How Could I Make A Basic Car Physics In Pygame?

I wanted to add a PyGame Sprite based answer to this question. Implementing this sort of thing as a sprite makes it easier to use the PyGame collision functions. For example, any number of CarSprites could be made, but their collision checked against the player's CarSrpite in a single call to groupcollide().

This implementation uses PyGame.math.Vector2() for velocity and position. This allows for a fairly simple turning and speed model utilising the Vector2's polar co-ordinate function. Initially this gave weird and confusing result... until I realised the Vector2.from_polar() required the angle in degrees. (Not radians unlike just about every other programming language function that takes angles.)

When the sprite is initially created, the code will make a lot of pre-rotated images. This does the smoothest turning at around 1 per degree (360), but if memory-usage was a issue, it could also be much less.

Anyway, the code is fairly self-explanatory. It requires a car_128.png image, and a background texture image road_texture.png. Please comment any questions.

import pygame
import math

# Window size
WINDOW_WIDTH    = 600
WINDOW_HEIGHT   = 600
WINDOW_SURFACE  = pygame.HWSURFACE|pygame.DOUBLEBUF|pygame.RESIZABLE


class CarSprite( pygame.sprite.Sprite ):
    """ Car Sprite with basic acceleration, turning, braking and reverse """

    def __init__( self, car_image, x, y, rotations=360 ):
        """ A car Sprite which pre-rotates up to <rotations> lots of
            angled versions of the image.  Depending on the sprite's
            heading-direction, the correctly angled image is chosen.
            The base car-image should be pointing North/Up.          """
        pygame.sprite.Sprite.__init__(self)
        # Pre-make all the rotated versions
        # This assumes the start-image is pointing up-screen
        # Operation must be done in degrees (not radians)
        self.rot_img   = []
        self.min_angle = ( 360 / rotations ) 
        for i in range( rotations ):
            # This rotation has to match the angle in radians later
            # So offet the angle (0 degrees = "north") by 90° to be angled 0-radians (so 0 rad is "east")
            rotated_image = pygame.transform.rotozoom( car_image, 360-90-( i*self.min_angle ), 1 )
            self.rot_img.append( rotated_image )
        self.min_angle = math.radians( self.min_angle )   # don't need degrees anymore
        # define the image used
        self.image       = self.rot_img[0]
        self.rect        = self.image.get_rect()
        self.rect.center = ( x, y )
        # movement
        self.reversing = False
        self.heading   = 0                           # pointing right (in radians)
        self.speed     = 0    
        self.velocity  = pygame.math.Vector2( 0, 0 )
        self.position  = pygame.math.Vector2( x, y )

    def turn( self, angle_degrees ):
        """ Adjust the angle the car is heading, if this means using a 
            different car-image, select that here too """
        ### TODO: car shouldn't be able to turn while not moving
        self.heading += math.radians( angle_degrees ) 
        # Decide which is the correct image to display
        image_index = int( self.heading / self.min_angle ) % len( self.rot_img )
        # Only update the image if it's changed
        if ( self.image != self.rot_img[ image_index ] ):
            x,y = self.rect.center
            self.image = self.rot_img[ image_index ]
            self.rect  = self.image.get_rect()
            self.rect.center = (x,y)

    def accelerate( self, amount ):
        """ Increase the speed either forward or reverse """
        if ( not self.reversing ):
            self.speed += amount
        else: 
            self.speed -= amount

    def brake( self ):
        """ Slow the car by half """
        self.speed /= 2
        if ( abs( self.speed ) < 0.1 ):
            self.speed = 0

    def reverse( self ):
        """ Change forward/reverse, reset any speed to 0 """
        self.speed     = 0
        self.reversing = not self.reversing

    def update( self ):
        """ Sprite update function, calcualtes any new position """
        self.velocity.from_polar( ( self.speed, math.degrees( self.heading ) ) )
        self.position += self.velocity
        self.rect.center = ( round(self.position[0]), round(self.position[1] ) )


### initialisation
pygame.init()
pygame.mixer.init()
window = pygame.display.set_mode( ( WINDOW_WIDTH, WINDOW_HEIGHT ), WINDOW_SURFACE )
pygame.display.set_caption("Car Steering")


### Bitmaps
road_image = road_image = pygame.image.load( 'road_texture.png' )
background = pygame.transform.smoothscale( road_image, ( WINDOW_WIDTH, WINDOW_HEIGHT ) )
car_image  = pygame.image.load( 'car_128.png' ).convert_alpha()


### Sprites
black_car = CarSprite( car_image, WINDOW_WIDTH//2, WINDOW_HEIGHT//2 )
car_sprites = pygame.sprite.Group() #Single()
car_sprites.add( black_car )


