On 14 August 2015 at 09:57, Luc Maisonobe <l...@spaceroots.org> wrote: > Le 13/08/2015 23:20, Monty Hall a écrit : >> Not exactly sure how it works. I need a BSP on short order. Given a set >> of polygons, I'd like a BSP generated. Please advise. Any working code on >> how to use it too? > > Hi Monty, > > Yes, BSP trees can be created from polygons in some cases, but I am not > sure what it does is what you want. So here is a description of what we > can do.
This looks like very useful information. It should be added to the Javadoc and/or user docs. > What I am refering to is that a BSP tree, in any supported topologies > and dimensions, can be built from a boundary representation. This means > that for building a BSP tree that represents a set of polyhedrons in 3D > space, the boundary representation is a set of 2D polygons that > represent the facets of the polyhedrons set. For example a 3D cube can > be defined using 6 2D squares that are embedded in the 3D space. > > There is one constructor that may be helpful to you for the > PolyhedronsSet class, in the > org.apache.commons.math3.geometry.euclidean.threed package. The > signature of this constructor is: > > PolyhedronsSet(List<Vector3D> vertices, List<int[]> facets, > double tolerance); > > The vertices list contains all the vertices of the polyhedrons, the > facets list defines the facets, as an indirection in the vertices list. > Each facet is a short integer array and each element in a facet array > is the index of one vertex in the list. So in our cube example, the > vertices list would contain 8 points corresponding to the cube > vertices, the facets list would contain 6 facets (the sides of the > cube) and each facet would contain 4 integers corresponding to the > indices of the 4 vertices defining one side. Of course, each vertex > would be referenced once in three different facets. > > Beware that despite some basic consistency checkings are performed in > the constructor, not everything is checked, so it remains under caller > responsibility to ensure the vertices and facets are consistent and > properly define a polyhedrons set. One particular trick is that when > defining a facet, the vertices *must* be provided as walking the > polygons boundary in *trigonometric* order (i.e. counterclockwise) as > seen from the *external* side of the facet. The reason for this is that > the walking order does define the orientation of the inside and outside > parts, so walking the boundary on the wrong order would reverse the > facet and the polyhedrons would not be the one you intended to define. > Coming back to our cube example, a logical orientation of the facets > would define the polyhedrons as the finite volume within the cube to be > the inside and the infinite space surrounding the cube as the outside, > but reversing all facets would also define a perfectly well behaved > polyhedrons which would have the infinite space surrounding the cube as > its inside and the finite volume within the cube as its outside! > > If you want to look at how it works, there is a test parser for PLY > file formats in the unit tests section of the library and some basic > ply files for a simple geometric shape (the N pentomino) in the test > resources. This parser uses the constructor defined above as the PLY > file format uses vertices and facets to represent 3D shapes. > > Hope this helps, > Luc > >> >> Thanks, >> >> M >> > > > --------------------------------------------------------------------- > To unsubscribe, e-mail: user-unsubscr...@commons.apache.org > For additional commands, e-mail: user-h...@commons.apache.org > --------------------------------------------------------------------- To unsubscribe, e-mail: dev-unsubscr...@commons.apache.org For additional commands, e-mail: dev-h...@commons.apache.org
