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Graphic design printing processes trapping chokes and spreads
Note: this was pulled from the May, 1993 issue of DT&G as
background on trapping. Its teachings are still true today. (be sure to see "Trapping with Photoshop" by Joyce
Oh, it's in vogue these days isn't it. Trap here. Trap there. All the computer
graphics experts and magazine writers showing off their divine knowledge, writing
about "trapping." Some people don't even know what trapping is. Some don't
even care. But the computer industry sure wants us to know – and they want us to
buy the latest and greatest software to prove it. Do I sound cynical? (He asks with
a sly grin.) Your first line of defense in avoiding trapping traps is to understand
What is "Trapping"
This process has been around nearly as long as offset lithography. More so before
the industry developed precision, high-tech printing presses.
__ When printing color, a separate impression is required
for each color. Many variables can cause the two colors to be slightly misaligned
or out of register. This is usually not an issue because the human eye cannot perceive
slight variations in positioning of objects.
__Trapping is a crucial issue however when the two colors
touch or overlap. (as in our illustration above) A slight misalignment in this case
can be very unsightly because the paper shines through where the colors don't meet,
and a dark ridge appears where the colors overlap too much. (See: Knockouts if you
don't know what we mean by "Knockout"
In the old days it was called "Choke & Spread"
I was taught mechanical trapping, by master cameraman, Jim Layman, at a very young
age. I was an apprentice in the camera department of a large printing company.
__ Chokes and spreads were created using optically-clear
mylar sheets of varying thickness, sandwiched between negative film and a positive
"dupe" film during a contact print exposure.
__ The positive was the "build-back" that
would later print the color to be trapped into the knocked-out background color.
Choking made the image smaller - "choking" the background into the positive
__ Spreading made the positive image larger - spreading
it into the background. The mylar caused the light in camera to refract at a slight
angle, resulting in an enlarged image.
__ Which film went on top determined choke or spread,
and the thickness of the mylar determined how much image change would take place.
This was relatively fool-proof because it was all done on a pin-registered contact
frame. The experts dictated which colors would choke and which would spread. Eventually
I got the hang of it.
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