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Integration of parametric functionality into a modeling engine: a universal approach by LEDAS
The vast majority of modern CAD and 3D modeling systems from desktop to high-end are built around a so-called modeling engine. The modeling engine provides the set of notions, objects, interfaces and data structures that permit all modules of CAD systems working on common ground of a common internal model. Furthermore, this model can be saved as a file and then be transferred to users of other CAM/CAE and CAD systems built around the same engine.
The model contains geometric and topological properties of a design: shape, sizes, relative location of parts, part-whole and other relations. The modeling engine handles the internal representation of the model, its creation, modification (including complex geometric transformations) and supports visualization of different kinds (projections, 3D rendering).
A modeling engine is a complicated piece of software that requires significant efforts for its development and testing. In the limited timeframes of the current software development process most CAD developers license third-party engines instead of creating their own. The best known modeling engines are: ACIS (from Spatial), Parasolid (from UGS) and Granite (from PTC). There are even open-source modeling cores like OpenCASCADE (currently supported and developed by Principia Research and Development).
A licensed engine reduces the software development timeframe and allows developers to concentrate on the functionality provided to customers rather than on the coding of complex algorithms like geometric transformations or 3D-rendering. Providers of modeling components have wide experience in algorithms and optimization and also support the latest hardware for the best possible performance. This provides CAD developers with the easiest way to follow the latest trends in algorithms and hardware and use them effectively.
However, some CAD systems are built around proprietary engines like CATIA from Dassault Systemes. Such systems have a long history and enormous efforts put into engine development. In return developers get precise control over the entire set of core features, low-level access to the model and thus can provide CAD-users with more precise control over the model. And of course since own engine is much more easily supported and extended, CAD developers don't need to wait for the next releases to get fixes for flaws that they find and implement requested functionality.
The constant increase of computing power makes many features seen recently only in high-end CADs now available even in desktop-level software. A good example is parametric (variational) design. Invented for large CADs like CATIA it is now used in mid-range systems like SolidWorks and SolidEdge and even in some dedicated versions of the desktop system AutoCAD. The idea behind parametric design is that the user may (implicitly or explicitly) link elements of the design to each other with constraints so that when some part of the design is altered those links are always taken into account (constraints satisfied). For example when some sizes or distances are modified then other sizes, distances and object positions may also change with respect to the earlier imposed constraints (or a diagnostic message will be shown stating that the modification leads to a contradiction, i.e. some constraint violation).
Most modeling engines don't support parametric functionality at all or the support is limited. As a rule, for full-feature parametric functionality another engine is required, a so-called parametric engine. To build such engine even more sophisticated knowledge is required. For this reason, there are not many marketed engines available to choose from. Two major providers are; the British company, D-Cubed (acquired by UGS in 2004), the developer of DCM (Design Constraint Manager) and secondly, the independent Russian company, LEDAS, with its LGS (LEDAS Geometric Solver): these engines are functionally very close. One may read about various aspects of functionality of parametric engines in other LEDAS publications, further on we will concentrate on integration of engines - modeling and parametric.
Parametric engines are model-independent and thus may be used with any modeling engine, but of course nowadays DCM is more closely related to Parasolid since both were developed and are distributed by UGS. In contrast, LEDAS is proud to be an independent vendor and to improve this position the company is releasing universal technology to simplify integration of LGS with design-related applications based on any modeling cores, licensed or proprietary.
Integration of parametric functionality into a system involves implementation of the following five interfaces between the parametric engine and modeling core (or the application itself):
- Call to parametric functionality from the application to enable an update of a model upon changes with regards to constraints.
- Data transfer from modeling to a parametric engine. This includes transfer of topological and geometric objects, relations and constraints on them and their parameters.
- Topological links between objects in a model to ensure model integrity.
- Update of parameters of objects within a modeling engine as a result of resolving constraints within a parametric engine. An update is needed only for object parameters (coordinates, distances) and the statuses of objects and constraints.
- Visualization of constraints. Dimensions and angles have natural representation, while parallelism or concentricity will be used for parametric functionality.
Integration of LGS with OpenCASCADE and ACIS engines showed that about 80% of the code does not depend on the engine used. It is needed to represent complex objects used in a modeling engine by basic geometric objects used in a parametric one. For example, a segment from a geometric model is presented inside a constraint solver as a line, two points and two incidence constraints.
The LEDAS Company developed an Integration Library based on this experience and built an entire integration technology around the library. No matter what engine is used in CAD or the 3D modeling system, no matter what version of LGS is used, the technology provides the easiest way to implement parametric functionality in the system, integrating LGS with the modeling engine and application.
The Integration Library contains the code common to all integrations and does not depend on the geometric engine, be it ACIS, Parasolid, Granite or a proprietary one. The library includes modeling of complex objects, means for the management of objects from models and to transfer results back to the application. The technology around the library significantly simplifies the development of engine-related code by arranging it in a comprehensive set of interfaces and specifying what is actually needed for the integration. The LEDAS technology supports interfaces 1, 2, 3 and 4 from the five listed above: only visualization of constraints is not covered due to its high dependence on application.
Different modeling engines have totally different structures of objects inside. For example, in OpenCASCADE geometric properties are just elements of a recorded history of the corresponding topological objects. Thus if we want to modify the geometry (as a result of constraint-related computations) we need to add new data to the history rather than modify any original objects that represent geometric properties. In other engines a geometric object might exist as a high-level entity and we need to alter only its properties to preserve all its references in the application. To handle all varieties of data representation a single library is not sufficient. So another important part of integration technology is a customizable generator of interface library templates. The interface library will contain a code to coordinate data representation between geometric and parametric engines. The integration library uses routines and objects from the interface library to access model engine data. The code of the interface library is written by the application developer, but instead of writing an original form himself, he only needs to fill-in routine templates produced by the generator. The semantics of the routines is fixed and well-documented. The actions of the generator are executed through a comprehensive wizard that helps to set up parameters like usage discipline of objects from the modeling engine, the version of LGS and even the possibility of test application generation.
Documentation is another vital part of the technology. A comprehensive manual simplifies the implementation of the code that depends on the modeling core and application. One more important component of the technology is a fully-functional sample that presents in detail the integration module for OpenCASCADE-based application, which will help developers to understand how to use the technology, what routines should be implemented and how. All software developers know that with a working example at hand; implementation of a similar code is much easier. Also a sample application for testing the integration module can be generated along with an interface library template. This test application not only shows how the integration code will be used within the application but it also allows the developer to load models written in a scripting language and save them in the modeling engine native format. Then the models can be loaded into a CAD application and analyzed. The test application itself does not work with the modeling engine directly, but uses the interface library. Such architecture allows the generation of test applications for virtually any engine.
Universal LGS integration technology has been available to existing and new LEDAS customers since March 2006. Please contact LEDAS Company for further information by e-mail: info@ledas.com. LEDAS experts will not only provide you with access to the integration technology, but can also assist you in selecting a configuration of LGS suited to your requirements and offer advice on the integration of LGS with the prevalent modeling engines ACIS, Parasolid and Granite.
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