[argyllcms] Re: Printer gamut

Milton Taylor wrote:

Having regenerated my profile for the i865, this time using the Canon's own profile as a starting point, and also putting some channel step wedges into the targets, I got some interesting results: 

Adding specific test patches for a profile can often be of benefit,
especially if the patch budget is limited (which it almost always is).

2. Examining the printed channel steps is quite revealing: red and green both range tonally quite well, but blue is a big problem. There's not that much tonal variation in it. Also, the profile maker came up with the warning about the gamut being non-monotonal. I had already suspected this was the case, but this just confirms it. (Could be the scanner too I guess, but less likely?) 

One problem with input device in general, is that most of them do
not "see" color the way we do (ie. their RGB filters are not
equivalent to the CIE standard observer). There are practical
reasons for this, both in terms of expense, and in terms
of optical efficiency (signal to noise ratio etc.).

This means that a scanner will see metamers where we don't,
and we will see metamers where it doesn't. Add into the
mix the difference in colorant spectral reflectance between
and IT8 photographic target, and inkjet ink, and you
have a recipe to explain some of the differences between
a scanner and a colorimeter. These spectral differences
are usually most pronounced at the blue end of things
(and the signal to noise ratio is usually lowest there),
so your observations are not that surprising.

4. A more general question that applies to any printer: assuming that a printer is reasonably well calibrated to start with, and can be trusted to be come up with decent greys when raw device values for each channel are equal (ignoring paper color and uneven spectrum lighting for the moment), is there any way to give more weight to those points at profile generation time, to avoid this sort of shift? Of course, it's possible that the error is coming from the flatbed...easy to tell: I'll just compare the readings of a true grey scale with a black-only grey-scale that comes from the printer. 

A simple way of weighting the result is to add more test patches
at the colors of interest. You can also duplicate them in
the measurement file as well, to get a similar effect.

Graeme Gill.


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