during my linearization development test I have discovered something strange in the Argyll chartread readings of black patches,
and I'm here to humbly ask for your kind collaboration to confirm if I'm really on to something or if, more simply, I'm missing something.
I have written a post in the color management section you can look at, if you wish,http://www.luminous-landscape.com/forum/index.php?topic=82133.0
that tell the whole story in more details, step by step (starting from a purplish cast in the icc black point).
I have already sent an e-mail to Graeme Gill, the Argyll author, shortly explaining what I have found, asking for an opinion that I will consider a reference, obviously.
After that I have decided to ask for replication of the issue and for this purpose I'm posting here a simple Argyll test procedure
to verify the Argyll "chartread" capability to read black patches in strip mode.
This test should be really simple to do, but it's best suited for people that already have a basic knowledge and experience using Argyll,
and it will really help me to understand what is happening.
So, let's start.
Argyll RGB printer profile "chartread" test with ColorMunki (but it could be done with other spectro if supported by Argyll)
(PC Win 7 64, Argyll 1.6.0)
My test was done using an Epson R3000 (OEM inks)
and Hahnemuehle Photo Rag Baryta 315 PK (no OBA's high quality cotton photo paper).
I have made a simple test target row of 15 patches including at least two black, one white and some saturated colors.
It will easy fit the long border of a single A4 sheet.
The test target was done using the following Argyll commands:
targen -v -d2 -G -B2 -e1 -f15 test
printtarg -v -iCM -h -T360 -p45x297 test
Note that A2 and A15 are the two black patches in the strip "test.tif" I have attached,
so, you don't need to do it.
Download the "Argyll_test.zip" file I have attached.
Print "test.tif" ensuring to disable color management in the application AND in the printer driver.
(I have used ACPU leaving 15mm margin in the left border in the Epson printer settings layout panel)
Wait at least 20 minutes to let ink settle a bit.
Open a command line (I assume Argyll is still installed and ready on your system) and put the test.ti1 and test.ti2 files I have provided
in a folder you will use for this test.
Read the row in strip mode with Argyll chartread and rename each time the "test.ti3" for further comparison
chartread -v -H -L -B -T0.4 test
rename test.ti3 teststripH.txt
chartread -v -L -B -T0.4 test
rename test.ti3 teststrip.txt
Now, read the row (or at least the A2 and A15 patches) in single spot mode and rename the "test.ti3" for further comparison
chartread -v -H -p -L -B -T0.4 test
rename test.ti3 testspotH.txt
chartread -v -p -L -B -T0.4 test
rename test.ti3 testspot.txt
Open the variuos .txt files above in a spreadsheet or in Notepad++,
look at the Lab values of the two black patches (A2 and A15) and compare it.
You will see that in strip mode the A2 and A15 black are measured with big a* and even bigger b* difference than in spot mode
and is evident that the readings done the -H (high resolution) engaged are the worst in both modes.
Finally, launch X-Rite ColorPicker (provided with ColorMunki) and read the same row in strip mode,
save it as "teststripCM" palette, look on screen at the Lab values of the A2 and A15 patches, write down it
and compare it with the ones already read with Argyll chartread.
Note that, here in ColorPicker, if you additionally read again the same A2 and A15 patches in spot mode (instead of strip mode)
you get nearly the same results (I saved them as "testspotCM"),
and both these readings are similar to the chartread readings made in spot mode (-p option).
Moreover, note that the majority of the colored patches does not appear to be so much different between chartread and ColorPicker (both in strip and spot mode),
so this issue affect mainly the black readings.
Please, try to replicate the test in your setup (with different printer/papers) and, maybe, share your findings.
It would be nice if someone would try to do the same test, by printing the same attached "test.tif" target
but using a i1Pro2 spectro (in UV-cut) instead of the ColorMunki to compare the chartread readings af the A2 A15 black patches
in strip/spot mode, with/without -H option engaged and, maybe, even in their X-Rite Colorpicker equivalent (which I ignore).
I have attached a zip file including my "test.tif" target you can directly use
and all the mentioned Argyll files:
test.ti1 test.ti2 teststripH.txt teststrip.txt testspotH.txt testspot.txt
plus the ColorPicker readings (teststripCM.txt and testspotCM.txt)
In addition, for your convenience, I have attached a pdf file showing all the measurements and highlighting the deviation for the blacks patches with different colors.
I will resume here the blacks reading and my comments:
L A B
chartread strip -H
A2 2.5 -2.1 4.3
A15 3.0 -2.0 5.2
A2 2.7 0.1 3.2
A15 3.0 0.1 3.6
chartread spot -H
A2 3.3 -0.7 1.6
A15 3.7 -1.0 2.7
A2 3.3 -0.2 0.4
A15 3.8 -0.2 0.7
A2 3.7 0.0 -0.5
A15 4.1 -0.1 0.0
A2 3.7 0.1 -0.3
A15 4.2 0.0 -0.1
The bottom line is that, at least in my setup, there is an evident problem for chartread to correctly read the black patches in strip mode with ColorMunki,
and the so often used -H mode should really be avoided, even in spot mode.
The only acceptable readings from chartread for the blacks are allowed in spot mode (-p) and without -H,
and this is a real pain imho...
Note that it is not an hardware issue of the ColorMunki, because the same strip if read in ColorPicker gives consistent and correct measurement values (both in strip and spot mode).
I have repeated the test several time, using different targets, and I observed always the same results.
Please, let me know your opinion.
Any comment is welcome and will be really appreciated.