Generate images of letters in different colors

imagegenerator.py

from subprocess import call
import os
import sys

n = int(sys.argv[1])
for i in range(n):
    directory = 'output/{0}'.format(i)
    if not os.path.exists(directory):
        os.makedirs(directory)
    for letter in 'ABCDEFGHIJKLMNOPQRSTUVWXYZ':
        label = 'label:{0}'.format(letter)
        filepath = '{0}/{1}.png'.format(directory, letter.lower())
        hue = int(float(i) / (n-1) * 255)
        color = 'hsb({0}, 200, 255)'.format(hue)
        call(['convert', '-gravity', 'center', '-background', color, '-fill', 'black', '-size', '350x350', '-pointsize', '300', label, filepath])