The Merriam-Webster dictionary describes transparency as “the quality or state of being transparent” while transparent means “to show through”.

Transparency in PDF files

PDF 1.4 (the file format supported by Acrobat 5) was the first version of PDF that supported transparency.

It obviously isn’t a coincidence that this PDF release can to market at the same time that Adobe Illustrator 9 was launched. Transparency was ‘the’ big new feature in Illustrator. Other applications like Corel Draw supported transparency already for years but had to resolve to a trick to generate a printable file: when creating an EPS file or printing a drawing, the application had to remove or calculate the transparency effects. This process is called ‘flattening’.

The challenge with transparency

Transparency is a very complex technology. The Adobe technical documentation on it is over 100 pages long.

The big stumbling block is the fact that a PDF file can contain transparent objects with different color spaces. Adding a drop shadow to a spot color element that sits on top of a CMYK background (or vice versa) is an example of a design that challenges the RIP or workflow that needs to process the job. Making an RGB image slightly translucent on top of a CMYK background is another example.

On top of that, any software processing transparency needs to work out how to best handle group of objects that are transparent. Should each object in turn be blended into the background or should it be done on the entire group.

Is there transparency in this file?

If you have Adobe Acrobat Professional, it is easy to detect if a PDF contains transparency. The preflight engine of recent versions can list all transparent objects in a file. Below is a screen capture of the preflight window of Acrobat Professional 8.

Other preflight engines like PitStop Professional can also be used to detect transparency.

Transparency in source files
To designers it is not always obvious when their files contain transparency. InDesign has a very nice mechanism for this. Pages that contain transparent objects have a checkerboard pattern in the pages list. InDesign indicates this per spread, even if only one of the two pages contains transparency. In the example below page 2, page 3 or both pages contain transparency.

These are the actions that add transparency to a file:

• Making an object transparent in a lay-out or design application (obviously).
• Adding drop shadows to objects.
• Feathering objects.
• Placing native files which contain transparency from Adobe Illustrator, InDesign or Photoshop.
• Dragging & dropping or copying & pasting transparent objects from applications like Adobe Illustrator to Adobe InDesign.

Tips & tricks to avoid having issues with transparency

You can download the Adobe designers guide to transparency here. Quark have a similar document for QuarkXPress 7 transparency.

Other sources of information

Adobe have produced an excellent white paper on the use of transparency in print production. They also have a separate guide on flattening in Illustrator 10.