Source code

The code for the complete game, together with the required media files, can be found in the Gosu distribution of your choice ('examples/Tutorial.rb').

If you have installed Gosu via RubyGems, the examples are in your 'gems' folder together with the rest of the library. For example, on OS X 10.5, the examples can be found in /Library/Ruby/Gems/1.8/gems/gosu-<version>/examples.

If you have not installed Gosu via RubyGems, you have to copy gosu.so (or gosu.bundle, respectively) into the examples directory, then run Tutorial.rb.

If you don't have an editor that supports direct execution (TextMate, SciTE…), cd into the directory and run it via ruby Tutorial.rb.

1. Overriding Window's callbacks

The easiest way to create a complete Gosu application is to write a new class that derives from Gosu::Window (see the reference for a complete description of its interface). Here's how a minimal GameWindow class might look like:

require 'gosu'

class GameWindow < Gosu::Window
  def initialize
    super(640, 480, false)
    self.caption = "Gosu Tutorial Game"
  end

  def update
  end

  def draw
  end
end

window = GameWindow.new
window.show

The constructor initializes the Gosu::Window base class. The parameters shown here create a 640x480 pixels large, non-fullscreen—that's what the "false" stands for—window. Then it changes the window's caption, which is empty until then.

update() and draw() are overrides of Gosu::Window's member functions. update() is called 60 times per second (by default) and should contain the main game logic: move objects, handle collisions, etc.

draw() is called afterwards and whenever the window needs redrawing for other reasons, and may also be skipped every other time if the FPS go too low. It should contain the code to redraw the whole screen, and no logic whatsoever.

Then follows the main program. A window is created and its show() member function is called, which does not return until the window has been closed by the user or its own code. Tada - now you have a small black window with a title of your choice!

2. Using Images

class GameWindow < Gosu::Window
  def initialize
    super(640, 480, false)
    self.caption = "Gosu Tutorial Game"
    
    @background_image = Gosu::Image.new(self, "media/Space.png", true)
  end

  def update
  end

  def draw
    @background_image.draw(0, 0, 0);
  end
end

Gosu::Image#initialize takes three arguments. First, like all media resources, it is tied to a window (self). All of Gosu's resources need a Window for initialization and will hold an internal reference to that window. Second, the file name of the image file is given. The third argument specifies whether the image is to be created with hard borders. See BasicConcepts for an explanation.

As mentioned in the last lesson, the window's draw() member function is the place to draw everything, so this is the place for us to draw our background image.

The arguments are almost obvious. The image is drawn at (0;0) - the third image is the Z position; again, see BasicConcepts.

Player & movement

Here comes a simple player class:

class Player
  def initialize(window)
    @image = Gosu::Image.new(window, "media/Starfighter.bmp", false)
    @x = @y = @vel_x = @vel_y = @angle = 0.0
  end

  def warp(x, y)
    @x, @y = x, y
  end
  
  def turn_left
    @angle -= 4.5
  end
  
  def turn_right
    @angle += 4.5
  end
  
  def accelerate
    @vel_x += Gosu::offset_x(@angle, 0.5)
    @vel_y += Gosu::offset_y(@angle, 0.5)
  end
  
  def move
    @x += @vel_x
    @y += @vel_y
    @x %= 640
    @y %= 480
    
    @vel_x *= 0.95
    @vel_y *= 0.95
  end

  def draw
    @image.draw_rot(@x, @y, 1, @angle)
  end
end

There are a couple of things to say about this:

• Player#accelerate makes use of the offset_x/offset_y functions. They are similar to what some people use sin/cos for: For example, if something moved 100 pixels at an angle of 30°, it would pass offset_x(30, 100) pixels horizontally and offset_y(30, 100) pixels vertically.
• When loading BMP files, Gosu replaces 0xff00ff (fuchsia; that really ugly pink) with transparent pixels.
• Note that draw_rot puts the center of the image at (x; y) - not the upper left corner as draw does!
• The player is drawn at z=1, i.e. over the background (obviously). We'll replace these magic numbers with something better later.
• Also, see the RubyReference for all drawing methods and arguments.

Integrating Player with the Window

class GameWindow < Gosu::Window
  def initialize
    super(640, 480, false)
    self.caption = "Gosu Tutorial Game"
    
    @background_image = Gosu::Image.new(self, "media/Space.png", true)

    @player = Player.new(self)
    @player.warp(320, 240)
  end

  def update
    if button_down? Gosu::Button::KbLeft or button_down? Gosu::Button::GpLeft then
      @player.turn_left
    end
    if button_down? Gosu::Button::KbRight or button_down? Gosu::Button::GpRight then
      @player.turn_right
    end
    if button_down? Gosu::Button::KbUp or button_down? Gosu::Button::GpButton0 then
      @player.accelerate
    end
    @player.move
  end

  def draw
    @player.draw
    @background_image.draw(0, 0, 0);
  end

  def button_down(id)
    if id == Gosu::Button::KbEscape
      close
    end
  end
end

As you can see, we have introduced keyboard and gamepad input! Similar to update() and draw(), Gosu::Window provides two member functions button_down(id) and button_up(id) which can be overriden, and do nothing by default. We do this here to close the window when the user presses ESC. (For a list of predefined button constants, see RubyReference). While getting feedback on pushed buttons is suitable for one-time events such as UI interaction, jumping or typing, it is rather useless for actions that span several frames - for example, moving by holding buttons down. This is where the update() member function comes into play, which only calls the player's movement methods. If you run this lesson's code, you should be able to fly around!

