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:
1. I think the profile gamut now is a broader and smoother
representation than what I had earlier. (I also dropped back to -qm -r
0.8). So posterisation in the sRGB hue/sat rainbows due to gamut
clipping time is not nearly as bad, and at perceptual intent it's now
much better than my earlier result which lost the best greens.
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?)
3. The greys are less neutral now...the profile is pushing them too far
off course into green territory. It raises another question in my mind
about how the printer decides when to use 'black' ink instead of a mix
of CYM. I would say now that it's not using black at all.
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.
I accept that I'm fighting an up-hill battle here, as obviously the
RGB->CMYK algorithm in the printer is going interfere a lot, and using
the flatbed is a poor way to reliably measure colors this way, but I'll
keep pushing the envelope as far as it can go, for the pure fun of it of
course.
Milt
