Computer-generated Mondrian art #2


This is the second post on generating images which resemble Mondrian paintings. The first post described a procedural algorithm; here I'll introduce an object-oriented approach.

From a geometrical point of view, the images we wish to create consist of intersecting lines on a canvas delimiting rectangular areas which are filled with one of four or so colours. In this implementation, we will create objects to represent the lines, the filled areas and the canvas itself. To be a little bit more general, we will allow the lines to adopt any orientation (rather than just horizontal and vertical) but will only allow a maximum of two lines to intersect at a given point.

First of all we need a class to represent two-dimensional vectors and perform basic operations such as addition on them. We will need the vector dot product and vector cross product as well (see below):

class Vector:
    """ A lightweight vector class in two-dimensions. """

    def __init__(self, x, y):
        self.x, self.y = x, y

    def __add__(self, other):
        return Vector(self.x + other.x, self.y + other.y)

    def __sub__(self, other):
        return Vector(self.x - other.x, self.y - other.y)

    def __mul__(self, lam):
        """ Multiply the vector by a scalar, lam. """
        return Vector(lam * self.x, lam * self.y)
    def __rmul__(self, lam):
        return self.__mul__(lam)

    def __eq__(self, other):
        return self.x == other.x and self.y == other.y
    def __neq__(self, other):
        return not self == other
    def __hash__(self):
        """ To keep Vector hashable when we define __eq__, define __hash__. """
        return self.x, self.y

    def __str__(self):
        return '({}, {})'.format(self.x, self.y)

    def dot(self, other):
        """ Dot product, a.b = ax.bx + """
        return self.x * other.x + self.y * other.y

    def cross(self, other):
        """ z-component of vector cross product, a x b = - ay.bx. """
        return self.x * other.y - self.y * other.x

Hence, for example:

In [2]: a = Vector(2,3)

In [3]: b = Vector(-1,2)

In [4]: print(a+b)
(1, 5)

In [5]: print (5*a)
(10, 15)

In [6]:
Out[6]: 4

In [7]: a.cross(b)
Out[7]: 7

In [8]: b.cross(a)
Out[8]: -7

Lines will be represented by a further class: a line segment is defined here by a vector, $\boldsymbol{p}$, to its start point and a second vector, $\boldsymbol{r}$ from its start point to its end point. Any point on the line can therefore be identified by the vector $\boldsymbol{p} + u\boldsymbol{r}$ with $0\le u\le 1$. We implement methods to determine whether two line segments are colinear or parallel and where (if at all) they intersect:

EPS = 1.e-12

class Line:
    """ A simple class representing a line segment between two points in 2D. """

    def __init__(self, p, r):
        p is the start point vector, r is the vector from this start point
        to the end point.


        self.p, self.r = p, r

    def from_endpoints(self, p, q):
        """ Create and return the Line object between points p and q. """

        return Line(p, q-p)

    def __str__(self):
        return '{} -> {}'.format(self.p, (self.p + self.r))

    def intersection(self, other):
        Return the vector to the intersection point between two line segments
        self and other, or None if the lines do not intersect.


        p, r = self.p, self.r
        q, s = other.p, other.r

        rxs = r.cross(s)
        if rxs == 0:
            # Line segments are parallel: no intersection
            return None
        u = (q - p).cross(r) / rxs
        t = (q - p).cross(s) / rxs

        if -EPS <= t <= 1+EPS and -EPS <= u <= 1+EPS:
            # We have an intersection!
            return p + t * r

        # Line segments are not parallel but don't intersect
        return None

    def get_point_on_line(self, u):
        """ Return the vector to the point on the line defined by p + ur. """
        return self.p + u * self.r

    def is_parallel(self, other):
        """ Are the lines self and other parallel? """

        return abs(self.r.cross(other.r)) < EPS

    def is_colinear(self, other):
        Are the lines colinear (have the same start and end points, in either
        order)? """

        return (self.is_parallel(other) and
                abs((self.p - other.p).cross(self.r)) < EPS)

Note that because of the finite precision of floating point arithmetic, it is necessary to compare some float quantities for equality within some tolerance defined by the quantity EPS: a and b are considered equal if they differ by less than this amount, which is set to generously as EPS = 1.e-12 in my code.

