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GDL - Geometric Description Language
In computer-aided design, Geometric Description Language (GDL) is the programming language of ArchiCAD library parts. GSM is the file format of these CAD objects.
These objects are similar to blocks in AutoCAD, but unlike blocks, these are parametric, and the 2D and 3D features are connected, so in any view one can get the correct visualization (for example a side view on the section, top view on plan, and perspective in the 3D view). GDL scripts define an ArchiCAD library part in its main roles, these are 3D model, 3D model projected to section/elevation or to 2D plan, 2D plan view, user interface display and behaviour and listing quantities.
All versions of the ArchiCAD contain their own default libraries, also objects like furniture, windows, doors, trees, people, cars, construction elements, etc.
There are several commercial websites for selling high-detailed intelligent parametric objects, which are perhaps better and more variable than the default libraries, for example there are a lot of environmental objects like plants, people or garden elements, which are not included in the original software.
Source: https://en.wikipedia.org/wiki/Geometric_Description_Language
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