Re: [Gimp-print-devel] Strategy for profiling Epson R280 with Ultrachrome K3 inks

[Hal V. E. <hv...@as...>]

On Friday 08 August 2008 03:36:18 am Marko Milisavljevic wrote:
> I tried printing CMYK but can't figure out how. I think the correct
> procedure, on a Mac at least is to take a CMYK image and print it
> while also selecting "CMYK" in Gutenprint driver settings. This
> appears to be printing RGB, since K gradient looks indistinguishable
> from gray CMY gradient. 

You need to make sure that the CMYK file you are printing actually has a K 
channel.  Most generic RGB -> CMYK conversions will use only process colors 
(IE. no K channel).  But you also need to remember that GutenPrint does use 
the K channel when it does it's RGB to printer seperations when you are 
printing an RGB image so even if there is a K channel being printed with your 
CMYK file this many not be too different from what GutenPrint is sending to 
the printer.  In addition if process grays are a close match to a K then you 
may not be able to tell the difference between a process gray gradiant and a K 
gradiant with out the use of a measurement device.  Do you have one?

> How do you make Gutenprint work in CMYK? 

You send it a CMYK file and you set the colormode in GutenPrint to CMYK.  You 
need to make sure colorsync is not messing with your stuff.

> At
> least from Photoshop.

I don't know what Photoshop does.  Photoshop was designed for Windows and Mac 
and originally both of these only had RGB print drivers.  GutenPrint is closer 
to being a RIP than a printer driver although you have to use third party 
front ends to fully utilize this functionality.  You might want to have a look 
at the PhotoPrint front end for GutenPrint.  It is specifically for printing 
images to a GutenPrint device and gives you complete control over how color 
management is used and how this interacts with GutenPrint assuming that you 
scan get colorsync to not cause problems.

Any image type (even RGB) -> CMYK device profile -> device specfic CMYK.

If you are using a CMYK profile to transform the image the file that the print 
driver gets is CMYK no matter what color space was used for the original.  Of 
course the original image file must be tagged with a profile or you will need 
to somehow provide this information during the transform.    

I don't know how ColorSync handles the steps for applying the color 
transformation but it must do it before it passes the image to GutenPrint if 
you are going to use it to handle this functionality.  On a Linux machine we 
would use PhotoPrint, CinePaint, Scribus, some other CM aware software or a 
command line utility to create the device specific CMYK file which is then 
handed to GutenPrint.   With apps like PhotoPrint, CinePaint or Scribus the 
transformation steps are more or less hidden and there is an automated hand 
off of the device specific file to GutenPrint.

When you generate your profiling target with targen it will be a CMYK target 
if you are going to create a CMYK profile.  You will print it directly to the 
device through GutenPrint using the same settings as you will use for printing 
your images in the final printing work flow (IE. all printer driver settings 
other than paper size and feed options must be the same and so must the 
paper).  You must make sure that ColorSync/CUPS/photoshop do not cause any 
transformations to occur to the profiling target on it's way to GutenPrint.  
Since GutenPrint (at least the versions that I have) does not include any 
cupsICCProfile entries in it's PPDs I would think that you don't have to worry 
about CUPS causing any issues with this.   Since your printer is "customized" 
if there are cupsICCProfile entries in the PPD you should remove them since 
these will clearly be incorrect with the new inks.

From my experience CinePaint and PhotoPrint will only print 16 bit/channel 
CMYK files and 8 bit/channel files must be up converted before either of these 
will accept them for printing.  I am not sure if this is a GutenPrint 
limitation or a limitation in the printing dialogs for these other 
applications.  But I suspect that GutenPrint only accepts 16 bit/channel CMYK 
files.  Robert is this correct?  If that is correct then you must make sure 
that the CMYK files coming out of the device transform are 16 bit/channel.  
Since you are using 16 bit/channel images this is likely not an issue for your 
normal work flow.  But targen creates 8 bit/channel target images and these 
will need to be converted to 16 bit/channel before they are sent to 
GutenPrint.  Not a big deal but I though that I would warn you up front.

One other ArgyllCMS thing.  ArgyllCMS has the ability to produce profiling 
targets that are optimized for a specific device if there is already an 
existing profile for that device.  The way this works is that you first create 
a profile using a relatively small number of patchs (perhaps 300 to 500 - IE. 
one or two pages depending on your spectrophotometer).   Using a small number 
of patches makes it fairly quick to to get this first profile and it should 
give OK results even with this limited numbr of patches if you should need to 
use it. Then you use that profile as input into targen which will look at the 
profile to find areas where it shows signs of having issues such as non-
linearities and in those areas it will generate more patches so that the 
profiling software can get a better handle on what is happening in those 
places.  For this second pass you will want to generate a bigger patch set of 
perhaps 2000 to 3000 patches.  This will take a lot more effort to measure but 
should result in a very high quality profile.

Hal