org.apache.commons.math3.geometry.partitioning

## Class BSPTree<S extends Space>

- java.lang.Object
- - org.apache.commons.math3.geometry.partitioning.BSPTree<S>

Type Parameters:
  `S` - Type of the space.

    public class BSPTree<S extends Space>
    extends Object

This class represent a Binary Space Partition tree.

BSP trees are an efficient way to represent space partitions and to associate attributes with each cell. Each node in a BSP tree represents a convex region which is partitioned in two convex sub-regions at each side of a cut hyperplane. The root tree contains the complete space.

The main use of such partitions is to use a boolean attribute to define an inside/outside property, hence representing arbitrary polytopes (line segments in 1D, polygons in 2D and polyhedrons in 3D) and to operate on them.

Another example would be to represent Voronoi tesselations, the attribute of each cell holding the defining point of the cell.

The application-defined attributes are shared among copied instances and propagated to split parts. These attributes are not used by the BSP-tree algorithms themselves, so the application can use them for any purpose. Since the tree visiting method holds internal and leaf nodes differently, it is possible to use different classes for internal nodes attributes and leaf nodes attributes. This should be used with care, though, because if the tree is modified in any way after attributes have been set, some internal nodes may become leaf nodes and some leaf nodes may become internal nodes.

One of the main sources for the development of this package was Bruce Naylor, John Amanatides and William Thibault paper Merging BSP Trees Yields Polyhedral Set Operations Proc. Siggraph '90, Computer Graphics 24(4), August 1990, pp 115-124, published by the Association for Computing Machinery (ACM).

Since:
  3.0
Version:
  $Id: BSPTree.java 1416643 2012-12-03 19:37:14Z tn $

### Nested Class Summary

Nested Classes
`static interface` `BSPTree.LeafMerger<S extends Space>` This interface gather the merging operations between a BSP tree leaf and another BSP tree.

### Field Summary

Fields
`private Object` `attribute` Application-defined attribute.
`private SubHyperplane<S>` `cut` Cut sub-hyperplane.
`private BSPTree<S>` `minus` Tree at the minus side of the cut hyperplane.
`private BSPTree<S>` `parent` Parent tree.
`private BSPTree<S>` `plus` Tree at the plus side of the cut hyperplane.

### Constructor Summary

Constructors
Constructor and Description
`BSPTree()` Build a tree having only one root cell representing the whole space.
`BSPTree(Object attribute)` Build a tree having only one root cell representing the whole space.
`BSPTree(SubHyperplane<S> cut, BSPTree<S> plus, BSPTree<S> minus, Object attribute)` Build a BSPTree from its underlying elements.

### Method Summary

Methods
`private void` `chopOffMinus(Hyperplane<S> hyperplane)` Chop off parts of the tree.
`private void` `chopOffPlus(Hyperplane<S> hyperplane)` Chop off parts of the tree.
`private void` `condense()` Perform condensation on a tree.
`BSPTree<S>` `copySelf()` Copy the instance.
`private SubHyperplane<S>` `fitToCell(SubHyperplane<S> sub)` Fit a sub-hyperplane inside the cell defined by the instance.
`Object` `getAttribute()` Get the attribute associated with the instance.
`BSPTree<S>` `getCell(Vector<S> point)` Get the cell to which a point belongs.
`SubHyperplane<S>` `getCut()` Get the cut sub-hyperplane.
`BSPTree<S>` `getMinus()` Get the tree on the minus side of the cut hyperplane.
`BSPTree<S>` `getParent()` Get the parent node.
`BSPTree<S>` `getPlus()` Get the tree on the plus side of the cut hyperplane.
`boolean` `insertCut(Hyperplane<S> hyperplane)` Insert a cut sub-hyperplane in a node.
`void` `insertInTree(BSPTree<S> parentTree, boolean isPlusChild)` Insert the instance into another tree.
`BSPTree<S>` `merge(BSPTree<S> tree, BSPTree.LeafMerger<S> leafMerger)` Merge a BSP tree with the instance.
`private BSPTree<S>` `merge(BSPTree<S> tree, BSPTree.LeafMerger<S> leafMerger, BSPTree<S> parentTree, boolean isPlusChild)` Merge a BSP tree with the instance.
`void` `setAttribute(Object attribute)` Associate an attribute with the instance.
`BSPTree<S>` `split(SubHyperplane<S> sub)` Split a BSP tree by an external sub-hyperplane.
`void` `visit(BSPTreeVisitor<S> visitor)` Visit the BSP tree nodes.

