X3D Schematron Validation and Quality Assurance (QA)

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

Summary | Motivation | Conversion | Resources | References | Contact

🔖 Summary

XML Schematron is a language for making assertions about the presence or absence of patterns in linked XML documents.

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.

X3D Schematron validation rules have greater expressive power than DOCTYPE and Schema constraints, and are most effective when applied following initial DOCTYPE and Schema validation.

🔖 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)

🔖 Conversion

The software setup for the conversion process is a bit involved, but the end result is straightforward. The following diagram shows the relationship between original source Schematron rules, X3D input scenes, and text output reports.

🔖 Resources

Example test results created during the build process are maintained in build.schematron.log.txt. Many errors are shown that confirm various checks are working OK.

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

• The X3D Validator helps ensure the correctness of X3D scenes and metadata by providing a server-based suite of quality-assurance (QA) tests. It is a server-based application that includes the X3D Schematron checks.
• X3D schematron rules (X3dSchematronValidityChecks.sch) and corresponding autogenerated X3D schematron stylesheet, maintained under version control with an open-source license (.txt).
• A pair of Ant build files (buildSchematronDevelopment.xml and build.xml) are used to process and test these rules. Details for this approach are described in Using Schematron for Ant by example.
• Specific outputs for this example include Schematron Validation Report Language (SVRL) intermediate output (.svrl) showing each rule tested, SvrlReportText.xslt stylesheet that distills .svrl output from XML format into a simple text file, and filtered output (.svrl.txt) for a plain-text summary of results..
• X3D-Edit provides this capability to X3D authors (found under X3D, Quality Assurance menu).
• TestSchematronDiagnostics.x3d (.html) is a nonviewable test file with numerous deliberate errors that is used to generate comprehensive X3D validation results.

🔖 References

Schematron is a language for making assertions about the presence or absence of patterns in linked XML documents.

The following references describe Schematron in further detail.

• ISO/IEC 19776-1, X3D XML Encoding, Annex C X3D XML Schematron provides informative guidance in the relevant International Standard for X3D.
• 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.
• New book: Schematron: A Language for Validating XML by Erik Siegel was released November 2022 by XML Press.
• Schematron source on github (updated X3D Schematron 22 October 2017)
• An older Schematron page: "An XML Structure Validation Language using Patterns in Trees"
• Wikipedia: Schematron
• Specification ISO/IEC 19757-3:2006: Information technology -- Document Schema Definition Language (DSDL) -- Part 3: Rule-based validation -- Schematron
• Jing (alternate) RELAX NG validator in Java is used to validate the schematron rules documents
• 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
• Best Practice: Use Multiple Schema Languages by Roger L. Costello

Of related interest: X3D DTDs and Schemas are maintained online and linked via the X3D Resources page. Example X3D DTD and schema usage tips are further documented as part of the X3D Scene Authoring Hints.

🔖 Contact

Updated: 30 March 2024
Maintained by Don Brutzman (brutzman at nps.edu). Comments & suggestions are welcome.

Revisions are found under Sourceforge version control and available online at http://www.web3d.org/x3d/tools/schematron/X3dSchematron.html