BLIS-XML

BLIS-XML is a methodology for encoding EXPRESS based information in XML format.

Overview

BLIS-XML is a methodology for encoding EXPRESS based information in XML format. A BLIS-XML schema can be automatically generated from an EXPRESS schema. It is both read and write compatible with the original EXPRESS schema.

The motivation for developing BLIS-XML is to enable the use of IFC model data in applications that can read/write data in XML format, but not EXPRESS format. A typical example of such an application is a web browser with XML support (such as Microsoft's Internet Explorer 5). In the near future XML support will be a standard feature in almost all software. This is not the case with EXPRESS data formats. BLIS members are not making judgements about this, but must address the business reality.

The BLIS-XML methodology results in early bound schemas, which means that there will be different schemas for different purposes. Another approach would be to have just one generic schema that can be used for virtually anything; the XML language itself is an example of such a generic schema.

The BLIS-XML methodology can be currently used in 3 different scenarios.

    1. A full EXPRESS data model is converted to BLIS-XML .
    2. A valid subset (view) of an EXPRESS data model is converted to BLIS-XML
      EXPRESS data models generally contain rules establishing a minimal framework that has to be included in a valid data exchange. In addition the data exchange may contain optional parts of the model that may internally contain similar rules. A BLIS-XML schema generated from such a view enables XML based data exchange that is valid from the viewpoint of the original EXPRESS data model. .
    3. A single concept from a EXPRESS data model is converted to BLIS-XML
      EXPRESS data models may contain concepts that are well defined and self sufficient, but according to the rules of the model can't be used independently in data exchange. By converting such a concept into BLIS-XML it can be used independently in data exchange or the concept can be embedded into other XML models in various data exchange scenarios.

The EXPRESS schemas are based on inheritance where each element inherits all the properties of its parent (e.g. in the IFC schema a door inherits all attributes of building element). BLIS-XML only contains the inherited attributes that are relevant to each model view context. Filtering of the inherited attributes is done when the view is defined. This dramatically reduces the complexity of the schema and makes it easier to understand and implement.

BLIS-XML also includes selected INVERSE attributes in the XML files. INVERSE attributes are not included in the STEP Part 21 file format. They are recalculated by the toolbox of the receiving system. Since for example the IFC model relies heavily on objectified relationships, INVERSE relationships are often the only way to find some object properties. An example of this is the fire rating of a door in the IFC model. EXPRESS based tools have methods for querying the INVERSE relationships. XML based tools do not. Consequently, including the relevant INVERSE relationships make the model data much easier to use in XML based tools.

Models originally designed in EXPRESS, generally do not convert well to XML representation. This is because EXPRESS allows many different overlapping and even conflicting data structures in the same model. The IFC schema has for example a containment, a relative placement and various grouping (group, zone, system etc.) views of the same data. By contrast, XML models are most efficient when structured according to single, simple hierarchical structure. Since BLIS-XML is one-to-one compatible with the EXPRESS schema the relationships in the XML files use a system called "idref" instead of the hierarchical structure typical of native XML schemas. A consequence of this is that XML data manipulation techniques, such as XSL scripts will be more challenging than with native XML schemas. However, the BLIS-XML methodology is still much simpler than the approaches defined in STEP Part 28.

File sizes for data files using BLIS-XML are up to 2.5 times larger than the equivalent Part 21 encoding. Those who are familiar with XML will recognize that this is extremely efficient, when compared to other EXPRESS to XML encoding approaches.

Support for BLIS-XML has been implemented by Secom Co. Ltd., in it's IFCsvr component, which is being used in software products by Microsoft, Timberline, Pacific Northwest National Labs and Lawrence Berkeley National Lab.

 

Acknowledgements

BLIS-XML schemas

Full models

 BLIS-XML for IFC 2x
 Date  05.12.2000
 Type  Full model
 EXPRESS  Schema  IFC 2x
 Name  IFC2X_FINAL
 Author  IAI (International Alliance for Interoperability)
BLIS-XML  Schema  BLIS-XML for the complete IFC R2x model
 Name  BLIS-XML for IFC 2x
 Author  BLIS -project
 Schema file  - BLIS_XML_FOR_IFC_2x.xml (355 KB)
 - BLIS_XML_FOR_IFC_2x.zip (28 KB)
 Documentation  Please refer to the original IFC R2x documentation
  (NOTE: Linked to the IAI UK web site at http://cig.bre.co.uk/iai_uk)
 Description  This BLIS-XML schema was automatically generated from the EXPRESS schema.

 

Model views

 BLIS-XML for IFC R2
 Date  30.11.2000
 Type  Model view
 EXPRESS  Schema  IFC Release 2.0
 Name  IFC20_LONGFORM
 Author  IAI (International Alliance for Interoperability)
BLIS-XML  Schema  BLIS-XML for the BLIS views of the IFC R2.0 model
 Name  BLIS-XML for IFC R2
 Author  BLIS -project
 Schema file  - BLIS_XML_FOR_IFC_R2.xml (80 KB)
 - BLIS_XML_FOR_IFC_R2.zip (7 KB)
 Documentation  Please refer to the BLIS view documentation
 Description This BLIS-XML schema contains the composit of all views defined by the BLIS -project for the IFC R2.0 model. The decision was made to generate only one BLIS-XML schema that can be used with any of the views rather than to compile a separate schema for each view. Consequently additions to any of the views will result in a new BLIS-XML schema for the R2.0 views. BLIS-XML provides backward compatibility with previous versions of this schema, because the content model is open and clients using the an older version will simply ignore the added data.

 

Model concepts

 BLIS-XML for IFC 2x Classification
 Date  30.11.2000
 Type  Model concept
 EXPRESS  Schema  IFC 2x
 Name  IFC2X_FINAL
 Author  IAI (International Alliance for Interoperability)
BLIS-XML  Schema  BLIS-XML for the classification concept defined in IFC 2x
 Name  BLIS-XML for IFC 2x Classification
 Author  BLIS -project
 Schema file  - BLIS_XML_for_IFC_2x_Classification.xml (6 KB)
 - BLIS_XML_for_IFC_2x_Classification.zip (1 KB)
 Documentation The documentation for this BLIS-XML schema uses the definitions that were provided for elements and attributes in the original IFC 2x documentation by the IAI (International Alliance for Interoperability)
 Description This BLIS-XML schema contains the concept of classification as defined in the IFC 2x model. The classification concept defines a generic way for attaching classification information to objects.
This concept is best examined by starting at the IfcClassificationReference element.

 

BLIS-XML data files

 simple2_001204.xml
 Date  30.11.2000
 Schema  BLIS-XML for IFC R2
 File  simple2_001204.zip (12 KB)
 Description Simple one storey building with walls, columns, spaces, doors, windows, openings. The file contains the IFC containment structure, groups and references between doors/windows and spaces. The file also contains the construction types / project types for building elements
  - Image in PDF format

Copyright © 2000 VTT Building Technology
Author: Jiri Hietanen (jiri.hietanen@qpartner.com)
All rights reserved.