Re: Decent results with Gutenprint - the poor man's RIP.
Re: Decent results with Gutenprint - the poor man's RIP.
- Subject: Re: Decent results with Gutenprint - the poor man's RIP.
- From: Roy Harrington <email@hidden>
- Date: Wed, 21 Jun 2006 10:28:42 -0700
On Wednesday, June 21, 2006, at 06:18 AM, Roberto Michelena wrote:
...
I've always wondered why no one uses a "lots of points" linearization.
Linearization is so important that doing it with 21 points (5% steps),
then have one point with measuring error, or just happening to hit a
small local artifact such as the dithering treshold you mention, and
bum! you get bad results.
With nowadays fast spectros, I'd rather have a 256-point linearization
target, then average values to smooth out the curve.
Or at least, 60 values, just the same as the 21-point curve but adding
a -1% and +1% to each point. So something like 0,
(4,5,6),(9,10,11),(14,15,16)...and so on up to (94,95,96)(99,100).
Then consider the x% point to be a weighted average of its reading
plus the two neighbours.
That should get rid of measurement errors and also give local
disturbances a more proper weight.
I think too many points for linearization has several pitfalls. As
you've
noted you need to smooth out the little bumps due to noise. But how
much
smoothing should you do? If you have values for 14,15,16,17 ... they
are
all different -- how much of the variation is noise and how much is real
difference? Sort of a circular argument: if you smooth the values
you are
depending on the underlying curve being smooth already and if the
underlying is smooth then you don't need so many points.
My current favorite is to use 21 points but print 4 patches and make 4
measurements of each. Averaging those 4 values reduces the noise.
(This is analogous to the multi-pass scanner method of reducing noise).
Of course this will lead you down deeper into the driver to make sure
the
smoothness is maintained. One ink and one drop size is pretty simple
but
making smooth transitions between multiple drop sizes and multiple
shades
of ink is critical. I have spent a lot of time figuring out how to do
these
transitions -- one setup has 3 drop sizes and 7 different shades of
gray.
There's no way for a linearization step to repair bumpy curve.
Roy
....
-- Roberto Michelena
Infinitek
Lima, Peru
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Roy Harrington
email@hidden
Black & White Photo Gallery
http://www.harrington.com
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