QTR-Quadtone RIP

That last 5% from 95 to 100 seems to be the most prone to difficulties.
The various paper and ink combinations seem to react differently when 
they
get close to overloading.  I haven't found a consistent way to predict 
what's
happening there.

As you know the three main variables that you can experiment with are
BOOST, LIMIT and OVERLAP.  I think there's a tendency to use too much
boost.  It brings the extra black in very steeply at the end and can 
give the
jumps in density.  I would typically use only 10% to 15% more that the 
ink
limit.  For instance if LIMIT=60 use between BOOST=66 and BOOST=69.
Something like 80 or 90 is going to be too much.   Another idea if you 
really
want to increase black ink is to up the LIMIT as well as the BOOST.  
What
this does is increase the amount of black for its whole range rather 
than
just at the end (which the BOOST does).  So in the example rather than
LIMIT=60 and BOOST=72 which will be steep at the end, use LIMIT=66
and BOOST=72.   It will spread out the black increase.

The other big issue is what happens to the gray inks at the end.   In 
general
the amount of gray plunges rapidly to allow the increase in black 
without
overloading the paper.  Sometime the extra gray seems to increase dMax 
and
sometimes decrease it.  While the OVERLAP effects the whole range it
has a particlar effect at step 100.  It basically gives the amount of 
residual
gray ink still being output.  OVERLAP=0 means no gray at all at 100,
OVERLAP=20 will mean 20% of gray ink is still output.   I've found that 
sometimes
keeping a little gray at the end controls the steep black -- i.e. 
improves the
linearity.  OVERLAPS of 10 to 20 seem appropriate.

You really have to see what's happening on the particular ink/paper 
combo
to see what helps.  None of these ideas are absolute recommendations.

Finally there's adding more steps in the linearization.  Usually 21 is 
fine, but
the 51 can be used.  You can actually use any number of steps as long as
they are evenly spaced.  The "evenly spaced" can be an issue once you 
get
higher up because of the 8-bit gray values.  E.g. 100 steps would have 
some
steps of 2 and some of 3.  You can fix this in two ways.  One is to 
pick the right
number of steps -- 86 steps is exactly 3 units at a time.  The other is 
to use a
16-bit gradient (PS CS only), posterize, and then convert to 8-bit.  PS 
will
dither the intermediate values.  The other thing you have to be careful 
about
with a lot of steps is that the MUST be increasing -- no dips allowed :)

Roy


On Saturday, May 7, 2005, at 08:27  PM, Tyler Boley wrote:

> --- In QuadtoneRIP@yahoogroups.com, "ccolbertbw" <ccolbert@u...> wrote:
> ...
>> For me, it is more difficult to get  a smooth well-linearized
>> transition from 95  or so up to that final dmax....
>
>> ...Having all the source code, you can see
>> what is going on. Its simple and direct.
>
> I wish I had the knowledge to work with it at that level.
>
>>
>> I just find it a bit frustrating that the last little bit of fine
> tuning is
>> fiddly and would like to figure out a way to make it less so.
>
> Regarding this and the first comment, I think more
> measurement/correction points in that area would help. With
> StudioPrint, I will see differences in linearizations between 2
> different papers between 95% and 100% untill I do an 80 patch
> linearization. Iterative linearizations would not help this if there
> are no sample points in that area.
> Obviously this is a rare problem, and 21 steps is working well for
> most situations. I did run into very similar results once though with
> QTR on a particular ink setup, and I'm going to start from scratch on
> that one.
> Otherwise, I'm not sure what the solution would be. Start with heavy
> limiting so things are behaving, then creep up the K boost?
>
> Tyler
>
>
>
>
>
>
> Yahoo! Groups Links
>
>
>
>
>
>
-
Roy Harrington
roy@...
Black & White Photo Gallery
http://www.harrington.com