A computer implemented method comprising: receiving, at a processor (502, 604), a first data model of an object; generating using the processor (502, 604), a second data model, wherein generating the second data model comprises: generating, for each of a 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; and 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; Characterized in that: wherein generating an octree comprises forming a serialised octree representation of the object such that, in the serialised octree representation of the object, the nodes are ordered such that in each octree, (i) a node representing a volume which includes a layer of the object to be generated earlier in an intended order of object generation precedes a node representing a volume which consists of layer(s) of the object to be generated subsequently; (ii) for nodes representing a volume which includes a given layer of the object, nodes at a coarser volumetric resolution precede nodes at a finer volumetric resolution; and (iii) nodes representing a volume which includes a given layer and being of the same resolution are ordered according to a predetermined location encoding pattern. receiving, at a processor (502, 604), a first data model of an object; generating using the processor (502, 604), a second data model, wherein generating the second data model comprises: generating, for each of a 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; and 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; Characterized in that: wherein generating an octree comprises forming a serialised octree representation of the object such that, in the serialised octree representation of the object, the nodes are ordered such that in each octree, (i) a node representing a volume which includes a layer of the object to be generated earlier in an intended order of object generation precedes a node representing a volume which consists of layer(s) of the object to be generated subsequently; (ii) for nodes representing a volume which includes a given layer of the object, nodes at a coarser volumetric resolution precede nodes at a finer volumetric resolution; and (iii) nodes representing a volume which includes a given layer and being of the same resolution are ordered according to a predetermined location encoding pattern. generating, for each of a 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; and 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; Characterized in that: wherein generating an octree comprises forming a serialised octree representation of the object such that, in the serialised octree representation of the object, the nodes are ordered such that in each octree, (i) a node representing a volume which includes a layer of the object to be generated earlier in an intended order of object generation precedes a node representing a volume which consists of layer(s) of the object to be generated subsequently; (ii) for nodes representing a volume which includes a given layer of the object, nodes at a coarser volumetric resolution precede nodes at a finer volumetric resolution; and (iii) nodes representing a volume which includes a given layer and being of the same resolution are ordered according to a predetermined location encoding pattern. (i) a node representing a volume which includes a layer of the object to be generated earlier in an intended order of object generation precedes a node representing a volume which consists of layer(s) of the object to be generated subsequently; (ii) for nodes representing a volume which includes a given layer of the object, nodes at a coarser volumetric resolution precede nodes at a finer volumetric resolution; and (iii) nodes representing a volume which includes a given layer and being of the same resolution are ordered according to a predetermined location encoding pattern.