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 a respective further feature for the design in the local coordinate system of each base in accordance with at least one mathematical function having base identification numbers representative of the respective bases as a variable of the function, 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 the value of the base identification number thereof and such that at least one of the positions and orientations of the further features relative to their local coordinate systems changes in accordance with the values of the base identification numbers.