W1512 Colorful Turtles

Within these castle walls be forged Mavens of Computer Science ...

— Merlin, The Coder

Hawaii Turtle

Prerequisites

• W1511 Walking Turtles

Background

Turtle graphics enable us to not only lift and drop the pen, but also to change the pen color and thickness. Using these tools and prior knowledge, we can generate a wide variety of colorful and complex patterns.

Prepare

Create a new Scenes shell project within your Experiences directory:

ty-cam@codermerlin:~$  cd ~/Experiences

ty-cam@codermerlin:~/Experiences$  git clone https://github.com/TheCoderMerlin/ScenesShellBasic W1512

Enter the Sources/ScenesShell directory of the new project:

ty-cam@codermerlin:~/Experiences$  cd W1512/Sources/ScenesShell/

Run the program. ty-cam@codermerlin:~/Experiences/W1512/Sources/ScenesShell$  run

Ensure that you are logged on to the wiki. Then, click on the Tools menu followed by right-clicking on IGIS and selecting the menu item Open in New Window or Open in New Tab.

You'll know you're successful if you see the title bar change to "Coder Merlin: IGIS". (The browser window will be blank because we haven't added any graphics yet.)

Helpful Hint

It's useful to bookmark this page in your browser.

Experiment

Stop the running program. Return to the console and press CONTROL-C

First Steps

Open the file Background.swift in emacs.

Add a new property to the Background class:

class Background : RenderableEntity {

    var didDraw = false

    init() {
        // Using a meaningful name can be helpful for debugging
        super.init(name:"Background")
    }
}

This will enable us to keep track of whether or not the turtle completed its drawing mission.

Add a new method (below init) as follows:

    override func render(canvas:Canvas) {
        if let canvasSize = canvas.canvasSize, !didDraw {

            let turtle = Turtle(canvasSize:canvasSize)
            turtle.forward(steps:100)
            turtle.right(degrees:90)
            turtle.forward(steps:100)
            turtle.right(degrees:90)
            turtle.forward(steps:100)
            turtle.right(degrees:90)
            turtle.forward(steps:100)
            turtle.right(degrees:90)
            canvas.render(turtle)

            didDraw = true
        }
    }

The conditional that we added will evaluate to true only if the size of the canvas is available and we didn't yet draw with the turtle. If the consequent is executed, the very last statement will set the flag didDraw to true, ensuring that the consequent executes only once.

The method creates a new Turtle. (Remember that the initial position of the turtle is home. In the home position, the turtle is located in the center of the canvas and pointed up (northward). We then:

1. Tell the turtle to move forward 100 steps
2. Turn right (clockwise) 90 degrees
3. Tell the turtle to move forward 100 steps
4. Turn right (clockwise) 90 degrees
5. Tell the turtle to move forward 100 steps
6. Turn right (clockwise) 90 degrees
7. Tell the turtle to move forward 100 steps
8. Turn right (clockwise) 90 degrees

Remember to save the file, then suspend emacs.

Run the program and view in a browser before continuing.

It appears that several of those steps were repeated. As we've learned, a better option to organize this code would be to use a loop. Let's refactor our code as follows:

    override func render(canvas:Canvas) {
        if let canvasSize = canvas.canvasSize, !didDraw {

            let turtle = Turtle(canvasSize:canvasSize)
            for _ in 1 ... 4 {
                turtle.forward(steps:100)
                turtle.right(degrees:90)
            }
            canvas.render(turtle)

            didDraw = true
        }
    }

Remember to save the file, then suspend emacs.

Run the program and view in a browser before continuing.

While this is an improvement, we can do better. Let's refactor some more and move the interesting functionality to a separate function and invoke that function from render.

    func renderSquare(turtle: Turtle) {
        for _ in 1 ... 4 {
            turtle.forward(steps: 100)
            turtle.right(degrees: 90)
        }
    }

    override func render(canvas:Canvas) {
        if let canvasSize = canvas.canvasSize, !didDraw {

            let turtle = Turtle(canvasSize: canvasSize)
            renderSquare(turtle: turtle)
            canvas.render(turtle)

            didDraw = true
        }
    }

Remember to save the file, then suspend emacs.

