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RadianceML and CHML

Extendable Geometric Models in XML

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RadianceML stands for Radiance Markup Language. It is an XML-application and an attempt to wrap the scene description language as used by the lighting simulation system Radiance in a markup language. So far, xslt-stylesheets for the translation of a subset of the entities defined in the native scene description language are available. The current development can be accessed by the project page of RadianceML at sourceforge.

One of the key problems in CHML development is to provide interfaces between geometric data and its generators as well as an extensible format to store this data and can link to other types of information. In lighting simulation, this describes much of the typical workflow at the beginning, usually referred to as import. The model data, mostly generated from CAD and modelling software, is linked to application specific information, such as material properties and the description of light sources. It is not surprising that after years of development, the native Radiance scene description language is quite evolved in describing this kind of enriched geometry, as well as supported by lots of relieable tools to support this import. Still, it is application-specific, bound to light simulation and more or less a one-way path. This results in a model that can not be extended by other information than those primitives defined in the language (besides the option to add comments, but which can not be bound to objects in the scene). Also the export of the once completed model is almost impossible, besides 2d-representations the only supported export is to mgf, the material and geometry format, from which an export to the rather obsolete vrml is possible as well as to inventor scene descriptions (which are almost identical to vrml-1) thanks to the tool mgf2inv. Mgf2inv is part of the mgf library. One further interesting approach to export from mgf is Renderpark, which reads mgf and can export rendered scnes to more recent vrml worlds.

Being able to write Radiance scenes in XML means that those can be accomplied by other data that can be evalated by applications that exceed the field of light simulation, without loosing any content of the model. As a broad range of import tools for Radiance is already available, we can even use Radiance scenes just as an intermediate format to get 3d models to XML. The result will be a three-dimensional surface model, with the possibility to add all kinds of information. A valuable feature of XML is its ability to combine different namespaces in one document, so the different types of information do not conflict. Still it is rather easy to read and translate these XML models for all kinds of applications, as this can be done by the very powerful transformations which can be programmed in xslt. An export from RadianceML to X3D, the new standard for 3d data on the web and successor of vrml, can be implemented in xslt.

Last not least defining XML formats for application specific models is an important step to develop generic, application independent descriptions of geometric structured information, as such standards are highly dependent on interfaces that allow easy connection to existing applications. In XML, these interfaces are well prepared by the availability of xslt.

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