Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/commons-math
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Gilles
commit
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@ -51,7 +51,7 @@
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</p>
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<p>
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All these regions share common features. Regions can be defined from several non-connected parts.
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As an example, one PolygonsSet instance in Euclidean 2D (i.e. one the plane) can represent a region composed
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As an example, one PolygonsSet instance in Euclidean 2D (i.e. the plane) can represent a region composed
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of several separated polygons separate from each other. They also support regions containing holes. As an example
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a SphericalPolygonsSet on the 2-Sphere can represent a land mass on the Earth with an interior sea, where points
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on this sea would not be considered to belong to the land mass. Of course more complex models can also be represented
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@ -59,11 +59,12 @@
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separate islands, some of which having lakes, which may have smaller island ... In the infinite Euclidean spaces,
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regions can have infinite parts. for example in 1D (i.e. along a line), one can define an interval starting at
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abscissa 3 and extending up to infinity. This is also possible in 2D and 3D. For all spaces, regions without any
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boundaries are also possible so one can define the whole space or the empty region. The classical set operations
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are available in all cases: union, intersection, symmetric difference (exclusive or), difference, complement.
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There are also region predicates (point inside/outside/on boundary, emptiness, other region contained). For some
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regions, they can be constructed directly from a boundary representation (for example vertices in the case of 2D
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polygons, both on the Euclidean space or on the 2-Sphere). Some geometric properties like size, or boundary size
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boundaries are also possible so one can define the whole space or the empty region.
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</p>
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<p>
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The classical set operations are available in all cases: union, intersection, symmetric difference (exclusive or),
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difference, complement. There are also region predicates (point inside/outside/on boundary, emptiness,
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other region contained). Some geometric properties like size, or boundary size
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can be computed, as well as barycenters on the Euclidean space. Another important feature available for all these
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regions is the projection of a point to the closest region boundary (if there is a boundary). The projection provides
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both the projected point and the signed distance between the point and its projection, with the convention distance
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@ -80,7 +81,7 @@
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Vector2D</a> and <a href="../apidocs/org/apache/commons/math4/geometry/euclidean/threed/Vector3D.html">
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Vector3D</a> provide simple vector types. One important feature is
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that instances of these classes are guaranteed
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to be immutable, this greatly simplifies modelling dynamical systems
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to be immutable, this greatly simplifies modeling dynamical systems
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with changing states: once a vector has been computed, a reference to it
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is known to preserve its state as long as the reference itself is preserved.
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</p>
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@ -119,7 +120,7 @@
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can build a rotation from any of these representations, and any of
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these representations can be retrieved from a <code>Rotation</code>
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instance (see the various constructors and getters). In addition, a
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rotation can also be built implicitely from a set of vectors and their
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rotation can also be built implicitly from a set of vectors and their
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image.
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</p>
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<p>
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@ -184,7 +185,7 @@
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on the 1-sphere (i.e. the one dimensional circle corresponding to the boundary of
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a two-dimensional disc) and the 2-sphere (i.e. the two dimensional sphere surface
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corresponding to the boundary of a three-dimensional ball). The main classes in
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this package corresopnd to the region explained above, i.e.
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this package correspond to the region explained above, i.e.
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<a href="../apidocs/org/apache/commons/math4/geometry/spherical/oned/ArcsSet.html">ArcsSet</a>
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and <a href="../apidocs/org/apache/commons/math4/geometry/spherical/twod/SphericalPolygonsSet.html">SphericalPolygonsSet</a>.
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</p>
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@ -232,6 +233,89 @@
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internal nodes may become leaf nodes and some leaf nodes may become
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internal nodes.
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</p>
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</subsection>
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<subsection name="11.5 Regions">
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<p>
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The regions in all Euclidean and spherical spaces are based on BSP-tree using a <code>Boolean</code>
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attribute in the leaf cells representing the inside status of the corresponding cell
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(true for inside cells, false for outside cells). They all need a <code>tolerance</code> setting that
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is either provided at construction when the region is built from scratch or inherited from the input
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regions when a region is build by set operations applied to other regions. This setting is used when
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the region itself will be used later in another set operation or when points are tested against the
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region to compute inside/outside/on boundary status. This tolerance is the <em>thickness</em>
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of the hyperplane. Points closer than this value to a boundary hyperplane will be considered
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<em>on boundary</em>. There are no default values anymore for this setting (there was one when
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BSP-tree support was introduced, but it created more problems than it solved, so it has been intentionally
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removed). Setting this tolerance really depends on the expected values for the various coordinates. If for
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example the region is used to model a geological structure with a scale of a few thousand meters, the expected
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coordinates order of magnitude will be about 10<sup>3</sup> and the tolerance could be set to 10<sup>-7</sup>
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(i.e. 0.1 micrometer) or above. If very thin triangles or nearly parallel lines occur, it may be safer to use
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a larger value like 10<sup>-3</sup> for example. Of course if the BSP-tree is used to model a crystal at
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atomic level with coordinates of the order of magnitude about 10<sup>-9</sup> the tolerance should be
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drastically reduced (perhaps down to 10<sup>-20</sup> or so).
