1. A computer-aided design apparatus for the design of three-dimensional objects to be fabricated, the apparatus comprising: a processor and a computer usable medium having computer readable program instructions embodied therein, the computer readable program instructions executable by the processor to so as to provide: a base generator operable to generate a plurality of bases in a reference coordinate system, each base comprising a local coordinate system; and a feature generator operable to generate at least one respective further feature for the design in the local coordinate system of each of a plurality of respective bases in accordance with a mathematical function having as a variable thereof base identification numbers representative of the plurality of respective bases in which the at least one respective further feature will be generated, such that each further feature has at least one of a position and orientation relative to the local coordinate system of the base in which it is located which depends upon a value of the base identification number thereof, and such that at least one of the position and orientation of the further feature of each of the plurality of bases relative to its local coordinate system changes in accordance with the value of the respective base identification number.