The Truth About Transparency


Illustrator contains several features that use transparency, including the ability to specify blend modes and opacity masks with the Transparency palette, and effects like Feather and Drop Shadow. Transparency as a feature in Illustrator (and InDesign as well) requires closer attention when it comes to printing documents. In fact, Adobe Illustrator CS2, Adobe InDesign CS2, and Adobe Acrobat 7.0 all use the same methods to print with transparency, so the concepts that you learn here apply to all of those applications as well.

Although you may have heard that printing with transparency is problematic, the reality is that a lot has changed since transparency was first introduced in Illustrator 9. Once you understand what happens to a file with transparency and you learn about a few simple settings, you won't have to worry about running into printing issues when you're using transparency features.

In truth, transparency has always been aroundin raster formin Photoshop. The only difference now is that you can apply these effects in vector form and still edit them late in your workflow. At the end of the day, these transparency effects will become rasterized, leaving you with the same result as if you had done everything in Photoshop. In any case, let's take a closer look at what transparency is and how it works.

Understanding Transparency Flattening

Let's start with a simple fact: PostScript doesn't understand transparency. As you probably know, PostScript is the language that printers and RIPs speak. Native transparency is understood only by PDF language version 1.4 or higher (present in Acrobat 5 and Illustrator 9).

In order to print objects with transparency, Illustrator must "translate" any transparent artwork into a language that PostScript understands. This translation process is called transparency flattening.

The process of flattening is simple, and Illustrator follows two cardinal rules when performing flattening on a file:

  1. All transparency in the file must be removed.

    If you've used Photoshop before, you may be familiar with the term flattening, which combines all layers in a document. Although similar in concept, transparency flattening is different.



  2. In the process of performing Rule #1, the appearance of the file cannot change.

Both of these rules are followed during the flattening process, with no exception. Obviously, all transparency has to be removed because PostScript doesn't know what transparency is. Additionally, if removing the transparency would result in your file changing in appearance, that would mean you could design something in Illustrator that couldn't be printed, which doesn't make sense either. If you think about it, if you're removing transparency from the file and you're also keeping the visual appearance of the object, something has to give, and that something is the editability of your file. Let's take a look at an example of this.

An Example of Flattening

We'll draw two different-colored circles, one overlapping the other, and we'll set the top circle to Multiply (Figure 11.10). The nice thing about transparency is that you can move the top circle around or change its color and any overlapping areas will simply multiply. The problem is that PostScript doesn't know what transparency is and doesn't know how to print that overlapping area, so transparency flattening is required.

Figure 11.10. By setting the top circle to the Multiply blend mode, you can see through it to the circle below, even with Opacity set to 100%.


Select both circles, choose Object > Flatten Transparency, and click OK (don't worry about the dialog, we'll get to that later). The file is now flattened. Does it look any different? It can't, because of Rule #2, but the file now no longer contains any transparency and can be printed on a PostScript device. The difference is that the file is no longer editable as it was before it was flattened. Upon selecting the circles, you'll find that the two transparent circles have now been broken up into three individual opaque shapes (Figure 11.11).

Figure 11.11. Once the objects are flattened, the artwork is split up into individual opaque pieces, called atomic regions.


This flattening process happens every time you print something with transparency. However, the flattening happens in the print stream, not to your actual Illustrator file. When you choose to print a file, Illustrator flattens a copy of your file and sends the flattened file to the printer, while leaving your document intact. It wouldn't be good if simply printing a file rendered it uneditable. In our example, we specifically flattened the file using the Flatten Transparency function to see the results, but under normal circumstances, you would not flatten the transparency manuallyIllustrator would do that for you automatically at print time.

So we can now understand that when you print a file with transparency, this flattening process occurs so that a PostScript printer can print the file correctly, and that this process happens on the way to printer, so your Illustrator file is not affected in any way.

Flattening also happens whenever you save or export your file to a format that doesn't understand transparency. For example, EPS (which is PostScript) and PDF 1.3 do not support transparency.



This example of the two overlapping circles is a very simple case of flattening. However, there are other examples that can display certain side effects. Let's explore such a case.

Flattening with Rasterization

As in the previous example, create two overlapping circles and set the top circle to Multiply. Fill each circle with a linear gradient, but in one of the circles, apply the gradient on a 45-degree angle. The result is two circles with gradients, but the area in which these two shapes overlap appears as two gradients traveling in different directions (Figure 11.12).

Figure 11.12. This figure shows two overlapping circles, each filled with a gradient on a different angle.


When this file is flattened, you know that the result will be three separate shapes as in the previous example; however, there's a difference. Although gradients can be preserved in vector form, there's no way to describe a crisscross gradient like you see in the overlapping area as a vector. Because of Rule #2, Illustrator is not allowed to change the appearance of your file during flattening, and so Illustrator's only course of action is to turn that overlapping area into a raster image.

