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

[Chris Murphy]

On May 17, 2011, at 5:23 PM, Alastair M. Robinson wrote:

> 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.

When I suggest DeviceN not possible, I'm referring to the smaller consumer inkjets. I don't know that there is actually a DeviceN path. I know there is one for the 17" and wider Epson printers. But for the R1900, R2000, WorkForce printers, Picture Mates, etc. I don't know that they accept anything other than sRGB. Same for the $50 HP's and Canon's.

Also, my understanding is that with Epson printers, the proprietary driver's sRGB option utilizes internal custom calibration tables, per printer, that allow very good consistency printer to printer of the same model. Whereas when not driven with the proprietary driver, this calibration information is not available and the repeatability between prints is not nearly as good. So again, I wonder if there is a way to access this sRGB entry path to the printer for open drivers, or if open drivers are stuck only using DeviceN?


> 
>> 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.

Is this hand-tuned parameterized curve simpler, or more complex than, either the ICC output device profile format (which has lots of stuff in it, LUTs, multiple curves, matrices); or the ICC devicelink profile format?

> 
> > 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!

Yeah so I'm curious if the base, always in place conversion, is CMYK -> DeviceN. 


> 
>> 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.

Right.

My brain is presently plugged up on how to do this, but I wonder how to have cake and eat it too. i.e. make it easier for the Gutenprint developers, and users who don't use custom ICC profiles, those users who do use custom RGB ICC profiles, and those who would use custom CMYK ICC profiles, and then bury it all behind an default interface that does the right thing for all of them. But it uses ICC profiles instead of this classic proprietary Epson, Canon, HP black box LUT...might be pie in the sky but I'm still wondering.


Chris Murphy