Digital BW, The Print

Steve Kale wrote:

>For those gurus who understand greyscale partitioning way better than I do I have the 
>following question.  Let's say you printed QTR's ink separation page and determined that you 
>would limit the K ink at 50% but noticed that your LK and LLK inks continued to show solid 
>increases in density well above 50%.  One could either limit each ink at 50% and calculate the 
>partitioning points based on this ink limit or decide to limit K at 50% but run LK/LLK at a 
>higher ink limit and calculate partitioning points based on this mix of limits.  Is there 
>anything to be gained by the latter?  I was thinking that perhaps one would ordinarily prefer 
>to run inks close to their "peak" densities in order to get a less "dotty" greyscale. Does this 
>make sense or does it not really matter?
>
>Cheers
>
>Steve
>
>  
>
It is flexible in the way QTR does it so you can decide what is more 
important. Roy's QTR may be more intelligent on ink amount than what I 
have done with another RIP and a more less DIY quad ink set + odd 
choices for the distribution of that inkset on the RIP's channels.

For example there's more than the Max-out point to decide where K's ink 
limit should be. Bleeding can be the limiting factor instead of the 
highest density and the ink limit should be put lower to keep contrasty 
detail possible in the shadows while sacrificing some density. With 
Photo Black the heavy gloss differential/bronzing usually lies at the 
same point of Max-out as the density drops with the loss of gloss, it is 
easier there. I also think that if the factors above allow the use of 
the Max-out patch you might gain some density by taking a few 
percentages above that patch to even out any reduction by the 
linearisation - profiling step. That's more a suspicion than a fact but 
I have done that on the Wasatch SoftRip and I didn't see strange 
deformations in the linearisation curve near the deepest black (for 
example a heavy cut in the curve just below black) .

If there's a boost with the next grey ink on the black ink you may 
consider to limit the K ink a bit more as bleeding could increase, the 
ink limit of the next grey ink will not influence the boost as it is 
just a small percentage of that ink. Why using a boost when it doesn't 
increase density and may cause bleeding ?  Not all printers print a 
smooth BO at the highest density, my 9000 with MIS Eboni doesn't, so I 
have PK running as the next grey ink and  get the black smoother that 
way. Not really a boost but good for the quality.  BTW, the MIS Photo 
Black on PhotoRag has a very short  usable grey range at 100% inklimit, 
Dmax starts early and bleeding too.

At the partioning points of the grey inks (quad sets) you may get 
bleeding problems (loss of contrasty details) faster while using all the 
steps of your lighter greys (say an ink limit up to 75%) than when you 
switch to a darker grey ink at 60%. Especially with black lines in 
highlights you may see that problem, the medium of the light grey ink 
dissolves the black ink at the edges. On the other hand the lighter grey 
ink will keep more gloss in gloss printing and a lower ink limit may 
give the visible dots of the next darker grey ink.

Toner ink is another factor. One Blue cool toner like I have does 
influence the darker warm grey patches much less than the lighter grey 
patches. The toner curve then has to be very progressive to get a smooth 
hue over the entire range. That's impossible as the bleeding gets too 
high at 70-80%. The result is that cool on this printer can't be that 
cool. The only other choice is two blue toners, one darker and more 
saturated, the other lighter and less saturated. With Sepia it goes 
better as it suits the warmer grey inks better and the saturation can be 
higher without the dots getting visible. I have mixed both for this 
printer so this isn't a general issue and a none issue on the 9600's etc 
with the UC inks.

I'm using the Wasatch SoftRip targets with the contrasty white and black 
lines in the patches to find out where bleeding is acceptable and where 
it is too high. I should adapt the QTR targets with similar lines but it 
works different there. With the typically CMYK based SoftRip I have 
ramps for Eboni + Middle Grey ink (Mm), Photo Black + Light Grey ink 
(Cc) and cool + blue toner in K + Y.  The ink limit + linearisation 
target has the ramps for the CMYK channels so two grey ones in my case + 
two toner ones. Next to that I have two build ups: of all the grey inks 
= CcMm and CcMm+K (and should have one for CcMmY but can't because that 
triggers black generation so doesn't work with that CMYK RIP). In the 
build ups there are contrasty white and black lines to check bleeding. 
In the first tests I check for bleeding in the single channels and take 
the inklimits per channel (overall number). In the second test with the 
first ink limits + partition curves I check for bleeding in the build 
ups and linearise on that target if acceptable. If not I adjust 
inklimits individually with the correction curves. The next target can 
be used for linearisation then. On top of that a Lab like curve was 
added (to my eyes).

Roy's QTR is probably more intelligent in ink amount control however it 
would be good to add the channel build ups to the target (selecting the 
channels by the user) and have the black and white lines running through 
the patches. I have not had time to check the latest incarnation of QTR 
and maybe this all has been added already.

Ernst