DrawBot: create a simple loop of nested ellipses

Ellipses.py

# Result:
# http://dailydrawbot.tumblr.com/post/160364438359/ellipses

def drawEllipses(cx, cy, rx, ry, dx, dy, n, bw=0):
    for i in range(n):
        fill(bw)
        oval(cx - rx, cy - ry, 2 * rx, 2 * ry)
        rx -= dx
        ry -= dy
        bw = 1 - bw

canvasSize = 500
numFrames = 50
numEllipses = 14
radius = 0.97 * canvasSize/2
maxStep = radius / (numEllipses - 0.5)

for frame in range(numFrames):
    t = frame / numFrames
    newPage(canvasSize, canvasSize)
    frameDuration(1/20)
    rect(0, 0, canvasSize, canvasSize)

    a = 2 * pi * t
    tx = (1 + sin(a)) / 2
    ty = (1 + sin(a + pi/2)) / 2
    drawEllipses(canvasSize/2, canvasSize/2, radius, radius,
        tx * maxStep, ty * maxStep, numEllipses, 1)

saveImage("Ellipses.gif")