Koch snowflake in Python with numpy and pygame

Question

I've drawn a Koch snowflake with Python 3. I'm new to the Python world and would appreciate all feedback on how to make the code more Pythonic as I'm not used to these idioms, styles and modules. All other feedback is of course also greatly appreciated.

The snowflake is constantly redrawn in preparation for the future. This main loop is based on the pygame.draw example but feels a bit clunky with done=False;while not done etc.

import pygame
import numpy
from math import pi, sin, cos

# some constants
FPS = 10
WINDOW_SIZE = [400,400]
LINE_WIDTH = 1
MIN_LINE_LENGTH = 1 # when to terminate recursion

# main loop
def main():
    # init pygame and window
    pygame.init()
    screen = pygame.display.set_mode(WINDOW_SIZE)
    # loop until user exits
    done = False
    clock = pygame.time.Clock()
    while not done:
        # limit frame rate
        clock.tick(FPS)
        # check if user exited
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                done=True
        # clear screen
        screen.fill([255,255,255])
        # calculate the three corners of the snowflake,
        # an equilateral triangle centered on the screen
        screen_center = numpy.array([ WINDOW_SIZE[0]/2, WINDOW_SIZE[1]/2 ])
        radius = WINDOW_SIZE[1]/2 # not really radius? but distance from center to corner
        snowflake_top   = screen_center + vector_from_polar( radius, pi/2 )
        snowflake_left  = screen_center + vector_from_polar( radius, pi/2 + 2*pi/3 )
        snowflake_right = screen_center + vector_from_polar( radius, pi/2 + 4*pi/3 )
        # draw the snowflake
        draw_koch_line( screen, snowflake_top,   snowflake_left  )
        draw_koch_line( screen, snowflake_left,  snowflake_right )
        draw_koch_line( screen, snowflake_right, snowflake_top   )
        # flip buffers
        pygame.display.flip()


# vector_from_polar: constructs a vector from its angle and mangitude
def vector_from_polar( magnitude, angle ):
    return magnitude * numpy.array( [ numpy.cos( angle ), numpy.sin( angle ) ] )

# draw_koch_line: (approximately) draws a Koch line between the points specified
def draw_koch_line(screen, line_start, line_end ):
    if numpy.linalg.norm( line_end - line_start ) / 3 < MIN_LINE_LENGTH:
        # last iteration: draw the line
        pygame.draw.line(screen, [0,0,0], line_start, line_end, LINE_WIDTH )
    else:
        # find the normal to this line
        line_normal = numpy.array([
                line_end[1]-line_start[1],
                line_start[0]-line_end[0] ])
        # find the three points of the "triangle" used to define the segments
        triangle_left  = 2/3*line_start + 1/3*line_end # 1/3 of this line
        triangle_right = 1/3*line_start + 2/3*line_end # 2/3 of this line
        triangle_top   = 1/2*line_start + 1/2*line_end + \
                         numpy.sqrt(3)/2/3 * line_normal # point "above" the line
        # recurse for each segments
        draw_koch_line(screen, line_start,     triangle_left)
        draw_koch_line(screen, triangle_left,  triangle_top)
        draw_koch_line(screen, triangle_top,   triangle_right)
        draw_koch_line(screen, triangle_right, line_end)


main()

Answer 1

This is good code — I only have a couple of minor points.

1. Since nothing changes or animates, there's no need to have a top-level loop or a clock. You can just draw the snowflake and then wait for the user to quit (calling event.wait instead of event.get so that the program isn't busy doing nothing).

2. It's clearer to write pygame.Color('white') and pygame.Color('black') instead of [255, 255, 255] and [0, 0, 0].

3. Instead of:

   # find the normal to this line
   line_normal = numpy.array([
           line_end[1]-line_start[1],
           line_start[0]-line_end[0] ])
   

   consider using the cross product:

   line_normal = numpy.cross(line_end - line_start, [0, 0, 1])
   

4. There's repeated code in the computation of the triangle points that could be factored out into a function:

   def interpolated(x, start, end):
       """Return vector interpolated between start and end by x."""
       return (1 - x) * start + x * end
   

   and then:

   triangle_left  = interpolated(1/3, line_start, line_end)
   triangle_right = interpolated(2/3, line_start, line_end)