X3D Schematron Validation and Quality Assurance

Don Brutzman (brutzman at nps.edu)

X3D Schematron helps authors by providing quality assurance to ensure intended results in X3D scenes.

Summary

X3D Schematron provides an additional form of XML validation for X3D scenes. Authors can use X3D Schematron reports to verify semantic correctness and detect internal-consistency problems.

Motivation

Schematron is a language for making assertions about patterns and internal consistency found in XML documents. If assertions for correct behavior fail, then error and warning messages are provided.

Reporting detected inconsistencies and error conditions enables authors to correct Extensible 3D (X3D) Graphics scene problems. Quality assurance to improve correctness can work on many levels, both syntactic and semantic.

The Extensible Markup Language (XML) encoding for .x3d scenes has offered three primary ways for checking the syntactic correctness of scenes:

• XML well-formed checks
• X3D document type definition (DTD or DOCTYPE) validation grammar ensures proper parent-child element relations and attribute naming
• X3D Schema validation grammar also ensures proper type-checking of most attribute values

An X3D Schematron ruleset is now available as a fourth approach to allow checking a wide variety requirements that are specific to X3D. These quality-assurance checks go beyond the capabilities of DTD or XML Schema grammar-based checking. Rule examples include:

• DEF must precede USE
• Interpolator key and keyValue arrays must have the same length
• Numeric values for index arrays are within bounds
• Plus many others, since X3D has many requirements for internal consistency

Another relevant description for Schematron:

"Schematron is a small language for making assertions about the presence or absence of patterns in XML documents using XPaths. It is can be categorized as a schema language, a rules language, or a constraint language. Schematron is design to co-exist and augment with existing schema languages, providing a convenient way to easily express and test business and structural constraints."   (Schematron Wiki)

X3D Schematron Resources

The following resources can be used to provide Schematron support for X3D.

• X3D schematron rules and corresponding autogenerated X3D schematron stylesheet, maintained under version control with an open-source license (.txt).
• A pair of Ant build files (build.xml and buildSchematronDevelopment.xml) are used to process and test these rules. Details for this approach are described in Using Schematron for Ant by example.
• X3D-Edit provides this capability to X3D authors (found under X3D, Quality Assurance menu).
• TestSchematronDiagnostics.x3d (.html) is a test file with numerous deliberate errors that is used to generate comprehensive X3D validation results.
• Specific outputs for this example include Schematron Validation Report Language (SVRL) output (.svrl) showing each rule tested, plus filtered output (.svrl.txt) for a plain-text summary of results. SvrlReportText.xslt converts .svrl output from XML into a simple text file.
• Of related interest: X3D DTDs and Schemas are maintained online and linked via the X3D Resources page. Example X3D DTD and schema usage is documented as part of the X3D Scene Authoring Hints.

References

The following resources describe Schematron.

• Schematron home page includes links to the latest Schematron conversion stylesheets, which can translate a Schematron rule file into an Extensible Stylesheet Language for Transformations (XSLT) stylesheet containing XML Path Language (XPath) expressions. Since style capabilities using XSLT and XPath are widely implemented, this approach makes it easy to port Schematron rules into different applications.
• Another Schematron page
• Schematron Wiki
• Wikipedia: Schematron
• Specification ISO/IEC 19757-3:2006: Information technology -- Document Schema Definition Language (DSDL) -- Part 3: Rule-based validation -- Schematron
• Schematron "O'Reilly short cut" by Eric van der Vlist (March 2007) provides an excellent tutorial on how to design and test Schematron rules
• Schemas: stereotypes, archetypes or prototypes? by Rick Jelliffe

Updated: 27 February 2010
Maintained by Don Brutzman (brutzman at nps.edu). Comments & suggestions are welcome.
Online at http://www.web3d.org/x3d/tools/schematron/X3dSchematron.html