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Graphic design printing processes trapping chokes and spreads

Adobe Illustrator 9 - Tips n Tricks for Graphic Designers and Desktop Publishers

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 Evans.)
Dangerous Subject

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 the concepts.

Graphic design printing processes trapping chokes and spreadsWhat is "Trapping" all about?"
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" and "Build-Back")

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.

Graphic design printing processes trapping chokes and spreads

Choking made the image smaller - "choking" the background into the positive image.

__ 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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