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by Jerry
Laiserin, FAIA
Getting CAD documents, specifications
text, or rendered images out of the digital realm and onto
the physical media of bond, vellum, or Mylar is no trivial
task. Successful printing and plotting require a coordinated
assortment of software for preprocessing the digital material
to be printed, hardware for the physical output, and optional
add-on software and third-party services.
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Photo: ©
National Reprographics
Cumbersome old blueprint machines
have given way to sleeker output devices like the HP Designjet
1050c Plus. |
Photo: © Hewlett-Packard |
How it works: Dots and lines
Clicking the printer icon on a computer
screen launches a complex sequence of internal software preparations.
The process starts with a program called a printer (or plotter)
driver, which harnesses the electronic bits whizzing around
inside the computer and aligns them in a printer-specific
way— different from the way that bits are displayed on
screen. The vectors, or x-y lines, that make up a CAD file
or a Postscript (PS) file for desktop publishing must be converted
into a raster, the arrangement of horizontal rows of dots
that most plotters and printers use to put pigment on paper.
Along with this raster image processing (RIP), other software
for anti-aliasing smoothes what would otherwise be stair-step
“jaggies” where curved or angled lines and text
characters span the printer’s rows of raster dots. If
a color file is being printed, the computer’s internal
red-green-blue (RGB) color representation gets translated
into a grayscale for black-and-white printers, or into the
cyan-magenta-yellow-black (CMYK) color representation that
most color plotters and printers use.
CAD output imposes additional demands. All the popular architectural
CAD programs store design information in an internal database
or model; each has its own means of extracting, exporting,
or reporting scaled orthogonal views (plans, elevations, and
sections) of the model onto paper-friendly arrangements called
viewports, layouts, or paper space. Most CAD program output
settings include pen tables, lists that correlate on-screen
line weights to the thickness of printed lines (equivalent
to the width of technical drafting pens).
Printing and plotting consumes lots of memory and processing
power, which is handled by the hardware in a computer, a printer/plotter,
or a network device called a print/plot server. Many printer/plotters
have computer processors and memory built in; some even have
their own internal hard drives to handle multiple print jobs.
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