The architectural direction is towards an enclosed form that could be rotationally molded using thermoplastics, as shown above. The form must have a planar equator that splits the geometry into two molds, perpendicular to the equator at all points.
The form begins with a 2D sketch of the equator spline and spine.
Next, the midpoint of the spine is used to generate a sectional spline. The height above and below the equator are functions of equator's area divided by its length. Equators with a higher ratio of area to length will generate taller sections, while equators with a lower ratio will generate shorter sections. The section is tangent to a vertical line when it hits the equator.
The equator spline must be duplicated and divided into two half-equator splines in a second sketch.
A multi-section surface is generated by moving the section spline along the two half-equator splines. This surface generation technique sometimes has difficulties with the very ends of the surface.
The surface is split along the equator. A new sketch is placed on the plane tangent to the surface at the apex of the section spline.
A door geometry is sketched and constrained to the orientation of the spine.
The door is extruded.
The two surfaces are then trimmed. A planar surface is offset from the equator is added to both halves to indicate the form edge.
Wild variations may break the model in different ways. Sometimes the method of generating the multi-section surface cannot handle the equator profile. In other cases, the door extrusion matches the surface in unexpected ways. It was very difficult to manage the behavior of the split equator profiles, as they would frequently break or reverse, resulting in only three points for the section spline.
