NAME

Algorithm::QuadTree - A QuadTree Algorithm class in pure Perl.

SYNOPSIS

    use Algorithm::QuadTree;

    # create a quadtree object
    my $qt = Algorithm::QuadTree->new(-xmin  => 0,
                                      -xmax  => 1000,
                                      -ymin  => 0,
                                      -ymax  => 1000,
                                      -depth => 6);

    # add objects randomly
    my $x = my $tag = 1;
    while ($x < 1000) {
      my $y = 1;
      while ($y < 1000) {
        $qt->add($tag++, $x, $y, $x, $y);

        $y += int rand 200;
      }
      $x += int rand 100;
    }

    # find the objects enclosed in a given region
    my $r_list = $qt->getEnclosedObjects(400, 300,
                                         689, 799);

DESCRIPTION

Algorithm::QuadTree implements a quadtree algorithm (QTA) in pure Perl. Essentially, a QTA is used to access a particular area of a map very quickly. This is especially useful in finding objects enclosed in a given region, or in detecting intersection among objects. In fact, I wrote this module to rapidly search through objects in a Tk::Canvas widget, but have since used it in other non-Tk programs successfully. It is a classic memory/speed trade-off.

Lots of information about QTAs can be found on the web. But, very briefly, a quadtree is a hierarchical data model that recursively decomposes a map into smaller regions. Each node in the tree has 4 children nodes, each of which represents one quarter of the area that the parent represents. So, the root node represents the complete map. This map is then split into 4 equal quarters, each of which is represented by one child node. Each of these children is now treated as a parent, and its area is recursively split up into 4 equal areas, and so on up to a desired depth.

Here is a somewhat crude diagram (those diagrams might not appear unless you run pod2text):

                   ------------------------------
                  |AAA|AAB|       |              |
                  |___AA__|  AB   |              |
                  |AAC|AAD|       |              |
                  |___|___A_______|      B       |
                  |       |       |              |
                  |       |       |              |
                  |   AC  |   AD  |              |
                  |       |       |              |
                   -------------ROOT-------------
                  |               |              |
                  |               |              |
                  |               |              |
                  |      C        |      D       |
                  |               |              |
                  |               |              |
                  |               |              |
                   ------------------------------

Which corresponds to the following quadtree:

                    __ROOT_
                   /  / \  \
                  /  /   \  \
            _____A_  B   C   D
           /  / \  \
          /  /   \  \
    _____AA  AB  AC  AD
   /  / \  \
  /  /   \  \
AAA AAB AAC AAD

In the above diagrams I show only the nodes through the first branch of each level. The same structure exists under each node. This quadtree has a depth of 4.

Each object in the map is assigned to the nodes that it intersects. For example, if we have a rectangular object that overlaps regions AAA and AAC, it will be assigned to the nodes ROOT, A, AA, AAA and AAC. Now, suppose we want to find all the objects that intersect a given area. Instead of checking all objects, we check to see which children of the ROOT node intersect the area. For each of those nodes, we recursively check their children nodes, and so on until we reach the leaves of the tree. Finally, we find all the objects that are assigned to those leaf nodes and check them for overlap with the initial area.

CLASS METHODS

The following methods are public:

Algorithm::QuadTree->new(options)

This is the constructor. It expects the following options (all mandatory) and returns an Algorithm::QuadTree object:

-xmin

This is the X-coordinate of the bottom left corner of the area associated with the quadtree.

-ymin

This is the Y-coordinate of the bottom left corner of the area associated with the quadtree.

-xmax

This is the X-coordinate of the top right corner of the area associated with the quadtree.

-ymax

This is the Y-coordinate of the top right corner of the area associated with the quadtree.

-depth

The depth of the quadtree.

$qt->add(ID, x0, y0, x1, y1)

This method is used to add objects to the tree. It has to be called for every object in the map so that it can properly assigned to the correct tree nodes. The first argument is a unique ID for the object. The remaining 4 arguments define the outline of the object. This method will recursively traverse the tree and add the object to the nodes that it overlaps with.

NOTE: The method does NOT check if the ID is unique or not. It is up to you to make sure it is.

$qt->delete(ID)

This method deletes the object specified by the given ID, and unassigns it from the tree nodes it was assigned to before.

$qt->getEnclosedObjects(x0, y0, x1, y1)

This method returns an <anonymous list> of all the objects that are assigned to the nodes that overlap the given area.

$qt->setWindow(x0, y0, scale)

This method is useful when you zoom your display to a certain segment of the map. It sets the window to the given region such that any calls to add or getEnclosedObjects will have its coordinates properly adjusted before running. The first two coordinates specify the lower left coordinates of the new window. The third coordinate specifies the new zoom scale.

NOTE: You are free, of course, to make the coordinate transformation yourself.

$qt->resetWindow()

This method resets the window region to the full map.

BUGS

None that I am aware of. Please let me know if you find any.

INSTALLATION

Either the usual:

        perl Makefile.PL
        make
        make install

or just stick it somewhere in @INC where perl can find it. It is in pure Perl.

AUTHOR

Ala Qumsieh aqumsieh@cpan.org

COPYRIGHTS

This module is distributed under the same terms as Perl itself.