Polygon


Martin McBride, 2020-08-18
Tags geometry polygon
Categories generativepy generative art

The Polygon class draws a polygon.

A polygon is defined by a list of points. The polygon is formed by joining the points with straight lines.

It is also possible to use bezier curves rather than straight lines to join the points. A shape can be formed from any combination of straight lines and curves.

There is also a polygon function that just creates a polygon as a new path.

See the polygons tutorial for examples.

Polygon class methods

The Polygon class inherits add, fill, stroke, fill_stroke, path, clip and other methods from Shape.

It has additional methods:

  • of_points
  • open

of_points

Creates a polygon based on a list of points.

of_points(points)
Parameter Type Description
points sequence of points A sequence of line or curve specifiers.

To define a simple polygon with straight sides, points should just be a list of points, like this:

[(300, 100), (300, 150), (400, 200), (450, 100)]

This will create a polygon with 4 vertices, at the points (300, 100), (300, 150), (400, 200), and (450, 100).

Alternatively, it is possible to specify that some of the sides are bezier curves rather than straight lines, like this:

[(1, 4.5), (1, 2.5), (2, 3, 3, 4, 4, 2.5), (4, 4.5)]

In this case, the third item is 6 elements long. This means that the second and third points are connected by a bezier curve (rather than a straight line) with values:

(1, 2.5), (2, 3), (3, 4), (4, 2.5)

See the composite paths tutorial for more details.

The polygon will be closed by default. To create an open polygon, call the open method.

open

Creates an open polygon, rather than a closed polygon.

open()

Calling this method will cause the final polygon to be open - the last point will not be connected to the first point (see the example). To create a closed polygon, simply don't call this method.

polygon function

Adds a polygon as a new path, without the need to create a Polygon object in code.

polygon(ctx, points, closed=True)
Parameter Type Description
ctx Context The Pycairo Context to draw to
points list of (number, number) A sequence of (x, y) tuples, giving the position of each vertex of the polygon.
closed boolean Optional, set to False to create an open polygon

Example

Here is some example code that draws polygons using the class and the utility function. The full code can be found on github.

from generativepy.drawing import make_image, setup
from generativepy.color import Color
from generativepy.geometry import polygon, Polygon

'''
Create polygons using the geometry module.
'''

def draw(ctx, width, height, frame_no, frame_count):
    setup(ctx, width, height, width=500, background=Color(0.8))

    # The polygon function is a convenience function that adds a polygon as a new path.
    # You can fill or stroke it as you wish.
    polygon(ctx, ((100, 100), (150, 50), (200, 150), (200, 200)))
    ctx.set_source_rgba(*Color(1, 0, 0))
    ctx.fill()

    Polygon(ctx).of_points([(300, 100), (300, 150), (400, 200), (450, 100)]).open().stroke(Color('orange'), 10)

make_image("/tmp/geometry-polygon.png", draw, 500, 500)
If you found this article useful, you might be interested in the book Computer Graphics in Python or other books by the same author.

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