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Collision::2D - Continuous 2d collision detection


  use Collision::2D ':all';
  my $rect = hash2rect ({x=>0, y=>0, h=>1, w=>1});
  my $circle = hash2circle ({x=>0, y=>0, radius => 1});
  my $collision = dynamic_collision ($rect, $circle);
  #When your geometric objects do not move, it is static.
  #Collision::2D is also capable of dynamic collisions, eith moving entities.
  my $roach = hash2circle ({x=>-1, y=>-12, radius => .08, xv = 3, yv => 22});
  my $food = hash2circle ({x=>0, y=>3, radius => .08, xv=>-6});
  my $co2 = dynamic_collision ($roach, $food);
  if ($co2){
     print "collision is at t=" . $co2->time . "\n"
     print "axis of collision is (" . join(',', @{$co2->axis}) .")\n";
  #we can also detect whether points collide with circles and rects. 
  #these entities collide at around y=20000, x=10000, t=100:
  my $tiny_rect = hash2rect {x=>15000-.00005, y=>30000-.00005, h=>.0001, w=>.0001, xv=>-50, yv=>-100};
  my $accurate_bullet = hash2point { x=>-40000, y=>80100, xv=>500, yv=> -601};
  my $strange_collision = dynamic_collision ($accurate_bullet, $tiny_rect, interval=>400);


Collision::2D contains sets of several geometrical classes to help you model dynamic (continuous) collisions in your programs. It is targeted for any game or other application that requires dynamic collision detection between moving circles, rectangles, and points.


Typically, collision detection in games and game libraries tends to be static. That is, they only detect overlap of motionless polygons. This is somewhat simple, but naive, because often the developer may want a description of the collision, so that he may implement a response.

Supply Collision::2D with any 2 moving entities (rects, circles, and points) and an interval of time and it will return a Collision::2D::Collision object. This $collision has attributes ->time and ->axis, which describe when and how the collision took place.


Initially, I implemented point-rect and point-circle. I used these to compose the other types of detection.

Circle-circle is just an extension of point-circle, and it reduces to a single point-circle detection.

Circle-rect and may use a bunch of calls to point-collision routines. This is a worst case, though. If both entities stay entirely separate on either dimension, no such calculation is required. If they intersect at t=0, it returns the null collision, with no axis involved.

Rect-rect operates independently of point operations.

In any case, if one entity is observed to remain on one side of the other, then we can be certain that they don't collide.



Detects collisions between 2 entities. The entities may be any combination of rects, circles, and points. You may specify a time interval as an keyed parameter. By default, the interval is 1.

 my $circle = hash2circle ({x=>0, y=>0, yv => 1, radius => 1});
 my $point = hash2point ({x=>0, y=>-2, yv => 2});
 my $collision = dynamic_collision ($circle, $point, interval => 4);
 #$collision->time == 1. More on that in L<Collision::2D::Collision>.
 #$collision->axis ~~ [0,1] or [0,-1]. More on that in L<Collision::2D::Collision>.
 print 'whoops' unless intersection ($table, $pie);

Detects overlap between 2 entities. This is similar to dynamic_collision, except that time and motion is not considered. intersection() does not return a Collision::2D::Collision, but instead true or false values.

hash2circle, hash2point, hash2rect
 my $circle = hash2circle ({x=>0, y=>0, yv => 1, radius => 1});

These takes a hash reference, and return the appropriate entity. The hash typically includes absolute coordinates and velocities. For hash2circle, it takes radius. For hash2rect, it takes h and w.

obj2circle, obj2point, obj2rect
 my $circle = hash2circle ($game_sprite);

These takes an object with the appropriate methods and return the appropriate entity. ->x(), ->y(), ->xv(), and ->yv() must be callable methods of the $object. For obj2circle, it takes radius. For obj2rect, it takes h and w.


Normalize your 2d vectors

 my $vec = [3,4];
 my $nvec = normalize_vec($vec);
 # $nvec is now [3/5, 4/5]


Collision::2D doesn't export anything by default. You have to explicitly define function names or use the :all tag.


 *point-point collisions? Don't expect much if you try it now.
 *either triangles or line segments (or both!) to model slopes.
 *Something that can model walking on mario-style platformers.
 **maybe entities should be linked to whatever entities they stand/walk on?
 **How should entities fit into 'gaps' in the floor that are their exact size?


Zach P. Morgan, <zpmorgan at>

Stefan Petrea <>

Kartik Thakore <>


Many thanks to Breno G. de Oliveira and Kartik Thakore for their help and insights.


This program is free software; you can redistribute it and/or modify it under the terms of either: the GNU General Public License as published by the Free Software Foundation; or the Artistic License.

See for more information.