Re: [OT] Dot shape prevalence
Re: [OT] Dot shape prevalence
- Subject: Re: [OT] Dot shape prevalence
- From: "RS" <email@hidden>
- Date: Mon, 10 Oct 2005 08:22:20 -0700 (PDT)
- Importance: Normal
A few comments:
1. Abe, I'd like to know more about how you came up with your current
screening parameters (dot shape, line screen, screen angles). Did you
conduct a series of press runs and find that having magenta and yellow
separated by 45 degrees produced the best image, or was that an assumption
from the beginning?
2. Like Richard and Martin, it has been my understanding that putting
magenta at the 45 degree angle was to keep this fleshtone-important ink at
the angle that the eye perceives as the smoothest, not to keep it
separated from the yellow angle by 45 degrees. Indeed, Heidelberg's
"Introduction to Screening" document you referenced makes no mention of
the importance of keeping the magenta and yellow angles separated by 45
degrees. Although this document describes several screen systems that have
yellow and magenta separated by 45 degrees, there are several other
systems that don't--M & Y are separated by 15 degrees in IS Y fine; 18.4
degrees in RT Classic; and 26.6 degrees in RT Y 45 fine (the guides notes
this system as "well suited for reproducing skin tones").
NOTE: This screening guide can be found at
http://www.heidelberg.com/wwwbinaries/bin/files/dotcom/en/products/prinect/screening_technology_eng.pdf
3. The screen system that you use appears to be IS Y30. The Heidelberg
guide notes that this is more suited for flexography or screen printing,
however. While I imagine it can be used for sheetfed printing (which I'm
assuming you do), I'm curious to know how how printed pieces compare when
you use a different screen system.
4. Page 7 of the Heidelberg screening document to which you referred has
an illustration that shows where moire occurs between two screens. As the
screen angles become closer together, the frequency with which moire
occurs increases. The IS Y30 screen system you use puts yellow at an angle
that is separated from black and cyan by only 15 degrees, thereby
increasing the frequency of moire between yellow and cyan, and yellow and
black. At first glance, this seems like a heavy sacrifice just to keep
yellow and magenta separated by 45 degrees, but, again, I'd like to hear
your first-hand experiences from your tests.
5. I'm surprised, as is Richard, that you're concerned with seeing a
"saw-tooth" effect when black is at angles other than 45 degrees. Even if
you had substantial black coverage, at a 240 line screen I'd imagine the
dots would be so small that any such artifacts would be quite difficult to
see.
email@hidden
-------
Abe Hayhurst wrote:
I actually use Heidelberg's Irrational Screening with a round dot at 240
linescreen.
The reason behind placing magenta at 45 degrees is so that you have 45
degrees between the angle of the yellow and the angle of the magenta
(with yellow normally being at zero degrees). The problem with black at
a non-45 degree angle is that you can see saw-tooth. I want the best of
both worlds, so I use a screen/angle set that puts cyan at 15 degrees,
magenta at 75 degrees, yellow at 30 degrees, and black at 45 degrees. So
I get 45 degrees between magenta and yellow and keep black at 45
degrees.
-------
Richard Corbett wrote:
You must be talking about image setters because on drum scanner output many
trade scanning houses produced their magenta at 45 degrees because this is
the least angle noticeable to the eye. There should not be any saw tooth on
a black at either 75 or 15 degrees and as the black is the lightest lay down
of colour - under normal circumstances - many printers in the UK used to ask
for those angles.
The angle of the yellow is almost immaterial as the eye cannot see the
colour as dots anyway and, at least in my experience, it never caused any
screen clash with any other colour.
-------
Martin Orpen wrote:
I thought that it was because 45° was the least distracting to the eye and
therefore much better to use for magenta than black in skin tone.
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