3, Simple animations

First, we are going to get rid of the magic numbers for Z positions from now on by replacing them with the following constants:

module ZOrder
  Background, Stars, Player, UI = *0..3
end

What is an animation? A sequence of images - so we'll use Ruby's built in Arrays to store them. (For a real game, there is no way around writing some classes that fit the game's individual needs, but we'll get away with this simple solution for now.)

Let's introduce the stars which are the central object of this lesson. Stars appear out of nowhere at a random place on the screen and live their animated lives until the player collects them. The definition of the Star class is rather simple:

class Star
  attr_reader :x, :y
  
  def initialize(animation)
    @animation = animation
    @color = Gosu::Color.new(0xff000000)
    @color.red = rand(255 - 40) + 40
    @color.green = rand(255 - 40) + 40
    @color.blue = rand(255 - 40) + 40
    @x = rand * 640
    @y = rand * 480
  end

  def draw  
    img = @animation[Gosu::milliseconds / 100 % @animation.size];
    img.draw(@x - img.width / 2.0, @y - img.height / 2.0,
        ZOrder::Stars, 1, 1, @color, :additive)
  end
end

Since we don't want each and every star to load the animation again, we can't do that in its constructor, but rather pass it in from somewhere else. (The Window will load the animation in about three paragraphs.)

To show a different frame of the stars' animation every 100 milliseconds, the time returned by Gosu::milliseconds is divided by 100 and then modulo-ed down to the number of frames. This very image is then additively drawn, centered at the star's position and modulated by a random colour we generated in the constructor.

Now let's add easy code to the player to collect away stars from an array:

class Player
…
  def collect_stars(stars)
    stars.reject! do |star|
      Gosu::distance(@x, @y, star.x, star.y) < 35
    end
  end
end

Now let's extend Window to load the animation, spawn new stars, have the player collect them and draw the remaining ones:

class Window < Gosu::Window
  def initialize
    super(640, 480, false)
    self.caption = "Gosu Tutorial Game"
    
    @background_image = Gosu::Image.new(self, "media/Space.png", true)
    
    @player = Player.new(self)
    @player.warp(320, 240)

    @star_anim = Gosu::Image::load_tiles(self, "media/Star.png", 25, 25, false)
    @stars = Array.new
  end

  def update
    ...
    @player.move
    @player.collect_stars(@stars)
    
    if rand(100) < 4 and @stars.size < 25 then
      @stars.push(Star.new(@star_anim))
    end
  end

  def draw
    @background_image.draw(0, 0, ZOrder::Background)
    @player.draw
    @stars.each { |star| star.draw }
  end
…

Done! You can now collect stars.

4. Text and sound

Finally, we want to draw the current score using a bitmap font and play a 'beep' sound every time the player collects a star. The Window will handle the text part, loading a font 20 pixels high:

class Window < Gosu::Window
  def initialize
    …
    @font = Gosu::Font.new(self, Gosu::default_font_name, 20)
  end

  …

  def draw
    @background_image.draw(0, 0, ZOrder::Background)
    @player.draw
    @stars.each { |star| star.draw }
    @font.draw("Score: #{@player.score}", 10, 10, ZOrder::UI, 1.0, 1.0, 0xffffff00)
  end
end

What's left for the player? Right: A counter for the score, loading the sound and playing it.

class Player
  attr_reader :score

  def initialize(window)
    @image = Gosu::Image.new(window, "media/Starfighter.bmp", false)
    @beep = Gosu::Sample.new(window, "media/Beep.wav")
    @x = @y = @vel_x = @vel_y = @angle = 0.0
    @score = 0
  end

  …

  def collect_stars(stars)
    stars.reject! do |star|
      if Gosu::distance(@x, @y, star.x, star.y) < 35 then
        @score += 10
        @beep.play
        true
      else
        false
      end
    end
  end
end

As you can see, loading and playing sound effects couldn't be easier! See the RubyReference for more powerful ways of playing back sounds - fiddle around with volume, position and pitch.

That's it! Everything else is up to your imagination. If you can't imagine how this is enough to create games, see if you can find useful source code from the GosuUsers page.