A customizable Ebbinghaus illusion

import math

def svg_preamble(width=600, height=450):
    return ('<svg width="{}" height="{}"'
          ' xmlns="http://www.w3.org/2000/svg"'
          ' xmlns:xlink= "http://www.w3.org/1999/xlink">'.format(width,height))

def ebbinghaus(filename='ebbinghaus.svg', width=600, height=450, r=35,
                 r_smaller=15, r_larger=50, nsmall=10, nlarge=6,
                 dist_ratio_small=1.2, dist_ratio_large=1.5,
                 centre_colour='tomato', surrounding_colour='silver',
                 separation=350):
    """The Ebbinghaus illusion.

    Write an SVG image of the Ebbinghaus circles illusion with the following
    parameters:

    filename: the name of the SVG file to write.
    width, height: the size of the SVG image in pixels.
    r: the radius of the central circle.
    r_smaller: the radii of the small surrounding circles.
    r_larger: the radii of the large surrounding circles.
    nsmall: the number of small circles.
    nlarge: the number of large circles.
    dist_ratio_small: the distance between the edge of the central circle and
        the centre of each small circle expressed as a ratio of the small
        circle radius (should be greater than 1 or the circles will overlap).
    dist_ratio_large: the distance between the edge of the central circle and
        the centre of each large circle expressed as a ratio of the large
        circle radius (should be greater than 1 or the circles will overlap).
    centre_color: a colour specification (HTML name or hex string) for the
        colour of the central circles.
    surrounding_color: a colour specification (HTML name or hex string) for the
        colour of the surrounding circles.
    separation: the separation of the central circles in pixels.


    """

    # Centres of the circles surrounded by larger and smaller circles.
    x1, y1 = (r_larger + r) * 2, height/2
    x2, y2 = x1 + separation, y1

    def svg_circle(r, x0, y0, s_class):
        """A circle at (x0, y0) with radius r and class s_class."""

        return ('<circle cx="{:.1f}" cy="{:.1f}" r="{:.1f}" class="{}"/>'
                        .format(x0, y0, r, s_class))

    def arrange_circles(cr, r, x0, y0, n, s_class):
        """Arrange circles on the circumference of a larger circle.

        Arrange n circles of radius r on a circle of radius cr, centered
        at (x0, y0) and attach style class s_class to them."""

        for i in range(n):
            th = 2 * math.pi * i / n
            x = x0 + cr * math.cos(th)
            y = y0 + cr * math.sin(th)
            print(svg_circle(r, x, y, s_class), file=fo)

    with open(filename, 'w') as fo:
        print(svg_preamble(), file=fo)
        print('<style>\n    /* <![CDATA[ */', file=fo)
        print('      .inner {{fill: {};}}'.format(centre_colour), file=fo)
        print('      .smaller, .larger {{fill: {};}}'.format(
                                             surrounding_colour), file=fo)
        print('    /* ]]> */\n</style>', file=fo)

        print(svg_circle(r, x1, y1, 'inner'), file=fo)
        print(svg_circle(r, x2, y2, 'inner'), file=fo)

        arrange_circles(r + r_larger*dist_ratio_large, r_larger, x1, y1,
                        nlarge, 'larger')
        arrange_circles(r + r_smaller*dist_ratio_small, r_smaller, x2, y2,
                        nsmall,'smaller')

        print('</svg>', file=fo)

ebbinghaus('ebbinghaus1.svg', dist_ratio_small=1.5, dist_ratio_large=1.7,
           separation=330)

ebbinghaus('ebbinghaus2.svg', dist_ratio_small=3, dist_ratio_large=1.7,
           separation=330)