### Main Loop
clock = pygame.time.Clock()
done = False
while not done:

    # Handle user-input
    for event in pygame.event.get():
        if ( event.type == pygame.QUIT ):
            done = True
        elif ( event.type == pygame.VIDEORESIZE ):
            WINDOW_WIDTH  = event.w
            WINDOW_HEIGHT = event.h
            window = pygame.display.set_mode( ( WINDOW_WIDTH, WINDOW_HEIGHT ), WINDOW_SURFACE )
            background = pygame.transform.smoothscale( road_image, ( WINDOW_WIDTH, WINDOW_HEIGHT ) )
        elif ( event.type == pygame.MOUSEBUTTONUP ):
            # On mouse-click
            pass
        elif ( event.type == pygame.KEYUP ):
            if ( event.key == pygame.K_h ):  
                print( 'meep-meep' )
            elif ( event.key == pygame.K_r ):  
                print( 'resersing' )
                black_car.reverse()
            elif ( event.key == pygame.K_UP ):  
                print( 'accelerate' )
                black_car.accelerate( 0.5 )
            elif ( event.key == pygame.K_DOWN ):  
                print( 'brake' )
                black_car.brake( )

    # Continuous Movement keys
    keys = pygame.key.get_pressed()
    if ( keys[pygame.K_LEFT] ):
        black_car.turn( -1.8 )  # degrees
    if ( keys[pygame.K_RIGHT] ):
        black_car.turn( 1.8 )

    # Update the car(s)
    car_sprites.update()

    # Update the window
    window.blit( background, ( 0, 0 ) ) # backgorund
    car_sprites.draw( window )
    pygame.display.flip()

    # Clamp FPS
    clock.tick_busy_loop(60)

pygame.quit()

car_128.png (Source: https://openclipart.org )

road_texture.png

Here is the improved code:

import pygame, math
pygame.init()

window = pygame.display.set_mode((600,600))
pygame.display.set_caption("car game")
img = pygame.image.load("1.png")

class Car:
    def __init__(self, x, y, height, width, color):
        self.x = x - width / 2
        self.y = y - height / 2
        self.height = height
        self.width = width
        self.color = color
        self.rect = pygame.Rect(x, y, height, width)
        self.surface = pygame.Surface((height, width)) # 1
        self.surface.blit(img, (0, 0))
        self.angle = 0
        self.speed = 0 # 2

    def draw(self): # 3
        self.rect.topleft = (int(self.x), int(self.y))
        rotated = pygame.transform.rotate(self.surface, self.angle)
        surface_rect = self.surface.get_rect(topleft = self.rect.topleft)
        new_rect = rotated.get_rect(center = surface_rect.center)
        window.blit(rotated, new_rect.topleft)

white = (255, 255, 255)
car1 = Car(300, 300, 73, 73, white) # 4
clock = pygame.time.Clock()

runninggame = True
while runninggame:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            runninggame = False
    
    pressed = pygame.key.get_pressed()
    car1.speed *= 0.9 # 5
    if pressed[pygame.K_UP]: car1.speed += 0.5 # 6
    if pressed[pygame.K_DOWN]: car1.speed -= 0.5 # 6

    if pressed[pygame.K_LEFT]: car1.angle += car1.speed / 2 # 7
    if pressed[pygame.K_RIGHT]: car1.angle -= car1.speed / 2 # 7
    car1.x -= car1.speed * math.sin(math.radians(car1.angle)) # 8
    car1.y -= car1.speed * math.cos(math.radians(-car1.angle)) # 8
    
    window.fill((0, 0, 0)) # 9
    car1.draw()
    pygame.display.flip()
    clock.tick(60) # 10

pygame.quit()

Some things to notice:

1. I created a new surface to use to draw the picture. This makes it easier to rotate it.
2. I created a speed variable for the car, to store its speed. I use it later for momentum.
3. The draw function rotates the image anticlockwise, because that's how Pygame works. Check out the code that I used.
4. The car dimensions that I used are 73, 73. Make this the width and height of your picture, otherwise the car won't turn properly.
5. I decrease the speed ever so slightly, so that when you don't press forward, the car goes on for a bit.
6. When the car moves forward and backward, its maximum speed is 5 pixels per frame. (Because 5 * 0.9 + 0.5 = 5.)
7. The angle that the car turns depends on the speed.
8. Here is the trigonometry that I was trying to say earlier. Because math.sin and math.cos use radians, I have to convert from degrees to radians.
9. I filled the screen with black so you wouldn't see the earlier frames.
10. The clock.tick is used to keep it from going too fast, and it means "a maximum of 60 frames per second".

I hope you understand everything.