TweetFollow Us on Twitter

Sep 00 Online

Volume Number: 16 (2000)
Issue Number: 9
Column Tag: MacTech Online

PDF and XML

by Jeff Clites <online@mactech.com>

Last month we covered Adobe's Portable Document Format (PDF), focusing on how it relates to Quartz, Apple's new imaging model. In brief, PDF originated as a simplification of PostScript, retaining PostScript's primitive graphics operators while discarding its programming-language constructs and adding file and document structure specifications. Quartz (specifically, the Core Graphics Rendering API) is again based on this same set of operators, making it natural to "record" graphics operations into a PDF file, and just as natural to "play back" a PDF into a series of native drawing instructions. At its simplest, PDF is the new PICT; more interestingly, the Quartz imaging model is at the center of all 2-D graphics on Mac OS X, providing a centralized facility for rending drawing commands from different APIs (such as QuickDraw) into different output formats, be they destined for the screen, a printer, or a file.

Of course, part of the beauty of Quartz is that it frees the programmer from having to worry about the details of this process. At the same time, Quartz is certain to increase the popularity of PDF, and in particular expand its use beyond just a format for traditional documents. Accordingly, it will be to a programmer's advantage to know as much as possible about PDF, and to be aware of its strengths and its weaknesses.

As touched on above, PDF defines a file format in addition to a graphics model. In the abstract, a PDF file describes a tree of objects, with a significant separation between document content and document layout. This should send off bells in a developer's head, because it sounds similar to XML, and it's natural to wonder how deep this connection is-to ask questions like, "can a PDF document be represented in XML." The short answer is "probably not", but it's interesting to investigate the parallels between the two formats.

Intersections with XML

PDF and XML are similar in that they define a file structure which is designed to encapsulate a wide range of data in a fairly generalized, hierarchical fashion. Although PDF is designed to be extensible, it does define an interpretation for the information it contains, and it's not clear how well current PDF-rendering applications would handle PDF documents with content which they don't recognize. XML is at the other extreme. At its core, it says nothing about the semantics of the data which it can contain, and it's often used as a format for information which isn't naturally thought of as a "document." But given its generality, it would certainly be possible to devise an XML-based format to encapsulate page-descriptions in a manner similar to PDF. On the other hand, there are several facilities of PDF which are not easily mimicked using XML-features dealing more with practical performance issues than with conceptual structure.

PDF was designed to be a final format, so that PDFs represent finished documents, rather than in-progress works (such as word-processing documents) which will be extensively changed. Still, it is possible to make limited modifications to PDFs, and interestingly this can be done by appending the "change" information to the end of a PDF, without requiring the entire document to be rewritten. This makes it convenient to prepare an initial document and at a later stage add annotations or hyperlinks. This approach also provides a measure of safety, as previous versions of a document can be recovered simply by truncating the changes off the end, and modifications cannot cause complete corruption of the base document. This also means that it is possible to modify large documents without large resource requirements.

Despite XML's flexibility, it isn't possible to create a well-formed XML document by appending information directly to another document, because of the requirement that there be a single root element. (It is possible to work around this limitation, but only by splitting the document into multiple files.) Additionally, PDF documents frequently encapsulate binary data (such as images or compressed text), and it is not convenient to embed such data into XML documents directly-XML is a text-based format, and binary data could be interpreted as markup, or mangled if the document is converted to a different character encoding. XML-based formats traditionally handle this by storing the data in a separate file which is then referenced from the base document, just as images are included in HTML files. This is less convenient than PDF's single-file approach. (It would be possible to include binary data in XML documents by converting it into a text-based representation, such as Base-64 encoding, but this tends to offset the benefits of compression.) Finally, PDF has a higher structural flexibility, in that logical containment is not always represented by physical containment. In other words, structures which logically contain other objects may do so by referencing the objects by name, whereas in XML such containment is almost always represented by physically nesting elements. This flexibility allows the same PDF to be represented in different ways, so that for example a PDF file may be optimized for page-at-a-time delivery over the internet, or alternatively it could be created in a single-pass by a printer driver.

