[argyllcms] Re: Profiling flexo presses
- From: Graeme Gill <graeme@xxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Tue, 23 Aug 2005 11:31:31 +1000
Roberto Michelena wrote:
Building a PCS->device table from target readings (device), basically
loses the accuracy of the original data and replaces it by
interpolated (thus less accurate) data situated at regular intervals
of PCS space (which the readings were not). As you don't know in
advance which points in PCS space you'll need to render, you just
select a "grid" and render it.
Afterwards when you want to render some PCS colors to device space,
you interpolate between those gridpoints that were already
interpolated from device space readings. Double loss of precision.
If instead of that, you keep the original readings (device->pcs table)
and when you want to find a device value for an original PCS point,
you do a reverse interpolation (I know what I want to say, but doubt
this is proper wording!) in the device space, you have much more
precision.
That's what the Imation CFM did (although I never saw evidence of
better quality!), and that's one of the options of Argyll CMM.
Well, technically no, Argyll always has "one level of
lost precision". There are ways of directly interpolating
scattered data (typical of chart readings) directly (e.g.
Delaunay Tessellation, inverse distance techniques etc.)
and in some situations (typically with RGB or CMY devices),
it might be possible to measure every value of a regular
test grid, allowing the clut table to be filled directly.
It may seem that such approaches would give the best,
most accurate results, but in practice this may not
be the case. The reasons are inaccuracy in the measurements,
sampling "noise", and not using "typical device behaviour"
information.
Direct interpolation techniques typically behave somewhat
simplistically between the sample points, and represent the
sample points exactly. If the sample points have some
inaccuracy (which will almost certainly be the case), then
this noise is represented exactly. An interpolation technique
that smoothes the sample data slightly may be able to reduce
the inaccuracy "noise", as well as giving a more pleasing
result due to better smoothness. The location of the sampling
points and the (often) linear or arbitrary interpolation between
them, causes a form of "stepping" or unrealistic transition
behaviour between the sample points, resulting in a sort
of sampling "noise". The relatively fixed or arbitrary interpolation
between the sampling points ignores any knowledge about the
typical, reasonable or desirable behaviour of the device between
sampling points. In addition there are issues with trying
to get reasonable extrapolated results, something that
often crops up in scanner profiling.
In practice therefore, I found it better to suffer one loss
of precision in resampling and interpolating from the measured
sample points (which can be unconstrained in their location,
adding flexibility), while overall gaining a better result.
It's also a necessary step in representing the device behaviour
in ICC profile format. In practice, the per channel device
and PCS curves can be "extracted" from the scattered sample
data, before generating the regular grid interpolation.
Yes, the creation of the B2A table represents a further
loss of accuracy, which direct inversion of the A2B table
avoids (icclink -G etc.).
Graeme Gill.
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