Printer Imaging Characteristics - Xerox DocuPrint Network Printer Series Guide Manual

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Printer imaging characteristics

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To preserve image quality when printing on 300 dpi printers,
regenerate the bitmaps at 300 dpi. The 600 dpi printers can accept
jobs with resources in either 300 or 600 dpi. See "Maintaining image
quality" in the "PCL5e" chapter for information about the
recommended attribute settings to use when generating a document
on your system.
Some manufacturers have added hardware imaging enhancements
to 300 by 300 dpi printers that effectively boost the apparent dpi by
micro-controlling the laser beam. The function is called post-
enhancement because it is performed by the hardware after the
PostScript interpreter has imaged into the frame buffer. The frame
buffer is still only 300 by 300 dpi, but the hardware is controlling the
laser beam to produce smoother edges.
Other manufacturers have added pre-enhancements that involve the
PostScript interpreter. In these printers, the frame buffer is actually
larger because the PostScript interpreter is effectively rendering at a
higher resolution. The hardware performs some of the halftoning
function to provide finer halftoning, resulting in picture output that
looks smoother.
You can only accurately compare two different printers if both are
using the same enhancement techniques or if all enhancement
techniques are turned off.
Most xerographic printers do not image perfect circular dots with
even dry ink distribution. The dry ink "spread" is typically Gaussian,
with dry ink density being the greatest at the center of the dot and
fading off toward the edges.
The terms "write-white" and "write-black" are broad generalizations.
The specific print imaging characteristics of some products may not
represent other products with the same write-white or write-black
classification. For example, the HewlettPackard (HP) LaserJet and
the LaserWriter II NTX use a write-black laser printer engine that
uses the laser beam to "write" where dry ink should go. Therefore,
single pixel dry ink dots are approximately round and are visible on
an HP LaserJet. In contrast, write-white laser printer engines "write"
the dry ink free areas, which can create single dots that have four
concave sides.
It is important to distinguish the fast scan direction, which is the
direction the laser beam moves across the page before jumping back
to do the next line. Remember that a laser printer scans a page in a
raster method, just like a television set. Fast scan directions are not
the same for a LaserWriter and the Xerox engine.
In general, when turning on the same pixels, the write-white engine
produces output that appears lighter than output produced by a write-
black engine. Xerox device-parameterized PostScript interpreters
are able to partially compensate for this by rendering thicker strokes
and fonts. Device-dependent PostScript cannot be compensated for
because it is sending user-created bit images instead of
mathematical descriptions of the desired output.
•
User-defined PostScript fonts—Some user-defined fonts use
bitmaps that are tuned to write-black engines. These fonts
appear light on certain write-white printers.
XEROX DOCUPRINT NPS GUIDE TO USING PAGE DESCRIPTION LANGUAGES