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 the Tk::Canvas
manpage 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 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.