Overprinting
While transparency and overprint are two distinct things, transparency will often affect overprint. In the process of flattening, overprint instructions are preprocessed when overprinted objects interact with transparent objects. This ensures that overprint settings are reliably processed when a file is flattened. The visual appearance of the objects involved does not change during flattening, nor does the amount of ink per plate.
Overprinting instructions are preprocessed by the flattener when:
• Overprinting objects are transparent (not 100% opaque).
• Overprinting objects are part of or within 1/72” of transparent objects or groups.
• Simulate Overprints is selected on the Advanced tab of the Print dialog (for use when printing proofs to low-end composite devices that cannot otherwise simulate overprints).
Note:
Changing overprinting instructions at the
RIP can affect not only objects set to overprint, but flattened transparency as well.
In these cases, the overprinting instructions for the objects are executed and the results have the same appearance as the original, but in a flattened form. When Preserve Overprints is selected for separated output, flattening does not occur for objects that are not involved in transparency, even when they are overprinted.
Overprint preview: It is always a good idea to check the results of settings before progressing with print output. To see an “ink preview” that approximates how blending, transparency, and overprinting will appear in color-separated output, choose View > Overprint Preview, which displays the results of overprinted colors. You can also use a desktop proofer to simulate overprinted colors by choosing “Simulate” overprints in the Advanced tab of the
Print dialog when doing composite output. When creating separations, you should not select
“Simulate.” Rather, using the “Preserve” option means Illustrator CS will correctly separate spot colors and overprinting.
Flattening transparency
When printing a file or converting it into a format that does not support native transparency instructions, all transparent objects in a file as well as objects that interact with transparency must be flattened. The process of flattening removes the live transparency effects and creates individual objects that take on the same appearance as the original transparent objects, but in a format that can be understood by the printing device or file format.
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Overlapping transparent art is divided into atomic regions when flattened, and live transparency effects are removed.
(On the right, regions have been shifted to show the different atomic regions.)
During flattening, all overlapping areas of a group of objects affected by transparency, are broken down into separate opaque pieces (atomic regions) that may include vectors, raster data, or both. Once flattened, the original objects can no longer be modified, so it is best to retain live transparency until the last possible point in the workflow.
Flattening occurs when you do any of the following:
• Use the Print dialog to print a file that contains transparency.
• Save a file that contains transparency in a legacy format that does not support live transparency such as Illustrator 8 or earlier, Illustrator 8 EPS or earlier, or PDF 1.3 (Acrobat 4 compatibility).
• Export a file containing transparency to a vector format that does not understand transparency (e.g., EMF or WMF).
• Copy and paste transparent artwork from Illustrator to another application with both the
AICB and Preserve Appearance options chosen.
• Use Object > Flatten Transparency (although it is recommended that you flatten at the time of printing rather than earlier in the process).
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Tr a n s pa re nc y f l at te ne r pre s e t s
To streamline the flattening process and ensure consistent results in similar conditions,
Illustrator CS includes three standard preset values for flattener settings, and the ability to create your own for different types of printing devices and kinds of output. Custom presets appear in the Print dialog box or EPS/PDF Export dialog boxes. The flattener settings include resolution and other output controls. To manage these presets, choose Edit >
Transparency Flattener Presets.
Use Edit > Transparency Flattener Presets to manage your presets. When you choose to create or edit a preset, this dialog box is used to select the flattener options.
• Raster/Vector Balance: To specify the amount of rasterization that is to be performed on the artwork, position the slider anywhere between 0% and 100% . In a service provider workflow, there is little reason to use a value lower than 100. Vector files tend to be smaller than raster.
• Line Art and Text Resolution: This resolution setting is used when text and vector artwork is rasterized as a result of its interaction with transparency. In most cases, this
value should not be changed. Higher resolutions may improve flattener quality, but will result in larger file sizes.
• Gradient and Mesh Resolution: For objects that use these effects, including drop shadows and feathering, a setting of 300 dpi is sufficient in most cases.
• Convert All Text to Outlines: This option converts all text in the document to outlines which may slightly fatten the type, but makes the width consistent. Use this setting to avoid type getting partially converted to outlines, which could result in different type thicknesses.
• Convert All Strokes to Outlines: To ensure that the width of all strokes in the artwork stays consistent, select this option, which converts all strokes in the document to filled outlines. This will usually slightly thicken the appearance of strokes.
• Clip Complex Regions: This option is only available when the Raster/Vector slider is set to less than 100. This setting decreases the probability of getting color stitching in the output. Stitching is the artifacts that potentially appear between atomic regions. (When overlapping transparent objects are flattened, each discrete shape that results from the overlapping objects is called an atomic region.) When complex areas get rasterized during the flattening process, it normally happens in a “blocky” fashion that can cut through objects, such that part of an object can be rasterized while the rest remains in vector form. Stitching results from software trying to anti-alias the edge of atomic regions. It can cause white or black lines to appear on screen, although they don’t typically appear in high-resolution print output. Selecting Clip Complex Regions decreases the probability of stitching by ensuring that the raster/vector boundaries always fall along existing natural object paths.
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F l at te ne r P re v ie w pa le t te
Whenever creating color separations with a file containing transparency, you should carefully examine which objects will be affected by overprinting and flattening before printing.
To make this process easy and efficient, Illustrator CS provides previewing capabilities that allow you to zoom in and see which objects will be affected by flattening and overprinting
(among other things) given the current flattener settings. Potential problems can be found and corrected before they become costly.
