pygame的/ Python的。 左移动似乎比右移动快。 为什么？

Question

I've coded a bit c++ and java during the past few years and now i've started with python. The goal is to make a game. I've noticed, however, that at the basic animation part of a player moving left and right, my left "speed" is faster than the right "speed". Although my "speed" values are exactly the same.

The update function:

def update(self,dt,game):
    last=self.rect.copy()


    self.time+=dt


    if self.left:
       self.rect.x-=300*dt
    if self.right:
       self.rect.x+=300*dt

I've noticed that if i change the left speed to 250, instead of 300, it will run the same. For example if i press both left and right keys the player will stay on the exact same spot. However if i have the left and right speed both at 300. The player will move slowly to the left.

My game loop:

while 1:
        dt=clock.tick(60)
        if dt>1000/60.0:
            dt=1000/60.0

        for event in pygame.event.get():
            if event.type==pygame.QUIT:
                return
            if event.type==pygame.KEYDOWN and event.key==pygame.K_ESCAPE:
                return
            if event.type==pygame.KEYDOWN and event.key==pygame.K_LEFT:
                self.player.left=True
                self.player.image=self.player.leftimages[1]
            if event.type==pygame.KEYDOWN and event.key==pygame.K_RIGHT:
                self.player.right=True
                self.player.image=self.player.rightimages[1]
            if event.type==pygame.KEYUP and event.key==pygame.K_LEFT:
                self.player.left=False
                self.player.image=self.player.leftimages[0]
            if event.type==pygame.KEYUP and event.key==pygame.K_RIGHT:
                self.player.right=False
                self.player.image=self.player.rightimages[0]

        self.tilemap.update(dt/1000.0,self)
        screen.fill((0,0,0)) 
        self.tilemap.draw(screen)
        pygame.display.update()

I'm using a tmx library from Richard Jones to be able to import my map made in tiled map editor. I hope this is sufficient information to answer my weird question. I've asked my other programming friend and he finds it weird.

Some code might be off, beacause i've just copy pasted from my original code that has some more stuff into it. But i've extracted the most basic parts that control the movement.

Thank you for your time!

Answer 1

Have you tried printing the value of dt from your update function? If the speed is different when you move left from when you move right, then that is what must be changing (backed up by you saying that holding left and right at the same time results in no movement).

Based on my own testing, it looks like pygame.time.Clock.tick() returns an integer value representing the number of milliseconds that have passed since the last call. Where you attempt to correct this value ( dt=1000/60.0 ), you will be getting a floating point value. I suspect that for some reason, the timer is off when either pressing left or right, and your attempts to correct it manually are causing a different dt value to be passed in to the update function during these situations.

The pygame documentation suggests pygame.time.Clock.tick_busy_loop() if you need a more accurate (but CPU intensive) timer.

http://www.pygame.org/docs/ref/time.html#pygame.time.Clock.tick_busy_loop

Answer 2

Instead of calculating the change in time since the last frame and then basing how far the player will move off of that, you should call pygame.time.Clock.tick once a frame. Then you can assume that the time between frames will be the same every time, so dt will always be the same.

As of now if there is a drop in the fps, everything else in your program will slow down, but your player will start to jump ahead faster than everything else. Rather than relying on time elapsed, it's usually best to depend on the fps.