Select both circles, choose Object > Flatten Transparency, and click OK. You'll find that although the file looks the same, it is now made up of two vector shapes and a raster image in the middle. Illustrator creates a vector mask for the middle shape so that the file will print correctly (raster images are always rectangular in shape). It's important to point out that Illustrator didn't raster the entire file, it only rasterized the portion of the file that could not be preserved in vector form (Figure 11.13).

Figure 11.13. Where appearance can't be preserved in vector form, Illustrator converts parts of a file into a raster.


At this point, a question should be forming in your brain: If part of the file is now a raster image, what is the resolution of that raster? Patience, my young Padawan, we'll get to that soon. Here's a review of what we've learned to this point:

  1. Transparency flattening is required to correctly print a file with transparency to a PostScript device.

  2. Transparency flattening happens automatically, in the print stream, when you print a file with transparency from Illustrator CS2, InDesign CS2, or Acrobat or Reader 7.0.

  3. Transparency flattening may cause certain parts of a vector file to become rasterized to prevent a file from changing in appearance.

Printing Files with Transparency from QuarkXPress

You can now begin to understand at a basic level why some people have problems printing files with transparency. If you save a PDF from Illustrator and place that file into QuarkXPress, there is still native transparency in that file. Although Quark can place PDF files, QuarkXPress doesn't translate the file when sending the page to the printer, and so the transparency flattening never occurs. The result is transparency commands being sent to a PostScript printer that doesn't understand them, and what shows up on the printed page is anyone's guess.

In Chapter 12, Saving and Exporting Files, you'll explore the different file formats that Illustrator CS2 can save with, and you'll learn to understand the correct formats to use when printing from QuarkXPress and other applications.


Two Levels of Rasterization

In the previous example, where two vector shapes resulted in a portion of that file becoming rasterized, Illustrator had no choice but to rasterize the middle region because there was simply no other way to preserve the appearance in vector form. This is one level of rasterization.

However, there is another case where a second level of rasterization may occur, even if the appearance of a file could be preserved in vector form. Before printing a file, Illustrator analyzes the entire document and looks for complex regions where there are many overlapping objects (which would result in a large number of atomic regions). Illustrator may then choose to rasterize those complex regions for performance reasons. Although we've been trained to think vector objects are simpler than their bitmapped counterparts, try to imagine an Illustrator graphic filled with many overlapping objects with transparency applied (Figure 11.14). Although it may only seem like several objects at first glance, once those objects are broken up into atomic regions, we may be looking at thousands of vector shapes, which can take a long time to process and print (Figure 11.15). In those cases, Illustrator can save precious RIP and processing time by rasterizing these complex regions.

Figure 11.14. Using the Symbol Sprayer tool, you can easily create a file that contains many overlapping shapes. Using the Symbol Screener tool, you can also make some of these symbols transparent.


Figure 11.15. Even though you may have started with a small number of objects, the resulting number of atomic regions due to flattening can be extremely large.


As far as the first level of rasterization goes, we really have no choice but to allow Illustrator to rasterize objects where it needs to. What we can do, however, is learn how to build files that work around this issue (see "Object Stacking Order and Transparency Flattening," later in this section). With regard to the second level of rasterization, we can control how liberal Illustrator is when looking for complex regions. In fact, we can even disable this second level of rasterization altogether. Finally, with either level of rasterization, Illustrator always gives us total control over how these areas are rasterized.

Understanding the Transparency Flattener Settings

As mentioned earlier in this chapter, Illustrator has three Transparency Flattener presets that you can choose from in the Advanced panel of the Print dialog. These settings control how files with transparency are flattened at print time. To access these settings, choose Edit > Transparency Flattener Presets and click the New button to define a new preset. Let's explore the settings in the Transparency Flattener Preset Options dialog box (Figure 11.16).

Figure 11.16. You can define your own custom flattener settings, or your printer or service provider can define one for you.


Raster/Vector Balance: This slider is what controls how liberal Illustrator is when looking for complex regions to rasterize (what we've defined above as the second level of rasterization). A number closer to zero (0) gives Illustrator more freedom to rasterize at will, resulting in faster print times. Moving the slider closer to 100 results in fewer rasterized areas, but longer print times. At the 100 setting itself, Illustrator does not rasterize any parts of the file for performance reasons, effectively disabling the second level of rasterization altogether. The High Resolution flattener preset uses this setting. In cases where files are taking extremely long to print (or crashing the RIP altogether), adjusting this slider to a slightly lower setting helps.

Line Art and Text Resolution: In cases where Illustrator is going to rasterize line art or text, you can specify a resolution that results in good-looking, sharp output. You'll notice that the High Resolution flattener setting specifies a resolution of 1200 ppi, ensuring that text elements and vector objects still have nice, clean, sharp edges in final output.

Gradient and Mesh Resolution: Because gradients and meshes are continuous tones in nature, they don't require a resolution as high as line art or text. In fact, anything twice your line screen is probably getting thrown out anyway. Therefore, Illustrator uses this setting to rasterize elements that can afford to be set at a lower resolution. You'll notice that the High Resolution flattener preset uses a value of 300 ppi.

Convert All Text to Outlines: In cases where text is going to be rasterized, chances are that the rasterized text looks a bit chunkier than regular vector text. To compensate for this, you can turn on this option to convert all text to outlines, giving a consistent chunkier look to all of your text. If you use the method described later in this chapter to move text onto its own layer, you'll rarely need to concern yourself with this setting.

Convert All Strokes to Outlines: Similar to the previous setting, this compensates for disparity between vector and rasterized strokes by converting all strokes to outlines.

Clip Complex Regions: We mentioned that InDesign can look for complex areas of a file and rasterize them for performance reasons. However, we know that raster images are always rectangular in shape, which means it's possible for "innocent" parts of your file to become rasterized simply because they fall into the rectangular bounding box of the area that is complex. More often than not, this results in stitching, or noticeable boxes and color shifts. The Clip Complex Regions option avoids this issue by creating a clipping mask around any rasterized complex region (so the rectangular-shaped raster is masked by the vector outline of the object). As you can probably understand, this makes for even more complex files and can result in longer print times as well. This option is turned on in the Medium Resolution flattener preset but isn't applicable at all in the High Resolution preset because no complex regions are rasterized at all with that setting (as it has a Raster/Vector Balance setting of 100).

The two resolution settings in the flattener controls are used whenever vector objects are forced to become rasters during the flattening process. However, Live Effects, like Feather and Drop Shadow, use the Document Raster Effects Resolution setting to determine their resolution.



Object Stacking Order and Transparency Flattening

When rasterization occurs during transparency flattening, the last thing you want to see turning into a raster is text. That's because you always want text to be clean and sharp in your printouts. Even at the High Resolution setting, where text is rasterized at 1200 ppi, that resolution is still less than half of what most image setters set text withusually upward of 2400 ppi.

Although it's true that under certain circumstances rasterization must occur in order to print a file and maintain its appearance, the way you build your files can affect how often this happens. Let's look at a simple example that clarifies this.

Draw a circle and add a drop shadow to it by choosing Effect > Stylize > Drop Shadow. As you learned in Chapter 7, the Drop Shadow effect is a raster-based effect, and when transparency is flattened, the drop shadow becomes rasterized. Switch to the Type tool, create some text, and position the text near the drop shadow (Figure 11.17). With the text still selected, choose Object > Arrange > Send to Back.

Figure 11.17. Placing text near an object is common, especially when you're adding captions or credit text near photographs.


Now select both the circle and the text, choose Object > Flatten Transparency, and click OK. Upon close inspection, you'll see that a portion of the text was rasterized. This happened because the text was below the drop shadow in the stacking order, and to maintain the file's appearance when the drop shadow was rasterized, Illustrator had to include part of the text in the drop shadow's bounding area (Figure 11.18).

Figure 11.18. In order to maintain the appearance of the file, Illustrator rasterized the text that was behind the drop shadow.


Choose Edit > Undo to go back before you applied the Flatten Transparency function and select the text object. Choose Object > Arrange > Bring to Front. Select both the circle and the text, choose Object > Flatten Transparency, and click OK. In this case, the text, which was above the drop shadow in the stacking order, was not affected at all and was not rasterized (Figure 11.19).

Figure 11.19. If the text appears above the shadow in the stacking order, the text is not rasterized during flattening.


When using transparency features in Illustrator, it's important to make sure that text always appears above objects with transparency to avoid unwanted rasterized text issues. Of course, some designs call for text to appear beneath transparent objects, and in those cases, you don't have much of a choice.

Does My File Contain Transparency?

Not every document needs flatteningonly those with transparency in them. The tricky thing is that transparency can be introduced into an Illustrator document in any of several different ways:

  • You apply a blend mode or an Opacity value other than 100% from the Transparency palette.

  • You apply the Effect > Stylize > Drop Shadow feature.

  • You apply the Effect > Stylize > Feather feature.

  • You apply the Effect > Stylize > Outer Glow.

  • You apply any "below the line" Photoshop effect from the Effect menu.

  • You place a PDF file that contains transparency.

  • You place a native Photoshop file that contains transparency.

Previewing Transparency Flattener Results Before You Print

It would be helpful to know if the document you're working on uses transparency or is even going to require any of the two levels of rasterization we spoke of earlier. You can use the Flattener Preview palette (Window > Flattener Preview) to tell if a document has transparency effects in it, as well as to preview areas that will become rasterized in the flattening process.

