Turtles in generativepy


Martin McBride, 2022-03-07
Tags generativepy tutorial path
Categories generativepy generativepy tutorial

This tutorial covers the basics of using Turtle objects in generativepy.

This provides a simple implementation of turtle graphics. This uses an imaginary turtle that can move around, drawing a line as it goes. The basic operation of a turtle are:

  • forward - move forward by a specified amount, drawing a line.
  • move - move forward by a specified amount, without drawing a line.
  • left, right - turn through a specified angle to the left or right.

This style of drawing allows complex geometric shapes to be drawn without needing to worry about coordinate geometry and all the trigonometry it requires.

The turtle has a few enhanced features:

  • set_heading - set an absolute direction.
  • move_to - move to an absolute position.
  • push and pop - allows the turtle to do some drawing then return to its original position and direction. This can be done recursively, for example to draw trees.
  • set_style - set the line style.

Turtle example code

Here is a sample Python program for drawing with the turtle. The code is explained later in this article:

from generativepy.drawing import make_image, setup, ROUND
from generativepy.color import Color
from generativepy.geometry import Turtle
import math

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

    turtle = Turtle(ctx)
    turtle.move_to(100, 100).forward(50).left(math.pi / 2).forward(50).left(math.pi / 4).forward(50)

    turtle = Turtle(ctx)
    turtle.move_to(200, 300).set_style(Color('green'), line_width=5, dash=[10]) \
        .push().forward(100).pop() \
        .push().left(3 * math.pi / 4).forward(100).pop() \
        .right(3 * math.pi / 4).forward(100)

    turtle = Turtle(ctx)
    turtle.move_to(350, 100).right(math.pi / 2).set_style(Color('red'), line_width=5, dash=[10], cap=ROUND).forward(100)
    turtle.set_style(Color('blue'), line_width=2, dash=[]).forward(100)
    turtle.set_style(Color('black'), line_width=4, dash=[15]).forward(100)

make_image("turtle.png", draw, 500, 500)

This code is available on github in tutorial/shapes/turtle.py.

Here is the resulting image:

Explanation

The first part of the code creates the thin, black line at the top of the image:

    turtle = Turtle(ctx)
    turtle.move_to(100, 100).forward(50).left(math.pi / 2).forward(50).left(math.pi / 4).forward(50)

Here is what the code does:

  • Moves to the point (100, 100),
  • Draws line of length 50. The turtle initially points to the right, so the line is horizontal.
  • Turns the turtle through pi/2 radians (90 degrees) left, so the turtle is pointing up the page. It then draws a line of length 50.
  • Turns the turtle through pi/4 radians (45 degrees) left, so the turtle is pointing diagonally to the upper-left. It then draws a line of length 50.

Unlike other shapes, the turtle draws as it goes along, so there is no need to call stroke.

The next part of the code creates the dashed green Y shape:

    turtle = Turtle(ctx)
    turtle.move_to(200, 300).set_style(Color('green'), line_width=5, dash=[10]) \
        .push().forward(100).pop() \
        .push().left(3 * math.pi / 4).forward(100).pop() \
        .right(3 * math.pi / 4).forward(100)

This code uses push and pop to draw three lines that all start at (200, 200):

  • Moves to the start position and sets a green, dashed line style.
  • Pushes. Draws a line of length 100 (in the default direction, to the right). The pops, restoring the turtle to its previous state.
  • Pushes. Turns 135 degrees left, draws a line of length 100 from position (200, 200) towards the upper-left. The pops, restoring the turtle to its previous state.
  • This is the last item so we don't bother pushing. Turns 135 degrees right, draws a line of length 100 from position (200, 200) towards the lower-left.

The final code illustrates styling:

    turtle = Turtle(ctx)
    turtle.move_to(350, 100).right(math.pi / 2).set_style(Color('red'), line_width=5, dash=[10], cap=ROUND).forward(100)
    turtle.set_style(Color('blue'), line_width=2, dash=[]).forward(100)
    turtle.set_style(Color('black'), line_width=4, dash=[15]).forward(100)

In this case, we draw 3 line segments, changing the line style each time - red dashed, blue solid, black dashed.

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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