Ellipse


Martin McBride, 2020-08-26
Tags geometry ellipse arc sector segment
Categories generativepy generative art

The Ellipse class draws ellipses, arcs, sectors and segments.

An ellipse is similar to a circle, but it has two radii, one in the x direction and one in the y direction.

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

Ellipse class methods

The Ellipse class inherits add, fill, stroke, and fill_stroke from Shape.

It has an additional method:

  • of_center_radius
  • as_arc
  • as_sector
  • as_segment

of_center_radius

Creates a ellipse based on the centre point and the two radii.

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

as_arc

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

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:

Ellipse(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.

as_segment

Modifies a ellipse, to show only a segment. An segment is the part of the ellipse 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.

as_sector

Modifies a ellipse, 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.

ellipse function

Adds a ellipse as a new path, without the need to create a Ellipse object in code.

ellipse(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 ellipse.
radius_x number A number, giving the x radius of the ellipse.
radius_y number A number, giving the y radius of the ellipse.

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

Example

Here is some example code that draws ellipses 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 Ellipse, ellipse

'''
Create ellipses using the geometry module.
'''

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

    # The ellipse function is a convenience function that adds a ellipse as a new the path.
    # You can fill or stroke it as you wish.
    ellipse(ctx, (1, 1), 0.7, 1.1)
    ctx.set_source_rgba(*Color(1, 0, 0))
    ctx.fill()

    # Ellipse objects can be filled, stroked, filled and stroked.
    Ellipse(ctx).of_center_radius((2.5, 1), 0.7, 0.3).fill_stroke(Color(0, 0, 1), Color(0), 0.05)
    Ellipse(ctx).of_center_radius((4, 1), 0.7, 0.3).as_arc(0, 1).stroke(Color(0, 0.5, 0), 0.05)

    Ellipse(ctx).of_center_radius((1, 2.5), 0.7, 0.3).as_sector(1, 3).stroke(Color('orange'), 0.05)
    Ellipse(ctx).of_center_radius((2.5, 2.5), 0.7, 0.3).as_sector(2, 4.5).fill(Color('cadetblue'))
    Ellipse(ctx).of_center_radius((4, 2.5), 0.7, 0.3).as_sector(2.5, 6).fill_stroke(Color('yellow'), Color('magenta'), 0.05)

    Ellipse(ctx).of_center_radius((1, 4), 0.7, 0.3).as_segment(1, 3).stroke(Color('orange'), 0.05)
    Ellipse(ctx).of_center_radius((2.5, 4), 0.7, 0.3).as_segment(2, 4.5).fill(Color('cadetblue'))
    Ellipse(ctx).of_center_radius((4, 4), 0.7, 0.3).as_segment(2.5, 6).fill_stroke(Color('yellow'), Color('magenta'), 0.05)

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

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