""" (C) Steven Byrnes, 2014. This code is released under the MIT license http://opensource.org/licenses/MIT  This code should work in Python 2.7 or 3.3. It requires imagemagick to be installed; that's how it assembles images into animated GIFs. """  from __future__ import division, print_function  import pygame as pg from math import pi from cmath import exp  import subprocess, os directory_now = os.path.dirname(os.path.realpath(__file__))  frames_in_anim = 200 animation_loop_seconds = 5 #time in seconds for animation to loop one cycle  bgcolor = (255,255,255) #white  circle_radius = 12  img_height = 300 img_width = 1000  # pygame draws pixel-art, not smoothed. Therefore I am drawing it # bigger, then smoothly shrinking it down final_width = int(round(0.3 * img_width)) final_height = int(round(0.3 * img_height))  # Constants and function for calculating electron motion def coef(n):     """ f(t) = sum of Re[coef(n) * e^(int)] """     return 0 if n % 2 == 0 else -1j / n  def exact(phase):     """ exact square wave function """     if phase == 0 or phase == pi:         return 0     if (phase % (2*pi)) < pi:         return pi/4     return -pi/4  center_x = 500 scale = 450 # scale multiplies all horizontal distances  def draw_arrow(surf, tail_xy, head_xy, width=2, color=(0,0,0)):     """     draw a horizontal arrow     """     # tail_xy and head_xy are 2-tuples. Unpack them first     tail_x, tail_y = tail_xy     head_x, head_y = head_xy     assert head_y == tail_y     h = 16 # arrowhead height     b = 18 # arrowhead half-base     if tail_x < head_x:         # rightward arrow         triangle = [(head_x, head_y),                     (head_x - h, head_y - b),                     (head_x - h, head_y + b)]     else:         # leftward arrow         triangle = [(head_x, head_y),                     (head_x + h, head_y - b),                     (head_x + h, head_y + b)]     pg.draw.line(surf, color, (tail_x, tail_y), (head_x, head_y), width)     pg.draw.polygon(surf, color, triangle, 0)  def main():     """ function for creating the animated GIF """     # Make and save a drawing for each frame     filename_list = [os.path.join(directory_now, 'temp' + str(n) + '.png')                          for n in range(frames_in_anim)]           for frame in range(frames_in_anim):         phase = 2 * pi * frame / frames_in_anim                  # initialize surface         surf = pg.Surface((img_width,img_height))         surf.fill(bgcolor)                  # draw all the arrows         x_now = center_x         y_now = 30         for n in range(1,13,2):             new_term = (scale * coef(n) * exp(n * 1j * phase)).real             draw_arrow(surf, (x_now, y_now), (x_now + new_term, y_now), width=8)             x_now += new_term             y_now += 30                  # draw the two circles         pg.draw.circle(surf, (100,0,100),                        (int(round(x_now)),int(round(y_now + 10))),                        circle_radius, 0)         pg.draw.circle(surf, (50,50,150),                        (int(round(center_x + scale * exact(phase))),                             int(round(y_now + 40))),                        circle_radius, 0)          # scale down then save the surface         shrunk_surface = pg.transform.smoothscale(surf, (final_width, final_height))         pg.image.save(shrunk_surface, filename_list[frame])      seconds_per_frame = animation_loop_seconds / frames_in_anim     frame_delay = str(int(seconds_per_frame * 100))     command_list = ['convert', '-delay', frame_delay, '-loop', '0'] + filename_list + ['anim.gif']     # Use the "convert" command (part of ImageMagick) to build the animation     subprocess.call(command_list, cwd=directory_now)     # Earlier, we saved an image file for each frame of the animation. Now     # that the animation is assembled, we can (optionally) delete those files     if True:         for filename in filename_list:             os.remove(filename)     return  main()