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docs/modules/box2d.md

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----
-icon: package-dependencies
-label: box2d
----
-
-!!!
-This module was moved to `carrotlib`, i.e. not a builtin module anymore.
-!!!
-
-[Box2D](https://box2d.org/) by Erin Catto, the world's best 2D physics engine.
-All platforms are supported, including desktop, mobile and web.
-
-## Overview
-
-The `box2d` module in pkpy provides a high-level, also simplified, interface to Box2D engine, which is suitable for most use cases.
-There are two classes in `box2d` module: `World` and `Body`.
-
-`World` is the world of Box2D, it is the container of all `Body`s.
-In most cases, you only need one `World` instance.
-`World` class provides methods to create, destroy and query `Body`s
-and also methods to step the simulation.
-
-A `Body` instance is a physical entity in the world.
-A `Body` can only have one shape at a time.
-For example, a circle, a rectangle, a polygon, etc.
-You are allowed to change the shape of a `Body` at runtime.
-`Body`s can be static, dynamic or kinematic.
-A static `Body` is not affected by forces or collisions.
-A dynamic `Body` is fully simulated.
-A kinematic `Body` moves according to its velocity.
-`Body` class provides methods to set its properties, such as position, velocity, etc.
-It also provides methods to apply forces and impulses to it.
-
-!!!
-A `box2d.Body` in pkpy is an unified wrapper of Box2D's `b2Body`,
-`b2Shape` and `b2Fixture`.
-It hides the details of Box2D's API and provides a high-level interface.
-!!!
-
-## APIs
-
-https://github.com/pocketpy/pocketpy/blob/main/include/typings/box2d.pyi
-
-## Example
-
-### Simple simulation
-
-```python
-import box2d
-from linalg import vec2
-
-# create a world
-world = box2d.World()
-
-# create a body with a box shape
-body = box2d.Body(world)
-body.set_box_shape(1, 1)
-
-# give it a velocity
-body.velocity = vec2(1, 0)
-
-# step the simulation
-for i in range(30):
-    world.step(1/30, 8, 3)
-    print(i, body.position)
-```
-
-```
-0 vec2(0.033, 0.000)
-1 vec2(0.067, 0.000)
-2 vec2(0.100, 0.000)
-3 vec2(0.133, 0.000)
-4 vec2(0.167, 0.000)
-5 vec2(0.200, 0.000)
-6 vec2(0.233, 0.000)
-7 vec2(0.267, 0.000)
-8 vec2(0.300, 0.000)
-9 vec2(0.333, 0.000)
-10 vec2(0.367, 0.000)
-11 vec2(0.400, 0.000)
-12 vec2(0.433, 0.000)
-13 vec2(0.467, 0.000)
-14 vec2(0.500, 0.000)
-15 vec2(0.533, 0.000)
-16 vec2(0.567, 0.000)
-17 vec2(0.600, 0.000)
-18 vec2(0.633, 0.000)
-19 vec2(0.667, 0.000)
-20 vec2(0.700, 0.000)
-21 vec2(0.733, 0.000)
-22 vec2(0.767, 0.000)
-23 vec2(0.800, 0.000)
-24 vec2(0.833, 0.000)
-25 vec2(0.867, 0.000)
-26 vec2(0.900, 0.000)
-27 vec2(0.933, 0.000)
-28 vec2(0.967, 0.000)
-29 vec2(1.000, 0.000)
-```
-
-### Two bodies with collision
-
-```python
-import box2d
-from linalg import vec2
-
-world = box2d.World()
-
-"""
-   12/s
-B ----->          A
--|-|-|-|-|-|-|-|-|-
-0 1 2 3 4 5 6 7 8 9
-"""
-
-# body_a is a static body with a box shape at (9, 0)
-body_a = box2d.Body(world)
-body_a.type = 0     # static type
-body_a.set_box_shape(0.5, 0.5)
-body_a.position = vec2(9, 0)
-
-class Node:
-    def on_box2d_contact_begin(self, other):
-        print('body_a collides with body_b!!')
-
-    def on_box2d_pre_step(self):
-        pass
-
-    def on_box2d_post_step(self):
-        pass
-
-# body_b is a dynamic body with a circle shape at (0, 0)
-body_b = box2d.Body(world, Node())
-body_b.set_circle_shape(0.5)
-body_b.position = vec2(0, 0)
-
-# give body_b a velocity and it will collide with body_a at some point
-body_b.velocity = vec2(12, 0)
-
-for i in range(30):
-    world.step(1/30, 8, 3)
-    print(i, body_b.position)
-```
-
-```
-0 vec2(0.400, 0.000)
-1 vec2(0.800, 0.000)
-2 vec2(1.200, 0.000)
-3 vec2(1.600, 0.000)
-4 vec2(2.000, 0.000)
-5 vec2(2.400, 0.000)
-6 vec2(2.800, 0.000)
-7 vec2(3.200, 0.000)
-8 vec2(3.600, 0.000)
-9 vec2(4.000, 0.000)
-10 vec2(4.400, 0.000)
-11 vec2(4.800, 0.000)
-12 vec2(5.200, 0.000)
-13 vec2(5.600, 0.000)
-14 vec2(6.000, 0.000)
-15 vec2(6.400, 0.000)
-16 vec2(6.800, 0.000)
-17 vec2(7.200, 0.000)
-18 vec2(7.600, 0.000)
-19 vec2(8.000, 0.000)
-body_a collides with body_b!!
-20 vec2(7.999, 0.000)
-21 vec2(7.998, 0.000)
-22 vec2(7.998, 0.000)
-23 vec2(7.997, 0.000)
-24 vec2(7.997, 0.000)
-25 vec2(7.996, 0.000)
-26 vec2(7.996, 0.000)
-27 vec2(7.996, 0.000)
-28 vec2(7.996, 0.000)
-29 vec2(7.996, 0.000)
-```
-
-## Caveats
-
-You should set the shape of the body first before accessing fixture properties.
-```python
-class Body:
-    ...
-
-    # fixture settings
-    density: float
-    friction: float
-    restitution: float
-    restitution_threshold: float
-    is_sensor: bool
-```
-
-```python
-import box2d
-world = box2d.World()
-body = box2d.Body(world)
-
-body.is_sensor = True       # this will raise an error
-```
-
-The correct usage is:
-```python
-body = box2d.Body(world)
-body.set_box_shape(1, 1)    # set shape first
-
-body.is_sensor = True       # OK
-```