >iPFValidate.txt
Line 1: First of all we create a target with RGB values . which is fine,
providing these are printed as is, with no color management.
Line 3: The target is created. Still fine.
Line 4: We now convert the target through the profile using RelCol.
Line 6: We read the printed target using Chartread (this is equivalent to
using spotread, so there is no white point compensation at this stage)
Line 7: We get the Lab values of the target passed through the profile.
This is equivalent to using xicclu, I think.
Line 8: colverify only gives a valid comparison between the simulated target
and the scanned target because of the -N flag, which normalises the whites.
If what I'm saying here is correct (which would be quite a fluke :-)), then
we need to do the equivalent of the -N to the spotread data AND to the
rendered image data before comparing them.
A few days ago I suggested an ArgyllWiki to Graham (which I hope he will
pick up on), precisely for questions like these, which are not answered in
the Argyll documentation and which really need a color expert to give an
answer / solution to. At this point I don't even know if the command lines
I have above are correct or not.
Robert
_____
From: argyllcms-bounce@xxxxxxxxxxxxx [mailto:argyllcms-bounce@xxxxxxxxxxxxx]
On Behalf Of Brad Funkhouser
Sent: 02 November 2014 15:15
To: argyllcms@xxxxxxxxxxxxx
Subject: [argyllcms] Re: Question regarding White Point and spotread
With a conversion into printer space using relative colorimetric with BPC, I
wouldn't expect the Lab value to be the same after the conversion (unless
the paper white exactly matched D50 L100 0 0 and the paper/ink black exactly
matched D50 L0 0 0).
At one point I was really confused by the following... Photoshop's info box
data on the Lab value of a color changes depending on how "Color
Settings/Conversion Options" is set.
I filled an image with 90 -14 87 Lab value (in mode Lab color) then
converted to a printer space using relative colorimetric with BPC. When the
"Color Settings/Conversion Options" is set to "Relative Colorimetric with
BPC" the Lab value reported in the info box is still 90 -14 87, but when
that setting is changed to "Absolute Colorimetric", the reported Lab value
in the info box changes to 87 -12 78. If I printed that color and measured
with spotread, I would expect to get a close match to the Absolute
Colorimetric value.
Once converted to printer space, does your reported Lab value change with
these two different Conversion Option settings?
I'm still back on CS2, so yours might be different.
Thanks.
- Brad
From: argyllcms-bounce@xxxxxxxxxxxxx [mailto:argyllcms-bounce@xxxxxxxxxxxxx]
On Behalf Of robert@xxxxxxxxxxxxxxxxxx
Sent: Sunday, November 02, 2014 7:41 AM
To: argyllcms@xxxxxxxxxxxxx
Subject: [argyllcms] Re: Question regarding White Point and spotread
I've repeated the spotread using the scanning rig and the results are pretty
much the same.
If I do repeated tests after setting a reference, the dE94 starts off at
around .005 and creeps up to around .02 over 15 or so readings. Moving the
instrument changes the reading up to .25 max (over a 3cm square. Changing
to the reflective read adaptor (without recalibrating) doesn't seem to
change things much (still around 0.3 max).
Going back to my original question: could the error I'm seeing have to do
with the white point? If I measure the paper white using spotread I get a
value of 97.74, 0.2, -0.59, whereas xicclu gives 100.000000, -0.000062,
0.000060 for RGB of 1,1,1 (or a dELab of about 2.3).
Robert
_____
From: argyllcms-bounce@xxxxxxxxxxxxx [mailto:argyllcms-bounce@xxxxxxxxxxxxx]
On Behalf Of Brad Funkhouser
Sent: 02 November 2014 12:54
To: argyllcms@xxxxxxxxxxxxx
Subject: [argyllcms] Re: Question regarding White Point and spotread
Have you spotread the patch 10 to 15 times in succession, to see your
measurement variability as the lamp heats up? Also move the read point
around on the patch to mitigate small differences in direct reflections
caused by texture? And even a tiny difference in instrument height above
the patch will change luminosity reading. Is your spotread setup truly
identical to strip reading of original target? Are you pressing down more,
or less for different readings, etc.
When I experimented with all these factors with i1pro, I was (wrongly)
expecting near perfection, and was surprised by the degree of variability.
- Brad
On Nov 2, 2014, at 3:59 AM, <robert@xxxxxxxxxxxxxxxxxx> wrote:
Hi Brad,
Between print and profiling I left the print overnight (minimum 16 hours).
After the spot test I left a couple of hours. I've just redone the
measurement (so about 16 hours again) and the values are now:
88.01, -13.56, 87.23 (so a bit worse).
BTW . this may (or may not) be relevant: the profile was made using
i1Profiler (with 2584 patches) and not Argyll.
Robert
_____
From: argyllcms-bounce@xxxxxxxxxxxxx [mailto:argyllcms-bounce@xxxxxxxxxxxxx]
On Behalf Of Brad Funkhouser
Sent: 02 November 2014 00:59
To: argyllcms@xxxxxxxxxxxxx
Subject: [argyllcms] Re: Question regarding White Point and spotread
Curious... how long did the inks dry between print and measurement of the
profiling target? And between print and measurement of the spot color test?
Thanks.
- Brad
On Nov 1, 2014, at 4:31 PM, <robert@xxxxxxxxxxxxxxxxxx> wrote:
I wonder if you would be kind enough to clarify something for me?
I'm trying to do a spot color test from a document through to print, and
this is what I'm doing (using Photoshop and Argyll):
1. The spot color has a Lab value of 90, -14, 87 in Photoshop.
2. I convert the document to the print destination space (relative
colorimetric with BPC).
3. After conversion, the spot color is RGB 240, 247, 52 (or 0.941176,
0.968627, 0.203922). The Lab value from Photoshop is still 90, -14, 87, as
expected.
4. xicclu (rel. col.), with the RGB colors above through the profile
(forwards), gives Lab 90.33650, -14.520704, 87.115660. I assume that
Photoshop is effectively doing the same as xicclu but is rounding the
values.
5. I also tried fakeread (rel.col) which gives me exactly the same Lab
values as xicclu.
6. I print the image with no color management.
7. spotread gives me Lab values of 88.987, -13.637, 87.268. This is a
dE-Lab of about 1.6 compared to the xicclu reading.
The dE-Lab error seems too high as I have only just calibrated the printer
(iPF6400) and profiled the paper (Canson Baryta, so good paper).
colverify has an option to normalise each file's readings to white XYZ, but
xicclu, fakeread and spotread have no such adjustment.
I would have thought that the paper white would need to be taken into
account in comparing the spotread value to the image Lab value. Should the
paper white be measured and the spotread value normalised? If so, how
should this be done?
I appreciate your help.
Robert
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