Re: Problems profiling 6 colour printers? (WAS: A new myth > Rant)

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<x-flowed>At 12:09 PM +0100 7/31/01, Rew, Alan wrote:
>
>Can one of our CM gurus please comment on this - if the light and dark inks
>(e.g. the C and c inks on my 1200) can produce  <different> hues on a paper,
>doesn't this make profiling such a printer fundamentally difficult, or maybe
>impossible sometimes?

Difficult, yes.

>This sounds like a theoretical nightmare, especially
>as the relative proportions of the light & dark inks will vary depending on
>the precise hue to be printed.

It can be twisted and I mean that literally.

In theory, the light c and m inks would be the same hue and when 
blended in the driver/RIP, would not enter into the CM equation. In 
reality the blending itself is rarely taken into consideration except 
in some RIPs where you can control how they blend.

Ultimately it is important to remember that ICC profiling is based on 
the appearance of colors and how they relate to the RGB, CMYK, 
CcMmYK, CMYKOG or whatever ink combinations that may create them. If 
we built absolutely massive profiles where each color was sampled & 
stored along with the ink formulation that produced it then the 
number of channels controlling the printer would not be too much of 
an issue (although black generation is something that will always be 
nice to have control over).

In reality we don't have that many values. Profiles are built from 
200-1000 (typically) patches that are interpolated into 3-D tables 
(for output at least). Even a small table with 15 divisions per side 
has 15x15x15 (>3000) values in it. So the profiling software 
interpolates to create the table. Then we come along with a color 
that doesn't fall right on a grid line (with a printer gamut 
containing 500,000 to 1,500,000 possible Lab values it's not too 
hard) and the CMM is forced to interpolate again. Now, if you had a 6 
channel printer, you would have more criss-crossing lines in the 
table to "guess" at the best ink combination for the color. If you 
only had 3 lines to choose from you could not possibly get as close.

I described it a while back in relation to an octopus marionette. 
Ideally, you would want to have an 8 or 9 string control for that 
octopus but that could get unwieldy (not being as well endowed as our 
undersea friends in the arms department). You could control the 
marionette with three strings and probably make a pretty convincing 
octopus, but the more strings the better.

Strange, late-night ramblings I know but I hope I shed a little light on it.


Regards,

Steve Upton

+--------------------------------------------------+
  Steve Upton
      upton@chromix.com

             CHROMiX / Profile Central

       www.chromix.com   www.profilecentral.com
          p:206-985-6837   f:206.526.8278
      9594 1st Ave NE #390 Seattle, WA 98115

  Color Store...Remote Profiling...Media Management
+--------------------------------------------------+

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