Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@y..., "darrelleifert" 
<darrelleifert@y...> wrote:
<snip>

> Now the "newbie" questions:
> 
> 1)  How is what is essentially a CMY breakdown initially created 
> within an RGB curves workspace?

Each RGB curve controls the output of one of the gray inks.  Here are 
a couple of paragraphs from the MIS-FS Partitioned Quad workflow 
document in the Files section of this forum:  

Files > Ink Sets, reviews and techniques > MIS FS > 1160 Partitioned 
Workflow  

What is a Partitioned Quad Workflow?
An RGB curve set is applied to the image and partitions (or assigns) 
each of the four inks to applicable overlapping ranges of grayscale 
values.  Thus the lightest ink prints the lightest portions of the 
gray scale etc. yielding essentially dotless lighter tones using the 
Epson driver.  Examples of partitioned workflows include those 
developed by Paul Roark, Chris Brandon, and others.   Lumped 
workflows, such as John Woolf's or Jerry Nivins' where the C, M, Y 
position inks are lumped or mixed together in approximate equal 
proportions across the grayscale range, yield essentially a two-tone 
ink system –- black and gray.  These lumped workflows can produce 
fine quality prints, however they are not dotless in the lighter 
tones (especially noticeable under a loupe) and sometimes show 
banding in the shadow areas.

Ink Positions
The Full Spectrum inkset uses the same ink positions as the 
PiezographyBW inkset.  Each FS ink bottle is labeled "Cyan" 
(C),  "Magenta" (M), "Yellow" (Y), or Black (K).  The C-ink is dark 
gray, the M-ink is medium gray, and the Y-ink is light gray.   As a 
labeling QC check, you may wish to smear some ink from each bottle 
onto a white sheet of paper with a q-tip and let dry.  If using a CFS 
or CIS system, make sure to connect the system tubes to the correct 
ink bottle.  The tubes running to the installed "color" cartridge 
should deliver  C, M, Y ink from left to right (as you stand in front 
of the printer). The K-ink tube goes to the black cartridge.  

Note:  The single curve of a lumped workflows controls all three of 
the gray inks--hense "lumped" or "nonpartitioned".  The black ink in 
both kinds of workflow is controlled directly by the Epson driver.  
The black ink is turned on when all three other inks are greater than 
about 50%.

> 2)  When the stepwedge with CMY transfer curves applied is printed 
> and, say, the 70% through 85% band and the 40% through 50% bands   
> are a solid gray, how is (are) the RGB curve(s) tweaked so that the 
> solid blocks resolve into distinct 5% bands?  

I am not sure I understand your question, but here goes... The 21 
steps (5% increments) of the stepwedge represent 21 points on a 
continious curve connecting the dots (I hope the FBI and CIA aren't 
reading (;>)) ).  If you printed a grayscale ramp, theoretically all 
shades of gray should be printed. Your example would represent 2 
bands of the continious grayscale ramp and would create a histogram 
that shows two large steps 40 to 50% and 70 to 85%. They would not 
resolve into 5% bands. An example of what a 21 stepwedge histogram 
looks like is shown in the profiles_make2.pdf document referenced 
eariler in this thread.

> Can the stepwedge be printed, 
> scanned, and some kind of histogram readout give a clue as to how  
> the RGB curves can be adjusted to achieve the 5% separation?  
> Put another way, is there a feedback loop that will provide 
> corrective information on shaping the correct partitioned curves   
> for each printer / paper combination, or is it massive trial and   
> error?

This process of scanning, using a histogram (Picture Windows Curves) 
or a spectorphotometer/densiometer to evaluate the curves, tweaking 
the curves, and reprinting is described in the profiles_make2.pdf 
document located in:

Files > Ink Sets, reviews and techniques > MIS FS.

Unfortunately there typically is the need to make quite a few 
iterations.


Good Luck.

Jeff Randall