zooming in and out with pygame

Question

I'm attempting to make it so that you can zoom in and zoom out in my pygame project. It is working fine except when I zoom in, the screen seems to be zooming towards the top left and then readjusting itself after (making it so that the player is on the center of the screen). When I zoom out, it simply does the same with the opposite corner, can someone explain me why is it doing so? Here is my code:

import pygame
import random 
import sys

from pygame.locals import *

WINDOW_SIZE = (600, 400)
FPS = 60


# Initialize pygame and create window
pygame.init()
pygame.mixer.init()
screen = pygame.display.set_mode(WINDOW_SIZE)
zoom = 2
display = pygame.Surface((int(WINDOW_SIZE[0] / zoom), int(WINDOW_SIZE[1] / zoom)))
pygame.display.set_caption('Game')
clock = pygame.time.Clock()

player_img = pygame.image.load('player.png').convert()
player_img.set_colorkey((255, 255, 255))
player_rect = pygame.Rect(0, 0, 5, 16)

grass_img = pygame.image.load('grass.png').convert()
dirt_img = pygame.image.load('dirt.png').convert()

# 0 = Air
# 1 = Grass
# 2 = Dirt

game_map =[['0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0'],
           ['0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0'],
           ['0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0'],
           ['0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0'],
           ['0','0','0','0','0','0','1','1','1','1','1','1','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0'],
           ['1','1','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','1','1','0','0','0','0','0','0','0','0','0'],
           ['2','2','1','1','1','1','1','1','1','1','1','1','1','1','1','1','1','2','2','0','0','0','1','1','1','1','1','0'],
           ['2','2','2','2','2','2','2','2','2','2','2','2','2','2','2','2','2','2','2','0','0','0','2','2','2','2','2','0'],
           ['0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','2','2','2','0','0'],
           ['0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','2','2','0','0','0'],
           ['0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','2','0','0','0'],
           ['0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0'],
           ['0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0']]


def collision_test(rect,tiles):
    collisions = []
    for tile in tiles:
        if rect.colliderect(tile):
            collisions.append(tile)
    return collisions
 
def move(rect, movement, tiles):
    collision_direction = {'up': False, 'down': False, 'right': False, 'left' : False}
    
    rect.x += movement[0]
    collisions = collision_test(rect, tiles)
    for tile in collisions:
        if movement[0] > 0:
            rect.right = tile.left
            collision_direction['right'] = True
        if movement[0] < 0:
            rect.left = tile.right
            collision_direction['left'] = True
            
    rect.y += round(movement[1])
    collisions = collision_test(rect, tiles)
    for tile in collisions:
        if movement[1] > 0:
            rect.bottom = tile.top
            collision_direction['down'] = True
        if movement[1] < 0:
            rect.top = tile.bottom
            collision_direction['up'] = True
            
    return rect, collision_direction

mouvement_speed = 2
moving_right = False
moving_left = False
moving_up = False
moving_down = False

ctrl_pressed = False
equals_pressed = False
minus_pressed = False
zero_pressed = False

player_y_momentum = 0

air_timer = 0

scroll = [0, 0]


running = True
# Game loop
while running:
    for event in pygame.event.get():
        if event.type == QUIT:
            running = False
        if event.type == KEYDOWN:
            if event.key == K_RIGHT or event.key == K_d:
                moving_right = True
            if event.key == K_LEFT or event.key == K_a:
                moving_left = True
                
            if event.key == K_UP or event.key == K_w:
                if air_timer < 3:
                    player_y_momentum = -4
                    
            if event.key == K_DOWN or event.key == K_s:
                moving_down = True

            if event.key == K_RCTRL or event.key == K_LCTRL:    
                ctrl_pressed = True

