Ping pong clone collision detection

Question

It's my first actual game-making attempt. I know code could be much better with classes or functions but I am trying to make the code simple (and shorter)

i am having problems with the collision detection of the ball and the bar/paddle. The ball sticks to the bar then carries on in the same direction. I tried some pygame collision functions and they didn't work cause i was using the surface method instead of the rect method. My question is:

how do make the ball bounce off the bar/paddle? and/or Which method or function should i be using?

#!\user\bin\ env python
import pygame, sys
from pygame.locals import *
from sys import exit
pygame.init()
#game constants
fps = 60
clock = pygame.time.Clock()
#Colors
BLACK = (  0,   0,   0)
WHITE =  (255, 255, 255)
BLUE =    (  0,   0, 255)
GREEN = (  0, 255,   0)
RED =      (255,   0,   0)

size = window_WIDTH, window_HEIGHT = 800,600
window = pygame.display.set_mode( size , pygame.RESIZABLE)
window.fill(BLACK)
ball = pygame.Surface((25, 25))
ball = ball.convert()
ball.fill(WHITE)
ball_x, ball_y = (window_WIDTH/2 -12), (window_HEIGHT/2 -12 )
ball_speed_x = 7  #ball speed = 20
ball_speed_y = 7

bar = pygame.Surface((15, 90))
bar.fill(WHITE)
bar1 = bar.convert()
bar2 = bar.convert()

bar_speed = 0
bar1_x,bar1_y = (50), (window_HEIGHT/2-60)

bar2_x,bar2_y = (window_WIDTH - 50), (window_HEIGHT/2-60)

middle_line = pygame.Surface((2,window_HEIGHT))
middle_line.fill(WHITE)
middle_line = middle_line.convert()

running = True
while running:
    for event in pygame.event.get():
        if ( event.type == pygame.QUIT ) or ( event.type == pygame.KEYDOWN and event.key == pygame.K_ESCAPE):
            running = False
        if event.type == pygame.KEYDOWN:
            if event.key == K_UP:
                bar_speed -= 20
            if event.key == K_DOWN:
                bar_speed += 20
        if event.type == pygame.KEYUP:
            if event.key == pygame.K_UP:
                bar_speed = 0
            if event.key == pygame.K_DOWN:
                bar_speed = 0

#since i don't know anything about collision, ball hitting bars goes like this.
    #colisions with the walls (veritical walls for bars)
    if bar1_y >= (window_HEIGHT - 90) or bar1_y <= 0:
        bar_speed = 0
    if bar2_y >= (window_HEIGHT - 90) or bar2_y <= 0:
        bar_speed = 0

    # collisions with bars (ball)
    if ball_x <= (bar1_x+15):
        if ball_y >= bar1_y:
            if ball_y <= (bar1_y + 45):  #checks if ball y cord is between bar y cord and bar length
                # To right direction up
                #ball_x += (bar1_x +15)
                #ball_x += ball_speed_x
                #ball_speed_y *= -1
                ball_y *= -1
                ball_x = (bar1_x +15)
                ball_speed_y = -ball_speed_y
                ball_speed_x = ball_speed_x

                # To right direction down
            if ball_y +45 <= (bar1_y + 90):#half down bar make y cord negative
                #ball_speed_y = ball_speed_y
                #ball_x += (bar1_x +15)
                #ball_x += ball_speed_x
                ball_y *= -1
                ball_x = (bar1_x +15)
                ball_speed_y = -ball_speed_y
                ball_speed_x = ball_speed_x

    if (ball_x+25) >= bar2_x:
        if ball_y >= bar2_y:
            if ball_y <= (bar2_y + 44):  #checks if ball y cord is between bar y cord and bar length
                # To right direction up
                #ball_x = (bar1_x )#+15)
                #ball_x -= ball_speed_x
                ball_y *= -1
                ball_x = (bar1_x +15)
                ball_speed_y = -ball_speed_y
                ball_speed_x = -ball_speed_x

            if ball_y +45 <= (bar2_y + 90):#half down bar make y cord negative
                #ball_speed_y = ball_speed_y
                #ball_x = (bar2_x )#+15)
                #ball_x = ball_speed_x
                ball_y *= -1
                ball_x = (bar1_x +15)
                ball_speed_y = -ball_speed_y
                ball_speed_x = -ball_speed_x

    #collisions of ball with up down walls
    if ball_y == 0:
        if ball_x > window_WIDTH/2:
            ball_speed_y = ball_speed_y
            ball_speed_x = ball_speed_x
        if ball_x < window_WIDTH/2:
            ball_speed_y = ball_speed_y
            ball_speed_x *= -1

    if ball_y == window_HEIGHT:
        if ball_x > window_WIDTH/2:
            ball_speed_y *= -1
            ball_speed_x = ball_speed_x
        if ball_x < window_WIDTH/2:
            ball_speed_y *= -1
            ball_speed_x *= -1
    #AI player
    if ball_x >= window_WIDTH/2:
        if not bar2_y == ball_y + 7.5:
            if bar2_y < ball_y + 7.5:
                bar2_y += bar_speed
            if  bar2_y > ball_y - 42.5:
                bar2_y -= bar_speed
        else:
            bar2_y == ball_y + 7.5
    if bar1_y >= 420.: bar1_y = 420.
    elif bar1_y <= 10. : bar1_y = 10.
    if bar2_y >= 420.: bar2_y = 420.
    elif bar2_y <= 10.: bar2_y = 10.

    bar1_y+= bar_speed
    ball_x += ball_speed_x
    ball_y += 1


    window.fill(BLACK)
    window.blit(middle_line,(window_WIDTH/2,0))
    window.blit(bar1,(bar1_x, bar1_y))
    window.blit(bar2,(bar2_x,bar2_y))
    window.blit(ball, (ball_x, ball_y))

    clock.tick(fps)
    pygame.display.update()
pygame.quit()

P.S I want the final game to below 100 lines of code

Answer 1

I suggest you use pygame's Rect, Sprite and Group class for a number if reasons:

• these make it easy to check for collisions with pygame.sprite.spritecollide
• it's also easy to use pixel-perfect collision with pygame.sprite.collide_mask
• checking if the ball bounces of the top or bottom of the screen also becomes dead easy, using Rect.top and Rect.bottom
• preventing the paddles going out of screen is as simple as calling Rect.clamp_ip

Here's a basic 2-player pong that should provide a good starting point. Note how simple the code is:

import pygame
import math
pygame.init()
screen = pygame.display.set_mode((640, 480))
clock = pygame.time.Clock()

# some helpful vector math functions
def normalize(v):
    vmag = magnitude(v)
    return [v[i]/vmag  for i in range(len(v))]
def magnitude(v):
    return math.sqrt(sum(v[i]*v[i] for i in range(len(v))))
def add(u, v):
    return [ u[i]+v[i] for i in range(len(u)) ]

class Paddle(pygame.sprite.Sprite):
    def __init__(self, start_pos, up_key, down_key):
        pygame.sprite.Sprite.__init__(self)
        # the image is just a white rect
        self.image = pygame.surface.Surface((20, 100))
        self.image.fill(pygame.color.Color('White'))
        self.image.set_colorkey(pygame.color.Color('Black'))
        self.rect = self.image.get_rect(topleft=start_pos)
        # using a mask so we can use pixel perfect collision
        self.mask = pygame.mask.from_surface(self.image)
        self.up_key, self.down_key = up_key, down_key
    def update(self, pressed):
        if pressed[self.up_key]:   self.rect.move_ip(0, -3)
        if pressed[self.down_key]: self.rect.move_ip(0,  3)
        # keep the paddle inside the screen
        self.rect.clamp_ip(pygame.display.get_surface().get_rect())

class Ball(pygame.sprite.Sprite):
    def __init__(self, start_pos):
        pygame.sprite.Sprite.__init__(self)
        # the image is just a white ball
        self.image = pygame.surface.Surface((20, 20))
        self.rect = self.image.get_rect(center=start_pos)
        pygame.draw.circle(self.image, pygame.color.Color('White'), self.image.get_rect().center, 10)
        self.image.set_colorkey(pygame.color.Color('Black'))
        # using a mask so we can use pixel perfect collision
        self.mask = pygame.mask.from_surface(self.image)
        # the vector we use to move the ball
        self.move_v = (1, 0.7)
        # store the absolute position in self.pos
        # because a rect can only use integers
        self.pos = self.rect.center
    def update(self, pressed):
        # check if the ball collides with any other sprite
        collide = [s for s in pygame.sprite.spritecollide(self, self.groups()[0], False, pygame.sprite.collide_mask) if s != self]
        if collide:
            # warning: this does not handle the case of the ball hits 
            # the top or bottom of the paddle, only the sides.
            self.move_v = [-self.move_v[0], self.move_v[1]]

        # check if the ball would go out of screen
        display = pygame.display.get_surface().get_rect()
        if self.rect.top < display.top and self.move_v[1] < 0 or \
           self.rect.bottom > display.bottom and self.move_v[1] > 0:
            self.move_v = [self.move_v[0], -self.move_v[1]]

        # apply a constant speed and update the position
        move_vector = [c * 4 for c in normalize(self.move_v)]
        self.pos = add(self.rect.center, move_vector)
        self.rect.center = map(int, self.pos)

player1 = Paddle((30,  190), pygame.K_w , pygame.K_s)
player2 = Paddle((590, 190), pygame.K_UP, pygame.K_DOWN)
ball = Ball(screen.get_rect().center)
sprites = pygame.sprite.Group(player1, player2, ball)

while True:
    for event in pygame.event.get():
        if event.type == pygame.QUIT: break
    else:
        pressed = pygame.key.get_pressed()
        sprites.update(pressed)
        screen.fill(pygame.color.Color('black'))
        sprites.draw(screen)
        pygame.display.flip()
        clock.tick(60)
        continue
    break
pygame.quit()

Answer 2

I think you are on the right track with collision process. Though there is some parts which are hard to understand, I think collision of the ball in up,down wall works fine.

Now you should take out x,y coordinate of the paddle/bar and consider collision of the ball and paddle now. Actually you probably want to make paddle "movable" so the x, y coordinates of paddle should be saved somewhere, transformed by the movement of paddle you make.

And like you did in collision - you should transform the direction (y or x) by -1