In an example, a method includes receiving a first data model of an object to be generated in additive manufacturing, at a processor. Using the processor, a second data model may be determined. Determining the second data model may include generating for each of plurality of contiguous, non-overlapping sub-volumes of a volume containing the object, a sub-volume octree characterising the sub-volume and having a root node. Determining the second data model may further include generating a volume octree characterising the volume containing the object, the volume octree characterising in its lowest nodes the root nodes of the sub-volume octrees.