FOP

So despite the current popularity of XML, it isn't likely that PDF is going to be superceded any time soon. So where do PDF and XML intersect? Well, as we observed before, it's natural to think of XML as unformatted data, and to think of PDF as an output format. The preferred way to get from XML to something with formatting is by way of XSL Transformations (XSLT). In the case of XML-to-PDF transformation, there's a tool to help with the process, FOP. (It's part of the Apache XML project.) To use FOP, you first use an XSLT processor to convert your XML document into a tree of formatting objects, which may itself be represented as an XML document. This is where you determine the form of your final document. Since, as mentioned above, XML documents are traditionally devoid of formatting information and are often viewed as pure data, any decisions about how this information will be presented must be encapsulated in the style sheet. Once this is done, and you have your tree of formatting objects, you feed this into FOP, which produces your final PDF. FOP is very much a work in progress, and does not yet support all of the formatting objects defined in the XSL specification, but even as-is it appears quite useful. IBM has an informative tutorial on transforming XML documents. (A free registration is required to access the tutorial.) It discusses using FOP to create PDF documents, and in addition shows you how to generate SVG (Scalable Vector Graphics), which is useful for creating things like charts and graphs from XML-encapsulated data.

OmniPDF

Finally, while you're playing with PDF, be sure to check out OmniPDF if you are running Mac OS X. It's a very cool PDF viewer. It's still under development, but it's Cocoa-native (and hence Mac-OS-X-native), and it really shows off the power of Quartz, as it uses Core Graphics Rendering to do its magic. (OmniPDF is from the Omni Group, who also created OmniWeb, which is currently the only Cocoa-native web browser available. You should check it out also-it's a refreshing alternative, and it has many fun features which set it apart from your usual browser choices.)

 
AAPL
$102.50
Apple Inc.
+0.25
MSFT
$45.43
Microsoft Corpora
+0.55
GOOG
$571.60
Google Inc.
+2.40

MacTech Search:
Community Search:

Software Updates via MacUpdate

Skype 6.19.0.450 - Voice-over-internet p...
Skype allows you to talk to friends, family and co-workers across the Internet without the inconvenience of long distance telephone charges. Using peer-to-peer data transmission technology, Skype... Read more
VueScan 9.4.41 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
Cloud 3.0.0 - File sharing from your men...
Cloud is simple file sharing for the Mac. Drag a file from your Mac to the CloudApp icon in the menubar and we take care of the rest. A link to the file will automatically be copied to your clipboard... Read more
LibreOffice 4.3.1.2 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
SlingPlayer Plugin 3.3.20.505 - Browser...
SlingPlayer is the screen interface software that works hand-in-hand with the hardware inside the Slingbox to make your TV viewing experience just like that at home. It features an array of... Read more
Get Lyrical 3.8 - Auto-magically adds ly...
Get Lyrical auto-magically add lyrics to songs in iTunes. You can choose either a selection of tracks, or the current track. Or turn on "Active Tagging" to get lyrics for songs as you play them.... Read more
Viber 4.2.2 - Send messages and make cal...
Viber lets you send free messages and make free calls to other Viber users, on any device and network, in any country! Viber syncs your contacts, messages and call history with your mobile device,... Read more
Cocktail 7.6 - General maintenance and o...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
LaunchBar 6.1 - Powerful file/URL/email...
LaunchBar is an award-winning productivity utility that offers an amazingly intuitive and efficient way to search and access any kind of information stored on your computer or on the Web. It provides... Read more
Maya 2015 - Professional 3D modeling and...
Maya is an award-winning software and powerful, integrated 3D modeling, animation, visual effects, and rendering solution. Because Maya is based on an open architecture, all your work can be scripted... Read more

