Category 16,  Topic 20
Message 191       Wed Jun 10, 1992
ISD [Nathan]                 at 16:35 EDT

The entire color issue is becoming one of training, patience and
understanding. I am indeed fortunate to have Mario handy so that I do not
appear unduly ignorant. :-) Every now and then I will glue Mario in front of
the computer screen and bring you information like the following, unless you'd
all prefer I did not. Personally, while this information gets a tad technical,
it is definitely a prerequisite for learning to be able to totally control
your color environment using the magnificent tool which is CALAMUS SL. :-)

       The Priciples behind Supercells in Imagesetting.
                    by Mario Georgiou

This subject has probably been one of the most puzzling of  all for most
people entering the field of Desktop design and colour.

The simplest way to explain the supercell concept is to look at how this
technology is being adapted to the field of Color  Separation and Printing.

In traditional colour separation an image or piece of art will be scanned and
colour separated photographically. To produce the necessary plates for colour
process printing. The plates will  have different angles for each colour layer
so that as they are  overlayed they will produce supplementary colours. This
process  is called Additive Colour.  This subject can be further explored by
picking up a good book on four colour process printing.

There is a lot of misinformation about how to apply this  technology to colour
separation. The problem is apparent when you attempt to accurately reproduce
the screen angles that a  traditional COLOUR SEPARATION house can do using
cameras. The Black and Yellow layers provide no real problems as they can be
easily adapted to computer based technologies. The Black film requires an
angle of 45 degrees and the yellow and angle of 0 or 90 degrees.

The yellow supercells can map on a 1:1 basis with a printer seeing as most
graphics are handled as bitmaps in horizontal and vertical at the printer end.
Then this equates to one cell per the square root of the bit plane intensity
range:

An eight bit supercell produces a range of 257 tones which equates to (2 to
the power of 8) + 1 intensity levels. Therefore this  value; 256, taken to its
square root produces the number 16.  A 16 by 16 cell in the yellow plane that
lies at the screen angle  of 0 or 90 degrees. To produce the same 16 by 16
cell in the black plane of the colour sep. would require the mapping of this
cell to a 45 degree angle requiring a supercell of approximately 22 by 22 now
then try to apply this theory to angles other than these last two examples.
The cyan plate is at 15 or 105 degrees and the magenta plate is at 75 degrees.
To accurately map both these  screens to the printer grid would result in
errors that cannot allow the screens to work as they should, causing moires
which are mismatches in overlapping screens.

The Raster Generator module in Calamus SL allows you to specify  the line
angles and densities you require for any element on a  page. The module will
also give you feedback, based upon the  resolution of your output device as to
how much a variation  off the true you will get when you try to specify
specific line densities. Each line density will have its own variation from
the true. The yellow plate will always match what you specify based upon the
the resolution you specify (more on this later). The Black will also present
fewer problems than the remaining two process colors.

The Raster Generator module allows you to push the accuracy of the screen
angles by using the arrows next to your angle settings. The thing to remember
is that the larger the number you select the longer  output time will be, and
the larger the supercell.

DO NOT SPECIFY A VALUE SO LARGE THAT YOU GET TWO OR THREE SUPER CELLS PER
INCH.

Keep your numbers low and use your screen to preview your output.
