[Openicc] linearisation targets, Argyll, G7

[Jan-Peter Homann]

Hello Graeme, hello list
I don´t think, that a linearization for a shared visual appearance is 
only useful for set up CMYK printing presses.
Especially in in the Inkjet area, we have a lot different printers and 
media. If the the linearization targets for all of this printers / ink / 
paper combination lead to to similar gradation and gray balance, 
workflows with standard profiles will be much more stable and predictable-

Most users will not need individual profiles if the usage of a standard 
profile for a paper class on a linearized device will lead to the 
expected gradation and gray balance.

As many printers have very strange curves in the pure linear state, the 
creation of 1D LUTs for a shared visual appearance is an iterative 
process. Black is calibrated by firstly printing a pure black scale. 
Based on the maximum darkness, which is reachable on the device, the 
appropriate target curve for the linearization is choosed.

For the CMY-scale a scale with different near neutral CMY patches is 
printed and measured or visually compared to reference print samples.
The linearization goal could e.g. be, that C50 M40 Y40 should lead to 
L*57 a*0 and b*-1.
If e.g. the CMY patch C30 M25 Y30 leads to L*57 a*0 and b*-1, the 
correction-curve for linearization should correct C50->30 M40->25 and 
Y40->30.

As the eye is very sensible for the neutrals, a re-linearization / 
calibration to a good gray balance and gradation will do the job, most 
users expect for a good printer-setup.

Both the basic linearization and also the re-linearization/calibration 
could be done either by measurements or by visual comparisons with 
reference print samples. As I described in my last e-mail.

As IDEAlliance and GRACoL have made a lot of material for the 
implementation of G7 for press calibration available, it is easy to 
adapt this methodology for inkjet linearization / calibration. Please 
note, that the in the original G7 / GRACoL guidelines, the target curves 
for the gradation of the K- and CMY-scale are defined in densities,
The documentation for the current GRACoL and SWOP charaterization-data 
at http://www.npes.org/standards/tools.html shows, that also L* values 
could be used in target curves.

If anyone is interested to make more real world tests concerning a G7 
based ink linearization with GutenPrint, you could contact me off-list.

Regards
Jan-Peter




Graeme Gill wrote:
> Jan-Peter Homann wrote:
>
>
>   
>> 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.
>>     
> [ie. this type of calibration seems to be very narrowly
>   useful for setting up a CMYK printing press to print
>   separated jobs, rather than something needed for
>   an electronic printing device.]
>
>   
>> 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.
>>     
>
> Generic GCR algorithms give very poor gamuts in the dark saturated
> regions (ie. K = min(CMY) etc.)
>
>   
>> 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.
>>     
>
> Unnecessary with Argyll, since both the test chart and profile
> can have ink limits down to 0 imposed on them (although naturally
> the gamut suffers badly at TAC's below 200).
>
>   
>> 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.
>>     
>
> I think this is outside the scope of print drivers for
> electronic printers as well, and is very much in the
> realm of CMYK proofing, or press re-targeting.
>
>   
>> 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.
>>     
>
> Argyll currently uses the CGATS format for the display calibration
> curves. The main attraction about using (say) an ICC device link
> as a format for calibration curves is that they can then be linked
> into a color transformation link path using current tools.
>
>   
>> 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.
>>     
>
> I gather this is a means to enable calibration without an instrument ?
> Interesting idea.
>
> Graeme Gill.
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>
>   


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