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</p>
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<p>
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The recommended way to build regions is to start from basic shapes built from their boundary representation
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and to use the set operations to combine these basic shapes into more complex shapes. The basic shapes
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that can be constructed from boundary representations must have a closed boundary and be in one part
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without holes. Regions in several non-connected parts or with holes must be built by building the parts
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beforehand and combining them. All regions (<code>IntervalsSet</code>, <code>PolygonsSet</code>,
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<code>PolyhedronsSet</code>, <code>ArcsSet</code>, <code>SphericalPolygonsSet</code>) provide a dedicated
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constructor using only the mandatory tolerance distance without any other parameters that always create
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the region covering the full space. The empty region case, can be built by building first the full space
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region and applying the <code>RegionFactory.getComplement()</code> method to it to get the corresponding
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empty region, it can also be built directly for a one-cell BSP-tree as explained below.
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</p>
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<p>
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Another way to build regions is to create directly the underlying BSP-tree. All regions (<code>IntervalsSet</code>,
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<code>PolygonsSet</code>, <code>PolyhedronsSet</code>, <code>ArcsSet</code>, <code>SphericalPolygonsSet</code>)
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provide a dedicated constructor that accepts a BSP-tree and a tolerance. This way to build regions should be
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reserved to dimple cases like the full space, the empty space of regions with only one or two cut hyperplances.
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It is not recommended in the general case and is considered expert use. The leaf nodes of the BSP-tree
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<em>must</em> have a <code>Boolean</code> attribute representing the inside status of the corresponding cell
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(true for inside cells, false for outside cells). In order to avoid building too many small objects, it is
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recommended to use the predefined constants <code>Boolean.TRUE</code> and <code>Boolean.FALSE</code>. Using
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this method, one way to build directly an empty region without complementing the full region is as follows
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(note the tolerance parameter which must be present even for the empty region):
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</p>
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<source>
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PolygonsSet empty = new PolygonsSet(new BSPTree<Euclidean2D>(false), tolerance);
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</source>
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<p>
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In the Euclidean 3D case, the <a href="../apidocs/org/apache/commons/math4/geometry/euclidean/threed/PolyhedronsSet.html">PolyhedronsSet</a>
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class has another specific constructor to build regions from vertices and facets. The signature of this
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constructor is:
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</p>
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<source>
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PolyhedronsSet(List<Vector3D> vertices, List<int[]> facets, double tolerance);
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</source>
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<p>
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The vertices list contains all the vertices of the polyhedrons, the facets list defines the facets,
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as an indirection in the vertices list. Each facet is a short integer array and each element in a
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facet array is the index of one vertex in the list. So in our cube example, the vertices list would
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contain 8 points corresponding to the cube vertices, the facets list would contain 6 facets (the sides
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of the cube) and each facet would contain 4 integers corresponding to the indices of the 4 vertices
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defining one side. Of course, each vertex would be referenced once in three different facets.
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</p>
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<p>
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Beware that despite some basic consistency checks are performed in the constructor, not everything
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is checked, so it remains under caller responsibility to ensure the vertices and facets are consistent
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and properly define a polyhedrons set. One particular trick is that when defining a facet, the vertices
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<em>must</em> be provided as walking the polygons boundary in <em>trigonometric</em> order (i.e.
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counterclockwise) as seen from the *external* side of the facet. The reason for this is that the walking
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order does define the orientation of the inside and outside parts, so walking the boundary on the wrong
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order would reverse the facet and the polyhedrons would not be the one you intended to define. Coming
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back to our cube example, a logical orientation of the facets would define the polyhedrons as the finite
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volume within the cube to be the inside and the infinite space surrounding the cube as the outside, but
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reversing all facets would also define a perfectly well behaved polyhedrons which would have the infinite
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space surrounding the cube as its inside and the finite volume within the cube as its outside!
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</p>
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<p>
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If one wants to further look at how it works, there is a test parser for PLY file formats in the unit tests
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section of the library and some basic ply files for a simple geometric shape (the N pentomino) in the test
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resources. This parser uses the constructor defined above as the PLY file format uses vertices and facets
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to represent 3D shapes.
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</p>
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</subsection>
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</section>
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</body>
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@ -119,6 +119,7 @@
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<li><a href="geometry.html#a11.2_Euclidean_spaces">11.2 Euclidean spaces</a></li>
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<li><a href="geometry.html#a11.3_n-Sphere">11.3 n-Sphere</a></li>
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<li><a href="geometry.html#a11.4_Binary_Space_Partitioning">11.4 Binary Space Partitioning</a></li>
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<li><a href="geometry.html#a11.5_Regions">11.5 Regions</a></li>
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</ul></li>
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<li><a href="optimization.html">12. Optimization</a>
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<ul>
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