The check confirms that the faces close up, that the edges lie on the surfaces they bound, and that nothing intersects itself. Every shape made by the solid namespace should pass; a failure means that an operation went wrong, usually a boolean between faces that nearly but not quite coincide, and the shape should not be written or drawn. The empty shape is valid.

A test of a part can assert it after the booleans that made it.

 S = subtract (solid.box (20, 20, 20), ...
               translate (solid.sphere (8), [10, 10, 20]));
 isvalid (S)
 ⇒ 1

Source Code: solid.Shape