Python - animated line - (Stackoverflow)

# date: 2024.06.22
# [Why does my python script not access global variables properly? - Stack Overflow](https://stackoverflow.com/questions/78654048/why-does-my-python-script-not-access-global-variables-properly)

import cv2
import numpy as np
import random

# --- constants ---

WIDTH = 1000
HEIGHT = 1000
FRAMES = 6000
FPS = 60

# --- functions ---

def modify_positions(x1, y1, x2, y2):
    while True:
        mx1 = x1 + random.randint(-5, 5)
        my1 = y1 + random.randint(-5, 5)
        mx2 = x2 + random.randint(-5, 5)
        my2 = y2 + random.randint(-5, 5)

        if 0 <= mx1 < WIDTH and 0 <= my1 < HEIGHT and 0 <= mx2 < WIDTH and 0 <= my2 < HEIGHT:
            x1 = mx1
            y1 = my1
            x2 = mx2
            y2 = my2
            break

    return x1, y1, x2, y2

# --- main ---

x1 = random.randint(0, WIDTH - 1)
y1 = random.randint(0, HEIGHT - 1)
x2 = random.randint(0, WIDTH - 1)
y2 = random.randint(0, HEIGHT - 1)

frame = np.zeros((HEIGHT, WIDTH, 3), dtype=np.uint8)

video_name = 'output_video.avi'
fourcc = cv2.VideoWriter_fourcc(*'XVID')
video_writer = cv2.VideoWriter(video_name, fourcc, FPS, (WIDTH, HEIGHT))

video_writer.write(frame)

for frame_num in range(1, FRAMES):

    # display some progress
    if frame_num % 10 == 0:
        #print('.', end='', flush=True)
        print(f'{frame_num:05}', end='\r', flush=True)

    current_frame = np.copy(frame)

    x1, y1, x2, y2 = modify_positions(x1, y1, x2, y2)

    affected_pixels = []
    dx = abs(x2 - x1)
    dy = abs(y2 - y1)
    sx = 1 if x1 < x2 else -1
    sy = 1 if y1 < y2 else -1
    err = dx - dy

    temp_x = x1
    temp_y = y1
    while True:
        affected_pixels.append((temp_x, temp_y))

        if temp_x == x2 and temp_y == y2:
            break

        e2 = err * 2

        if e2 > -dy:
            err -= dy
            temp_x += sx

        if e2 < dx:
            err += dx
            temp_y += sy

    # invert_affected_pixels
    for x, y in affected_pixels:
        current_frame[y, x] = 255 - current_frame[y, x]  # Invert pixel color

    video_writer.write(current_frame)
    frame = current_frame

video_writer.release()

print(f"Video saved as {video_name}")