Iterative profile corrections
Iterative profile corrections
- Subject: Iterative profile corrections
- From: "James B. Reswick, Jr." <email@hidden>
- Date: Sun, 1 Dec 2002 00:29:07 -0800
- Organization: Reswick Consulting
Hi:
I am trying to complete my workflow for performing iterative profile
corrections, and am unsure how to proceed. Below, is the direction I have
taken to get to where I am now, and I would greatly appreciate it if anyone
could help me to move forward.
(I use both Spectrum Pro 4.0 or QImage 2003 for printing, both can have color
management shut off)
Print 'i1 RGB 1.5 ref' target with no color management.
Read target with EyeOne, entering data into ProfileMaker 4.1.
Save target data from ProfileMaker as text spectral data file.
Build RGB profile for RGB printer driver with ProfileMaker.
Read target again with Measure Tool 4.1 to create text Lab data file.
Build a Lab TIFF file from the Lab data file created in Measure Tool with Logo
ColorLab, and convert image to 630x560 pixels.
Load and convert the Lab TIFF file to an RGB TIFF file using the newly created
profile in Photoshop, Absolute Colormetric, and save.
Print the newly made target with no color management.
Read the newly printed target with the Measure Tool, and save the text Lab
data file.
Load the both the Lab data file used to create the new target, and the Lab
data file read from the newly printed target, into MS Excel as tab delimited.
In Excel, build a third table by subtracting the new Lab data from the
original Lab data for each patch to show the difference.
Sample of a patch (many better, a few worse):
Original Lab data:
XYZ_X XYZ_Y XYZ_Z Lab_L Lab_a Lab_b
13.26 12.98 34.46 42.73 4.96 -48.25
Converted and reprinted Lab data:
XYZ_X XYZ_Y XYZ_Z Lab_L Lab_a Lab_b
14.88 15.37 37.63 46.14 0.36 -46.82
Difference:
XYZ_X XYZ_Y XYZ_Z Lab_L Lab_a Lab_b
-1.62 -2.39 -3.17 -3.41 4.6 -1.43
So, now I can see how well the profile is performing for this patch (I think),
and where it is off. My next move is to adjust this patch on my original
target appropriately (along with all the rest of the patches respectively) and
reprint it again without color management, so that when it is read into
ProfileMaker, the adjusted patches will compensate for the errors and cause
ProfileMaker to create a corrected profile.
Ideally, to make my corrections, I would modify the original RGB text
reference file that accompanies the RGB TIFF target by an amount appropriate
to the errors indicated, and build a new TIFF target in Logo ColorLab.
Unfortunately, this involves working between device independent and device
dependent color spaces, which implies the use of profiles when they are not
desired (I think).
An alternative might be to take the text Lab data file read from the original
target with the Measure Tool, apply the corrections, build a Lab TIFF file in
Logo ColorLab, and print the corrected target without color management, to
then be read by ProfileMaker. Unfortunately, we are again moving between
device independent and device dependent color spaces to print a Lab TIFF in
RGB space, which again implies the use of profiles when they are not desired.
So, I thought I would just take the corrected text Lab data file and load it
directly into ProfileMaker to create the corrected profile. ProfileMaker 4.1
saves its data in a text spectral format, and when I loaded my corrected Lab
file instead, I got a profile that was too far off to be usable. I'm not sure
if this relates to the differences in how ProfileMaker 4.1 builds profiles
from the two different types of data files, or if there is some other flaw in
my process. I thought that maybe my corrections were in the wrong direction,
but my corrections are relatively small, and the profile errors from the
corrected profile are relatively large.
Does anyone have any ideas on how I can proceed? Are there folks out there
doing successful iterative profile corrections, who could share their workflow
with me?
Thanks,
Jim
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