Re: [OT] Dot shape prevalence
Re: [OT] Dot shape prevalence
- Subject: Re: [OT] Dot shape prevalence
- From: "RS" <email@hidden>
- Date: Thu, 6 Oct 2005 08:16:47 -0700 (PDT)
- Importance: Normal
Roger,
Heidelberg's Delta RIP documentation recommends using their "smooth
elliptical" dot. About the ellipitcal dot, the "Delta Technology Screen
Frequencies" manual notes the following (I'll send you the PDF):
(from pp. 1-10 to 1-11):
The Elliptical dot shape is the screen dot recommended for offset printing.
This dot starts off as an almost round dot in the highlight area and
becomes increasingly elliptical. When the dots first join at 44%, they
become somewhat rhombic. After the second time they join at 61%, rhombic
shapes are first created, then they become elliptical in the shadows,
round holes appear.
In offset printing, there is a density jump where the dots touch each
other which in the case of an elliptical dot is divided up into two
sections. This reduces the effect and makes it easier to control by means
of gradation curves.
This is the ideal dot shape for offset printing. This dot shape is also
recommended for screen printing, letterpress prinitng and O/G conversion
[whatever the hell that is].
(from p. 1-26):
Because the dot joints are distributed, the dot gains are also
distributed. For that reason it is far less critical than it would be for
round-square [euclidian] dot shapes.
About the euclidian dot, the manual notes the following:
(from pp. 1-11 to 1-12):
The Round-Square dot shape is the classic dot shape for offset printing
which stems from the glass engraving screen mentioned at the beginning of
this book. In PostScript, this dot shape is also known as a euclidian dot.
This dot starts off as an almost round dot in the highlight area, becomes
increasingly square in the midtone. Round holes appear in the shadows. The
dots join together at 50% and are slightly staggered to smoothen the
density jump. This also makes it easier to control by means of gradation
curves.
This dot shape is used frequently for motifs as in this example where the
density jump resulting from printing is used to increase the contrast in
midtone. However, it makes better sense to set the contrast by changing
the gradation curve in the image editing system and then to use the
elliptical dot for exposure.
To a certain extent, this dot is still also used in traditional printing
houses where a change in production methods (changing the process
calibration, quality control, etc.) would involve too many complicated
steps. A switchover is not absolutely necessary since smooth vignettes can
also be produced with this dot shape. [Note: the version I have was
published in 1999.]
(from p. 1-24):
Four-sided dot joints are the only disadvantage of this dot shape, because
they can result in a clearly visible dot gain when printing, which can be
critical in the case of gradations.
I'd be happy to email the PDF of this manual to anyone else who is
interested. It includes descriptions, illustrations, and recommended uses
of many other dot shapes as well. [Note: what Heidelberg calls a diamond
screen is stochastic.]
This manual also goes on to describe the many screen systems available to
users of the Delta RIP. These screen systems are separate from dot shape
and control only screen angles relative screen frequencies (i.e., some
colors have slightly different line screens; this is discussed below).
Some of these screen systems, including "IS Classic" (described as the
"conventional offset screen system" on p. 2-4), swaps the black and
magenta screen angles. The goal, ostensibly, is to put ink most critical
to fleshtones (magenta) on the angle most pleasing to the eye (45
degrees). This is best suited for jobs when three conditions are met: 1)
fleshtones or other midtone magenta-rich elements are present; 2) these
elements are the most critical parts of the piece; and 3) there is
relatively little (skeleton) black. As described and illustrated in "Print
Pubilshing: A Haden Shop Manual," halftones made with a 45 degree screen
angle yield the smoothest, least jarring image. It may, therefore, make
sense to put the darkest ink (black) on the smoothest looking angle, but
since black is often used as a skeleton for detail and shadow the 45
degree angle is more often reserved for an ink with more coverage. As
higher line screens are used the benefit of swapping screen angles will be
of less importance, however, as the individual dots become more difficult
to discern.
Many of these screen systems also change the screen frequencies of one or
more inks to reduce moire. IS Classic, for example, increases the
requested screen frequency of yellow by a factor of 1.06, and in "IS Y
fine" yellow has a relative screen frequency of 1.41. This significantly
affects dot gain, for which separate calibartion curves need to be applied
when making plates.
email@hidden
-----------------
I'm doing an informal survey about which halftone dot shape has gained
widespread acceptance in printing these days? Is it Eucledian or Elliptical?
I suspect Eucledian is probably more widespread because it is the default
dot shape in most PostScript RIP. But I can't say for sure.
And what is the current practice when printing portraits and skintone: I
gather that placing the magenta in the angle of the black gives "better"
results. Is there any truth to this? Or is that all taken care by the more
advanced screening techniques such as Irrational Tangent?
I was searching through the SWOP 10th Edition booklet in the hope that it
would have a recommendation on screening but, alas, nothing. Unless I missed
it :(
Regards,
Roger Breton | Laval, Canada | email@hidden
http://pages.infinit.net/graxx
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