When you create artwork in Illustrator (most commonly), or text layout documents in Quark XPress or InDesign then you will select your colours from a Palette of swatches. Although this issue can be a problem in Photoshop, this warrants a more detailed explanation as the applications are slightly different, and you select your colours generally from a mixture of RGB or CMYK.
These swatches have Attributes – one of these is Overprint.
To understand why this can be an issue in print, you have to consider that in full-colour printing, the 4 plates (C, M, Y & K) are laid down onto the paper seperately.
Examine the very useful picture from the Adobe article on overprinting:
In the left example, all the colours are set to the default (Knockout). The red negates the colours in the background. In the right example, the red is set to overprint. The colours show through; the ink values are additive.
This can cause issues, for example if you wish to print white text on top of black, but the white is set to Overprint, then the additive nature of the process will mean the entire black solid is printed, and then the white is “overprinted” – effectively making it invisible.
Normally this is not an issue, but designers can make changes to this setting without knowledge of the implications, and in some cases, the program will make this setting for you.
A prime example is 100% K (pure black). This is by default set to knockout in Photoshop, but Registration is set to Overprint in Illustrator, and 100% K is set to overprint in InDesign.
The reason for this is as follows. Because the 4 plates are laid down seperately, there may be a slight variance in the positioning of these plates. When black is set to Knockout, you might find that the white area behind it doesn’t line up perfectly. For very small text this can be especially problematic. Consider the following example, of black text (100% K) printed on top of magenta. In both the Knockout and Overprint example, you’ll see how the Magenta plate and Black plate will be generated by your printer on output.
The closeup of the Knockout plate shows the issue that can be created, and the reason why Overprint is used for black or dark text over coloured backgrounds. If the two plates aren’t perfectly lined up (or even if they are, simply at the printing dot level) you can see a little outline of white.
So we can see that in this instance, overprint is more desirable. Your software will try to simplify these decisions and make the correct ones on your behalf.
However if you then take the overprint text, and change it to white, it will retain its attributes and become transparent when printed, but not on screen.
This is the primary issue with overprinting – if its set when it shouldn’t be, items can “disappear completely” from your print job.
How to Check for Overprint
Most programs have a setting called Overprint Preview. This will show your document exactly how it will appear when litho printed. This is a good way of examining the document for issues.
What it will not pick up is if you have used Spot Colours, and some are set to overprint (especially on white), and some are not. RIP software (that converts print files to colour seperations) will pick up the standard spots and convert them to CMYK but will assume the overprinted spots are to be printed seperately – they will again appear transparent.
Examining the Appearance will show you where this has been done. Examining Attributes will allow you to switch it off.