蒙版

蒙版是二值化的图像，每个像素的值只能是0或1。

mask(蒙版)的用途：

• 碰撞检测
• 部分着色

案例

和字母的碰撞检测

当玩家碰到字母$\alpha$时，改变玩家颜色为绿色，否则为红色。
注意：我们希望碰到字母$\alpha$时才变色，而不是碰到字母$\alpha$所在的矩形就变色。

# collision
	if pygame.sprite.spritecollide(player.sprite,obstacle,False):
		if pygame.sprite.spritecollide(player.sprite,obstacle,False,pygame.sprite.collide_mask):
			player.sprite.image.fill('green')
		else:
			player.sprite.image.fill('red')

这里我们先用普通的精灵碰撞检测（矩形碰撞），如果发生了矩形碰撞，在使用蒙版碰撞将pygame.sprite.collide_mask作为参数传入pygame.sprite.spritecollide。
因为矩形检测比蒙版检测速度快很多，这样做可以减少蒙版检测的次数，提高效率。

https://www.pygame.org/docs/ref/sprite.html#pygame.sprite.spritecollide

完整代码：

import pygame, sys 

class Player(pygame.sprite.Sprite):
	def __init__(self):
		super().__init__()
		self.image = pygame.Surface((40,40)) 
		self.image.fill('red')
		self.rect = self.image.get_rect(center = (300,300))
		self.mask = pygame.mask.from_surface(self.image)

	def update(self):
		if pygame.mouse.get_pos():
			self.rect.center = pygame.mouse.get_pos()

class Obstacle(pygame.sprite.Sprite):
	def __init__(self):
		super().__init__()
		self.image = pygame.image.load('alpha.png').convert_alpha()
		self.rect = self.image.get_rect(center = (400,400))
		self.mask = pygame.mask.from_surface(self.image)
			
# pygame setup
pygame.init()
screen = pygame.display.set_mode((800,800))
clock = pygame.time.Clock()

# group setup
player = pygame.sprite.GroupSingle(Player())
obstacle = pygame.sprite.GroupSingle(Obstacle())

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

	screen.fill('white')
	obstacle.draw(screen) 

	# updating and drawing 
	player.update()
	player.draw(screen)
	
	# collision
	if pygame.sprite.spritecollide(player.sprite,obstacle,False):
		if pygame.sprite.spritecollide(player.sprite,obstacle,False,pygame.sprite.collide_mask):
			player.sprite.image.fill('green')
		else:
			player.sprite.image.fill('red')
	
	pygame.display.update()
	clock.tick(60)

图片轮廓

将字母$\alpha$的轮廓变成橘色。
我们需要：

1. 从surface 创建mask.
2. 将mask变成surface.
3. 使用2中的surface创建轮廓(silhouette)
4. 显示轮廓。
   

import pygame,sys

pygame.init()
screen = pygame.display.set_mode((800,800))
clock = pygame.time.Clock()

# creating the obstacle
obstacle_surf = pygame.image.load('alpha.png').convert_alpha()
obstacle_pos = (100,100)
# 1. surface -> mask
obstacle_mask = pygame.mask.from_surface(obstacle_surf)

# 2. mask -> surface 
new_obstacle_surf = obstacle_mask.to_surface()
#  与colorkey相同的像素变透明
new_obstacle_surf.set_colorkey((0,0,0))

# 4. filling in the surface with a color
surf_w,surf_h = new_obstacle_surf.get_size()
for x in range(surf_w):
	for y in range(surf_h):
		if new_obstacle_surf.get_at((x,y))[0] != 0:
			new_obstacle_surf.set_at((x,y),'orange')

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

	screen.fill('grey')

	# complex way for a outline
	offset = 4
	screen.blit(new_obstacle_surf,(obstacle_pos[0] + offset,obstacle_pos[1])) # right
	screen.blit(new_obstacle_surf,(obstacle_pos[0] - offset,obstacle_pos[1])) # left
	screen.blit(new_obstacle_surf,(obstacle_pos[0], obstacle_pos[1] - offset)) # top
	screen.blit(new_obstacle_surf,(obstacle_pos[0], obstacle_pos[1] + offset)) # bottom
	screen.blit(new_obstacle_surf,(obstacle_pos[0] + offset, obstacle_pos[1] - offset)) # topright
	screen.blit(new_obstacle_surf,(obstacle_pos[0] + offset, obstacle_pos[1] + offset)) # bottomright
	screen.blit(new_obstacle_surf,(obstacle_pos[0] - offset, obstacle_pos[1] + offset)) # bottomleft
	screen.blit(new_obstacle_surf,(obstacle_pos[0] - offset, obstacle_pos[1] - offset)) # topleft


	screen.blit(obstacle_surf,obstacle_pos)

	# simple way to create an outline from a mask
	# for point in obstacle_mask.outline():
	# 	x = point[0] + obstacle_pos[0]
	# 	y = point[1] + obstacle_pos[1]
	# 	pygame.draw.circle(screen,'red',(x,y),1)

	pygame.display.update()
	clock.tick(60)

重叠部分着色

1. 使用overlap_mask 获得重叠部分的mask
2. 重新设置重叠部分mask的颜色。

import pygame,sys

pygame.init()
screen = pygame.display.set_mode((800,800))
clock = pygame.time.Clock()

# ship setup
ship_surf = pygame.image.load('ship.png').convert_alpha()
ship_overlay_surf = pygame.image.load('ship.png').convert_alpha()
ship_rect = ship_surf.get_rect(center = (300,300))
ship_mask = pygame.mask.from_surface(ship_surf)

# obstacle setup
obstacle_surf = pygame.image.load('alpha.png').convert_alpha()
obstacle_pos = (100,100)
obstacle_mask = pygame.mask.from_surface(obstacle_surf)

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

	screen.fill('white')

	# obstacle 
	screen.blit(obstacle_surf,obstacle_pos)
	
	# moving part
	if pygame.mouse.get_pos():ship_rect.center = pygame.mouse.get_pos()
	screen.blit(ship_surf,ship_rect)
	
	# mask coloring 
	offset_x = obstacle_pos[0] - ship_rect.left
	offset_y = obstacle_pos[1] - ship_rect.top
	if ship_mask.overlap(obstacle_mask,(offset_x,offset_y)):
		new_mask = ship_mask.overlap_mask(obstacle_mask,(offset_x,offset_y))
		new_surf = new_mask.to_surface()
		new_surf.set_colorkey((0,0,0))

		surf_w, surf_h = new_surf.get_size()
		for x in range(surf_w):
			for y in range(surf_h):
				if new_surf.get_at((x,y))[0] != 0:
					new_surf.set_at((x,y),'orange')
		screen.blit(new_surf,ship_rect)


	pygame.display.update()
	clock.tick(60)