Modern DTP applications allow designers to vary the degree of the transparency of objects from 100 per cent opacity (opaque) to zero per cent opacity (transparent). When an object's opacity is decreased the underlying artwork becomes visible through the object.
Transparency is not really a new option, but in the early
days of desktop publishing, such special effects as
transparent overlays and soft drop shadows required
software such as Adobe PhotoShop to export flattened layers
and transparency to EPS or TIFF files.
Transparency is introduced into a layout when an object's
opacity is changed; when a drop shadow is introduced; when
feathering is is used; when blending modes are employed; or
when any of the effects in the transparency palette are
Transparent content in files can either be live or
flattened. Files that contain live transparency can be
opened in the originating (native) software and the
transparent objects can be modified. Native software that
supports live transparency includes PhotoShop, native
Illustrator, Illustrator EPS, native InDesign, Quark XPress
(version 7) and Adobe PDF 1.4, PDF 1.5, PDF 1.6 and PDF
Flattened transparency occurs when a file is exported to a
non-native format. Flattened file formats include
PostScript, EPS, DCS, Adobe PDF 1.3, GIF, JPEG, BMP, and
TIFF images created with PhotoShop prior to version six.
Files created using Acrobat Distiller do not contain live
transparency regardless of the PDF version selected.
Transparency is flattened when a PostScript file is
generated. Note that the Illustrator EPS format
transparency as is actually comprises of two files - one
native Illustrator format that only Illustrator can open
and the other in an EPS format that other applications can
import/place in which the transparency is flattened.
At the time of writing (May, 2007) most installed RIPs are
PostScript devices. As discussed above postScript does not
support live transparency. Therefore, flattened
transparency is required. Some RIPs flatten live
transparency prior to RIPping, others require only flattened
files to be presented. This year the Adobe Print Engine was
introduced upon which many future workflows and RIPs will
be based - but for now most PDF specifications will call
for PDF 1.3.
Headley Brothers PDF specification recommends that pages
are printed to PostScript and distilled to PDF using
Acrobat Distiller to PDF 1.3. However, designers often
prefer to export to PDF. Either way, the PDF 1.3
specification requires transparency to be flattened. The
flattener settings can be saved in a transparency flattener
preset (Edit>Transparency Flattener Presets). You can
then apply these settings when you print or export to PDF
1.3 (Acrobat 4) or even EPS formats.
>> About flattening