## 1 Introduction  
Additive manufacturing (AM) has garnered considerable research interest over the last decade and is considered an upcoming production process that can be used to realise new practical manufacturing. This paper describes geometric processing to derive final sections utilised by an additive machine controller directly from a CAD model. There is a common assumption that the additive manufacturing process chain is well known, but there are a number of variables that are less well understood. The common view of the chain from CAD to part is:  
1. CAD model
2. Approximation as STL (or maybe AMF)
3. Transfer to pre-processing software and correction, if necessary
4. Slicing and offsetting
5. Generating supports, if necessary, and tool paths
6. Slice transfer to additive machine
7. Part manufacture  
This chain has been established based on practical observations made, not theoretical information. Before describing the modern, planned chain proposed by [PERSON] [1], the normally assumed chain origins have been discussed. AM machines and graphics computers, which are forerunners to graphics cards, were introduced at around the same time. To understand why the stereolithography (STL) format has been used for the communication of solid data, the technological development made during this period should be examined. During this period, two main techniques existed for representing solids: constructive solid geometry (CSG) and boundary representation (BRep). Although these two uses very different methods for representation, the CSGmethod generated a BRep model for graphics because direct graphics from CSG models proved to be too slow. So, the boundary of a solid, essentially the \"skin\" between the inside and outside of a solid model was a common factor. However, there was then no agreed format for communicating boundary data to applications. Another solid modeling method, the Octree method, was used for communicating volumetric data but eventually the skin of the object, communicating using triangles, gained dominance as a graphics communication format [2]. The idea of producing a real physical model as a visualisation tool may have encouraged the use of triangles, in the form of an STL file [3], as a means of communication for additive manufacturing. However, the STL format is, and always has been, a format for graphics, not for communicating solid information, which is what is needed for additive processes. For graphics purposes it is unimportant that the triangles are unconnected, but for additive processes it is critical that the triangles match and that the whole set is closed. In graphics the triangles are used to colour parts of the screen, so connectivity is unimportant and visual gaps only an annoyance. Nor is the shape of the triangle critical because the normal vector is given explicitly and does not need to be calculated from the facet vertex positions, so avoiding the problem of degeneracy. However, slicing triangles can lead to numerical problems if they are long and thin. In addition, faceting of surface models which may not be closed and separate faceting of faces in solid models leads to problems of mismatched facets, again not a problem for graphics but a major problem for additive manufacturing. Dissatisfaction with STL has long been apparent. In 1994 [PERSON] [4] suggested using STEP [5] for shape communication. [PERSON] and [PERSON] [6] produced a survey of different formats. However, despite the problems of STL, its use has continued unabated. The new suggestion, AMF (Additive Manufacturing Format) [7] corrects some of the shortcomings, by adding material and colour information, but is still, essentially, a graphics format, unsuited for communicating solid model data. In the meantime, since the original additive machines appeared for 3D printing of solids, the use case of additive manufacturing has changed. The traditional use case was 3D visualisation models and prototype manufacture additive processes.