tim-mccurrach / geometrySVG

A python package for generating SVG files for use in school geometry problems.

A python package for generating SVG files for use in school geometry problems.

Introduction

This package was born out of a need to algorithmically create diagrams involving lines and polygons, with angles labeled on the diagrams. If not placed carefully text overlaps with lines which at best looks untidy and at worst un-readable. Even if text is not overlapping with lines, if it is not placed as close as possible to the point of the angle, the arc around the text can become very large. This package initially aims to adress this problem, and other typical school geometry features as it expands.

Quick start

After the package has been imported an instance of the SVGCanvas object must be created.

from geometrySVG import SVGCanvas
# Create canvas object specifying height and width
canvas = SVGCanvas(300,300,cart_coords=True)

An optional argument cart_coordsis provided since by default SVG measures coordinates (x,y) from the top-left in a similar fashion to HTML. cart_coords=True means that coordinates (x,y) are measured from the bottom-left as is normal when dealing with cartestian coordinates.

After defining a canvas we must now add points to the canvas. Points are not visible SVG elements, but references used to create SVG elements.

canvas.add_point(30, 40, "A")
canvas.add_point(160, 60, "B")
canvas.add_point(180, 190, "C")

Each point must be given a name, which will be a single character. Now that we have added some points we can use a variety of methods to create shapes on the diagram.

canvas.add_polygon("ABC")

Finally, when we have drawn our diagram, we can output the SVG for the whole canvas as a string, to use however you please.

# generate SVG 
result=canvas.generate_SVG(200, 200)
# do something with it
print(result)

Important note: This package was originally intended to create inline SVG in an HTML file, and does not include all of the style information. If you would like to write directly to an SVG file, use style_info=True e.g

result=canvas.generate_SVG(200, 200, style_info=True)
with open("output.svg", "w") as file:
    file.write(result)

Key Methods

The following methods are provided to add lines and shapes to the canvas:

• SVGCanvas.add_line(*points, **kwargs) Adds a single line to the canvas. There must be exactly 2 points given, otherwise an error is raised.

• SVGCanvas.add_lines(*points, **kwargs) Adds a series of lines that are joined end to end. E.g add_lines("ABC") is equivalent to add_line("AB") and add_line("BC")

• SVGCanvas.add_polyon(*points, name=None, **kwargs) This adds a closed polygon from the points. At least 3 points must be given to define a polygon otherwise an error is raised. The name attribute is provided in order to later access the polygon. It can be accessed as a property, to access the polygon methods

For example:

canvas.add_polygon("ABCDE", name="pentagon")
canvas.pentagon.add_angles()

• SVGCanvas.add_text(text, *point, **kwargs) Adds text at the given point.

Points In all methods points can either be provided as a single string, or separated into individual points. For example canvas.add_lines("ABC") is equivalent to canvas.add_lines("A","B","C")

Adding SVG attributes All methods have an optional **kwargs which can be used for adding SVG attributes to the elements. For example: canvas.add_polygon("ABC", fill="red") Many SVG attributes include hyphens. To use these, simply replace the hyphen with an underscore and it will be converted to a hyphen. For example to set stroke-width simply do: canvas.add_line("AB", stoke_width=4)

Adding angles

Angles can be added using the method add_angle for example:

canvas.add_angle("ABC", text="37", reflex=True, text_kwargs={font-size:12}, fill="blue")

As before SVG attributes can be added as key-word arguments. reflex and direction are optional arguments, however at least one must be specified otherwise an error is raised. They indicate whether the angle should be a reflex one, or if it should go clockwise or anticlockwise (from the line AB, to the line BC). As before key word arguments can be used to add SVG attributes, and there is the additional arguments text_kwargs, which accepts a mapping for any SVG attributes to be applied to the text inside an angle.

If a polygon has been created, it will have its own methods add_angle and add_angles. Both have an optional text argument, which in the case of add_angles must be an array (the position of the text within the array corresponds to the order in which the points were specified when the polygon was defined).

canvas.add_polygon("ABCD", name="square")
#Add a single angle
canvas.square.add_angle("B",text="90")
#Add in all the angles with labels
canvas.square.add_angles(text=['90','90','90','90'])

Example

The following shows how the package can be used to create a simple image that might be used in a typical maths question.

from geometrySVG import SVGCanvas

canvas = SVGCanvas(300, 300, cart_coords=True)

canvas.add_point(30, 40, "A")
canvas.add_point(160, 60, "B")
canvas.add_point(180, 190, "C")

canvas.add_polygon("ABC", name="triangle", fill="none", stroke="black")

canvas.triangle.add_angles(fill="blue",
                           text=["a-9\u00b0", "5a\u00b0", "a\u00b0"],  # \u00b0 is degrees
                           font_family="Times Roman")

a = canvas.generate_SVG(500, 500, style_info=True)

with open("output.svg", "wb") as file:
    file.write(a.encode('utf8'))  # utf8 encoding necessary due to degrees symbol

The resulting image is shown below: