Focus on PDF Layers (OCG)

Dave Wraight Planet PDF Contributing Editor

February 18, 2004

With the introduction of PDF version 1.5 came the concept of Layers. Layers or as they are more formally known Optional Content Groups (OCG) provide an excellent method for dealing with related content.

The concept of Layers isn't new, Illustration and CAD packages have been using Layers to separate content for many years, however the ability to have this functionality inside Acrobat/PDF provides developers and PDF authors alike with the kind of functionality we've become accustomed to, and made use of extensively.

Authoring Applications

An essential point to remember is that Acrobat is not able to set or create layers from existing content. This is largely due in part to the fact that Acrobat isn't an illustration package or indeed an authoring application. This means we need to rely on authoring applications to generate PDF files that can write PDF version 1.5 and also write OCGs/Layers.

At present the following applications support this functionality:

• Illustrator CS (v11)
• InDesign CS (v3)
• AutoCAD (w/Acrobat PDFMaker)
• Visio (w/Acrobat PDFMaker)
• AcroTips.com StacksOn for MSWord*
• ARTS PDF Stratify (Acrobat Plugin)*

The new Adobe Creative Suite translates directly the layers you have in your documents into layers in you PDF's, AutoCAD and Visio rely on Acrobat's PDFMaker (VBA) macro to to create the corresponding layers from the source documents.

StacksOn* is the first MSWord tool for creating Layers from Word source documents and ARTS PDF Stratify* is the only tool on the market for creating or rather defining layers from existing PDF content (it's a plugin for Acrobat).

* AcroTips is currently in the late stages of development of StacksOn -- the first tool for creating layers in Word that produce a layered PDF version. ARTS PDF (a division of BinaryThing) has also announced it will be releasing a new version of ARTS PDF Stratify which will let Acrobat users define layers from within Acrobat.

Next Steps

So this gives us an idea on how to create Layered PDFs, in our example I've created a PDF with a floor plan with several layers.

The potential of Layered PDF (OCGs) is huge, one of the best examples I've seen so far has been a company brochure with two layers. The page is initially blank with just the background images for the products, however when the PDF is opened a little bit of Javascript checks the language of the user and switches either the German Layer or English layer on -- these two layers contain the appropriate translated text for the product -- all in one PDF!

What we'll concentrate on in this article is how to manipulate OCGs with Javascript to truly make best use of this new technology.

Throughout this article I'll use the terms Layers and OCGs interchangeably.

Finding the Layers

When you open a PDF file that has Layers you'll notice a piece of cake (no really, a piece of cake) in the status bar at the bottom left of the screen. This icon indicates that the PDF you have open has special features and by clicking on the icon a dialog will display informing you what these special features are.

In our case the PDF has Layers (or Optional Content Groups), so the dialog let's us know that such a thing exists and how to show the layers panel.

So if we havent already got the Layers Panel open we can go into the 'view' menu and select 'Navigation Tabs' and then choose 'layers'.

You should be presented with the list of currently available Layers for this PDF. Just remember that Layers are page bound not Document bound, meaning each layer in the list belongs to a page somewhere in the document, using the properties menu you can actually elect to show only Layers for the current page or for the entire document.

Properties for Layers

The authoring application that created the PDF can set attributes for each Optional Content Group that can tell the viewing software (in our case Acrobat/Reader) how to deal with the OCG and also any other features that may or may not be Interface driven.

OCG Properties

The following properties can be defined when creating PDF's, the actual properties that are set are determined by the authoring application, some properties are user defineable and others are part of the PDF standard. (This list isn't complete)

var docOCGs = this.getOCGs();
for (var x=0; x < docOCGs.length; x++) 
{
	if(docOCGs[x].name == "names")
	{
		docOCGs[x].state = !docOCGs[x].state;
	}
}

// Visible
var docOCGs = this.getOCGs();
for (var x=0; x < docOCGs.length; x++) 
{
	docOCGs[x].state = true;
}

// InVisible
var docOCGs = this.getOCGs();
for (var x=0; x < docOCGs.length; x++) 
{
	docOCGs[x].state = false;
}