[argyllcms] Re: Whitepoint stored in Argyll-made ICC profiles
- From: Gerhard Fuernkranz <nospam456@xxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Sat, 08 Sep 2007 02:00:54 +0200
John Weissberg wrote:
> I have noticed that the value in the wtpt tag of an Argyll-made ICC
> profile varies dramatically depending on the quality level selected
> via the -q and whether input/output matrices are included via the -n
> tag. I would like to understand how it is that the wtpt is not
> invariant with respect to these tags:
When you fit a smooth regression model (smoothing splines in case of
Argyll) to the measurements, then the splines will rather pass smoothly
through the cloud of data points, but won't fit the data points exactly.
And the "white" data point is no exception; like any other data point it
may deviate more or less from the fitted curve. So for CMYK=[0,0,0,0]
the splines will not predict exactly the same L*a*b* numbers as measured
for the white patch(es). For relative colorimetric or perceptual intent
it is however very nasty, if CMYK=[0,0,0,0] does not map exactly to
paper white, as this would result in scum dots. So my understanding is
that Graeme "cheats" a little bit and does not put the measured white,
but the color predicted for CMYK=[0,0,0,0] by the regression model into
the wtpt tag of the profile, in order to optimize the relative
colorimetric and perceptual intent. In order to reduce the fitting error
of the regression model at/near white, you can populate your target with
more "white" patches (option "-e" of targen). Note that the dependency
is O(N²), i.e. to reduce the fitting error for white by a factor of two
you need to use 4 times as many white patches (this relationship will be
of course distorted somewhat by the outputs tables).
>> Measured whitepoint (from .ti3 file): 97.510 0.78000 -2.9600
>> Whitepoint from wtpt tag: 89.079722, 0.552168, -2.911983
This does really not look sane. Are you sure that the measurements are
sane? Is the printer well-behaved (i.e. well-linearized)? But without
looking at the .ti3 file it rather hard to get a clue what might be wrong.
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