Certain examples described herein relate to producing three-dimensional objects using additive manufacturing systems. These examples combine the use of material volume coverage vectors to define object data with octree data structures and models for tracking material placement error and available volumes. Material volume coverage vectors correspond to a volumes of three-dimensional objects and define a probabilistic distribution of materials available to an additive manufacturing system including combinations of said materials. In certain examples, a bottom level of the error tracking octree model is constructed to contain at least a portion of the data values for a set of obtained material volume coverage vectors. This is then used in an error distribution process to generate manufacturing control data for the additive manufacturing system.