An apparatus (100, 150) arranged to preprocess object data for the manufacture of a three-dimensional object, the apparatus comprising: a data interface (110, 160) arranged to receive object data (130, 155) representative of the three-dimensional object for manufacture; and a halftone generator arranged to apply a halftoning operation to the object data to output discrete material formation instructions for the manufacture of the three-dimensional object, wherein the apparatus is characterized by: a structural modifier (120, 170) arranged to operate on object data (130, 155) representing 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 being arranged such that they are separated by a shared contiguous interface, wherein the structural modifier (120, 170) is arranged to modify said object data (130, 155) by modulating the shared contiguous interface between the first and the second object portions, said modulating comprising: generating a halftone transition region for the shared contiguous interface, the halftone transition region comprising material composition placement instructions for manufacture of the three-dimensional object wherein volumes of the first production material composition are interleaved with volumes of the second production material composition; wherein application of the halftoning operation to the halftone transition region by the halftone generator generates discrete material formation instructions that define at least one of the first and the second object portions 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 portions. a data interface (110, 160) arranged to receive object data (130, 155) representative of the three-dimensional object for manufacture; and a halftone generator arranged to apply a halftoning operation to the object data to output discrete material formation instructions for the manufacture of the three-dimensional object, wherein the apparatus is characterized by: a structural modifier (120, 170) arranged to operate on object data (130, 155) representing 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 being arranged such that they are separated by a shared contiguous interface, wherein the structural modifier (120, 170) is arranged to modify said object data (130, 155) by modulating the shared contiguous interface between the first and the second object portions, said modulating comprising: generating a halftone transition region for the shared contiguous interface, the halftone transition region comprising material composition placement instructions for manufacture of the three-dimensional object wherein volumes of the first production material composition are interleaved with volumes of the second production material composition; wherein application of the halftoning operation to the halftone transition region by the halftone generator generates discrete material formation instructions that define at least one of the first and the second object portions 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 portions. a structural modifier (120, 170) arranged to operate on object data (130, 155) representing 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 being arranged such that they are separated by a shared contiguous interface, wherein the structural modifier (120, 170) is arranged to modify said object data (130, 155) by modulating the shared contiguous interface between the first and the second object portions, said modulating comprising: generating a halftone transition region for the shared contiguous interface, the halftone transition region comprising material composition placement instructions for manufacture of the three-dimensional object wherein volumes of the first production material composition are interleaved with volumes of the second production material composition; wherein application of the halftoning operation to the halftone transition region by the halftone generator generates discrete material formation instructions that define at least one of the first and the second object portions 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 portions.