Pygame line drawing example using Bresenham Algorithm

drawline.py

import pygame
from pygame.locals import *

WIDTH, HEIGHT = 100, 100
PIXEL_SIZE = 5 # should be bigger than 1
SCREEN_WIDTH, SCREEN_HEIGHT = WIDTH * PIXEL_SIZE, HEIGHT * PIXEL_SIZE

surf = pygame.Surface((SCREEN_WIDTH, SCREEN_HEIGHT))
surf.fill((0, 0, 0))

def put_pixel(x, y):
    global surf
    pygame.draw.rect(surf, (255, 0, 0),
                     (x * PIXEL_SIZE, y * PIXEL_SIZE, PIXEL_SIZE, PIXEL_SIZE))

def put_line(x0, y0, x1, y1):
    # referenced from http://www.poshy.net/java/graphic/linedraw4.htm

    dx = x1 - x0
    dy = y1 - y0

    if dy < 0:
        dy = -dy
        stepy = -1
    else:
        stepy = 1

    if dx < 0:
        dx = -dx
        stepx = -1
    else:
        stepx = 1

    dx <<= 2
    dy <<= 2

    put_pixel(x0, y0)

    if dx > dy:
        fraction = dy - (dx >> 1)
        while x0 != x1:
            if fraction >= 0:
                y0 += stepy
                fraction -= dx
            x0 += stepx
            fraction += dy
            put_pixel(x0, y0)
    else:
        fraction = dx - (dy >> 1)
        while y0 != y1:
            if fraction >= 0:
                x0 += stepx
                fraction -= dy
            y0 += stepy
            fraction += dx
            put_pixel(x0, y0)

pygame.init()
screen = pygame.display.set_mode((SCREEN_WIDTH, SCREEN_HEIGHT))
pygame.display.set_caption('Line Drawing')
clock = pygame.time.Clock()

running = True
while running:
    for event in pygame.event.get():
        if event.type == QUIT:
            running = False
        elif event.type == KEYDOWN:
            if event.key == K_ESCAPE:
                running = False
        elif pygame.mouse.get_pressed()[0] and event.type == MOUSEMOTION:
            x, y = event.pos[0] / PIXEL_SIZE, event.pos[1] / PIXEL_SIZE
            rel_x, rel_y = (int(event.rel[0] / float(PIXEL_SIZE)),
                            int(event.rel[1] / float(PIXEL_SIZE)))
            put_line(x - rel_x, y - rel_y, x, y)

    if running:
        clock.tick(60)

        screen.blit(surf, (0, 0))
        pygame.display.update()