Hang around a print shop long enough, and you’ll hear the term overprint. In the world of prepress, overprinting is a way to control how color separated plates interact with each other. A printing press imprints each color on a piece of paper, one after the other, as it runs through the press. Because of this process, you need to consider certain issues when making color separations.
For example, say you design some blue text over a yellow background. When those colors are separated and printed on press, the blue and yellow mix, resulting in green text on a yellow background. Therefore, under normal conditions, when pages are separated, color that appears underneath other objects is removed so that the color on top is unaffected. In this example, the blue text removes, or knocks out, the yellow background underneath it, allowing the blue to appear correctly when printed.
Overprinting, however, is a method of overriding a knockout and forcing overlapping colors to mix on press. In our example, setting the blue text to overprint means that the yellow background still appears behind it, and the result on press is green text on a yellow background (Figure 1).
Figure 1. The text on the left, by default, knocks out the background behind it. The text on the right is set to overprint, and the background behind it is unaffected.
You’d want to apply an overprint when you specifically want to mix colors on press. Some designers who work with low-budget jobs that print in two or three spot colors can simulate other colors by mixing those spot colors. Before transparency rolled around, designers would also specify overprints to simulate objects being transparent; you could also simulate shadows or shading by overprinting with black over other elements.
Overprinting is also essential when you’re creating plates for custom dyes and varnishes. For example, if you want to create a spot varnish for a particular photo, you need to create a spot color called Varnish and set it to overprint, because this allows the photo that appears beneath it to print (otherwise, the varnish knocks out the photo).
You can easily specify overprinting from the Attributes panel (Window > Attributes). With an object selected, you can force the fill, the stroke, or both to overprint. Remember that Illustrator also allows you to specify whether a stroke is painted in the centerline, inside, or outside a path, and you should be aware that if you overprint a stroke that’s on the inside or the centerline of a path, the stroke also overprints the fill of that object.
Handling the Limitations of Overprints
Let’s get technical for a moment. You’ll encounter some limitations when it comes to using overprints. First, whereas one color plate can overprint another, an overprint cannot overprint its own plate. For example, if you have a color that contains cyan and you set it to overprint over a background that contains cyan, you won’t get an overprint on the cyan plate.
Second, sometimes users specify overprinting for objects colored white. Usually, white is always a knockout (because it lets the white paper show through), and setting a white object to overprint would kind of defeat the purpose. However, these things do happen accidentally. You might have a logo that you created that’s colored black and that you’ve set to overprint. Then you might come upon a situation where you need a reverse (white) version of the logo, so you might just open the file, color it white, and save it with a different name, forgetting that you set the fill to overprint. This would most likely result in the file not printing properly, because either the white overprints (making it entirely transparent) or the RIP doesn’t process the file correctly.
Because overprints are really PostScript commands that you use when you’re printing color separations, you’ll always have a problem with displaying overprints onscreen or when you’re printing composite proofs to show a client. In the past, the only real way to proof overprints was by printing separations and creating a matchprint proof or by investing in expensive prepress plug-ins. More often than not, a designer would show a proof to a client and say, “It won’t look like this when it’s actually printed.” If only there were a better way…
Illustrator offers that better way. By choosing View > Overprint Preview, you can actually see on your monitor what the effects of overprint commands are. Additionally, in the Output panel of the Print dialog box, the Simulate Overprint option, when activated, prints composites as they will look with overprints applied. This is perfect for showing clients exactly what they are going to get. The Simulate Overprint option is also available in the Advanced panel of the PDF dialog box, so you can even show your client an accurate proof via PDF. You disable Simulate Overprint when you choose to print separations—it’s available only when you’re printing composites.
Handling Transparency Effects That Disappear or Print as White Boxes
You create some artwork that contains two spot colors (let’s say Pantone Blue 072 and Red 032). The logo has a drop shadow behind it, and you’ve correctly set the Illustrator Drop Shadow effect to use the Blue 072 spot color, not black. On the Illustrator artboard, the logo appears correctly against the spot color background (Figure 2).
Figure 2. In Illustrator, the Drop Shadow effect appears correctly against the spot color background.
Then you save the art as a PDF/X-1a file because it will be used in an ad and you want to make sure it will print correctly. Or you save your document using Acrobat 4 (PDF 1.3) compatibility. Alternatively, you save your file as an EPS file because maybe you’re required to place this logo into a QuarkXPress document. The point here to focus on is that you’re saving your file to a flattened format.
The “problem” is that when you open the PDF in Acrobat or Reader, or when you place the file into QuarkXPress or InDesign and print the file to your laser or ink-jet printer, it comes out looking incorrect—either the drop shadow disappears completely (Figure 3) or a white box appears where the transparent effect should blend into the background (Figure 4).
Figure 3. When saving the file from Illustrator and viewing or printing the art outside of Illustrator, a white box appears around the transparency effect.
Figure 4. When saving the file from Illustrator and viewing or printing the art outside of Illustrator, the transparency effect seems to disappear.
When you have a transparent effect, the result is a mixture of the inks. In this case, the shadow, which is Pantone Blue 072, blends right into the Red 032 background. By default, when one color sits on top of another color, a knockout occurs. In other words, the area beneath the top shape is removed from the lower object. Otherwise, the top color will print on top of the bottom color when the paper is run through the printing press, causing the two inks to mix. In the case of the red and blue colors, the result would be purple in appearance. However, in this case, where you want the drop shadow to blend into the background on press, you have to override that knockout by specifying an overprint.
The thing is, Illustrator already knows this, so no action is required on your part. When you print your file from Illustrator, all these settings are done automatically, so your file looks great when you print it—either as a composite or as separations. The same applies when you save your file from Illustrator as a native Illustrator file and place it into InDesign or when you create a PDF with Acrobat 5 compatibility (PDF 1.4) or newer.
But when you save your file to a format that doesn’t support transparency, Illustrator has to flatten the transparency. And in that process, Illustrator realizes that in order to preserve the spot colors so that they print in separations correctly, the drop shadow must be set to overprint the background color (in Illustrator CS4 and CS5, the spot color is set to overprint instead).
The problem is that overprint commands are honored only when you print your file as separations. When you are previewing your document onscreen or when you are printing a composite proof of your file, the overprint commands aren’t used, and either the result will be white where overprinting should occur or the transparency effect will simply disappear. The file will print correctly when you print as separations, because, at that time, the overprints are honored (as they should be).
In InDesign, choose View > Overprint Preview. This will allow you to view overprints on your screen. When printing composite proofs, select the Simulate Overprints box in the Output panel of the Print dialog box to get the correct appearance in your printouts.
In Acrobat or Reader, choose Always in the Use Overprint Preview popup menu (in the Page Display panel in Preferences) to view the file correctly on your screen. When printing composite proofs, choose Print, and then click the Advanced button. Then select the Simulate Overprinting box in the Output section of the dialog box. The file will then print with the correct appearance.