### Methods inherited from class java.lang.Object

`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

### Field Detail

#### cut

    private SubHyperplane<S extends Space> cut

Cut sub-hyperplane.

#### plus

    private BSPTree<S extends Space> plus

Tree at the plus side of the cut hyperplane.

#### minus

    private BSPTree<S extends Space> minus

Tree at the minus side of the cut hyperplane.

#### parent

    private BSPTree<S extends Space> parent

Parent tree.

#### attribute

    private Object attribute

Application-defined attribute.

### Constructor Detail

#### BSPTree

    public BSPTree()

Build a tree having only one root cell representing the whole space.

#### BSPTree

    public BSPTree(Object attribute)

Build a tree having only one root cell representing the whole space.

Parameters:
  `attribute` - attribute of the tree (may be null)

#### BSPTree

    public BSPTree(SubHyperplane<S> cut,
           BSPTree<S> plus,
           BSPTree<S> minus,
           Object attribute)

Build a BSPTree from its underlying elements.

This method does not perform any verification on consistency of its arguments, it should therefore be used only when then caller knows what it is doing.

This method is mainly useful kto build trees bottom-up. Building trees top-down is realized with the help of method `insertCut`.

Parameters:
  `cut` - cut sub-hyperplane for the tree
  `plus` - plus side sub-tree
  `minus` - minus side sub-tree
  `attribute` - attribute associated with the node (may be null)
See Also:
  `insertCut(org.apache.commons.math3.geometry.partitioning.Hyperplane)`

### Method Detail

#### insertCut

    public boolean insertCut(Hyperplane<S> hyperplane)

Insert a cut sub-hyperplane in a node.

The sub-tree starting at this node will be completely overwritten. The new cut sub-hyperplane will be built from the intersection of the provided hyperplane with the cell. If the hyperplane does intersect the cell, the cell will have two children cells with `null` attributes on each side of the inserted cut sub-hyperplane. If the hyperplane does not intersect the cell then no cut hyperplane will be inserted and the cell will be changed to a leaf cell. The attribute of the node is never changed.

This method is mainly useful when called on leaf nodes (i.e. nodes for which `getCut` returns `null`), in this case it provides a way to build a tree top-down (whereas the `4 arguments constructor` is devoted to build trees bottom-up).

Parameters:
  `hyperplane` - hyperplane to insert, it will be chopped in order to fit in the cell defined by the parent nodes of the instance
Returns:
  true if a cut sub-hyperplane has been inserted (i.e. if the cell now has two leaf child nodes)
See Also:
  `BSPTree(SubHyperplane, BSPTree, BSPTree, Object)`

#### copySelf

    public BSPTree<S> copySelf()

Copy the instance.

The instance created is completely independant of the original one. A deep copy is used, none of the underlying objects are shared (except for the nodes attributes and immutable objects).

Returns:
  a new tree, copy of the instance

#### getCut

    public SubHyperplane<S> getCut()

Get the cut sub-hyperplane.

Returns:
  cut sub-hyperplane, null if this is a leaf tree

#### getPlus

    public BSPTree<S> getPlus()

Get the tree on the plus side of the cut hyperplane.

Returns:
  tree on the plus side of the cut hyperplane, null if this is a leaf tree

#### getMinus

    public BSPTree<S> getMinus()

Get the tree on the minus side of the cut hyperplane.

Returns:
  tree on the minus side of the cut hyperplane, null if this is a leaf tree

#### getParent

    public BSPTree<S> getParent()

Get the parent node.

Returns:
  parent node, null if the node has no parents

#### setAttribute

    public void setAttribute(Object attribute)

Associate an attribute with the instance.

Parameters:
  `attribute` - attribute to associate with the node
See Also:
  `getAttribute()`

#### getAttribute

    public Object getAttribute()

Get the attribute associated with the instance.

Returns:
  attribute associated with the node or null if no attribute has been explicitly set using the `setAttribute` method
See Also:
  `setAttribute(java.lang.Object)`

#### visit

    public void visit(BSPTreeVisitor<S> visitor)

Visit the BSP tree nodes.

Parameters:
  `visitor` - object visiting the tree nodes

#### fitToCell

    private SubHyperplane<S> fitToCell(SubHyperplane<S> sub)

Fit a sub-hyperplane inside the cell defined by the instance.

