Question

I want to fade in a rectangle from alpha = 0 to alpha = 255 over 2 seconds. Using the following code, I am able to get the rectangle fade in but I don’t know to get the duration to be exactly (as exact as possible) 2 seconds.

pygame.init()

#frames per second setting
FPS = 30
fpsClock = pygame.time.Clock()

#Set up window
screen = pygame.display.set_mode((1280,800))
shape = screen.convert_alpha()

#Colors
alpha = 0
WHITE = (255, 255, 255,alpha)
BLACK = (0,0,0)

#Stimuli
stimulus = pygame.Rect(100,250,100,100)

def fade():
    """Fades in stimulus"""
    global alpha
    alpha = alpha + 5   <--- (Do I change this increment?)
    #Draw on surface object
    screen.fill(BLACK)
    shape.fill(BLACK)
    pygame.draw.rect(shape,(255, 255, 255,alpha),stimulus)
    screen.blit(shape,(0,0))
    pygame.display.update(stimulus)
    fpsClock.tick(FPS)

while True:
    fade()

Additionally, should I use the flags HWSURFACE and DOUBLEBUF if I am using fullscreen? Thanks.

Answer 1

I would try to measure actual time passed since the operation start and make alpha directly proportional to the remaining time.

Something along the lines:

import time

def fade():
    DURATION = 2.0 # seconds
    start_time = time.clock()
    ratio = 0.0 # alpha as a float [0.0 .. 1.0]
    while ratio < 1.0:
        current_time = time.clock()
        ratio = (current_time - start_time) / DURATION
        if ratio > 1.0: # we're a bit late
            ratio = 1.0
        # all your drawing details go in the following call
        drawRectangle(coordinates, alpha = 255 * ratio)
        fpsClock.tick(FPS)

This way, if tick works imperfectly (which it does), your alpha follows its imperfections, instead of accumulating the error. When 2 seconds are up, you’re guaranteed to have drawn a fully opaque rectangle.