Navigation :

Creating geometries

Creating a geometry for the linear medium

Octopus uses the CGAL library, which allows to parse standard geometry files, as used e.g. for 3D printing. Currently, we support only the OFF file format.

There are many software packages around to generate such files, but we recommend openSCAD and ctmconv which is part of OpenCTM.

In Debian style systems, these can be installed using


    apt-get install openscad
    apt-get install openctm-tools

OpenSCAD provides its own scripting language for creating geometries, which can be created in a GUI, but also can be rendered on the command line. The documentation of OpenSCAD can be found here.

Example: creating a simple lens with Openscad

Here is an example script for a lens, generated from the intersection of two spheres:


$fs=0.5;
$fa=0.5;
intersection(){
  translate([-22,0,0]) sphere(r=25);
  translate([ 22,0,0]) sphere(r=25);
};

For details on the syntax of this, see the OpenSCAD tutorials. This creates the following lens:

If you copy the above code into a file lens.scad, you can generate the required OFF file by the command:

openscad -o lens.off lens.scad

The `OFF` files, generated by openSCAD are malformed. If you want to further manipulate them with `ctmconv`, the header needs to be fixed:


    OFF 1162 2320 0
    1.81312 4.79828 5.90861
    1.73416 3.80751 6.86969
    1.98506 3.84776 5.90861
    1.72673 7.81373 -0.993913
    1.39732 8.75085 -0.993913
    1.41584 8.75777 0
    ...

The first line needs to be `OFF` only, and the first line of the data block needs to contain the number of vertices, planes and (optional) connections. You can add comments (starting with `#` of empty lines are allowed after the header).


    OFF
    # This is a lens (generated with OpenSCAD)

    1162 2320 0
    1.81312 4.79828 5.90861
    1.73416 3.80751 6.86969
    1.98506 3.84776 5.90861
    1.72673 7.81373 -0.993913
    1.39732 8.75085 -0.993913
    1.41584 8.75777 0
    ...