Pygame animating image by transforming it

Question

I am trying to create an animation of a rectangle in pygame but I struggle to animate it over the given frame-rate.

class Muscle(pygame.sprite.Sprite):
def __init__(self, screen, posX=200, posY=200, contraction=0.5, extension=1.5, length=10, thickness=5, min_len=0.2,max_len=1.5, power=20):
    super(Muscle, self).__init__()
    self.screen = screen
    self.contraction = contraction
    self.relaxation = relaxation
    self.length = length
    self.thickness = thickness
    if power < 1:
        self.power = 1
        power = 1
    else:
        self.power = power
    self.image = pygame.Surface((length, thickness))
    color_index = 5 * power
    self.image.fill((255, color_index, color_index))
    self.rect = self.image.get_rect()
    self.rect.x = posX
    self.rect.y = posY

def render(self):
    self.screen.blit(self.image, (self.rect.x, self.rect.y))

def contract(self):
    expected_width = self.length * self.contraction
    counter = 0
    while self.image.get_width() > expected_width:
        self.image = pygame.transform.scale(self.image, (self.rect.width, self.image.get_height()))
        self.rect.x += 1;
        self.rect.width -= 2
        self.render()
        counter += 1

def extend(self):
    expected_width = self.length * self.relaxation
    counter = 0
    while self.image.get_width() < expected_width:
        self.image = pygame.transform.scale(self.image, (self.rect.width, self.image.get_height()))
        self.rect.x -= 1;
        self.rect.width += 2
        self.render()
        #print("Relaxation:" + str(counter))
        counter += 1

If I call the contract and extend methods separately, they resize the image of the rectangle successfully, but I would like to perform it over time, without interrupting my main loop which draws the environment and the sprite.

Answer 1

The simplest solution would be to call extend or contract once per frame, but then the animation would be frame rate bound.

import pygame


class Muscle(pygame.sprite.Sprite):
    def __init__(self, screen, posX=200, posY=200, contraction=0.5,
                 extension=1.5, length=40, thickness=20, min_len=0.2,max_len=1.5, power=20):
        super(Muscle, self).__init__()
        self.screen = screen
        self.contracting = False  # To check if the muscle is contracting or extending.
        self.contraction = contraction
        self.relaxation = 1
        self.length = length
        self.thickness = thickness
        if power < 1:
            self.power = 1
            power = 1
        else:
            self.power = power
        self.image = pygame.Surface((length, thickness))
        color_index = 5 * power
        self.image.fill((255, color_index, color_index))
        self.rect = self.image.get_rect()
        self.rect.x = posX
        self.rect.y = posY

    # This method gets called every frame.
    def update(self):
        if self.contracting:
            self.contract()
        else:
            self.extend()

    def render(self):
        self.screen.blit(self.image, (self.rect.x, self.rect.y))

    def contract(self):
        expected_width = self.length * self.contraction
        counter = 0
        if self.image.get_width() > expected_width:
            self.image = pygame.transform.scale(self.image, (self.rect.width, self.image.get_height()))
            self.rect.x += 1
            self.rect.width -= 2
            self.render()
            counter += 1

    def extend(self):
        expected_width = self.length * self.relaxation
        counter = 0
        if self.image.get_width() < expected_width:
            self.image = pygame.transform.scale(self.image, (self.rect.width, self.image.get_height()))
            self.rect.x -= 1
            self.rect.width += 2
            self.render()
            counter += 1


def main():
    screen = pygame.display.set_mode((640, 480))
    clock = pygame.time.Clock()
    muscle = Muscle(screen)
    all_sprites = pygame.sprite.Group(muscle)

    done = False

    while not done:
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                done = True
            # Change the state of the muscle.
            elif event.type == pygame.KEYDOWN:
                if event.key == pygame.K_c:
                    muscle.contracting = True
            elif event.type == pygame.KEYUP:
                if event.key == pygame.K_c:
                    muscle.contracting = False

        all_sprites.update()
        screen.fill((30, 30, 30))
        all_sprites.draw(screen)

        pygame.display.flip()
        clock.tick(30)


if __name__ == '__main__':
    pygame.init()
    main()
    pygame.quit()

Alternatively, you can call extend or contract after some time interval. Check out these answers to see how you can implement a timer.