Why we use descreening
There may come a time when something important is printed, or displayed in a magazine
or newspaper that you want to scan into your computer to save or manipulate. This
may at first seem to be a simple process when you go about setting up your scan.
What could be easier than just putting the document into the scanner, and then scanning
it into a computer? Well many people don’t understand that the images that
appear so clean and crisp in the magazine or newspapers are printed using a method
known as halftoning. Those photos are printed using a series of overlapping dots
that fool your eyes into seeing more colors than are actually there. Because of
these overlapping dots, scanning these documents or images will produce something
known as Moiré pattern (pronounced more-ay).
A Moiré pattern happens when two identical patterns of lines, circles, or
array of dots are overlapped with imperfect alignment creating an interference pattern
(resembles an optical illusion). Scanning these images into the computer will result
in a wavy pattern that garbles the original image leaving you with a useless image.
What makes things worse is scanning the image or document slightly off alignment
can increase the effects of this wavy pattern, not only that, but the pitch of the
scanners sensors will enhances the pattern even more. This will happen with any
scanner, so don’t go blame it on the manufacturer just yet for the pitch of
the scanner sensors.
To counter this attack on our visual spectrum Art-Scan was designed with two counters
to this Moiré pattern. Art-Scan was designed with a descreening function
that when used removes the halftone dot pattern from the printed material during
scanning by defocusing the image. This effectively eliminates the Moiré pattern
and the color shifting patterns. The downside to this process is the image will
become slightly less detailed due to the blurring effect caused by the descreening
process. This is where Art-Scan’s Sharpen Image setting comes in. By setting
the unsharp mask after performing a descreening operation the image can be restored
to its original sharpness.
How to perform a descreening
1) Find the article either from a book or magazine or even a newspaper that
you want to save onto your computer. Clip the article from the document, or if it
is able to fit underneath the scanner bed lid slide the document onto the scanner
bed. For this example we will be using Art-Scan 5.0.
2) Go to your Start Button>click on Programs>click on
Art-Scan Pro 5.0>and then click on Art-Scan 5.0 via Jetsoft Viewing Booth
Pro 2.0(Acquire Edition) to open Art-Scan. If you are unsure if Art-Scan
is using the current scanner you wish to acquire the image from, hold the shift
button while you click to open Art-Scan 5.0. This will display the select source
dialog, then go ahead and select the current scanner you wish to work with.
In order to get an idea of where your document/image is placed on the
scanner bed go ahead and click on the ‘Preview’ button in order to get
the preview image saved in the image area. Keep repeating this step until the area
of the document or image is as squarely lined up parallel with the scanner bed side,
which is represented by the edges of the image preview area.
4) Change the scan mode now from RGB (default), to Gray Scale, and perform
another ‘Preview’. At this point I would also suggest clicking on the
‘Preferences’ option, then setting the “Target Resolution for
Preview Scan:” from its default 50 to 200dpi. That way the image being shown
in the preview area is displayed with better clarity to give you a better understanding
of what the finished image will look like.
7) You can now select the descreening option, but when you do make sure click
and hold on the button to bring up the advanced descreening options. The first thing
you should do is change the ‘Sample Resolution’ from its default 300dpi
to 600dpi, and then change ‘Type of Original’ to the type of document
you’re working with, anything from a newspaper to lithograph. In our example
we chose “Newspaper / Laser (65 106 lpi) 9x9”.
8) Since descreening the image will have the unfortunate effect of causing
a slight blur, it’s best to apply a sharpen effect to the image as well. By
applying the sharpen option the blur from the descreening will be instantly corrected,
and the image will again be as crisp and clean as the original was to our naked
eye. Since you may make any number of changes with the descreening options, you
too can click, and hold on the ‘Sharpen Image’ option to bring up the
advanced sharpening menu. From there you can change anything from the percentage
of intensity to the percentage of sharpening for shadows in the image. From the
menu you can perform a preview to see what the effect of changing the intensity,
or any other option will have on your image so you can judge which setting is best
for what you’re doing.
9) Finally all that is left is to click the scan button, and let Art-Scan
do the rest. The whole process may take a few moments to scan the image at a higher
resolution, and then apply both the Descreen and the Sharpening to the image, but
in the end we’re sure you’ll be pleased.