Re: [geometry] Transition from PolyhedronSet to ??? (RegionBSPTree3D ?)

[Sven Rathgeber]

Hi Matt,

thanks for the help. With your hint I could refactor a lot of the old

code. There are two questions left:

I used the PolyhedronSet to determine the intersections with a line.

First I got a Plane and then I had to find the intersection with the Plane.

My code makes use of both, the Plane and the intersection.

1. Can I use RegionBSPTree3D.linecast  for the intersection part ?

2. Is there a way to find out the intersected Plane ?


Regards,

Sven


On 6/11/20 3:57 AM, Matt Juntunen wrote:

Hi Sven,

No problem! To answer your question in the subject line, yes, RegionBSPTree3D 
is the replacement for the old PolyhedronsSet. They both represent 3D regions 
using BSP trees. As for your code example, you're probably going to want to use 
Planes.indexedConvexPolygons() [1] or Planes.indexedTriangles() [2] to create a 
list of ConvexPolygon3D instances and then use those to create a 
RegionBSPTree3D:

public static PolyhedronsSet createPrism(
     List<Position2D> basePolygon,
     int upperBound,
     int lowerBound) {

     List<Vector3D> vertices = createVertices(polygon, upperBound, lowerBound);
     int[][] facets = createFacets(polygon);

     List<ConvexPolygon3D> polys = Planes.indexedConvexPolygons(vertices, 
facets, PRECISION_CONTEXT);

     return RegionBSPTree3D.from(polys);
}

There is actually a unit test here [3] that does exactly this.

Note that if the region is convex and you have a large number of polygons to 
insert, the resulting tree will be highly unbalanced and will suffer from poor 
performance. To help with this, you can insert partitions into the BSP tree 
that do not affect the represented region but help keep the tree balanced and 
shallow. Here's an example:

public static PolyhedronsSet createPrism(
     List<Position2D> basePolygon,
     int upperBound,
     int lowerBound) {

     List<Vector3D> vertices = createVertices(polygon, upperBound, lowerBound);
     int[][] facets = createFacets(polygon);

     // compute the region bounds and centroid
     Bounds3D bounds = Bounds3D.from(vertices);
     Vector3D centroid = bounds.getCentroid();

     // create an empty region
     RegionBSPTree3D tree = RegionBSPTree3D.empty();

     // insert structural partitions into the tree (RegionCutRule.INHERIT) at 
the centroid; the
     // resulting tree has internal partitions but the represented region is 
still empty
     tree.insert(Planes.fromPointAndNormal(center, Vector3D.Unit.PLUS_X, 
PRECISION_CONTEXT).span(), RegionCutRule.INHERIT);
     tree.insert(Planes.fromPointAndNormal(center, Vector3D.Unit.PLUS_Y, 
PRECISION_CONTEXT).span(), RegionCutRule.INHERIT);
     tree.insert(Planes.fromPointAndNormal(center, Vector3D.Unit.PLUS_Z, 
PRECISION_CONTEXT).span(), RegionCutRule.INHERIT);

     // create and insert the region boundaries
     List<ConvexPolygon3D> polys = Planes.indexedConvexPolygons(vertices, 
facets, PRECISION_CONTEXT);
     tree.insert(polys);

     return tree;
}

I've created a RegionBSPTree.subdivided() factory method in my current working 
branch that abstracts this internal partitioning so this should be less verbose 
in the near future.

Let me know if you have any other questions or run into issues. I'm working now 
on a tutorial focusing on the BSP tree usage (GEOMETRY-95) so if you find 
anything that's confusing or messed up, I can try to address it.


Regards,
Matt

[1] 
https://github.com/apache/commons-geometry/blob/master/commons-geometry-euclidean/src/main/java/org/apache/commons/geometry/euclidean/threed/Planes.java#L295
[2] 
https://github.com/apache/commons-geometry/blob/master/commons-geometry-euclidean/src/main/java/org/apache/commons/geometry/euclidean/threed/Planes.java#L234
[3] 
https://github.com/apache/commons-geometry/blob/master/commons-geometry-euclidean/src/test/java/org/apache/commons/geometry/euclidean/threed/PlanesTest.java#L448


________________________________
From: Sven Rathgeber <sven.rathge...@web.de>
Sent: Wednesday, June 10, 2020 3:28 AM
To: Commons Developers List <dev@commons.apache.org>
Subject: [geometry] Transition from PolyhedronSet to ??? (RegionBSPTree3D ?)

Hi Matt,

first of all:

Thanks a lot for all your work (rewrite) on the library !!!!

Could you give me a hint how to make the transition from PolyhedronSet
to the new data structures ?

This is my current code:

public static PolyhedronsSet createPrism(
      List< Position2D > basePolygon,
      int upperBound,
      int lowerBound )
    {
      List< Vector3D > vertices = createVertices( polygon, upperBound,
lowerBound );
      int[][] facets = createFacets( polygon );

      return new PolyhedronsSet( vertices, Arrays.asList( facets ),
PRECISION_CONTEXT );
    }

Cheers. Sven


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