source: samples/sample-gradient.py

#!/usr/bin/env python
#
# Example on how to generate multiple linear gradiant circles in one picture,
# adding the data if present.
#
# Rick van der Zwet <info@rickvanderzwet.nl>


from PIL import Image
import ImageDraw
import numpy as np


def make_circle(draw, center, radius,colour=(0,255,0)):
  """ Cicle gradient is created by creating smaller and smaller cicles """
  (center_x, center_y) = center
  for i in range(0,radius):
    draw.ellipse(
      (center_x - radius + i,
       center_y - radius + i,
       center_x + radius - i,
       center_y + radius - i
      ),
      colour +(255 * i/(radius * 2),)
    )


class Picture():
  im = None
  def __init__(self, method, size):
    self.im = Image.new(method, size)

  def save(self,filename):
    self.im.save(filename)

  def add_circle(self, center, radius, colour):
    """ Adding a new cicle is a matter of creating a new one in a empty layer
    and merging it with the current one """

    im_new = Image.new("RGBA", self.im.size)
    draw = ImageDraw.Draw(im_new)
    make_circle(draw, center, radius, colour)
  
    data = np.array(self.im)
    data2 = np.array(im_new)
    
    # Add channels to make new images
    data3 = data + data2
    
    self.im = Image.fromarray(data3)


im = Picture("RGBA",(200,200))

im.add_circle((50,50),100,(0,255,0))
im.add_circle((150,150),100,(255,0,0))

filename = 'sample-gradient.png'
im.save(filename)
print "#INFO Output saved as '%s'" % filename