[OpenSCAD] Difference between modelling with Openscad and Freecad

arnholm at arnholm.org arnholm at arnholm.org
Wed Oct 2 07:55:05 EDT 2019

On 2019-10-02 12:42, Robin2 wrote:
> It seems to me there is a more fundamental difference. With Freecad 
> (and
> other similar CAD programs) you can define an object and later "ask
> questions" about that object. For example you can select a point that 
> is the
> corner of a cube and it will tell you the coordinates in 3D space. And 
> you
> can locate the surface of a face of the cube even though it may have 
> been
> rotated or scaled.
> However with Openscad the process of defining a model is 
> mono-directional.
> You define the object (such as a cube) and you can have no further
> interaction with it. You cannot "ask questions".
> Is this a fair assessment, or am I completely muddled?

It is a fair assessment.

OpenSCAD and similar programs are based on the Constructive Solid 
Geometry method ( 
https://en.wikipedia.org/wiki/Constructive_solid_geometry ), which is 
kind of an implicit way of defining a model, which makes it harder to 
refer to details in the model. Another thing is that the OpenSCAD 
language explicitly does not allow any model queries, this is a design 
choice. Another difference is that the OpenSCAD internal model 
representation is mesh based using CGAL, which is fundamentally 
different from the internal model representation in FreeCAD.

In FreeCAD, the internal representation is based on OpenCascade, which 
is a boundary representation model (BREP), and not a mesh model. The 
OpenCascade model is much more complex and more equivalent to 
traditional CAD programs. OpenSCAD (and AngelCAD) are not directly 
comparable because of the mesh representation.

Whether you can "ask questions" or not is related to the internal 
representation and difficulties of referring to parts of the model, but 
it is not the whole story. It is also a question of design philosophy.

Carsten Arnholm

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