            if event.key == K_EQUALS:
                equals_pressed = True

            if event.key == K_MINUS:
                minus_pressed = True

            if event.key == K_0:
                zero_pressed = True
                
        if event.type == KEYUP:
            if event.key == K_RIGHT or event.key == K_d:
                moving_right = False
            if event.key == K_LEFT or event.key == K_a:
                moving_left = False
            if event.key == K_UP or event.key == K_w:
                moving_up = False
            if event.key == K_DOWN or event.key == K_s:
                moving_down = False
            if event.key == K_EQUALS:
                equals_pressed = False
            if event.key == K_MINUS:
                minus_pressed = False
            if event.key == K_0:
                zero_pressed = False
            if event.key == K_RCTRL or event.key == K_LCTRL:    
                ctrl_pressed = False

    if equals_pressed and ctrl_pressed:
        zoom += 0.2
        print('ZOOMING IN')

    if minus_pressed and ctrl_pressed:
        zoom -= 0.2
        print('ZOOMING OUT')

    if zero_pressed and ctrl_pressed:
        zoom = 2
        print('RESET')
        
    try:   
        display = pygame.Surface((int(WINDOW_SIZE[0] / zoom), int(WINDOW_SIZE[1] / zoom)))
    except:
        zoom = 0.4

    display.fill((5, 195, 225))

    tile_rect = []
    y = 0
    for layer in game_map:
        x = 0
        for tile in layer:
            if tile == '1':
                display.blit(grass_img, (x * 16 - scroll[0], y * 16 - scroll[1]))
            if tile == '2':
                display.blit(dirt_img, (x * 16 - scroll[0], y * 16 - scroll[1]))
            if tile != '0':
                tile_rect.append(pygame.Rect(x * 16, y * 16, 16, 16))  
            x += 1
        y += 1

    scroll[0] += ((player_rect.x - int(WINDOW_SIZE[0]/ (zoom * 2)) + 2) - scroll[0]) / 12
    scroll[1] += ((player_rect.y - int(WINDOW_SIZE[1]/ (zoom * 2)) + 5) - scroll[1]) / 12
    
    player_mouvement = [0, 0]
    
    if moving_right:
        player_mouvement[0] += mouvement_speed
    if moving_left:
        player_mouvement[0] -= mouvement_speed
    if moving_down:
        player_mouvement[1] += 5    
    player_y_momentum += 0.2
    if player_y_momentum > 5:
        player_y_momentum = 5
        
    player_mouvement[1] += player_y_momentum
    
    player_rect, collision_direction = move(player_rect, player_mouvement, tile_rect)

    if collision_direction['down']:
        air_timer = 0
        player_y_momentum = 0
    else:
        air_timer += 1
        
    if collision_direction['up']:
        player_y_momentum = 0
                     
    display.blit(player_img, (player_rect.x - scroll[0], player_rect.y - scroll[1]))
    #pygame.draw.rect(display,(255, 255, 255),player_rect)

    #print(air_timer)      
    screen.blit(pygame.transform.scale(display,WINDOW_SIZE),(0,0))
    pygame.display.update()
    clock.tick(FPS)
    

pygame.quit()
sys.exit()

Answer 1

Your scroll calculation is a bit confusing and I am not quite sure what the calculation is exactly doing (and there are no comments to explain it). Clearly the intent is to scroll it to track the player and so adjust where the game_map gets drawn onto the display and where the player gets drawn onto it. You then scale() that to the screen to get your zoom effect

Though I cannot comment on exactly what the issue is with the scroll I could suggest a different and I believe simpler way to go about this. Draw your display without doing the scroll stuff. Then use subsurface() to grab a section of display centered on your player with the desired size to achieve the zoom effect you want. Then scale that subsurface and blit() it onto screen.

Although above I just said centered on your player for simplicity, you would have to adjust that if the player was too close to the edges of display at the desired zoom level.

This should simplify all the creation of the display and the only effort is in deciding where and how big the subsurface is within display to get the location and zoom factor that you want.

Answer 2

All that you need to do is lock the scroll towards the player by using

scroll[0] = player_rect.x - int(WINDOW_SIZE[0]/ (zoom * 2)) + 2
scroll[1] = player_rect.y - int(WINDOW_SIZE[1]/ (zoom * 2)) + 5