A well-formed shape is a complete, holistic shape. For example, a rubber-band stretched in the shape of a rectangle is a well-formed shape. However, a set of four lines meeting at four corners are not considered as a well-formed shape. For example, a set of four matchsticks put together in the shape of a rectangle does not create a well-formed shape, even if the corners are glued together.
In CAD software, shapes are built out of elements like points and lines. Such shapes are ill-formed shapes. Similarly, when a shape is extruded to form a volume, it can either be a well-formed object or not. Multiple ill-formed shapes come together to form the faces of the volume. In solid modeling software, if an object can further be subdivided into other geometrical shapes and elements, the objects are considered as ill-formed objects. However, the objects in the solid modeling software are holistic in nature and cannot be subdivided further geometrically. Therefore, these shapes are considered as well-formed objects.
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