Partical tutorial #8 from http://www.petercollingridge.co.uk/pygame-physics-simulation

partical_tutorial_8

import pygame
import random
import math

background_colour = (255,255,255)
(width, height) = (400, 400)
drag = 0.999
elasticity = 0.75
gravity = (math.pi, 0.002)
import cv2
fourcc = cv2.VideoWriter_fourcc(*'XVID')
out = cv2.VideoWriter('/home/sm/Desktop/tutorial_7.avi',fourcc, 48.0, (width, height))

def addVectors((angle1, length1), (angle2, length2)):
    x  = math.sin(angle1) * length1 + math.sin(angle2) * length2
    y  = math.cos(angle1) * length1 + math.cos(angle2) * length2
    
    angle = 0.5 * math.pi - math.atan2(y, x)
    length  = math.hypot(x, y)

    return (angle, length)

def findParticle(particles, x, y):
    for p in particles:
        if math.hypot(p.x-x, p.y-y) <= p.size:
            return p
    return None

def collide(p1, p2):
    dx = p1.x - p2.x
    dy = p1.y - p2.y
    
    dist = math.hypot(dx, dy)
    if dist < p1.size + p2.size:
        tangent = math.atan2(dy, dx)
        angle = 0.5 * math.pi + tangent

        angle1 = 2*tangent - p1.angle
        angle2 = 2*tangent - p2.angle
        speed1 = p2.speed*elasticity
        speed2 = p1.speed*elasticity

        (p1.angle, p1.speed) = (angle1, speed1)
        (p2.angle, p2.speed) = (angle2, speed2)

        p1.x += math.sin(angle)
        p1.y -= math.cos(angle)
        p2.x -= math.sin(angle)
        p2.y += math.cos(angle)

class Particle():
    def __init__(self, (x, y), size):
        self.x = x
        self.y = y
        self.size = size
        self.colour = (0, 0, 255)
        self.thickness = 0
        self.speed = 0
        self.angle = 0

    def display(self):
        pygame.draw.circle(screen, self.colour, (int(self.x), int(self.y)), self.size, self.thickness)

    def move(self):
        (self.angle, self.speed) = addVectors((self.angle, self.speed), gravity)
        self.x += math.sin(self.angle) * self.speed
        self.y -= math.cos(self.angle) * self.speed
        self.speed *= drag

    def bounce(self):
        if self.x > width - self.size:
            self.x = 2*(width - self.size) - self.x
            self.angle = - self.angle
            self.speed *= elasticity

        elif self.x < self.size:
            self.x = 2*self.size - self.x
            self.angle = - self.angle
            self.speed *= elasticity

        if self.y > height - self.size:
            self.y = 2*(height - self.size) - self.y
            self.angle = math.pi - self.angle
            self.speed *= elasticity

        elif self.y < self.size:
            self.y = 2*self.size - self.y
            self.angle = math.pi - self.angle
            self.speed *= elasticity

screen = pygame.display.set_mode((width, height))
pygame.display.set_caption('Tutorial 8')

number_of_particles = 5
my_particles = []

for n in range(number_of_particles):
    size = random.randint(10, 20)
    x = random.randint(size, width-size)
    y = random.randint(size, height-size)

    particle = Particle((x, y), size)
    particle.speed = random.random()
    particle.angle = random.uniform(0, math.pi*2)

    my_particles.append(particle)

selected_particle = None
running = True
while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False
        elif event.type == pygame.MOUSEBUTTONDOWN:
            (mouseX, mouseY) = pygame.mouse.get_pos()
            selected_particle = findParticle(my_particles, mouseX, mouseY)
        elif event.type == pygame.MOUSEBUTTONUP:
            selected_particle = None

    if selected_particle:
        (mouseX, mouseY) = pygame.mouse.get_pos()
        dx = mouseX - selected_particle.x
        dy = mouseY - selected_particle.y
        selected_particle.angle = 0.5*math.pi + math.atan2(dy, dx)
        selected_particle.speed = math.hypot(dx, dy) * 0.1

    screen.fill(background_colour)

    for i, particle in enumerate(my_particles):
        particle.move()
        particle.bounce()
        for particle2 in my_particles[i+1:]:
            collide(particle, particle2)
        particle.display()
    path = '/home/sm/Desktop/img.png'
    pygame.image.save(screen, path)
    img = cv2.imread(path)
    out.write(img)
    pygame.display.flip()