By clicking on the Refresh button in the palette, Illustrator highlights specific areas in your file in red, indicating where rasterization will occur. You can enlarge the palette to see a larger image, and you can also click with your mouse inside the preview area of the palette to zoom in closer to see more detail. From the Highlight pop-up menu, you can choose from a variety of items that Illustrator will preview. If all of the items listed in your Highlight pop-up are grayed out, that indicates that there is no transparency present in your file, and no flattening is necessary to print your file (Figure 11.20). For example, when you choose Transparent Objects, Illustrator shows you where all objects are on your page that use transparencyalthough those regions may not necessarily become rasterized. We also mentioned earlier that Illustrator looks for complex areas of a document; you can see where those areas are by choosing Rasterized Complex Regions in the pop-up (Figure 11.21). Additionally, the All Affected Objects option shows you all of the objects that may not be transparent themselves but that interact with transparency in some way (like the example we mentioned above with the drop shadow and the text: the text itself doesn't have transparency applied to it, but if the text appears below the drop shadow, the text must become rasterized to preserve the appearance).

Figure 11.20. If your file contains no transparency, you don't have to worry about the effects of flattening.


Figure 11.21. You can use the Flattener Preview palette to identify areas that Illustrator deems as complex regions, giving you a heads up for what areas will become rasterized.


To take advantage of all that the Flattener Preview palette can offer, adjust the different flattener settings and preview the resultsmaking changes or adjustments where necessaryall before you actually print the file. As an aside, InDesign CS2 and Acrobat 7.0 Professional also contain a similar Flattener Preview palette and identical flattener settings (in fact, it's the same underlying code).

What Kind of RIP Are You Using?

To throw yet another variable into the mix, the kind of printer or RIP that you use can also render different results. For the most part, any Adobe PostScript Language Level 3 device should be able to handle transparency without issue. Specifically, PostScript version 3015 (which appears in the latest versions of RIPs) has enhanced functionality to process files that have been flattened. It's important to remember that flattening has to occur for any RIP to understand how to print transparency. If your RIP can process PDF files, that doesn't necessarily mean that it can process PDF files with transparency in them. If you're in doubt, check with your RIP manufacturer to find out if transparency flattening can occur inside the RIP or if you need to print files from an Adobe application to flatten them.

There are some older print devices that are confused by the effects of flattening. For example, Scitex (since acquired by Creo) RIPs look at jobs that are printing and split up the vector and raster elements onto two layers. The rasterized content prints on a CT (Continuous Tone) layer at a lower resolution (like 300 dpi), and line art prints on a separate vector layer at a much higher resolution (like 2400 dpi). Because flattening could cause a vector object to be rasterized, the RIP only sees that raster as a CT image and prints it at the lower resolution. This might cause text that is rasterized to print with noticeably jagged edges. Scitex has since updated their RIPs to address this issue, but that doesn't automatically mean that everyone who owns a Scitex RIP has installed the update (or knows that it exists).

Bottom line, the best advice in any case is to talk with your printer. For any big job, most printers will be happy to run a test file for you to make sure everything will print correctly. Taking advantage of these opportunities will surely save you headaches when press deadlines loom. Adobe also has free specialized training materials for print service providers if your printer needs more information (found online at http://partners.adobe.com/asn/programs/printserviceprovider/index.jsp).


Printing with Confidence

You can avoid accidents by learning to anticipate possible problems. Now that you're aware of how transparency works, here are a few ways to ensure that you get the results you expect when you're printing from Illustrator.

  • Use the right flattener presetsLow Resolution, Medium Resolution, and High Resolution. For quick proofs to your laser printer, you can use the Low or Medium setting, but when you're printing to a high-end proofer or image setter, use the High setting. You'll find the Transparency Flattener settings in the Advanced panel of the Print dialog.

  • To avoid text becoming rasterized, create a new layer in your Illustrator file and place all of your text on that layer. As long as you keep that text layer as the top layer in your document, you won't have to worry about chunky or pixilated text due to rasterization.

  • A potential problem is that even if you, as a designer, are aware of transparency, there are still plenty of printers out there who aren't. If you are sending out a file and aren't sure who will be printing it or what they will be using to print it, you might consider sending the file as a PDF/X-1a file. See Chapter 12 for more information on PDF/X.

Designing with transparency allows you to create things that were previously prohibitive and difficult to implement, thus allowing you to save valuable time while being even more creative. Now that you know how transparency works and what's necessary to use it in your workflow, give it a test drive. You'll be happy you did.




Real World Adobe Illustrator CS2
Real World Adobe Illustrator CS2
ISBN: 0321337026
EAN: 2147483647
Year: 2003
Pages: 147
Authors: Mordy Golding

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