It is convenient to provide two methods of creating Line objects: by providing the vectors $\boldsymbol{p}$ and $\boldsymbol{r}$ as described, or by providing the line segment end points, $\boldsymbol{p}$ and $\boldsymbol{q} = \boldsymbol{p} - \boldsymbol{r}$: the latter function is defined as a classmethod. For example,

In  [9]: p1, r1 = Vector(-1, 2), Vector(4, -2)  # start point and direction

In [10]: l1 = Line(p1, r1)

In [11]: p2, q2 = Vector(-1, -1), Vector(3, 4)   # end points

In [12]: l2 = Line.from_endpoints(p2, q2)

In [13]: print(l1.intersection(l2))
(0.7142857142857142, 1.1428571428571428)

The algorithm used here will add lines by selecting random points on two different, randomly chosen lines and joining them. For each added line, we keep track of the polygons formed by the intersection of this line with the existing lines: any polygon which has exactly two of its edges intersected by the new line is split into two new polygons. We define a polygon (which will be convex) as an ordered sequence of its vertex vectors, from which it is easy to calculate an ordered sequence of its edges as Line objects: by being careful to maintain the ordering (say, clockwise) when a polygon is split we avoid double-counting the polygons. The necessary class is:

class Polygon:
    """ A small class to represent a polygon in two dimensions. """

    def __init__(self, vertices):
        Define the polygon from an ordered sequence of vertices and get its
        area and edges (as Line objects).


        self.vertices = vertices
        self.n = len(self.vertices)
        self.area = self.get_area()
        self.edges = self.get_edges()

    def get_area(self):
        """ Calculate and return the area of the polygon."""

        # We use the "shoelace" algorithm to calculate the area, since the
        # polygon has no holes or self-intersections.
        s1 = s2 = 0
        for i in range(self.n):
            j = (i+1) % self.n
            s1 += self.vertices[i].x * self.vertices[j].y
            s2 += self.vertices[i].y * self.vertices[j].x
        return abs(s1 - s2) / 2

    def get_edges(self):
        Determine an ordered sequence of edges for the polygon from its
        vertices as a list of Line objects.


        edges = []
        # Indexes of edge endpoints in self.vertices: (0,1), (1,2), ...,
        # (n-2, n-1), (n-1, 0)
        vertex_pair_indices = [(i,(i+1) % self.n) for i in range(self.n)]
        for (i1, i2) in vertex_pair_indices:
            v1, v2 = self.vertices[i1], self.vertices[i2]
            edges.append(Line.from_endpoints(v1, v2))
        return edges

    def split(self, intersections):
        """ Return the two polygons created by splitting this polygon.

        Split the polygon into two polygons at the points given by
        intersections = (i1, p1), (i2, p2)
        where each tuple contains the index of an edge, i, intersected at the
        point, p, by a new line.

        Returns: a list of the new Polygon objects formed.


        (i1, p1), (i2, p2) = intersections
        vertices1 = ([edge.p + edge.r for edge in self.edges[:i1]] +
                     [p1, p2] +
                     [edge.p + edge.r for edge in self.edges[i2:]])
        polygon1 = Polygon(vertices1)
        vertices2 = ([edge.p + edge.r for edge in self.edges[i1:i2]] +
                     [p2, p1])
        polygon2 = Polygon(vertices2)
        return [polygon1, polygon2]

    def __str__(self):
        return ', '.join([str(v) for v in self.vertices])

To improve the look of the image we will reject any lines which create polygons smaller than a certain area: the method get_area uses the shoelace algorithm to calculate this area.

Finally, the Canvas class defines methods for initializing the canvas, adding lines, maintaining the state of the Polygon objects (stored as a set), making the "painting" and writing the SVG file.

class Canvas:

    # Fill colours for polygons, and their cumulative probability distribution
    colours = ['blue', 'red', 'yellow', 'white']
    colours_cdf = [0.15, 0.3, 0.45, 1.0]
    def get_colour(self):
        Pick a colour at random using the cumulative probability distribution


        cprob = random.random()
        i = 0
        while Canvas.colours_cdf[i] < cprob:
            i += 1
        return Canvas.colours[i]

    def __init__(self, width, height):
        """ Initialize the canvas with a border around the outside. """

        self.width, self.height = width, height
        self.lines = []

        corners = Vector(0,0), Vector(0,1), Vector(1,1), Vector(1,0)
        self.add_line(Line(corners[0], Vector(0,1)))
        self.add_line(Line(corners[1], Vector(1,0)))
        self.add_line(Line(corners[2], Vector(0,-1)))
        self.add_line(Line(corners[3], Vector(-1,0)))

        self.polygons = {Polygon(corners)}

    def add_line(self, new_line):
        """ Add new_line to the list of Line objects. """

    def split_polygons(self, new_line):
        Split any Polygons which are intersected exactly twice by new_line.

