[argyllcms] Re: Printer gamut


How does the argyll software go about determining the extremities of a printer's gamut? 

By examining the profile.


Er, I should have been more specific....I meant at initial profile creation time with no initial reference point....but you've answered below.

> Assuming it has no 'starting point' profile when the
test charts are generated, unless it manages to generate a full range of RGB values that correspond to the printer's most saturated colours, I can't see how it would know? 

It doesn't need to know anything about the response of the device
to characterize it. The default chart explores the device values
full gamut. Any color has to be reachable that way. The recommended
chart algorithm evenly spaces test patches in device space. This
turns out to be a robust and reasonable approach when nothing is
known about the device response, and produces results that are hard
to improve on, even if something is known about the device.

> And since the device RGB values that
correspond to the most saturated colors could not be known, I imagine it would be easy to end up with very few patches that cover certain parts of the gamut that might be the critical parts, especially if the device gamut was rather non-uniform. 

Possibly, if the devices gamut maximum points do not correspond to
the device maximums. Such a device is not very well behaved though,
and can be difficult to impossible to profile. As long as such
non-monotonic response is at the end of each channel response,
Argyll should cope (but it's not something I've verified in detail).

Hmmm...that may be the nub of it. Given that it has an RGB interface, when I look at the printed rainbow chart (with raw RGB values going to the printer), what I see is that the deepest (actually perceptually darker) greens at L=50% and S=100% are not when RGB=0,255,0 but when RGB=0,255,80 or so. This is where the green hue is shifting over to cyan. This looks different to the rainbow as it's displayed to monitor with raw values.

Actually, I'm thinking now this isn't really right, and is demonstrating what you might expect of a cheap printer with cheap dye-based inks. Those deeper greens should be at L=20% S=100%, and not where they are. i.e.Green is changing from a lighter to a darker shade as one shifts across the hue spectrum...this is just wrong. But then I'm sure lots of compromises get made along these lines, and they just hope no-one notices or that the profile will straighten things out a bit. (I'm sure the average buyer of this class of printer would think it works great! Look at those lovely colours! ("Never mind if they're not quite the right colours...")

Well that explains a lot!

It's hard to make judgements about such things. The difference between a real
instrument and using a scanner could easily explain such a discrepancy.
(Personally, I wouldn't have tried more than about 1 sheet using a scanner
as an instrument). Your printer and the one the manufacturer profiled might
not be operating identically. I've also come across manufacturers profiles
that simply seem to have been manipulated, and their colorimetric table
values didn't seem to be "real".


I suppose you're right. I should be just satisfied to get any kind of half-reasonable result doing what I'm doing!

Cheers,
Milt


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