[FIXED] Why does pygame crash when I try to plot individual pixels to the screen for an image

Issue

I've been working on a piece of code that compares two images and tells me if they are similar, it also tells me which pixels are different in the image, after this it plots them into a pygame screen so that I can see which parts of the image are moving more clearly. The only problem is that it seems as if pygame cannot handle it or something and it crashes, no errors appear.

code:

import cv2
import pygame
from pygame.locals import *

lib = 'Map1.png'
lib2 = 'Map2.png'
lib3 = []
coordenatesx = []
coordenatesy = []

Read = list(cv2.imread(lib).astype("int"))
Read2 = list(cv2.imread(lib2).astype("int"))

counter = 0

for i in range(len(Read)):#y coords
    for j in range(len(Read[i])):#x coords
        blue = list(Read[i][j])[0]
        green = list(Read[i][j])[1]
        red = list(Read[i][j])[2]
        blue2 = list(Read2[i][j])[0]
        green2 = list(Read2[i][j])[1]
        red2 = list(Read2[i][j])[2]
        difference = (blue+green+red)-(blue2+green2+red2)
        lib3.append(difference)
        if difference <= 10 and difference >= -10:
            counter+=1
            coordenatesx.append(j)
            coordenatesy.append(i)


if counter >= (i*j)*0.75:
    print('They are similar images')
    print('They are different by:', str((counter / (i * j)) * 100), '%')
else:
    print('They are different')
    print('They are different by:', str((counter / (i * j)) * 100), '%')

pygame.init()

screen = pygame.display.set_mode((500,500))

while 1:
    screen.fill((20))
    for event in pygame.event.get():
        if event.type == QUIT or (event.type == KEYDOWN and event.key == K_ESCAPE):
            pygame.quit()
    for l in range(len(coordenatesx)):
        for v in range(len(coordenatesy)):
            pygame.draw.rect(screen, (blue, red, green), pygame.Rect(coordenatesx[l], coordenatesy[v], 1, 1))
    pygame.display.update()

image1:

image2:

Solution

Pygame didn't crash. You know how defining a Pygame window without calling the pygame.event.get() method would cause problems, right? Well, when you put

    for l in range(len(coordenatesx)):
        for v in range(len(coordenatesy)):
            pygame.draw.rect(screen, (blue, red, green), pygame.Rect(coordenatesx[l], coordenatesy[v], 1, 1))

into the while loop that's supposed to constantly call the pygame.event.get() method, you are dramatically slowing down the looping process.

To see this with your eyes, add a print() statement into the loop, and see how slow it prints:

while 1:
    screen.fill((20))
    print("Looping...")
    for event in pygame.event.get():
        if event.type == QUIT or (event.type == KEYDOWN and event.key == K_ESCAPE):
            pygame.quit()
    for l in range(len(coordenatesx)):
        for v in range(len(coordenatesy)):
            pygame.draw.rect(screen, (blue, red, green), pygame.Rect(coordenatesx[l], coordenatesy[v], 1, 1))
    pygame.display.update()

One fix is to move the pygame.event.get() call into the nested for loop (as well as the pygame.display.update() call if you want to see the updating):

while 1:
    screen.fill((20))
    for l in range(len(coordenatesx)):
        for v in range(len(coordenatesy)):
            for event in pygame.event.get():
                if event.type == QUIT or (event.type == KEYDOWN and event.key == K_ESCAPE):
                    pygame.quit()
            pygame.draw.rect(screen, (blue, red, green), pygame.Rect(coordenatesx[l], coordenatesy[v], 1, 1))
            pygame.display.update()

Answered By - Ann Zen

Answer Checked By - Marilyn (PHPFixing Volunteer)