Fitting is done by chopping off the parts of the sub-hyperplane that lie outside of the cell using the cut-hyperplanes of the parent nodes of the instance.

Parameters:
  `sub` - sub-hyperplane to fit
Returns:
  a new sub-hyperplane, guaranteed to have no part outside of the instance cell

#### getCell

    public BSPTree<S> getCell(Vector<S> point)

Get the cell to which a point belongs.

If the returned cell is a leaf node the points belongs to the interior of the node, if the cell is an internal node the points belongs to the node cut sub-hyperplane.

Parameters:
  `point` - point to check
Returns:
  the tree cell to which the point belongs (can be

#### condense

    private void condense()

Perform condensation on a tree.

The condensation operation is not recursive, it must be called explicitely from leaves to root.

#### merge

    public BSPTree<S> merge(BSPTree<S> tree,
                   BSPTree.LeafMerger<S> leafMerger)

Merge a BSP tree with the instance.

All trees are modified (parts of them are reused in the new tree), it is the responsibility of the caller to ensure a copy has been done before if any of the former tree should be preserved, no such copy is done here!

The algorithm used here is directly derived from the one described in the Naylor, Amanatides and Thibault paper (section III, Binary Partitioning of a BSP Tree).

Parameters:
  `tree` - other tree to merge with the instance (will be unusable after the operation, as well as the instance itself)
  `leafMerger` - object implementing the final merging phase (this is where the semantic of the operation occurs, generally depending on the attribute of the leaf node)
Returns:
  a new tree, result of `instance <op> tree`, this value can be ignored if parentTree is not null since all connections have already been established

#### merge

    private BSPTree<S> merge(BSPTree<S> tree,
                   BSPTree.LeafMerger<S> leafMerger,
                   BSPTree<S> parentTree,
                   boolean isPlusChild)

Merge a BSP tree with the instance.

Parameters:
  `tree` - other tree to merge with the instance (will be unusable after the operation, as well as the instance itself)
  `leafMerger` - object implementing the final merging phase (this is where the semantic of the operation occurs, generally depending on the attribute of the leaf node)
  `parentTree` - parent tree to connect to (may be null)
  `isPlusChild` - if true and if parentTree is not null, the resulting tree should be the plus child of its parent, ignored if parentTree is null
Returns:
  a new tree, result of `instance <op> tree`, this value can be ignored if parentTree is not null since all connections have already been established

#### split

    public BSPTree<S> split(SubHyperplane<S> sub)

Split a BSP tree by an external sub-hyperplane.

Split a tree in two halves, on each side of the sub-hyperplane. The instance is not modified.

The tree returned is not upward-consistent: despite all of its sub-trees cut sub-hyperplanes (including its own cut sub-hyperplane) are bounded to the current cell, it is not attached to any parent tree yet. This tree is intended to be later inserted into an higher level tree.

The algorithm used here is the one given in Naylor, Amanatides and Thibault paper (section III, Binary Partitioning of a BSP Tree).

Parameters:
  `sub` - partitioning sub-hyperplane, must be already clipped to the convex region represented by the instance, will be used as the cut sub-hyperplane of the returned tree
Returns:
  a tree having the specified sub-hyperplane as its cut sub-hyperplane, the two parts of the split instance as its two sub-trees and a null parent

#### insertInTree

    public void insertInTree(BSPTree<S> parentTree,
                    boolean isPlusChild)

Insert the instance into another tree.

The instance itself is modified so its former parent should not be used anymore.

Parameters:
  `parentTree` - parent tree to connect to (may be null)
  `isPlusChild` - if true and if parentTree is not null, the resulting tree should be the plus child of its parent, ignored if parentTree is null
See Also:
  `BSPTree.LeafMerger`

#### chopOffMinus

    private void chopOffMinus(Hyperplane<S> hyperplane)

Chop off parts of the tree.

The instance is modified in place, all the parts that are on the minus side of the chopping hyperplane are discarded, only the parts on the plus side remain.

Parameters:
  `hyperplane` - chopping hyperplane

#### chopOffPlus

    private void chopOffPlus(Hyperplane<S> hyperplane)

Chop off parts of the tree.

The instance is modified in place, all the parts that are on the plus side of the chopping hyperplane are discarded, only the parts on the minus side remain.

Parameters:
  `hyperplane` - chopping hyperplane