By Martin McBride, 2020-08-23
Tags: geometry circle arc sector segment
Categories: generativepy generative art

The Circle class draws circles, arcs, sectors and segments.

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

Circle class methods

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

It has an additional method:

  • of_center_radius
  • as_arc
  • as_sector
  • as_segment


Creates a circle based on the centre point and radius.

of_center_radius(center, radius)
Parameter Type Description
center (number, number) A tuple of two numbers, giving the (x, y) position of the centre of the circle.
radius number A number, giving the radius of the circle.


Modifies a circle, to show only an arc. An arc is part of the circumference of the circle.

as_arc(start_angle, end_angle)
Parameter Type Description
start_angle number A number, giving the start angle of the arc
end_angle number A number, giving the end angle of the arc

This is used as a modifier with of_center_radius, to draw just an arc. To draw an arc use:

Circle(ctx).of_center_radius((0, 0), 1).as_arc(0, 1).stroke(Color('black'), 0.1)

Angles are measured in radians. Angle zero lies along the positive x-axis, and the angle increases in the clockwise direction - note that this is opposite to the normal mathematical convention, where angle increases in the counterclockwise direction. The difference is due to the fact that y increases as you move down the image in generativepy.

If you are using a flipped coordinate system (see the setup function in the drawing module), the angle increases in the counterclockwise direction.

Since an arc is a line, you should normally use the stroke method to draw it. If you attempt to fill the arc, it will fill it as if it was a segment.


Modifies a circle, to show only a segment. An segment is the part of the circle that is cut off by a chord.

as_segment(start_angle, end_angle)

This function works in a similar way to the as_arc function, but it includes the area of the segment. You can fill or stroke the area, see the example below.


Modifies a circle, to show only a sector. An sector is a "pizza slice", like you would use in a pie chart,.

as_sector(start_angle, end_angle)

This function works in a similar way to the as_arc function, but it includes the area of the sector. You can fill or stroke the area, see the example below.

circle function

Adds a circle as a new path, without the need to create a Circle object in code.

circle(ctx, center, radius)
Parameter Type Description
ctx Context The Pycairo Context to draw to
center (number, number) A tuple of two numbers, giving the (x, y) position of the centre of the circle.
radius number A number, giving the radius of the circle.

The circle function doesn't allow you to create arcs, segments or sectors.


Here is some example code that draws circles 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 Circle, circle

Create circles using the geometry module.

def draw(ctx, width, height, frame_no, frame_count):
    setup(ctx, width, height, width=5, background=Color(0.8))

    # The circle function is a convenience function that adds a circle as a new the path.
    # You can fill or stroke it as you wish.
    circle(ctx, (1, 1), 0.7)
    ctx.set_source_rgba(*Color(1, 0, 0))

    # Circle objects can be filled, stroked, filled and stroked.
    Circle(ctx).of_center_radius((2.5, 1), 0.7).fill_stroke(Color(0, 0, 1), Color(0), 0.05)
    Circle(ctx).of_center_radius((4, 1), 0.7).as_arc(0, 1).stroke(Color(0, 0.5, 0), 0.05)

    Circle(ctx).of_center_radius((1, 2.5), 0.7).as_sector(1, 3).stroke(Color('orange'), 0.05)
    Circle(ctx).of_center_radius((2.5, 2.5), 0.7).as_sector(2, 4.5).fill(Color('cadetblue'))
    Circle(ctx).of_center_radius((4, 2.5), 0.7).as_sector(2.5, 6).fill_stroke(Color('yellow'), Color('magenta'), 0.05)

    Circle(ctx).of_center_radius((1, 4), 0.7).as_segment(1, 3).stroke(Color('orange'), 0.05)
    Circle(ctx).of_center_radius((2.5, 4), 0.7).as_segment(2, 4.5).fill(Color('cadetblue'))
    Circle(ctx).of_center_radius((4, 4), 0.7).as_segment(2.5, 6).fill_stroke(Color('yellow'), Color('magenta'), 0.05)

make_image("/tmp/geometry-circles.png", draw, 500, 500)

See also

If you found this article useful, you might be interested in the book NumPy Recipes or other books by the same author.

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