Methods and apparatus relating to representation of a three-dimensional object are described. In an example, a converter for converting input object data representing a model of a three-dimensional object into a compressed form is provided. The converter comprises a data receiving module to receive data representing object properties of a three-dimensional object to be generated by additive manufacturing apparatus and a data identifying module to identify data representing a continuous region of the object with a common property specification. The converter further comprises a sub-region defining module to determine, within the data representing the continuous region, a set of contiguous sub-regions and an object data module to represent the continuous region as the set of sub-regions.