Certain examples described herein relate to the preprocessing of object data for the manufacture of a three-dimensional object. In one case, the object data represents at least a first and a second object portion, the first object portion being associated with a first production material composition and the second object portion being associated with a second production material composition. The first and the second object portions may be arranged such that they comprise a contiguous interface. In certain described cases, the object data is modified by modulating the contiguous interface. Said modulating the contiguous interface may comprise modulating data defining at least one of the first and the second object parts such that a portion of at least one of the first and second production material compositions extends into object space occupied by a respective one of the second and first object parts. The resultant modified object data may be used to manufacture the three-dimensional object.