Collider

This is an easy to use swept collision detecition and response Python library for axis-aligned rectangles. It is ideal to use for platformer or tile-based games where full-featured physics engine such as Box2D or Bullet would be an overkill.

Features:

• minimalistic and easy-to-use,
• uses swept collision detection to prevent tunelling,
• optimized to reduce the number of collision tests
• includes some basic responses to collisions
• allows to add own collision responses

Where collider is NOT not suitable:

• realistic simulations
• games requiring shapes other than axis-aligned rectangles

Basic usage

In order to use collider, download and import collider.py file from the repository. The module contains a few classes which let you get startes.

Positions and boxes

All positions in the collider are represented using Vector2(x, y) namedtuple containing coordinates. The fields can be accessed just as a with regular tuple or throught named fields:

vec = Vector2(3.6, 4.2)
vec[0]  # == 3.6
vec.y  # == 4.2
x, y = vec  # x == 3.6, y == 4.2

Note that the collider uses floating point numbers to represent positions and dimensions, so the positions in the world is not limited by integer pixel coordinates. Further, it's usually advisable to make object position in the world independent of the rendering system.

The elements taking part in the collisions are Rectangle objects. They can be created using the position and dimensions

Rectangle(x, y, w, h)

These values can be accessed and changes directly from the object's fields x, y, w, h. Additionally, pos and size properties are available to get and set position and size using tuples.

Creating World

The World is the container for the rectangles. It acts as a scene for the rectangles resolving all movements and collisions. To create a new world use a default empty constructor

world = World()

You can create as many worlds as you want i.e. to represent different locations in your game. The objects can only collide with other objects in the same world.

Adding object

A new rectangle can be added to the world using

world.add(rect)

The library does not prevent adding the same rectangle to more than one world. However, note that the position is tied to the rectangle and any changes will be reflected in all worlds the rectangle was added to.

Removing object

Similarly, the rectangle can be removed from the world using

world.remove(rect)

Testing collisions

Before the object can be moved to a new position the collisions happening on the way need to be resolved. The movement of the object with collision resolving can be checked using

position, collisions = world.check_move(rect, target, filter)

The parameters that the method accepts are following:

param	type	description
rect	Rectangle	rectangle to be moved
target	Vector2	a new position for the rectangle
filter	function	function used to determine collision response

The value returned by check_move is a tuple (Vector2, List[Collision]) containing the final position of the rectangle and the list of collisions which occured on the way. Keep in mind that the final position will not necessarily be the same as the target position provided. Some of the collisions may change the trajectory of the object. Also, check_move will not attempt to modify any positions of the objects and it's up to the user to change it accordingly.

The filter function should accept two arguments, the rectangle currently being moved (this) and the one it collided with (other), and return the response type.

filter(this: Rectangle, other: Rectangle) -> str

If the returned value is None or "ignore" the calculation of collision between this and other will be skipped saving computation time. Other accepted response types are:

• "block" : the object will be blocked by the obstacle stopping at the point of contact. Useful for projectiles/arrows/bullets which does not penetrate throught objects.
• "pass" : the objects should pass through each other without changing the trajectory. The only difference between "pass" and "ignore" is that the collision is recorded. Useful for collisions with coins or other collectibles.
• "slide" : the object will slide along the side it collided with. Useful for collision betwen a hero and a wall as it allows walking along the wall instead of "sticking" to it.
• "bounce" : the object will bounce off the side it collided with. Useful for bouncing bullets or mirrors reflecting rays.
• "return" : the object will be reflected back in the direction it came from.

Collisions

The second parameter returned by check_move is the list of collisions encountered by the object on its way. The collisions are ordered by their occurence. Each collision object contains the following fields:

field	type	description
rect	Rectangle	The rectangle that the object collided with
distance	float	Number between 0 and 1 telling how far the object travelled along the straight line when the collision occurred. Used for internal sorting.
normal	Vector2	The normal to the collision side. Either -1, 0 or 1 along x or y direction.
touch	Vector2	Position of the moved object where the collision occurred.
reponse	str	The response type used to resolve the collision

The most useful fields are rect, which tells which rectangles the object collided with, and normal, which tells the direction the collision came form.

Advanced usage

(coming later)

Documentation

(coming later)