Have you ever viewed an Acrobat file that appeared to have several thin white outlined boxes in it? What you are seeing is an artifact (aka…product) of saving as or exporting to PDF from a software application. Fair warning: this is a complicated concept, so get ready for a lot of pictures with this posting.
Just for a starting point, take a look at the “Banner Art” screenshot above. The art was created in Illustrator and involves a photographic background overlayed by several vector objects with effects, drop shadows, and a white glow. Just to get a better idea of the structure of the art, let’s take a look at the art in Illustrator’s outline view (shortcut: command/control y) – see “Banner Art Outlines” screenshot below.
Now, let’s cut this up into slices. First, we’ll save as a PDF. And just to simplify matters, let’s save as a specific flavor of PDF, the PDF/X-1a:2001. (See “Save As PDF” screenshot below.)
Now, let’s open the PDF file in Illustrator. (See “Banner Art PDF” screenshot below.) You probably noticed several white lines in the art. How odd; they weren’t there before.
What you are seeing is a collection of image slices. When you save as or export to PDF, raster objects are created where transparent and semi-transparent objects and effects like drop shadows and glows overlay another object. This process is sometimes called stitching. Now, take a look at the “Banner Art PDF Outlines” screenshot below. This is what the file looks like in Illustrator’s outline view (shortcut: command/control y).
Now, why would Illustrator, Photoshop, Corel Draw, Quark, or InDesign, for that matter, do this? Image slices have a purpose. That purpose is to create a common, standardized file for every variety of computer application called a RIP (raster image processor). This standardized strategy enables RIPs to print to a wide variety of devices such as offset printing presses, plate-makers, large format printers, computer-driven photo images, and desktop printers.
You may have noticed the PDF/X1a is a relatively old standard, circa 2001. It may be old, but it’s still an enforced standard by the magazine and 4-color print industry. The reason for the longevity of the X/1a standard is some RIPs cannot interpret files which include transparent objects and effects. Before sending Acrobat files to your printer, it’s always a good idea to verify your save as or export settings.
One last word on slices before we close. The PDF process is automated and will produce redundant objects called clipping paths. A clipping path defines the visible boundary of an image slice whether the slice is vector or raster. Sometimes these redundant paths are out of registration or misaligned. This is called a slicing error. We always recommend checking your PDFs in Acrobat with the zoom tool (shortcut: command/control spacebar). If the slice lines disappear or remain as a single-pixel hairline when you zoom in, it is likely they will not print. If the white line grows as you zoom in, then you should either make a new PDF or try to edit your PDF with an application like Photostop or Illustrator.