        Returns the set of "old" Polygons split and a list of the "new"
        Polygons thus formed.

        new_polygons = []
        old_polygons = set()
        for polygon in self.polygons:
            intersections = []
            for i, edge in enumerate(polygon.edges):
                p = new_line.intersection(edge)
                if p:
                    intersections.append((i, p))
            if len(intersections) == 2:
                # this polygon is split into two by the new line

        return old_polygons, new_polygons

    def update_polygons(self, old_polygons, new_polygons):
        Update the set of Polygon objects by removing old_polygons and adding
        new_polygons to self.polygons.


        self.polygons -= old_polygons

    def get_new_line(self):
        """ Return a random new line with endpoints on two existing lines. """

        # Get a random point on each of any two different existing lines.
        line1, line2 = random.sample(self.lines, 2)
        start = line1.get_point_on_line(random.random())
        end = line2.get_point_on_line(random.random())
        # Create and return a new line between the points
        return Line.from_endpoints(start, end)

    def get_new_orthogonal_line(self):
        Return a new horizontal or vertical line between two existing lines.


        line1 = random.choice(self.lines)

        def get_xy(line):
            """ Return 'x' for horizontal line or 'y' for vertical line. """
            return 'x' if abs(line.r.y) < EPS else 'y'
        def get_other_xy(xy):
            """ Passed 'x' or 'y', return 'y' or 'x'. """
            return 'y' if xy == 'x' else 'x'

        # Is this a line in parallel to the x-axis or the y-axis?
        xy = get_xy(line1)
        other_xy = get_other_xy(xy)

        start = line1.get_point_on_line(random.random())
        c = getattr(start, xy)
        parallel_lines = []
        for line in self.lines:
            if line.is_colinear(line1):
            if get_xy(line) != xy:
                # This line is perpendicular to our choice
            c1, c2 = sorted([getattr(line.p, xy), getattr(line.p+line.r, xy)])
            if not c1 <= c <= c2:
        line2 = random.choice(parallel_lines)

        end = Vector(None, None)
        setattr(end, xy, getattr(start, xy))
        setattr(end, other_xy, getattr(line2.p, other_xy))

        return Line.from_endpoints(start, end)

    def make_painting(self, nlines, minarea=None, orthogonal=False):
        Make the "painting" by adding nlines randomly, such that no polygon
        is formed with an area less than minarea. If orthogonal is True,
        only horizontal and vertical lines are used.


        for i in range(nlines):
            while True:
                # Create a new line and split any polygons it intersects
                if orthogonal:
                    new_line = self.get_new_orthogonal_line()
                    new_line = self.get_new_line()
                old_polygons, new_polygons = self.split_polygons(new_line)

                # If required, ensure that the smallest polygon is at least
                # minarea in area, and go back around if not
                if minarea:
                    smallest_polygon_area = min(polygon.area
                                                for polygon in new_polygons)
                    if smallest_polygon_area >= minarea:

            self.update_polygons(old_polygons, new_polygons)

    def write_svg(self, filename):
        """ Write the image as an SVG file to filename. """

        with open(filename, 'w') as fo:
            print('<?xml version="1.0" encoding="utf-8"?>', file=fo)
            print('<svg xmlns=""\n'
                  '    xmlns:xlink="" width="{}"'
                  ' height="{}" >'.format(self.width, self.height), file=fo)

<style type="text/css"><![CDATA[

line {
    stroke: #000;
    stroke-width: 5px;
    fill: none;

</defs>""", file=fo)

            for polygon in self.polygons:
                path = []
                for vertex in polygon.vertices:
                    path.append((vertex.x*self.width, vertex.y*self.height))
                s = 'M{},{} '.format(*path[0])
                s += ' '.join(['L{},{}'.format(*path[i])
                                        for i in range(polygon.n)])
                colour = self.get_colour()
                print('<path d="{}" style="fill: {}"/>'.format(s, colour),

            for line in self.lines[4:]:
                x1, y1 = line.p.x * self.width, line.p.y * self.height
                x2, y2 = ((line.p + line.r).x * self.width,
                          (line.p + line.r).y * self.height)

                print('<line x1="{}" y1="{}" x2="{}" y2="{}"/>'
                                        .format(x1,y1,x2,y2), file=fo)

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

Example images:

nlines = 25
width, height = 600, 450
minarea = 5000 / (width * height)
canvas = Canvas(width, height)
canvas.make_painting(nlines, minarea, orthogonal=False)

Non-orthogonal Mondrian image

Or with orthogonal=True:

Orthogonal Mondrian image

Current rating: 3


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Alex Hall 6 years ago

This is tremendous.

I confess I only begin to follow what is going on, but I thought I'd pull it all together in one script and see if I can get it to work.

It seems to me some import statements are missing? The first I noticed is random. After I satisfied that (if I have correctly with just 'import random'), now it throws "NameError: name 'EPS' is not defined."

If you're inclined to comment at a GitHub issue I filed where I've horked your code :) that would be great!

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Alex Hall 6 years ago

I commented asking how to get EPS defined, and dug through this page and saw it has to be 1.e-12. I added that define at the top of the code, and now it works!

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christian 6 years ago

Thanks, Alex. I've edited in the definition of EPS to my code to make it clearer. The code is also available on my github page at

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Alex Hall 6 years ago

Thanks for pointing out the source--I had looked for any repo of it and didn't find it.

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