[argyllcms] Re: RGB Printer profiling and ColorSavvy CM2C

Alastair M. Robinson wrote: 
Is there any special reason for the apparently random placement of the colour 
patches on the
page?  (It makes scanning the patches with the colormouse an "interesting" 
task, since with no
software support, the XYZ values have to be added to the .ti3 file by hand...)


The randomization is there for a number of reasons, including
averaging out device non-uniformity, recognizing strips, and making
sure there is enough contrast between patches in a strip for
the instrument to recognize the patch boundaries. It can be
turned of by using the -r flag to printtarg.

Does Argyll have to generate the target itself, or can the patches be 
"hand-picked" or augmented?
I ask because in my experiments last night I got a remarkably good result with 
a mere 60 patches
- much smoother than either Prism or Print Profiler.


You can use any target you like really, you just have to get the final
device and instrument data into a .ti3 file.

What I'd like to do is use a sparse scattering of patches, and then augment 
this in the
"difficult" areas, adding some extra patches down the grey axis and in typical 
skin tones.


It's not really set up to do that kind of thing easily, since I was
aiming at strip or XY instruments rather than one patch at a time
type instruments. Don Bone had a interesting approach to that kind
of thing, generating new test patches based on previously read
patches, but it's not something I've pursued.

Finally, with both Argyll and Print Profiler (and to a much lesser extent, 
Prism as well), images
printed using the generated profile generally have very good hue and 
saturation, but need further
gamma correction to get the luminance looking "right" - in terms of Gutenprint 
controls,
Composite Gamma needs to be increased to 1.1 - 1.15 or so. Any idea what's 
causing this?


It could be anything really. Check your source profile (is it really
accurate for the material you are printing ?), intent (are you
creating a gamut mapped perceptual intent table and using it ?) and
viewing conditions (are they appropriate ?).

It depends how general Argyll's model is.  I'm assuming the fact that it can 
adjust the patches
based on the non-linearity measured in an existing profile means that the 
sampled data points
don't have to be equally spread.   The questions is, do they have to be some 
kind of
n-dimensional grid, or can they truly be scattered data points?


Argyll's profiling copes with scattered sample data. There are some
minimum requirements, but it should produce something with any
sort of reasonable sample set.

Gerhard Fuernkranz wrote: 
For example, to create the contents for the table in the .ti1 file to augment 
patches along the
gray axis for an RGB device you could do something like

$ perl -e 'for $i (20..99) { print "$i 0 0\n" }' | xicclu -pl -ir -fif 
profile.icc | awk '{ print
$8, $9, $10 }' | icclu -px -ir -ff profile.icc | awk '{ print ++i, 100*$1, 
100*$2, 100*$3,
100*$8,100*$9,100*$10 }'


Excel is really the tool alternative for someone not familiar
with perl, awk etc.

Generally, one should check the minimal patch size specified by the instrument 
manufacturer.
Accuracy may suffer if the patch is too small, even if the instrument still 
seems to just fit on
the patch.


Right, backscatter through the media can cause color outside the aperture
to influence the measured color.

Alastair M. Robinson wrote: 
Yep, those colours aren't easy to get right.  I've done some work on the 
curve-based colour
correction used in Gutenprint - and the real complication is that the printer's 
Cyan ink, and
more specifically, the dark cyans that a naive RGB->CMYK conversion produces 
are *way* outside
the monitor's gamut. It's very hard to create a simple model to compensate for 
that in a way that
 doesn't also desaturate the blues. Sounds like the driver you tried had 
similar issues...


If you're in a position to implement your own RGB/CMY to CMYK separation,
the argyll can generate a pretty good one using the fakeCMY utility to
create a cmy .ti3 from a cmyk profile, and then linking them to form
a CMY to CMYK device link separation.

My goal is only to have to do one pass though, because I want to be able to 
profile without
having physical access to the printers.  My goal, BTW, is "looks good", not 
"colorimetrically
perfect" - the ad-hoc curve-based correction currently used in Gutenprint is 
very difficult to
get right - especially for skin tones, hence my special interest in those 
colours...


The default "full spread" algorithm is ideal for this sort of purpose.

Graeme Gill.





Graeme Gill.

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