The Nature of Code閱讀心得與Python實作：4.4 A Particle Emitter

到目前為止，我們已經建立了描述單一粒子的Particle類別，並且能夠利用list這個資料結構，來處理由Particle類別產生的大量粒子物件。在這一節，我們要把這一些整合起來，建立一個Emitter類別。有了這個類別，就可以讓主程式寫起來長成這個樣子：

emitter = Emitter()

while True:
    :
    :
    emitter.run()

除此之外，也可以讓我們能夠比較輕鬆地一次處理多個由許多粒子構成的系統。

這個Emitter類別，就是在4.1節提到的ParticleSystem類別，但加入了一個新功能，用來設定產生粒子的位置。所以，這個Emitter類別，其實也就是4.2節提到的粒子的發射器。Emitter類別的程式碼如下：

class Emitter:
    def __init__(self, x, y, mass):
        self.mass = mass
        self.size = 16*self.mass
        
        self.particles = []
        
        # 發射器位置
        self.origin = pygame.Vector2(x, y)
        
    def add_particle(self):
        self.particles.append(Particle(self.origin.x, self.origin.y, self.mass))
        
    def run(self):
        for particle in self.particles:
            particle.apply_force(gravity)
            particle.update()    
        
        # 移除壽命已到的粒子
        self.particles = list(filter(lambda particle: not particle.is_dead(), self.particles))
    
        for particle in self.particles:
            particle.show()

要注意的是，在run()方法裡頭，作用在粒子上的重力gravity，是個全域變數；這樣子的寫法顯然不怎麼好。不過，在目前的架構下，好像也只能這樣寫了。

下面這個例子，就是利用Emitter類別，在畫面的指定位置，不斷地噴發出粒子。

Example 4.3: A Single Particle Emitter

# python version 3.10.9
import random
import sys

import pygame  # version 2.3.0


pygame.init()

pygame.display.set_caption("Example 4.3: A Single Particle Emitter")

WHITE = (255, 255, 255)

screen_size = 640, 360
screen = pygame.display.set_mode(screen_size)

FPS = 60
frame_rate = pygame.time.Clock()

gravity = pygame.Vector2(0, 0.05)

emitter = Emitter(320, 50, 1)

while True:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()
            sys.exit()
    
    screen.fill(WHITE)    

    emitter.add_particle()
    emitter.run()
        
    pygame.display.update()
    frame_rate.tick(FPS)

Exercise 4.3

從滑鼠游標發射粒子。

# python version 3.10.9
import random
import sys

import pygame  # version 2.3.0


pygame.init()

pygame.display.set_caption("Exercise 4.3")

WHITE = (255, 255, 255)

screen_size = 640, 360
screen = pygame.display.set_mode(screen_size)

FPS = 60
frame_rate = pygame.time.Clock()

gravity = pygame.Vector2(0, 0.05)

x, y = pygame.mouse.get_pos()
emitter = Emitter(x, y, 1)

while True:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()
            sys.exit()
    
    screen.fill(WHITE)    

    x, y = pygame.mouse.get_pos()
    emitter.origin = pygame.Vector2(x, y)
    
    emitter.add_particle()
    emitter.run()
        
    pygame.display.update()
    frame_rate.tick(FPS)

Exercise 4.4

修改Spaceship類別。完整的Spaceship類別程式碼以及主程式如下：

class Spaceship:
    def __init__(self, mass=1, heading=90):
        # 取得顯示畫面及其大小
        self.screen = pygame.display.get_surface()
        self.width, self.height = self.screen.get_size()
        
        self.mass = mass
        
        # 航向，單位是「度」
        self.heading = heading  # in degree
        
        # 起始位置在畫面中央
        x, y = self.width//2, self.height//2
        self.position = pygame.Vector2(x, y)
        
        # 最高速率
        self.top_speed = 5

        # 起始速度、加速度
        self.velocity = pygame.Vector2(0, 0)
        self.acceleration = pygame.Vector2(0, 0)

        # 設定spaceship所在surface的格式為per-pixel alpha
        self.surface = pygame.Surface((35, 30), pygame.SRCALPHA)
        
        # 在surface上畫出spaceship
        body = [(35, 15), (5, 0), (5, 30)]
        pygame.draw.polygon(self.surface, (0, 0, 0), body)    
        
        nozzle1 = pygame.Rect(0, 5, 5, 5)
        pygame.draw.rect(self.surface, (0, 0, 0), nozzle1)  
        
        nozzle2 = pygame.Rect(0, 20, 5, 5)
        pygame.draw.rect(self.surface, (0, 0, 0), nozzle2)  

