[Openicc] linearisation targets, Argyll, G7

[Jan-Peter Homann]

Hello list,
 >From my point of view, we should concentrate on tools, which can run 
both under LINUX and also with Mac OSX or windows.
Concerning linearization-targets as printable images, we should use a 
design and focus on instruments, which are supported by Argyll and 
OPTIONAL also by X-Rite colorport.
As output format CGATS (or ISO 12640) is fine, as also Graeme stated.

Concerning linearization, we have to look at following topics:

1) linearization of individual channels incl. inklimit
----------------
This is mainly done by targets representing steps of every channel.
Typical representation are 1D LUTS. We had the discussion about density 
vs. L*a*b* based linearization and there were several reasons to prefer 
the L*a*b* based variant.


2) linerization targets for a shared visual appearance
--------------
This step optimizes the 1D LUTS in case of CMY-gradation and CMY gray 
balance and also for K-gradation
This can be either done by spectral measurements but also by comparing 
printed samples with reference samples.

After a linearization for a shared visual appearance, the output of 
CMYK-data WITHOUT ICC colormanagement  looks very similar.
The usage of standard profiles delivered with the installer will good 
response and very often individual profiles are not necessary.

Currently, the best described method for a linearization based on a 
shared visual appearance is the G7 method.

very good targets both for visual judging and measurements can be 
downloaded from

http://www.hutchcolor.com/Images_and_targets.html
The website from Don hutcheson, who also invented the G7 methodology.
Please note, that this targets are only free for private use and it is 
not allowed to redistribute them without allowance from Don Hutcheson

3) Inklimiting for the dark areas
----------------------
This is applied after 1D LUT linearization and individual channel 
limits. It can be either done by some generic GCR algorithm or by the 
standard printer profile.
for papers. For some paper ink combinations, it is necessary to limit 
the total amount of Color (TAC) to 200% or even lower. In this case, the 
usual method is some generic GCR BEFORE the profiling target is printed. 
The printer profile itself will than be calculated with a higher TAC.

The Hutchcolor TAC_04.TIF is quite helpful for TAC evaluation of 
linearized printing systems

4) Re-Linearization or calibration
-------------------------
This steps brings defined ink /paper combination in a state to mimic a 
given reference.
This re-linearization could be done based on 1D LUTs for every channel 
and also via a Device-Link as 4D LUT.
High-end proofing solutions are using often a DeviceLink-based 
calibration based on charts between 300 and 600 patches.

But also re-linearization / calibration based on 1D LUTs for matching a 
shared visual appearance will bring a printing device with a specified 
ink/paper combination to well defined visual state.
Other as in step 1) which is necessary for creating the basic ink-limit 
and linearization from scratch for a completely new device/ink/paper 
combination, the re-linerization / calibration target is printed with 
standard 1D LUTs. The results are either measured with a 
spctrophotometer or compared visually with printed reference samples for 
manual adjustments.


5) linerization files in CGATS / ISO 12640 format
--------------------------
The CGATS-format  or ISO 12640 is an OS independent ASCII based format 
for color-data. It could easily adapted also to transport 1D LUT data.
This would allow e.g. to write export and import filter to manipulate 
printer linerization-data with  imaging applications, which can handle 
CMYK images and have a curve tool.  including a softproof of the 
manipularions.


6) Open Source Creation of reference print samples for visual 
(re-)linerization
-----------------------
The Epson Photo 2400 with Ultrachrome K3 Inks delivers very stable 
output results and is supported by GutentPrint. On a linearized and 
profiled device (maximum GCR), it is possible to produce print sample 
with defined L*a*b* values. This could be e.g. neutral gray samples with 
L* 80, L*50. L*30 and L*20. printed as square with 7x7 and a cut out 
hole in the middle. The reference samples are compared with 
(re-)linerization prints for K- and CMY-scales. The user reads the 
number of the patch, which matches best the reference sample and types 
it into the "(re-)linerization wizard". This application calculates the 
new linerization curve for the current device/paper/ink combination.

For shure, there are issues like e.g. dealing with paperwhite and 
optical brightners, which will need some finetuning of the process. But 
the idea to document the complete process of the creation for the 
reference samples, that every one, who has printer supported by 
Gutenprint, a spectrophometer and Argyll would be able to produce and 
sell such reference print samples.


Regards
Jan-Peter






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