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BIM Tools >> Parametric Features in ArchiCAD - with GDL
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Parametric Features in ArchiCAD - with GDL
The following is a temporary documentation will be revised soon. (Citation from Graphisoft Manual)
This technical report reviews parametric modeling capability of ArchiCAD. Followings are used as a criteria of parametric capability.
1) Relations within a object:
A. support for defining geometric relations between points, edges, surfaces
B. associating parameters with parameters of surfaces or lines
C. can parameters be non-shape properties? Material strength, surface area, etc.
2) Parameters in one object that refer to another object (like the wall ends ending at another specified wall)?
A. Object association.
i. For example, in revit, when the height of wall is changed, the position of a top slop associated that wall are also changed. Thus, it can be one criterion for parametric capability.
B. Object hierarchy.
i. For example, When a wall is deleted, windows belongs to the wall also should be deleted. It is also important criteria in parametric capability.
3) Constraints
A. Type of constraints.
i. Some cad system supports pre-defined constraints only. For instance, parallel, perpendicular, attach and so forth. However, some cad application such as digital project supports some script-language, which include mathematical formula; sin, cos.., equality and inequality sign, in the interface level, not API level.
4) Can a BIM tool support definition of new parametric objects?
a. using a Sketch tool, VBA or scripting language, or programming language?
ArchiCAD has its own geometry modeling language named as GDL. GDL is important to review the capability of parametric modeling because embedded architectural objects and user-defined objects are defined by using the GDL in the archiCAD. Thus, By investigating the GDL in more detail, we can have clear understanding on the parametric modeling capability of archiCAD.
In this report, first, embedded architectural objects such as wall and window are tested based on the criteria described above. And next, the GDL is reviewed to see the capability of the parametric modeling of ArchiCAD in more detail.
1. Relations within an object
A. support for defining geometric relations between points, edges, surfaces
i. One geometry shape can be generated through sequential order, which define initial points first and other geometry is produced based on these points. These points are named as hotspots. Hotspots is displayed on the 2D symbol, thus the hotspots can be used to modify initial points in order to change the final shape. Followings are the figure of 2D symbol with hotspots and the GDL script, which show how line and other geometries are defined by the initial points.
B. associating parameters with parameters of surfaces or lines
i. ArchiCAD doesn't support the parameters of a surface such as u,v parameter, control point, normal, tangent plane and principal curvature. These surface parameter might be calculated by using initial points which is used to define the surface, but it is impossible because ArchiCAD doesn't support mathematical function to calculate it.
C. Can parameters be non-shape properties? Material strength, surface area, etc
i. Any type of property can be a parameter, even non-shape properties can be defined as a parameter in ArchiCAD. However, this kind of parameters can not be used to control the shape. We can see the non-shape properties such as Group type and Object weight unit in the following figure.
2. Parameters in one object that refer to another object (like the wall ends ending at another specified wall)?
A. Object association.
i. In order to test the object association, the behavior of two wall objects and of column and beam are tested. As we can see the following figure, wall, column and beam does not have parametric association with other objects. It just moved separately.
B. Object hierarchy.
i. Hierarchical relation, for instance, the relation between wall and window or door, is supplied in ArchiCAD. Thus, when wall is deleted, the other objects belonged to the wall also are deleted. However, it has some limitation when one objects is belongs to two objects. For example the corner window, which belonged to two walls, is divided to two windows when the walls are separated instead of removed from the wall. Thus, when the walls are connected again. it still remains as two separated windows.
3. Constraints
A. Type of constraints.
Geometric constraints such as perpendicular, orthogonal, parallel relation between two geometry elements are not supported in the ArchiCAD.
However, user can define the geometric constraints by using GDL.
4. Can a BIM tool support definition of new parametric objects?
A. using a Sketch tool, VBA or scripting language, or programming language?
ArchiCAD has "geometry modeling language" named GDL. Following is the feature of GDL in the GDL manual.
- Because GDL has many 2D and 3D commands, you can make interesting objects that cannot be made with the Tools palette - they can be Complex and Curvy.
- Because you can define diameters, spacing, thickness, materials and pens, your objects can allow variations -- they can be Parametric.
- Because you specify elements in a GDL model by precise dimensions, angles or parameters, the objects will be exactly what you want them to be -- they can be Accurate.
- Because you can write IF statements, you can build rules of behavior into your objects, such as manufacturers' requirements, checks on incorrect parameters, self-sizing components etc. - they can be Smart.
- Because you can write routines that loop around and repeat actions many times, you can economically build large or repetitive structures that would be futile to build with the Tools palette - your objects can be Tools.
- Because you can rotate, slide and resize components in the model, your objects can change their shape - you can make Mechanisms
5. GDL
A. Sequential description of shape like a scene graph in OpenGL
i. GDL support point, line and primitive shape such as block, cylinder, sphere and cone.
ii. It supports transformation of shape. Translation and rotation can be used for transformation.
iii. It supports transformation stack like a OpenGL. Instead of push and pop matrix of OpenGL, it has DEL function.
B. Parametric representation of shape.
i. User can define parameter and a shape can be generated based on the parameter.
C. Hotsports as control points of shape
i. Hotspots can be defined as a graphical input for initial point.
ii. It shows up in the 2D symbol when insert the object to a project.
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