Adobe InDesign - Manual (Page 657)

Note:
In most cases, flattening produces excellent results when you use an appropriate predefined flattener preset, or create a preset with settings
appropriate for your final output. For a complete reference and troubleshooting guide on how transparency affects output, see the document
“Achieving Reliable Print Output with Transparency” (English only) on the Adobe website.
However, if your document contains complex, overlapping areas and you require high-resolution output, you can achieve more reliable print output
by following a few basic guidelines:
If you’re applying transparency to documents intended for high-resolution output, be sure to discuss your plans with your service provider.
Good communication between you and your service provider will help you achieve the results you expect.
Overprinting objects
Although flattened objects may look transparent, they are actually opaque and don’t allow other objects beneath them to show through. However, if
you don’t apply overprint simulation, the transparency flattener may be able to preserve basic overprinting of objects when exporting to PDF or
printing. In this case, recipients of the resulting PDF file should select Overprint Preview in Acrobat 5.0 or later to accurately view the results of
overprinting.
Conversely, if you apply overprint simulation, the transparency flattener provides a simulation of what the overprints look like, and this simulation
results in all opaque objects. In PDF output, this simulation converts spot colors to process color equivalents. Therefore, Simulate Overprint should
not be selected for output that will be color separated later.
Spot colors and blending modes
Using spot colors with certain blending modes sometimes produces unexpected results. This is because InDesign uses process color equivalents
on screen, but uses spot colors in print. In addition, isolated blending in an imported graphic could create knockouts in the active document.
If you use blending, check your design periodically using Overprint Preview in the View menu. Overprint Preview gives an approximation of how
spot inks that overprint or interact with transparent objects will appear. If the visual effect is not what you want, do any of the following:
Use a different blending mode or no blending mode. Avoid these blending modes when working with spot colors: Difference, Exclusion, Hue,
Saturation, Color, and Luminosity.
Use a process color where possible.
Blend space
If you apply transparency to objects on a spread, all colors on that spread convert to the transparency blend space you’ve chosen (Edit >
Transparency Blend Space), either Document RGB or Document CMYK, even if they’re not involved with transparency. Converting all the colors
results in consistency across any two same-colored objects on a spread, and avoids more dramatic color behavior at the edges of transparency.
Colors are converted “on the fly” as you draw objects. Colors in placed graphics that interact with transparency are also converted to the blend
space. This affects how the colors appear on-screen and in print, but not how the colors are defined in the document.
Depending on your workflow, do one of the following:
If you create documents for print only, choose Document CMYK for the blend space.
If you create documents for web only, choose Document RGB.
If you create documents for both print and web, decide which is more important, and then choose the blend space that matches the final
output.
If you create a high-resolution print piece that you’ll also publish as a high-profile PDF document on a website, you may need to switch the
blending space back and forth before final output. In this case, be sure to reproof the color on every spread that has transparency, and avoid
using the Difference and Exclusion blend modes—these modes can change the appearance dramatically.
Type
When type is close to transparent objects, it may interact with transparent objects in unexpected ways. For example, type that wraps around a
transparent object may not actually overlap the object, but the glyphs may be close enough to interact with the transparency. In this case, the
flattener may convert the glyphs to outlines, resulting in thickened stroke widths on the glyphs only.
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