Latest Forum Discussions

See All

Rhonna Designs Magic (Photography)
Rhonna Designs Magic 1.0 Device: iOS Universal Category: Photography Price: $1.99, Version: 1.0 (iTunes) Description: Want to sprinkle *magic* on your photos? With RD Magic, you can add colors, filters, light leaks, bokeh, edges,... | Read more »
This Week at 148Apps: August 25-29, 2014
Shiny Happy App Reviews   | Read more »
Qube Kingdom – Tips, Tricks, Strategies,...
Qube Kingdom is a tower defense game from DeNA. You rally your troops – magicians, archers, knights, barbarians, and others – and fight against an evil menace looking to dominate your kingdom of tiny squares. Planning a war isn’t easy, so here are a... | Read more »
Qube Kingdom Review
Qube Kingdom Review By Nadia Oxford on August 29th, 2014 Our Rating: :: KIND OF A SQUARE KINGDOMUniversal App - Designed for iPhone and iPad Qube Kingdom has cute visuals, but it’s a pretty basic tower defense game at heart.   | Read more »
Fire in the Hole Review
Fire in the Hole Review By Rob Thomas on August 29th, 2014 Our Rating: :: WALK THE PLANKUniversal App - Designed for iPhone and iPad Seafoam’s Fire in the Hole looks like a bright, 8-bit throwback, but there’s not enough booty to... | Read more »
Alien Creeps TD is Now Available Worldwi...
Alien Creeps TD is Now Available Worldwide Posted by Ellis Spice on August 29th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Dodo Master Review
Dodo Master Review By Jordan Minor on August 29th, 2014 Our Rating: :: NEST EGGiPad Only App - Designed for the iPad Dodo Master is tough but fair, and that’s what makes it a joy to play.   | Read more »
Motorsport Manager Review
Motorsport Manager Review By Lee Hamlet on August 29th, 2014 Our Rating: :: MARVELOUS MANAGEMENTUniversal App - Designed for iPhone and iPad Despite its depth and sense of tactical freedom, Motorsport Manager is one of the most... | Read more »
Motorsport Manager – Beginner Tips, Tric...
The world of Motorsport management can be an unforgiving and merciless one, so to help with some of the stress that comes with running a successful race team, here are a few hints and tips to leave your opponents in the dust. | Read more »
CalPal Update Brings the App to 2.0, Add...
CalPal Update Brings the App to 2.0, Adds Lots of New Stuff Posted by Ellis Spice on August 29th, 2014 [ permalink ] | Read more »

Price Scanner via MacPrices.net

Save up to $300 on an iMac with Apple refurbi...
The Apple Store has Apple Certified Refurbished iMacs available for up to $300 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free. These are the best prices on... Read more
The Rise of Phablets
Carlisle & Gallagher Consulting Group, a businesses and technology consulting firm focused solely on the financial services industry, has released an infographic depicting the convergence of... Read more
Eddy – Cloud Music Player for iPhone/iPad Fre...
Ukraine based CapableBits announces the release of Eddy, its tiny, but smart and powerful cloud music player for iPhone and iPad that allows users to stream or download music directly from cloud... Read more
A&D Medical Launches Its WellnessConnecte...
For consumers and the healthcare providers and loved ones who care for them, A&D Medical, a leader in connected health and biometric measurement devices and services, has launched its... Read more
Anand Lal Shimpi Retires From AnandTech
Anand Lal Shimpi, whose AnandTech Website is famous for its meticulously detailed and thoroughgoing reviews and analysis, is packing it in. Lal Shimpi, who founded the tech site at age 14 in 1997,... Read more
2.5GHz Mac mini, Apple refurbished, in stock...
The Apple Store has Apple Certified Refurbished 2.5GHz Mac minis available for $509, $90 off MSRP. Apple’s one-year warranty is included, and shipping is free. Read more
13-inch 2.5GHz MacBook Pro on sale for $999,...
B&H Photo has the 13″ 2.5GHz MacBook Pro on sale for $999.99 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Read more
Labor Day Weekend MacBook Pro sale; 15-inch m...
B&H Photo has the new 2014 15″ Retina MacBook Pros on sale for up to $125 off MSRP. Shipping is free, and B&H charges NY sales tax only. They’ll also include free copies of Parallels Desktop... Read more
Labor Day Weekend iPad mini sale; $50 to $100...
Best Buy has the iPad mini with Retina Display (WiFi models) on sale for $50 off MSRP on their online store for Labor Day Weekend. Choose free shipping or free local store pick up. Price is for... Read more
13-inch 1.4GHz MacBook Air on sale for $899,...
Adorama has the new 2014 13″ 1.4GHz/128GB MacBook Air on sale for $899.99 including free shipping plus NY & NJ tax only. Their price is $100 off MSRP. Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.