Understanding Overprints

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. Specifically, in a four-color process job, all of the colors that you use on a page are broken down into percentages of four colors: cyan, magenta, yellow, and black. 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, certain considerations need to be made when color separations are made.

Technical Knock Out

Take an example where you design some cyan text over a yellow background. When those colors are separated and printed on press, the cyan and yellow would mix, resulting in green text over 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 (Figure 1). In this example, the yellow color that appears behind the cyan text is removed or “knocked out,” allowing the cyan to appear correctly when printed.

Overprinting, on the other hand, is a method of overriding a knockout and forcing overlapping colors to mix on press.

Round One

Why overprint? 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. InDesign does allow you to create multi-colored inks (called Mixed Ink Swatch on the Swatch palette), and internally, InDesign simply overprints the colors that make up the mixed inks. Before transparency rolled around, designers would also specify overprints to simulate objects being transparent, and you could also simulate shadows or shading by overprinting with black over other elements. Overprinting is also essential when creating plates for custom dies and varnishes. For example, if you wanted to create a spot varnish for a particular photo, you’d create a color called “varnish” and set it to overprint, as this will allow the photo that appears beneath it to print (otherwise, the varnish would knock out the photo). You can easily specify overprinting directly within InDesign from the Attributes palette (Window > Attributes) (Figure 2). With an object selected, you can choose to force to overprint the fill, the stroke, or the gap (the spaces that appear between dashes in a dashed stroke). Remember that InDesign also allows you to specify whether a stroke is painted in the centerline, inside, or outside of 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 will also be overprinting the fill of that object.

The Blow by Blow

Let’s get technical for a moment. There are some limitations when it comes to using overprints. First of all, while 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. Here’s another issue: Sometimes users specify overprinting for objects colored Paper. Usually, Paper is always a knockout (as it lets the white paper show through), and setting a Paper object to overprint would kind of defeat the purpose. However, these things do happen accidentally. You might have a logo that you created in Illustrator 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 had set the fill to overprint. This would most likely result in the file not printing properly, as either the “paper color” will overprint (making it entirely transparent) or the RIP won’t process the file correctly.

Up Against the Ropes

Because overprints are really PostScript commands that are used when printing color separations, there is always a problem with displaying overprints on screen or when printing composite proofs to show a client. The only real way to proof overprints in the past 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 was a better way… InDesign offers that better way. By choosing View > Overprint Preview, you can actually see what the effects of overprint commands are, on your monitor (Figure 3).

Additionally, in the Output panel of the Print dialog there’s a Simulate Overprint option, which when activated, will print composites as they will look with overprints applied (Figure 4). 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 Export dialog, so you can even show your client an accurate proof via PDF. Simulate Overprint is disabled when choosing to print separations—it’s only available when printing composites. By default, InDesign globally sets black to overprint. That makes sense when you have a lot of text over colored backgrounds. However, if you need black to knock out instead you can turn off the Overprint Black Swatch at 100% in the General Preferences dialog. Or try this old trick to make an individual black object knock out: Set it to 99-percent black. On any real world device this ends up as solid black, but it knocks out whatever is beneath it. Remember that you can always use InDesign’s onscreen Separations Preview (Window > Output Preview > Separations) to view each individual plate and check for correct overprinting (Figure 5).

Float like a Butterfly, Sting like a Bee

While overprints are useful (and essential in some workflows), my advice is to talk to your printer before you use them, as some printers prefer to specify overprints themselves. In either case, be at ease knowing that your butterfly-InDesign-has all the tools necessary for you to be the king of the ring in professional design.