[Openicc] Printing targets: App or driver ? Profiling RGB or CMYK

[Alastair M. Robinson]

Hi,

On 17/05/11 23:41, Chris Murphy wrote:

> Some inkjet printers are CMYK, most have more than four colorants. But most of those consumer printers
> with more than four colorants don't provide access to individual channels, i.e. DeviceN not possible.

Epson inkjets at least - and I believe Canon too - are driven at a 
pretty low-level - Gutenprint is telling the printer precisely which ink 
drops to print at which sizes at which locations - so yes, DeviceN is 
available.  Its not being available on closed platforms is just a driver 
limitation.

> 2. Not clear to me is the exact nature of alternative paths to these printers, when available. Are they unspecified RGB?.
 > CMYK? Or DeviceN?

If you're talking about the path between Gutenprint and the printer, 
rather than the path between CUPS filters and Gutenprint, then *only* 
DeviceN is available.  Having said that, Gutenprint has support for 
limiting which channels are used, so you can print using only CMYK if 
you really want to.  There are other more exotic situations to handle, 
too, such as on the R800 and friends, CMYRB and only one of two 
available blacks, and with or without gloss optimiser!

> 3. If DeviceN is available, how is this utilized by Gutenprint? Is there first a CMYK to DeviceN LUT?

Hand-tuned parameterised curves, rather than a LUT, but broadly, yes.

 > And then there is a separate RGB to CMYK LUT? Is RGB is reached via 
concatenation of RGB-CMYK-DeviceN LUTs?
> Or is it a separately developed RGB-DeviceN LUT?

That I don't know - but it wouldn't surprise me if the answer to that 
varied between, say, a CMYKRB printer (R800 and friends), and a CcMmYK 
printer (random Photo printer of your choice) I think it would depend on 
the precise ink set supported by the printer in question.  CMYK -> 
CMYKRB is of course way more complicated than CMYK -> CcMmYK!

> What what is the basis for the RGB space? sRGB or other?

I'd say "de-facto sRGB" (as in, whatever looks about right when Robert's 
hand tuning the printer in question!) - the process isn't yet instrument 
based.

All the best
--
Alastair M. Robinson