pygame 笔记-10 摩擦力与屏幕环绕

多年前写过一篇 Flash/Flex学习笔记(25)：摩擦力与屏幕环绕，可惜的当时上传的flash，服务器后来无人维护，现在flash链接都失效了。本篇用pygame重新实现了一个：

原理是类似，但要注意的是：pygame中旋转的角度采用逆时针系统 ，即：逆时针方向旋转，角度为正，反之为负。所以在外理角度时，y轴方向的速度要取反。

素材图（2张）：

飞船熄火

飞船点火

需求：按向上键点火，飞船启动，一直加速；无按键时，飞船熄火，速度慢慢降下来（设置摩擦系数）；左右键控制转向；飞出屏幕时，从另一侧切回来。

代码：

 import os
 import pygame
 import sys
 import math

 pygame.init()

 clock = pygame.time.Clock()

 SIZE = WIDTH, HEIGHT = 400, 400
 GRAY = (200, 200, 200)
 RED = (255, 0, 0)
 screen = pygame.display.set_mode(SIZE)
 pygame.display.set_caption("ship")
 img_base_path = os.getcwd() + '/img/'

 class Ship(object):
     def __init__(self, img_base_path, screen):
         self.vx = 0
         self.vy = 0
         # 旋转角速度
         self.vr = 0
         # 推进力
         self.thrust = 0
         self.angle = 0
         self.show_flame = False
         self.scale = 1.0
         # 是否显示辅助边框
         self.show_rect = False

         self.img_src = pygame.image.load(img_base_path + 'ship.png')
         self.img_flame_src = pygame.image.load(img_base_path + 'ship_flame.png')

         self.img = self.img_src
         self.rect = self.img_src.get_rect()

         self.img_new = self.img
         self.rect_new = self.img_new.get_rect()

         self.rect = self.rect.move((WIDTH - self.rect.width) * 0.5, (HEIGHT - self.rect.height) * 0.5)

     def draw(self, screen):
         screen.blit(self.img_new, self.rect_new)
         if self.show_rect:
             pygame.draw.rect(screen, GRAY, ship.rect, 1)
             pygame.draw.rect(screen, RED, ship.rect_new, 1)

     def move(self):
         self.rect = self.rect.move(self.vx, self.vy)
         self.rect_new = self.rect_new.move(self.vx, self.vy)
         # 向左飞出边界
         if self.rect_new.right < 0 and ship.vx < 0:
             self.rect_new.left = WIDTH
             self.rect.left = WIDTH
         # 向右飞出边界
         if self.rect_new.left > WIDTH and ship.vx > 0:
             self.rect_new.right = 0
             self.rect.right = 0
         # 向下飞出边界
         if self.rect_new.top > HEIGHT and ship.vy > 0:
             self.rect_new.bottom = 0
             self.rect.bottom = 0
         # 向上飞出边界
         if self.rect_new.bottom < 0 and ship.vy < 0:
             self.rect_new.top = HEIGHT
             self.rect.top = HEIGHT

     def rotate_zoom(self):
         # rotozoom=旋转+缩放
         self.img_new = pygame.transform.rotozoom(self.img, self.angle, self.scale)
         self.rect_new = self.img_new.get_rect(center=self.rect.center)
         if math.fabs(self.angle) == 360:
             self.angle = 0

     def set_flame(self, show_flame=False):
         self.show_flame = show_flame
         if self.show_flame:
             self.img = self.img_flame_src
         else:
             self.img = self.img_src

 def get_speed(speed):
     if speed > 0:
         return math.ceil(speed)
     if speed < 0:
         return math.floor(speed)
     return speed

 ship = Ship(img_base_path, screen)
 ship.scale = 0.5
 ship.show_rect = True
 # 摩擦系数
 friction = 0.995
 while True:
     clock.tick(60)

     for event in pygame.event.get():
         if event.type == pygame.QUIT:
             sys.exit()
         elif event.type == pygame.KEYUP:
             # KEYUP时，熄火，动力归0
             ship.vr = 0
             ship.thrust = 0
             ship.set_flame(False)
         elif event.type == pygame.KEYDOWN:
             keys = pygame.key.get_pressed()
             if keys[pygame.K_LEFT]:
                 ship.vr = 5
             elif keys[pygame.K_RIGHT]:
                 ship.vr = -5
             if keys[pygame.K_UP]:
                 # 按向上键时，点火，动力为0.3
                 ship.set_flame(True)
                 ship.thrust = 0.3
             else:
                 ship.set_flame(False)

     # 将每一帧的底色先填充成白色
     screen.fill((255, 255, 255))

     pygame.draw.line(screen, GRAY, (0, HEIGHT / 2), (WIDTH, HEIGHT / 2), 1)
     pygame.draw.line(screen, GRAY, (WIDTH / 2, 0), (WIDTH / 2, HEIGHT), 1)

     ship.angle += ship.vr
     ax = math.cos(ship.angle * math.pi / 180) * ship.thrust
     # 注：pygame中，角度是逆时针转的，所以垂直加速度要取反
     ay = -1 * math.sin(ship.angle * math.pi / 180) * ship.thrust
     ship.vx += ax
     ship.vy += ay

     # 摩擦系数
     if math.fabs(ship.vx) > 0.001:
         ship.vx = ship.vx * friction
     if math.fabs(ship.vy) > 0.001:
         ship.vy = ship.vy * friction

     print("vx:", ship.vx)

     ship.rotate_zoom()
     ship.move()
     ship.draw(screen)

     # 更新画布
     pygame.display.update()

如果把背景变成黑色，辅助边框去掉，看上去更有漆黑宇宙的感觉:)

源代码地址： https://github.com/yjmyzz/pygame_tutorial/blob/master/move_02.py