The elements of design - Creative techniques

Creative techniques

http://en.wikipedia.org/wiki/Overprinting

Overprinting; Sees one ink print over another so that the two inks mix to create a new colour...

By default, when you print opaque, overlapping colours, the top colour knocks out the area underneath. You can use overprinting to prevent this knockout and make the top most overlapping printing ink appear transparent in relation to the underlying ink. The degree of transparency in printing depends on the ink, paper, and printing method used. Consult your print shop to determine how these variables will affect your final artwork.

You may want to overprint in the following situations:

• Overprint black ink to aid in registration. Because black ink is opaque (and usually the last to be printed), it doesn’t look much different when printed over a colour as opposed to a white background. Overprinting black can prevent gaps from appearing between black and coloured areas of your artwork.
• Overprint when the artwork does not share common ink colours and you want to create a trap or overlaid ink effects. When overprinting process colour mixes or custom colours that do not share common ink colours, the overprint colour is added to the background colour. For example, if you print a fill of 100% magenta over a fill of 100% cyan, the overlapping fills appear violet, not magenta.

After you set overprinting options, you should use the Overprint Preview mode (View >Overprint Preview) to see an approximation of how the overprinting colours will print. You should also carefully check overprinted colours on separated artwork using integral proofs (where each separation is shown in register on a single piece of paper) or overlay proofs (where the separations are shown in register on separate plastic sheets stacked on top of each other).

Set up overprinting

1. Select the object or objects that you want to overprint.
2. In the Attributes panel, select Overprint Fill, Overprint Stroke, or both.

Trapping

Good colour registration isn't always possible; gaps can appear when two inks that are to be printed as solid colours are placed next to each other. This is a problem that can be foreseen and resolved! Through the use of ink-trapping.

Overprint & Knockout,   

KNOCKOUT
pros: neither colour affects the other (if kept in registration)
cons: a swine to keep in perfect registration and may leave unprinted halo edges if not

OVERPRINTED
pros: no halo white edges when out of registration
cons: can change the colour of the overprinted colour, makes a thicker (2 layers of print) ‘dinnerplate’ finish

WITH TRAPPING
pros: no halo white edges when out of registration a little
cons: guessing how much trapping is desired, overlapping printing areas may change colour giving the appearance of an outline