Graphics
 

Adobe Illustrator CS5 : Understanding Overprints

11/28/2011 5:44:49 PM
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.

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. 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.

Trapped in a Corner

Those who work in packaging use overprints all the time to create traps—colors that share borders with other colors that overlap slightly. This is because commonly used package materials and the printing processes used (called flexographic printing, or flexo) don’t always result in perfect printing. Remember that the requirements for printing a couple hundred brochures and printing several million containers of milk can be quite different. The next time you see a bag of potato chips or a bottle of soda, take a close look at the label; you’ll be able to see the overprint traps. These are usually created in Illustrator by setting just the stroke to overprint.


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.

Previewing Overprints

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.

Although overprints are useful (and essential in some workflows), our advice is to talk to your printer before you use them, because some printers prefer to specify overprints themselves.

Handling Transparency Effects That Disappear or Print as White Boxes

Has the following scenario ever happened to you?

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.


The key items to focus on here are that you have used a transparent effect and you’ve used a spot color. Now, you’ll know what’s happening and what the solution is.

Note

Some RIPs have built-in settings to ignore overprints in files and instead use their own settings for overprints. This often results in output that isn’t desirable. You can easily fix these issues by instructing the RIP to honor the overprints in your files. For example, Rampage RIPs have a setting called Preserve Application Overprint that, when activated, results in perfect output.


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).

This issue is easy to solve in InDesign, Acrobat, or Reader:

Tip

If you’re using QuarkXPress (at least up through version 8.0) you really don’t have an option, because that program doesn’t allow you to simulate overprint commands when printing composite proofs. One workaround is to create two versions of your file: one that uses spot colors that will separate correctly when you print separations and another version where you’ve converted your spot colors to process colors. When you convert to process colors, you don’t need the overprints, and the file will print with the correct appearance on a composite proof.


  • 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.

 
Others
 
- Dreamweaver CS5 : Using Dreamweaver Templates - Enabling Repeating Regions
- Dreamweaver CS5 : Using Dreamweaver Templates - Making Attributes Editable
- Adobe Photoshop CS5 : Fixing Group Shots the Easy Way
- Adobe Photoshop CS5 : Fixing Reflections in Glasses
- Adobe After Effects CS5 : Color Matching (part 3)
- Adobe After Effects CS5 : Color Matching (part 2) - The Fundamental Technique - Dramatic Lighting & No Clear Reference
- Adobe After Effects CS5 : Color Matching (part 1) - The Fundamental Technique - Ordinary Lighting
- Adobe After Effects CS5 : Color Look Development
- Adobe Fireworks CS5 : Applying the Unsharp Mask Live Filter
- Adobe Fireworks CS5 : Adjusting brightness with the Dodge and Burn tools
- CorelDraw 10 : Adding Graphics to a Page (part 3) - Optimizing images for the Web
- CorelDraw 10 : Adding Graphics to a Page (part 2) - Exporting the rollover graphic
- CorelDraw 10 : Adding Graphics to a Page (part 1) - Creating Rollover Graphics
- CorelDraw 10 : Adding Text to a Web Page
- Adobe Flash Professional CS5 : Transforming Gradients and Bitmap Fills (part 2) - Adjusting scale with the Gradient Transform tool, Setting gradient fill overflow styles
- Adobe Flash Professional CS5 : Transforming Gradients and Bitmap Fills (part 1) - Adjusting the center point with the Gradient Transform tool, Rotating a fill with the Gradient Transform tool
- Adobe InDesign CS5 : Joining Anchor Points & Using the Smooth Tool
- Adobe InDesign CS5 : Splitting Paths
- Adobe Flash Catalyst CS5 : Using design-time data (part 2) - Add and remove elements in the repeated item
- Adobe Flash Catalyst CS5 : Using design-time data (part 1) - Replace design-time images & Edit design-time text
 
 
Most View
 
- Microsoft OneNote 2010 : Working with Links (part 2) - Creating a Link from a Picture,Modifying a Link in Your Notes, Removing a Link from Your Notes
- Sharepoint 2013 : Welcome to the Central Administration Web Site (part 1) - Application Management
- Windows Server 2012 Technology Primer : Windows Server 2012 Defined - Windows Server 2012 Under the Hood
- Windows 7 : Programming Drivers for the Kernel Mode Driver Framework (part 8) - File Create and Close Requests
- Windows Server 2012 : A complete virtualization platform (part 3) - Using PowerShell to configure the extensible switch
- Windows Server 2012 Technology Primer : Versions of Windows Server 2012
- Windows Small Business Server 2011 : Deploying Applications with Group Policy (part 2) - Creating a GPO for Software Deployment
- Microsoft Lync Server 2013 : Mediation Server Configuration
- Windows Server 2012 : Understanding the Modular Approach to Installing IIS 8
- Windows Server 2012 : Increase scalability and performance (part 2) - Network adapter hardware acceleration
 
 
Top 10
 
- Sharepoint 2013 : Developing Integrated Apps for Office and Sharepoint Solutions - The New App Model for Office
- Overview of Oauth in Sharepoint 2013 : Application Authorization - On-Premises App Authentication with S2S
- Overview of Oauth in Sharepoint 2013 : Application Authorization - Requesting Permissions Dynamically
- Microsoft Excel 2010 : Working with Graphics - Inserting a Diagram,Inserting an Object
- Microsoft Excel 2010 : Working with Graphics - Inserting WordArt, Using Smart Art in Excel
- Microsoft Excel 2010 : Working with Graphics - Using AutoShapes
- Overview of Oauth in Sharepoint 2013 : Application Authentication (part 2) - Managing Tokens in Your Application
- Overview of Oauth in Sharepoint 2013 : Application Authentication (part 1) - Using TokenHelper
- Overview of Oauth in Sharepoint 2013 : Creating and Managing Application Identities
- Overview of Oauth in Sharepoint 2013 : Introduction to OAuth