Mudanças entre as edições de "Dago:Algoritmo Quadtree"

Edição das 14h11min de 10 de maio de 2009

Índice

1 Descricão
2 Modo de usar
3 Descricao
4 Métodos da Classe
5 BUGS
6 INSTALLATION
7 Algoritmo QuadTree
8 AUTHOR
9 COPYRIGHTS

Descricão

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

Modo de usar

   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);

Descricao

Algorithm::QuadTree implements a quadtree algorithm (QTA) in pure Perl. Essentially, a I<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 L<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 I<AAA> and I<AAC>, it will be assigned to the nodes I<ROOT>, I<A>, I<AA>, I<AAA> and I<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 I<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.

Métodos da Classe

The following methods are public:

I<Algorithm::QuadTree>-E<gt>B<new>(I<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.

I<$qt>-E<gt>B<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 I<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 I<NOT> check if the ID is unique or not. It is up to you to make sure it is.

I<$qt>-E<gt>B<delete>(ID)

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

I<$qt>-E<gt>B<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.

I<$qt>-E<gt>B<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 B<add> or B<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.

I<$qt>-E<gt>B<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.

Algoritmo QuadTree

package Algorithm::QuadTree;

use strict;
use Carp;

our $VERSION = 0.1;

1;

###############################
# sub new() - constructor
#
# Arguments are a hash:
#
# -xmin  => minimum x value
# -xmax  => maximum x value
# -ymin  => minimum y value
# -ymax  => maximum y value
# -depth => depth of tree
#
# Creating a new QuadTree objects automatically
# segments the given area into quadtrees of the
# specified depth.
###############################
sub new {
    my $self  = shift;
    my $class = ref($self) || $self;

    my $obj   = bless {} => $class;

    $obj->{BACKREF} = {};
    $obj->{OBJECTS} = [];
    $obj->{ORIGIN}  = [0, 0];
    $obj->{SCALE}   = 1;

    my %args  = @_;

    for my $arg (qw/xmin ymin xmax ymax depth/) {
    unless (exists $args{"-$arg"}) {
        carp "- must specify $arg";
        return undef;
    }

    $obj->{uc $arg} = $args{"-$arg"};
    }

    $obj->_segment;

    return $obj;
}

###############################
# sub _segment() - private method
#
# This method does the actual segmentation
# and stores everything internally.
###############################
sub _segment {
    my $obj = shift;

    $obj->_addLevel(
            $obj->{XMIN},
            $obj->{YMIN},
            $obj->{XMAX},
            $obj->{YMAX},
            1,             # current depth
            0,             # current index
            undef,         # parent index
            );

}

###############################
# sub _addLevel() - private method
#
# This method segments a given area
# and adds a level to the tree.
###############################
sub _addLevel {
    my ($obj,
    $xmin,
    $ymin,
    $xmax,
    $ymax,
    $curDepth,
    $index,
    $parent,
    ) = @_;

    $obj->{AREA}    [$index] = [$xmin, $ymin, $xmax, $ymax];
    $obj->{PARENT}  [$index] = $parent;
    $obj->{CHILDREN}[$index] = [];
    $obj->{OBJECTS} [$index] = [];

    if (defined $parent) {
    push @{$obj->{CHILDREN}[$parent]} => $index;
    }

    return if $curDepth == $obj->{DEPTH};

    my $xmid = $xmin + ($xmax - $xmin) / 2;
    my $ymid = $ymin + ($ymax - $ymin) / 2;

    # now segment in the following order (doesn't matter):
    # top left, top right, bottom left, bottom right
    $obj->_addLevel($xmin, $ymid, $xmid, $ymax,  # tl
            $curDepth + 1, 4 * $index + 1, $index);
    $obj->_addLevel($xmid, $ymid, $xmax, $ymax,  # tr
            $curDepth + 1, 4 * $index + 2, $index);
    $obj->_addLevel($xmin, $ymin, $xmid, $ymid,  # bl
            $curDepth + 1, 4 * $index + 3, $index);
    $obj->_addLevel($xmid, $ymin, $xmax, $ymid,  # br
            $curDepth + 1, 4 * $index + 4, $index);
}

###############################
# sub add() - public method
#
# This method adds an object to the tree.
# The arguments are a unique tag to identify
# the object, and the bounding box of the object.
# It automatically assigns the proper quadtree
# sections to each object.
###############################
sub add {
    my ($self,
    $objRef,
    @coords,
    ) = @_;

    # assume that $objRef is unique.
    # assume coords are (xmin, ymix, xmax, ymax).

    # modify coords according to window.
    @coords = $self->_adjustCoords(@coords);

    ($coords[0], $coords[2]) = ($coords[2], $coords[0]) if
    $coords[2] < $coords[0];
    ($coords[1], $coords[3]) = ($coords[3], $coords[1]) if
    $coords[3] < $coords[1];

    $self->_addObjToChild(
              0,        # current index
              $objRef,
              @coords,
              );
}

