QMS 860, Руководство пользователя, Страница: 190 / 398

using 300 dpi is 53 lpi. This typically allows 33 different levels of
gray. You may change screen frequency through your application
or by using the PostScript setscreen operator. The QMS Crown
Technical Reference Manual, which is an optional purchase
available from your QMS vendor, contains information on
setscreen.
The extra gray levels available through higher resolution provide
a smoother shift from the darkest black to very light grays while
higher resolution provides sharper image outlines.
Screen Angle is the angle at which a halftone screen prints. The
default screen angle for your QMS 860 Print System is 45°. This is
the normal angle for a black and white printer. You can change the
screen angle through the PostScript setscreen operator. The QMS
Crown Technical Reference Manual, which is an optional purchase
available from your QMS vendor, contains information on
setscreen.
Gamma Correction Concept
Often in scanned images, gray scaling may be too light or dark to
show details clearly. Using gamma correction on a gray-scale
image is similar to using a graphic equalizer on a home stereo; the
range of gray levels is adjusted for the best possible image. This
sharpens overall midrange contrast while blacks (shadows) and
whites (highlights) keep their original values. Gamma correction
sets gray-scale values specified through the setgray PostScript
operator to your printer’s gray-scale values.
The gamma correction value provided through the printer control
panel refers to a constant value applied to the scan. Increasing the
gamma value lightens the image and decreasing the value darkens
the image.
Chapter 6
P r i n t Q u a l i t y 6 - 5