Generate "Ulam spiral (prime spiral)"

ulam_spiral.py

# -*- encoding: utf-8 -*-
from PIL import Image
from math import sqrt

Max = 333 
pix = 3 
filename = 'output.png'

f1 = lambda x,y: sorted(range(x-(y-1)*2+1-y, x-(y-1)*2+1),reverse=True)
isprime = lambda x: x != 1 and (x == 2 or not 0 in [x%y for y in range(2,int(sqrt(x))+1)])
ulam = []

for i in range(Max/2+1):
  ulam.append(f1((Max-2*i)**2,Max-2*i))

for i in range(1,Max/2+1):
  ulam.append(range((i*2+1)**2-i*2,(i*2+1)**2+1))

for i in range(1,len(ulam)-1):
  m = i if i < Max/2+1 else Max - i - 1
  for j in range(m):
    ulam[i].insert(j,ulam[i-1][j]+1)
  ulam[i] += reversed([ulam[i-1][Max-1-x]-1 for x in range(m)])

for i in range(len(ulam)):
  ulam[i] = map(isprime,ulam[i])

img = Image.new('RGB',(Max*pix,Max*pix),(255,255,255))
for y in range(len(ulam)):
  for x in range(len(ulam[i])):
    if ulam[y][x]: color = (0,0,0)
    else: color = (255,255,255)
    for a in range(pix):
      for b in range(pix):
        img.putpixel((x*pix+b,y*pix+a),color)
img.save(filename, 'PNG')