Remember to save the file, then suspend emacs.

Run the program and view in a browser before continuing.

Let's add a parameter to the function enabling us to make a square of any color and any size.

    func renderSquare(turtle: Turtle, color: Color, width: Int) {
        turtle.penColor(color:color)
        for _ in 1 ... 4 {
            turtle.forward(steps: width)
            turtle.right(degrees: 90)
        }
    }

    override func render(canvas:Canvas) {
        if let canvasSize = canvas.canvasSize, !didDraw {

            let turtle = Turtle(canvasSize:canvasSize)
            renderSquare(turtle:turtle, color:Color(.red), width:50)
            canvas.render(turtle)

            didDraw = true
        }
    }

Run the program and view in a browser before continuing.

First Pattern

Let's make good use of our function and invoke it in a loop, drawing 50 squares of different sizes.

    override func render(canvas:Canvas) {
        if let canvasSize = canvas.canvasSize, !didDraw {

            let turtle = Turtle(canvasSize:canvasSize)
            for i in 1 ... 50 {
                let width = i * 10
                paintSquare(turtle:turtle, color:Color(.red), width:width)
            }
            canvas.paint(turtle)

            didDraw = true
        }
    }

Run the project. View the results in the browser as you did earlier.

Now, let's change the colors in a specific pattern. To do so, we'll define an array to hold some colors, and then select each color in turn.

    override func render(canvas:Canvas) {
        if let canvasSize = canvas.canvasSize, !didDraw {

            let turtle = Turtle(canvasSize:canvasSize)
            let colors = [Color(.palegreen), Color(.mediumspringgreen), Color(.limegreen), Color(.lime)]
            var colorIndex = 0
            for i in 1 ... 50 {
                let width = i * 10
                let color = colors[colorIndex]
                paintSquare(turtle:turtle, color:color, width:width)
                colorIndex = (colorIndex + 1) % colors.count
            }
            canvas.paint(turtle)

            didDraw = true
        }
    }

Take careful note of how the colors are selected in turn. What prevents us from running off the right edge of the array?

Run the project. View the results in the browser as you did earlier.

More Interesting Patterns

What would happen if we add just one line of code that rotates the turtle a few degrees after we draw each square?

    override func render(canvas:Canvas) {
        if let canvasSize = canvas.canvasSize, !didDraw {

            let turtle = Turtle(canvasSize:canvasSize)
            let colors = [Color(.palegreen), Color(.mediumspringgreen), Color(.limegreen), Color(.lime)]
            var colorIndex = 0
            for i in 1 ... 50 {
                let width = i * 10
                let color = colors[colorIndex]
                paintSquare(turtle:turtle, color:color, width:width)
                colorIndex = (colorIndex + 1) % colors.count
                turtle.right(degrees:5)
            }
            canvas.paint(turtle)

            didDraw = true
        }
    }

Before running the program, give the above change some thought. What do you predict will happen? Draw your hypothesis on graph paper before proceeding.

Run the project. View the results in the browser as you did earlier.

Was your hypothesis correct? If not, why not?

Exercises

1. Produce a new pattern by repeatedly drawing a polygon (of no fewer than five sides) of various sizes at various angles. Define your own color scheme and rotate through the colors.

Supplemental Exercises

1. Before drawing your pattern, paint the canvas with an aesthetic background using the drawing primitives that were covered in earlier projects (e.g. Ellipses, Rectangles).

Key Concepts

• Beautiful patterns may be generated using simple polygons
  • Polygons may be drawn in various sizes
  • After drawing each polygon, the turtle's starting position and/or rotation is slightly modified, so that the subsequent polygon is subtly different
  • As more and more polygons are drawn, a pattern will emerge
• Defining functions to contain repeated code greatly aids comprehension
• A series of colors may be represented in an array
  • The expression index = (index + 1) % count is a very common paradigm for rotating through a series of elements in an array