Page 142 - Graphic Design and Print Production Fundamentals

5.2 Raster Image Processing

Wayne Collins

The raster image processor (RIP) is the core technology that does the computational work to convert
the broad range of data we use to create a computer graphic into the one-bit data that drives a physical
imaging device. Let’s examine the creation of a single character of the alphabet, or glyph. A font file
delivers PostScript language to the RIP that describes a series of points and vector curves between
those points to outline the letter A. The RIP has a matrix grid at the resolution of the output device and
computes which spots on the grid get turned on and which are turned off to create the shape of that letter
A on the output device. The spots on the grid can only be turned on or off — which is how binary data
is encoded — either as 0 or 1. The grid then acts as a switch to turn a mechanical part of the imaging
engine on or off.

With computer-to-plate technology for lithographic printing plate production, a laser is used to expose
an emulsion on a printing plate. Most plate-setters have a resolution of 2,000 to 3,000 lspi (laser spots
per inch). The RIP calculates all the spots that must be turned ‘on’ to create the graphic that will be
imaged on the printing plate. If the image fills a typical sheet-fed press, it is (30 inches x 3,000 lspi) x (40
inches x 3,000 lspi) = 1.08 trillion, which takes 10 gigabytes of computer memory to store and transfer.
A printing plate for flexographic print production is created by turning a laser on and off at a slightly
lower resolution. An inkjet printer uses the same RIP process to deliver the same one-bit data to each
inkjet nozzle for each colour of ink in the printer. Most inkjet engines have a resolution between 600
and 1,200 spots per inch — so the matrix grid is smaller — but if it is an eight-colour printer, the data
for all eight nozzles must be synchronized and delivered simultaneously. An electophotographic (Xerox)
printer usually has a resolution similar to an inkjet printer and utilizes a similar RIP process to change a
grid of electrostatic charges to positive or negative on an electrostatic drum that is the maximum media
size the machine can image. Each colour in the printer has a separate raster image that charges the drum
in the right spot to attract that colour of toner to that exact location. The data for each colour must be
synchronized for simultaneous delivery. The data must refresh the charge on the drum after each print in
order to pick up new toner. That is a very important fact to remember when we talk about personalizing
print with variable data later in this chapter.

This basic understanding of RIP’s place in a computer graphic workflow is essential to understanding
how to prepare files for, and manage, RIP resources. It is also essential in solving some of the common
problems we see in various RIPs. When we compare the two mass production imaging technologies,
lithography and flexography, to the personalized imaging technologies, electrophotography and inkjet,
we can identify some core similarities. In lithography and flexography, a high-powered laser is used to
alter a physical emulsion that is durable and finely grained enough to let the laser image a spot that is
one three-thousandth of an inch without affecting the spot of equal size beside it. We can reliably image
that spot in a serif of a glyph set in one point type or a hair on a face in a photo that is imaged with a
5 micron frequency modulated (FM) screening pattern. The mass production technology assures us that
the first print will be identical to the millionth print.

The raster grid of one-bit data that the RIP produces must be delivered to the imaging drum or the inkjet

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