###############################
# sub _addObjToChild() - private method
#
# This method is used internally. Given
# a tree segment, an object and its area,
# it checks to see whether the object is to
# be included in the segment or not.
# The object is not included if it does not
# overlap the segment.
###############################

sub _addObjToChild {
    my ($self,
    $index,
    $objRef,
    @coords,
    ) = @_;

    # first check if obj overlaps current segment.
    # if not, return.
    my ($cxmin, $cymin, $cxmax, $cymax) = @{$self->{AREA}[$index]};

    return if
    $coords[0] > $cxmax ||
    $coords[2] < $cxmin ||
    $coords[1] > $cymax ||
    $coords[3] < $cymin;

    # Only add the object to the segment if we are at the last
    # level of the tree.
    # Else, keep traversing down.

    unless (@{$self->{CHILDREN}[$index]}) {
    push @{$self->{OBJECTS}[$index]}  => $objRef;    # points from leaf to object
    push @{$self->{BACKREF}{$objRef}} => $index;     # points from object to leaf

    } else {
    # Now, traverse down the hierarchy.
    for my $child (@{$self->{CHILDREN}[$index]}) {
        $self->_addObjToChild(
                  $child,
                  $objRef,
                  @coords,
                  );
    }
    }
}

###############################
# sub delete() - public method
#
# This method deletes an object from the tree.
###############################
sub delete {
    my ($self,
    $objRef,
    ) = @_;

    return unless exists $self->{BACKREF}{$objRef};

    for my $i (@{$self->{BACKREF}{$objRef}}) {
    $self->{OBJECTS}[$i] = grep {$_ ne $objRef} @{$self->{OBJECTS}[$i]};
    }

    delete $self->{BACKREF}{$objRef};
}

###############################
#
# sub getEnclosedObjects() - public method
#
# This method takes an area, and returns all objects
# enclosed in that area.
#
###############################

sub getEnclosedObjects {
    my ($self,
    @coords) = @_;

    $self->{TEMP} = [];

    @coords = $self->_adjustCoords(@coords);

    $self->_checkOverlap(
             0,   # current index
             @coords,
             );

    # uniquify {TEMP}.
    my %temp;
    @temp{@{$self->{TEMP}}} = undef;

    # PS. I don't check explicitly if those objects
    # are enclosed in the given area. They are just
    # part of the segments that are enclosed in the
    # given area. TBD.

    return [keys %temp];
}

###############################
# sub _adjustCoords() - private method
#
# This method adjusts the given coordinates
# according to the stored window. This is used
# when we 'zoom in' to avoid searching in areas
# that are not visible in the canvas.
###############################
sub _adjustCoords {
    my ($self, @coords) = @_;

    # modify coords according to window.
    $_ = $self->{ORIGIN}[0] + $_ / $self->{SCALE}
    for $coords[0], $coords[2];
    $_ = $self->{ORIGIN}[1] + $_ / $self->{SCALE}
    for $coords[1], $coords[3];

    return @coords;
}

###############################
# sub _checkOverlap() - private method
#
# This method checks if the given coordinates overlap
# the specified tree segment. If not, nothing happens.
# If it does overlap, then it is called recuresively
# on all the segment's children. If the segment is a
# leaf, then its associated objects are pushed onto
# a temporary array for later access.
###############################
sub _checkOverlap {
    my ($self,
    $index,
    @coords,
    ) = @_;

    # first check if obj overlaps current segment.
    # if not, return.
    my ($cxmin, $cymin, $cxmax, $cymax) = @{$self->{AREA}[$index]};

    return if
    $coords[0] >= $cxmax ||
    $coords[2] <= $cxmin ||
    $coords[1] >= $cymax ||
    $coords[3] <= $cymin;

    unless (@{$self->{CHILDREN}[$index]}) {
    push @{$self->{TEMP}} => @{$self->{OBJECTS}[$index]};
    } else {
    # Now, traverse down the hierarchy.
    for my $child (@{$self->{CHILDREN}[$index]}) {
        $self->_checkOverlap(
                 $child,
                 @coords,
                 );
    }
    }
}

###############################
# sub ===setWindow()=== - public method
#
# This method takes an area as input, and
# sets it as the active window. All new
# calls to any method will refer to that area.
###############################
sub setWindow {
    my ($self, $sx, $sy, $s) = @_;

    $self->{ORIGIN}[0] += $sx / $self->{SCALE};
    $self->{ORIGIN}[1] += $sy / $self->{SCALE};
    $self->{SCALE}     *= $s;
}

###############################
# sub setWindow() - public method
# This resets the window.
###############################
sub resetWindow {
  my $self = shift;

  $self->{ORIGIN}[$_] = 0 for 0 .. 1;
  $self->{SCALE}      = 1;
}

AUTHOR

Ala Qumsieh I<aqumsieh@cpan.org>

COPYRIGHTS

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

Mudanças entre as edições de "Dago:Algoritmo Quadtree"

Edição das 14h11min de 10 de maio de 2009

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Métodos da Classe

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Algoritmo QuadTree

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