Certain examples described herein relate to the use of three-dimensional threshold matrices in the production of three-dimensional objects. In one case, data values for a three-dimensional matrix for use in halftoning are assigned based on a structural volume coverage representation. In certain described cases, the structural volume coverage representation defines a probabilistic distribution of at least two different structures available for the production of the three-dimensional object. A convex combination of the at least two different structures is determined, based on data values of the structure volume coverage representation. A data value may be assigned to the three-dimensional matrix based on the convex combination determined. The resultant three-dimensional matrix is used in a halftoning operation to control a structure distribution in the three-dimensional object.