I was reading about it in my illustrator book, and I heard printers refering to it, but for some reason I just cant grasp it.

Ahh trapping..... also known as Jedi mind trick.

To understand trapping you have to understand the printing process. Here's a quick primer

Most colour images are printed using a CMYK process.
It uses Cyan, Magenta, Yellow and Black (K)
Each ink is printed one at a time, one on top of the other (even on a multi colour press)
When the paper goes through the press it moves slightly from one colour to the next.
This causes mis-registration - colours not lining up
Mis-registration is what trapping tries to fix
To see mis-registration look at any colour ad in a newspaper or phonebook. You will see the colours don't fit exact.

To understand the next part you have to be part Jedi.
Imagine each of the four colours (cmyk) as a piece of paper.
Say you have a file with a black box and yellow background.
Your yellow paper would have a hole where the black box is and your black paper would just have a black box. Now if you place these two exactly on top of each other then no problem, but if you move the black paper slightly (like the paper going through the press) you end up with a white gap (paper showing through) along 1 or 2 side of the box depending on how the paper moves. To "fix" this you have to do one of the following.

The way trapping works:

There are two types, Overprint and Choke/Spread

Overprint is usually only used with black or very dark colours because it alters the way your file works. Using the example above, if you overprinted the black box your yellow page would be solid yellow and the black page the same. This mixes the black and yellow together producing black. If you were to do this with two other colours though, say yellow and blue then the box would end up green instead of blue.


Choke/Spread

Using the above example, a coke/spread would add a stroke to the black box. A choke would add a yellow overprinted stroke to the box while a spread would add a black overprinted stroke to the box. Using this method only the sections where the two objects overlap are overprinted so you don't get the green box as in the overprint sample. You would only get a slight green line around the box.


Warning Sometime it's best to use no trapping. If you had a yellow background and a blue box it would be best to use no trapping. Why you ask? Because the only purpose of trapping is to mask imperfections in the print process. If you stroked or chocked the blue box then you would get a fairly noticeable green line around the box. If you had no trapping you would have a small line of white. White is so close visually to (cmyk) yellow that the eye would never pick it up, but it would see the green for sure.


Misc bits:

Trapping has to be done on any print job where two or more colours meet. Good quality offset printers need about a .25 stroke, different processes need different amounts.

Some places do in-rip trapping which basically means it is done by machine when your file is being made into a plate or film.

Some programs have automatic trapping, like QuarkXpress
Quark will not trap linked files though, like Illustrator eps files.

Illustrator has a sort of automatic trapping but I always do it manually.

Remember to always backup your file before trapping. Overprint the wrong things and you whole file can be trashed.

Trapping is not needed on files to be colour laser printed as they print all 4 colours at the same time so the is very little mis-registration.


Good books:

Adobe print publishing guide - used to come with Pagemaker
Pocket guide to digital pre-press - old but good, about $20 Cnd
Pocket pal - has almost everything but trapping $30-40 Cnd or $12US


Guess this is why I'm a "Qualified Professional" :)

freakyclean

That must have taken you sone time to write (Ihave a feeling my mods are going to be sending me invoices soon).

The info actually helped me out too. :)

Thanks freaky. :)

Cheers.

Excellent! I love this place! Ask and ye shall receive. Thanks alot Freakyclean.