Methods, systems, and apparatus, including medium-encoded computer program products, for creating one or more gradients of different materials for a three dimensional (3D) surface model include, in one aspect, a system including: an additive manufacturing machine designed to use different materials in combination with each other when manufacturing objects; and means for creating a discretized gradient for a 3D surface model of an object, to be manufactured using the additive manufacturing machine, by inserting one or more 3D surfaces into the 3D surface model at specified locations, thereby creating a non-manifold version of the 3D surface model having multiple discrete volumetric regions, and assigning a material specification to each of the discrete volumetric regions, each of the material specifications being either a single one of the different materials or a specified combination of the different materials, which are usable by the additive manufacturing machine to manufacture the object.