Sieve of Eratosthenes image generator python code

#Made by /u/iamaquantumcomputer
#Preview to what output should look like: http://imgur.com/gallery/0gI8eK6
#usage from terminal: $ python sieve.py width height
#   example used to generate linked preview: $ python sieve.py 683 384
#relies on PIL (Python Imaging Library) to function

from PIL import Image
import sys

try:
    width = int(sys.argv[1])
    height = int(sys.argv[2])
except IndexError:
    width = 1080
    height = 768
except ValueError:
    width = 1080
    height = 768

def convert(num):
    x = (num-1)%width
    y = (num-1)/width

    return (x,y)

def plot(num):
    tup = convert(num)
    x = tup[0]
    y = tup[1]
    pixdata[x,y] = (0,0,0)

def calcsieve():
    plot(1)

    print "eliminating multiples of 2"
    for m in range(4,width*height+1,2):
        plot(m)
        #print "\tkilling " + str(m)
    for a in range(3,int((width*height)**.5)+1,1):
        coor = convert(a)
        if (pixdata[coor[0],coor[1]][0]==255):
            print "eliminating multiples of " + str(a)
            for b in range(3*a,width*height+1,2*a):
                #print "\tkilling " + str(b)
                plot(b)

img = Image.new("RGB", (width,height), (255,255,255))
pixdata = img.load()
calcsieve()
img.save("sieve.png", "PNG")