PyGame : How to use Box2D python package - part 002.

I've written a tutorial in the past about the python package called box2d, see this tutorial.

Today I will be more precise and I will show you a source code related to 2D simulation.

I installed the python package in the Fedora 35 Linux distribution with the DNF tool:

[root@fedora mythcat]# dnf search pybox2d
...
python3-pybox2d.x86_64 : A 2D rigid body simulation library for Python
[root@fedora mythcat]# dnf install python3-pybox2d.x86_64
Last metadata expiration check: 0:18:37 ago on Sun 16 Jan 2022 10:15:43 AM EET.
Dependencies resolved.
...
Installed:
  python3-pybox2d-2.3.2-17.fc35.x86_64                                          

Complete!

I created the working folders and the first python file named example001.py:

[mythcat@fedora ~]$ mkdir PyGameProjects
[mythcat@fedora ~]$ cd PyGameProjects/
[mythcat@fedora PyGameProjects]$ touch example001.py
[mythcat@fedora PyGameProjects]$ vi example001.py

It contains a default source code:

from Box2D import (b2PolygonShape, b2World)
# create word 
world = b2World()  
# set the world 
groundBody = world.CreateStaticBody(position=(0, -10),
                                    shapes=b2PolygonShape(box=(50, 10)),
                                    )
# create a dynamic body at position
body = world.CreateDynamicBody(position=(0, 4))

# add and set a box fixture onto it with a nonzero density, so it will move
box = body.CreatePolygonFixture(box=(1, 1), density=1, friction=0.3)

# use a time step of 1/60 of a second
timeStep = 1.0 / 60

# simulation scenario with 6 velocity/2 position iterations
vel_iters, pos_iters = 6, 2

# the game loop.
for i in range(60):
    # use step of simulation
    world.Step(timeStep, vel_iters, pos_iters)

    # clear body forces even I didn't apply any forces
    world.ClearForces()

    # print the position and angle of the body.
    print(body.position, body.angle)

The result of the run looks like this:

...
b2Vec2(1.8719e-08,1.01496) 6.208252216310939e-06
b2Vec2(1.90152e-08,1.01497) 4.9494738050270826e-06

For a more complex example we created two python files.

One is a class for a box and a file that calls the class.

The first file contains the following source code is named box.py:

import pygame
from Box2D import (b2EdgeShape, b2FixtureDef, b2PolygonShape, b2_dynamicBody,
                   b2_kinematicBody, b2_staticBody, b2World)

class Box:
    def __init__(self, x, y, l, world):
        self.x = x / l
        self.y = y / l
        self.w = .2
        self.h = .2

        self.world = world
        self.attachment = self.world.CreateDynamicBody(
            position=(self.x, self.y),
            fixtures=b2FixtureDef(
                shape=b2PolygonShape(box=(self.w, self.h)), density=0.4, friction = 0.4),)
                
    def display(self, screen):
        for body in self.world.bodies:
            for fixture in body.fixtures:
                shape = fixture.shape
                vertices = [(body.transform * v) * 20 for v in shape.vertices]
                pygame.draw.polygon(screen, 'azure', vertices)
                pygame.draw.polygon(screen, 'blue', vertices,width=3)

The file calling the class has the following source code:

import pygame
from box import Box
from Box2D import b2World

l = 20
fps = 60
frame_rate = 1.0 / fps

pygame.init()
screen = pygame.display.set_mode((640, 480))
pygame.display.set_caption("Physics")
clock = pygame.time.Clock()

# A list for all of our rectangles
list_boxes = []
world = b2World(gravity=(0, 9.8), doSleep=False)

close = False

while not close:
    
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            close = True
    
    screen.fill('white')

    click, _, _ = pygame.mouse.get_pressed()    
    if click == 1:
        x,y = pygame.mouse.get_pos()
        box = Box(x, y, l, world)
        list_boxes.append(box)

    for box in list_boxes:
        box.display(screen)

    world.Step(frame_rate, 10, 10)
    pygame.display.flip()
    clock.tick(fps)

pygame.quit()

The result of running this file looks like this screenshot with some squares moving to the bottom: