Shap3d03.dat, 3rd 'shape' file for CPO3D
An elliptical hole in an elliptical disc
A disc is set up in the xy plane, with an outer radius of 1 and a central hole of radius 0.25. The 'stretching' option is used to increase all x coordinates by a factor of 2, producing an elliptical disc with an elliptical hole.
Note that the x=y plane of reflection symmetry is not used because the stretching is neither perpendicular nor parallel to this plane.
The equation for the inner and outer bounding circles of the disc is
(x/a)**2 + (y/b)**2 = 1,
where a = b is the radius of the circle, 0.25 for the inner circle and 1 for the outer. After stretching the x coordinates by a factor of 2 the values of a are effectively increased by this factor, producing ellipses.