        # 噴嘴發射器位置
        nozzle1_location = pygame.Vector2(0, 5+5/2)
        nozzle2_location = pygame.Vector2(0, 20+5/2)
        
        # 建造發射器
        emitter1 = Emitter(nozzle1_location.x, nozzle1_location.y, 0.3)
        emitter2 = Emitter(nozzle2_location.x, nozzle2_location.y, 0.3)
        self.emitters = [emitter1, emitter2]
        
        # 中心點至噴嘴發射器向量，用於計算在不同航向時，發射器在顯示畫面上之位置
        ship_center = pygame.Vector2(35/2, 30/2)    
        nozzle1_vec = nozzle1_location - ship_center
        nozzle2_vec = nozzle2_location - ship_center
        self.nozzle_vecs = [nozzle1_vec, nozzle2_vec]
        
        self.thrusting = False

    def set_heading(self):
        keys = pygame.key.get_pressed()
        if keys[pygame.K_LEFT]:
            self.heading += 5
        elif keys[pygame.K_RIGHT]:
            self.heading -= 5        
     
    def generate_thrust(self):
        keys = pygame.key.get_pressed()
        if keys[pygame.K_z]:   
            self.thrusting = True
            thrust = pygame.Vector2.from_polar((1, self.heading))
            thrust.scale_to_length(0.1)
            thrust.y = -thrust.y
        else:
            self.thrusting = False
            # 沒有按下z鍵，減速到停下為止
            if spaceship.velocity.length() > 1e-5:
                thrust = -spaceship.velocity
                thrust.scale_to_length(0.05)
            else:
                thrust = pygame.Vector2(0, 0)
         
        return thrust   
         
    def apply_force(self, force):
        self.acceleration += force/self.mass
        
    def update(self):
        self.velocity += self.acceleration
        if self.velocity.length() < 1e-5:
            # 速度很小時，直接判定為靜止狀態
            self.velocity *= 0 
        else:
            # 限速
            if self.velocity.length() > self.top_speed:
                self.velocity.scale_to_length(self.top_speed)
        
        self.position += self.velocity
        
        self.acceleration *= 0
        
    def show(self):
        rotated_surface = pygame.transform.rotate(self.surface, self.heading)
        rect_new = rotated_surface.get_rect(center=self.position)
                
        # 把spaceship所在的surface貼到最後要顯示的畫面上
        self.screen.blit(rotated_surface, rect_new)  
        
        if self.thrusting:
            emitter_direction = pygame.Vector2.from_polar((1, self.heading+180))
            emitter_direction.y = -emitter_direction.y
    
            # 計算發射器在顯示畫面上之位置，並裝填及設定粒子之速度、壽命
            for i in range(2):
                self.emitters[i].origin = self.position + self.nozzle_vecs[i].rotate(-self.heading)
                self.emitters[i].add_particle() 
                self.emitters[i].particles[-1].velocity = 0.5*emitter_direction
                self.emitters[i].particles[-1].lifespan = 100
                
        for emitter in self.emitters:
            emitter.run()

    def check_edges(self):
        if self.position.x > self.width:
            self.position.x = 0
        elif self.position.x < 0:
            self.position.x = self.width
    
        if self.position.y > self.height:
            self.position.y = 0
        elif self.position.y < 0:
            self.position.y = self.height
            
            
# python version 3.10.9
import random
import sys

import pygame  # version 2.3.0


pygame.init()

pygame.display.set_caption("Exercise 4.4")

WHITE = (255, 255, 255)

screen_size = (640, 360)
screen = pygame.display.set_mode(screen_size)

FPS = 60
frame_rate = pygame.time.Clock()

gravity = pygame.Vector2(0, 0)

spaceship = Spaceship()

while True:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()
            sys.exit()
    
    screen.fill(WHITE)    

    spaceship.set_heading()
    
    thrust = spaceship.generate_thrust()
    spaceship.apply_force(thrust)

    spaceship.update()
    spaceship.check_edges()
    spaceship.show()
    
    pygame.display.update()
    frame_rate.tick(FPS)