#!/usr/bin/python """ random_walk.cgi A CGI script to generate an image of a random walk using the python PIL library. final version : a random walk, URL parameters and cookies, oh my. Options which can be set in the URL and remembered in a cookie : key description default ------------------ ----------------------------------------- ------- width window width in pixels 400 height window height in pixels 400 steplength length of random walk step in pixels 8 maxsteps maxium number steps (or stop at image edge) 1000 colorchangeperstep max (r,g,b) color shift per step 16 default if set (i.e. = 1), all other options are reset to their default values. Running it over the web : First visit it with arguments in the URL : http://....../random_walk.cgi?width=200&height=200 Then successive visits will keep the same size (storing data in a cookie) http://....../random_walk.cgi Running it from the command line : The HTTP headers won't be printed if the REQUEST_METHOD environment variable isn't set, so there are several ways it can be run. $ ./random_walk.cgi > rw.png # write an image to a file $ REQUEST_METHOD=GET ./random_walk.cgi # print HTTP headers and image This last approach could also be used to try to 'fake' a web request at the command line, by setting for environment variables like QUERY_STRING and HTTP_COOKIE explicitly. Jim Mahoney | cs.marlboro.edu | Sep 2012 | MIT License """ from PIL import Image, ImageDraw from random import randint from sys import stdout from math import sin, cos, pi import cgi import Cookie import os ## --- debugging ----------------------------------------------------- ## ## With these next two lines turned on, ## exceptions will be displayed in the returned web page. ## However, compile errors still crash the program - ## run it from the command line and/or look in the web server log. ## if False: import cgitb # CGI TraceBack cgitb.enable() ## ## To see the sort of run time error that cgitb can display, ## do something like like this to hrow a ZeroDivisionError exception. ## #if True: # a = 1/0 # stop execution with an exception ## ## Here's another debugging trick. ## Since the cgitb routine shows the value of passed arguments, ## you can pass a function anything you want to see, and then ## make it throw an exception. ## For example, if you turn on this definition, and enable cgitb, ## then invoking debug_show(some_variable) would display its value. ## if False: def debug_show(x): # use cgitb to see the value of x return 1/0 # some_variable = {'one':1, 'ten':10} # debug_show(some_variable) # These image options (i.e. arguments) can be set by a POST or GET # web request, and will be remembered with cookies. input_keys = ('width', 'height', 'steplength', 'maxsteps', 'colorchangeperstep') def default_args(): """Return the default arguments that specifiy the image to be drawn""" return { 'image_mode': 'RGB', 'background_color': (224, 224, 232), # (red,green,blue) from 0 to 255 'line_width': 2, # pixels (may be buggy and mean 4) 'width': 400, # of the image, in pixels 'height': 400, 'steplength': 8, # in pixels 'maxsteps': 1000, # walk stops at image edge or this 'colorchangeperstep': 16, # maximum delta (red, green, blue) } def args_from_cookie(args = {}): """ Return image arguments from cookie else from given args """ # For each argument (e.g. 'width') the corresponding cookie name # is preceded by "randomwalk_" (e.g. "randomwalk_width") cookie = Cookie.SimpleCookie() try: cookie.load(os.environ['HTTP_COOKIE']) for key in input_keys: # cookie[] returns a Morsel object; # see docs.python.org/library/cookie.html ; # its .value is a string, which is converted to integer. args[key] = int(cookie['randomwalk_' + key].value) except (KeyError, ValueError, TypeError): pass # Just keep going if there's an error. return args def args_from_GET_and_POST(args = {}): """ Return image arguments from GET or POST or from given args. But if 'default' is set, return default_args() """ # An example web request would be e.g. # http://..../random_walk.cgi?maxsteps=3&steplength=32 post_or_get = cgi.FieldStorage() if post_or_get.has_key('default'): return default_args() else: for key in input_keys: try: # Again, the .value field has the data. args[key] = int(post_or_get[key].value) except (KeyError, ValueError, TypeError): pass return args def set_cookie_string(args): """ Return the string for setting the cookie in the HTTPD response""" # e.g. 'Set-Cookie: randomwalk_width=32\r\nSet-Cookie: randomwalk_height=64' # # By default the cookie Path includes the full URL. # Note that this means if the script changes location, # there may be duplicate cookies stored with the same names. # cookie = Cookie.SimpleCookie() cookie.clear() # We're going to put in new values. for key in input_keys: cookie['randomwalk_' + key] = args[key] return cookie.output() def random_direction_step(distance): """ Return random direction (dx,dy) with (dx**2 + dy**2) ~ distance**2 """ angle = 2 * pi * randint(0,999)/1000.0 return int(distance * cos(angle)), int(distance * sin(angle)) def make_image(args={}): """ Return a PIL Image object representing a random walk. """ image = Image.new(args['image_mode'], (args['width'], args['height']), args['background_color']) draw = ImageDraw.Draw(image) start_point = (args['width']/2, args['height']/2) color = (randint(0,255), randint(0,255), randint(0,255)) for i in range(args['maxsteps']): delta = random_direction_step(args['steplength']) end_point = (start_point[0] + delta[0], start_point[1] + delta[1]) draw.line((start_point, end_point), fill=color, width=args['line_width']) dcolor = args['colorchangeperstep'] # The tuple() conversion in what follows is required by # draw.line() routine, which wants tuples, not lists. # I'm also using a 'python list comprehension' syntax. # For example, [i**2 for i in (2,3,4)] evaluates to [4, 9, 16]. color = tuple([(color[i] + randint(-dcolor, dcolor)) % 255 \ for i in (0,1,2)]) if end_point[0] < 0 or end_point[0] > args['width'] or \ end_point[1] < 0 or end_point[1] > args['height']: break start_point = end_point return image arguments = args_from_GET_and_POST(args_from_cookie(default_args())) if os.environ.has_key('REQUEST_METHOD'): # Is this a web request? print set_cookie_string(arguments) print "Content-Type: image/png\n" print make_image(arguments).save(stdout, 'PNG')