How to draw a moving circle in Pygame with a small angle at a low speed and blinking?

Question

The code below moves two balls on the screen. The first one is moved with an angle of 10 degree at a low speed with a good drawing quality. The second ball is moved with an

Answer 1

The issue is caused, because pygame.Rect stores integral coordinates:

The coordinates for Rect objects are all integers. [...]

The fraction component of delta_x and delta_y is lost at self.rect.move(delta_x, delta_y).

You have to use floating point numbers for the computation. Add an attribute self.pos, which is a tupe with 2 components an stores the center point of a ball:

self.pos = self.rect.center

Compute the position with the maximum floating point accuracy:

delta_x = self.speed * math.cos(self.angle)
delta_y = self.speed * math.sin(self.angle)
self.pos = (self.pos[0] + delta_x, self.pos[1] + delta_y)   

Update the self.rect.center by the round()ed position.

self.rect.center = round(self.pos[0]), round(self.pos[1])

self.pos is the "internal" position and responsible for the exact computation of the position. self.rect.center is the integral position and responsible to draw the ball. self.pos slightly changes in each frame. self.rect.center changes only if the a component of the coordinate has changed by 1.

Class Ball:

class Ball():
    def __init__(self,
                 screen,
                 color,
                 radius,
                 startX,
                 startY,
                 speed,
                 angle=45):
        super().__init__()
        self.screen = screen
        self.color = color
        rectSize = radius * 2
        self.rect = pygame.Rect(startX, startY, rectSize, rectSize)
        self.speed = speed
        self.angle = math.radians(angle)
        self.pos = self.rect.center

    def update(self):
        delta_x = self.speed * math.cos(self.angle)
        delta_y = self.speed * math.sin(self.angle)
        self.pos = (self.pos[0] + delta_x, self.pos[1] + delta_y)

        self.rect.center = round(self.pos[0]), round(self.pos[1])

        if self.rect.right >= self.screen.get_width() or self.rect.left <= 0:
            self.angle = math.pi - self.angle

        if self.rect.top <= 0 or self.rect.bottom >= self.screen.get_height():
            self.angle = -self.angle

    def draw(self):
        '''
        Draw our ball to the screen with position information.
        '''
        pygame.draw.circle(self.screen, self.color, self.rect.center, int(self.rect.width / 2))

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With this solution you can scale up the flops per second (clock.tick()) and scale down the speed by the same scale. This leads to a smooth movement without blinking.