Page 144 - Graphic Design and Print Production Fundamentals

132 Chapter 5. Pre-press
of values a byte can record is 256 — the number of combinations of on and off that the eight bits in the
byte can express. A computer records a byte of data for each primary colour (red, green, and blue —
RGB) for each detail in a photo, as a pixel (picture element), which controls the phosphors on electronic
imaging devices. A RIP must convert the eight-bit RGB values into the four primary printing ink colours
(cyan magenta, yellow, and black — CMYK). There are two distinct steps here: (1) conversion from
RGB to CMYK continuous tone data (24 bit RGB to 32 bit CMYK); and (2) continuous tone to one-
bit screening algorithms. We have to be in the output colour space before we can apply the one-bit
conversion. It converts the eight-bit tonal values into one-bit data by dividing the area into cells that can
render different sizes and shapes of dots by turning spots on and off in the cell. A cell with a grid that
is 10 laser spots wide by 10 laser spots deep can render different 100 dot sizes (10 x 10), from 1% to
99%, by turning on more and more of the laser spots to print. If we think back to the plate-setter for
lithographic platemaking, we know it is capable of firing the laser 2,000 to 3,000 times per inch. If the
cells making up our printing dots are 10 spots square, we can make dot sizes that have a resolution of
200 to 300 halftone screened dots in one inch. A RIP has screening (dot cell creation) algorithms that
convert the data delivered in RGB pixels at 300 pixels per inch into clusters of laser spots (dots) for each
printing primary colour (CMYK).

This description of how a RIP processes photographic data from a digital camera can help us understand
why it is important to capture and deliver enough resolution to the RIP. It must develop a detailed
representation of the photo in a halftone screened dot that utilizes all of the laser spots available. The
basic rule is: Required PPI = 2 x lines per inch (LPI) at final size. So if you need to print something
at 175 lines per inch, it must have a resolution of 350 pixels per inch at the final scaled size of the
reproduction. Use this rule if you are not given explicit direction by your print service provider. You can
use a default of 400 ppi for FM screening where lpi is not relevant.

WYSIWYG

It is important to know that each time we view a computer graphic on our computer screen, it is imaging
the screen through a RIP process. The RIP can change from one software program to another. This is
why some PDF files look different when you open them in the Preview program supplied with an Apple
operating system than they do when opened in Adobe Acrobat. The graphics are being processed through
two different RIPs. The same thing can happen when the image is processed through two different
printers. The challenge is to consistently predict what the printed image will look like by viewing it on
the computer screen. We use the acronym WYSIWYG (what you see is what you get) to refer to imagery
that will reproduce consistently on any output device. Designers have faced three significant challenges
in trying to achieve WYSISYG since the advent of desktop publishing in the early 1980s.

The first challenge was imaging typography with PostScript fonts. The second was colour managing
computer screens and output devices with ICC profiles. The third and current challenge is in imaging
transparent effects predictably from one output device to another. Font problems are still the most
common cause of error in processing client documents for all imaging technologies. Let’s look at that
problem in depth before addressing the other two challenges in achieving WYSIWYG.