Rapid Game Development with RUby and Gosu – Ruby Manor 4

1. Rapid Game
Development with
Ruby and Gosu
Belén Albeza
@ladybenko

2. Aren’t games coded in
C++?

3. Minecraft
(Java)

4. To the Moon
(RPG Maker)

5. So?
• Some games will require C++
• Some games won’t
• You can trade performance for:
• Better productivity (faster development,
prototypes to test ideas, etc.)
• Happiness :)

6. Prototyping
• One Game A Month
www.onegameamonth.com
• Experimental Gameplay
www.experimentalgameplay.com
• Ludum Dare
www.ludumdare.com

7. Introducing Gosu

8. What is Gosu?
• Gosu is a minimalistic 2D game library
www.libgosu.org
• Free, Open source (MIT License)
• Multiplatform (Win, OS X, Linux)
• Has bindings for Ruby and C++
• $gem install gosu

9. Gosu’s API is very small
• ~100 methods in 9 classes
• Gosu provides a way to:
• Create an OpenGL window
• Load and draw images and fonts
• Load and play sounds
• Gather player’s input

10. Show demo

11. Gosu 101
https://github.com/belen-albeza/gosu-rubymanor

12. The Game Loop
snippets/create_window.rb

13. Get player input
60 FPS
Update game
Draw game

14. require 'rubygems'
require 'gosu'
class Game < Gosu::Window
# ...
end
game = Game.new
game.show

15. class Game < Gosu::Window
def initialize
super(800, 600, false)
end
def draw # gets called every frame
end
def update # gets called every frame
end
def button_up(key) # callback
end
end

16. Images
snippets/draw_image.rb

17. Instance of Gosu::Window
# load
@img_bg =
Gosu::Image.new(self,‘space.png’)
# draw
@img_bg.draw(0, 0, 0)
@ship.draw_rot(400, 300, 0, 45)
# note: audio and fonts follow the same
# approach.

19. Input
snippets/input.rb

20. # callback for key up events
def button_up(key)
close if key == Gosu::KbEscape
end
# check if a key is being pressed
def update
if self.button_down?(Gosu::KbLeft)
move_left
end
end
Instance of Gosu::Window

21. Delta time
snippets/delta_time.rb

22. 4px / frame
= 12 px
4 px
4px
4 px
= 46 ms
13 ms
4px / frame @ 60 FPS
vs
240 pixels / second
16 ms
17 ms
240 px / second
= 11.04 px
3.12 px
3.84 px
4.08 px
= 46 ms
13 ms
16 ms
17 ms

23. def update_delta
current_time = Gosu::milliseconds /
1000.0
# tip: always cap your delta
@delta = [current_time - @last_time,
0.25].min
@last_time = current_time
end
# simple movement
@x += SHIP_SPEED * @delta
# with inertia
@speed_x += SHIP_ACCELERATION * @delta
@x += @speed_x * @delta

24. Distribution
• Mac: App wrapper with a Ruby on it
https://github.com/jlnr/gosu/wiki/RubyPackaging-on-OS-X
• Windows: OCRA https://github.com/jlnr/
gosu/wiki/Ruby-Packaging-on-Windows

25. Game Dev Techniques

26. Bounding boxes
•
Quick collisions, but not
very accurate
•
Shapes can be combined
to increase accuracy
•
Beware of rotations!
http://devmag.org.za/2009/04/13/basic-collisiondetection-in-2d-part-1/

27. Finite State Machines
•
•
•
Patrol
Easy to implement,
cheap, lots of uses...
AI: character behaviors
Scene stack
seeing player?
not seeing player?
out of attacking distance?
Chase
Attack
in attacking distance?
http://www.generation5.org/content/2003/
fsm_tutorial.asp

28. Tiles
•
•
Divide a level into a grid
•
Useful to save memory,
make a level editor,
implement simple
physics, etc.
Visual grid != Logic
grid... but we can map
them :)
http://www-cs-students.stanford.edu/~amitp/
gameprog.html#tiles

29. Path-finding
•
They are usually very
expensive... try to
minimise their use
•
Dijkstra is enough for
simple graphs (ie. an
adventure)
•
A* for everything else
(action RPG’s, strategy,
etc.)
http://theory.stanford.edu/~amitp/GameProgramming/

30. Scripting
• Scripting transforms a simple arcade level
into a mission or a quest (see Cave Story)
• Embed a VM into your engine (most
popular for games is Lua)... but Ruby is
already a script language :D
• Useful triggers: enter an area, exit an area,
talk to NPC, pick up item, kill an enemy,
etc.

31. event = {
:type => :talk_to,
:data => :friend
}
click
call
talk_to_friend

32. Scripting example
# this method is called when the event
# talk_to is triggered on the :pirate
# NPC
def talk_to_pirate
npc_say(:pirate, ‘Aaaarrrr’)
add_to_inventory(:rum)
end

33. Physics engine
•
Real physics for your
games! Done by smart
people! And free!
•
They are slow, so try to
minimise the amount of
physical entities
•
You need to map your
visual world into an
invisible physical world
(beware of units!)

34. Physics + Gosu
• Use Box2D (low-level) or Chipmunk
• Chipmunk integration tutorial at https://
github.com/jlnr/gosu/wiki/Ruby-ChipmunkIntegration

35. The Golden Rule of Game Dev
If you can fake it,
then fake it.

36. Resources
• Chingu: game framework for Gosu https://
github.com/ippa/chingu
• Creative Commons art: http://
www.lostgarden.com/search/label/free
%20game%20graphics
• More: http://www.libgosu.org/cgi-bin/mwf/
board_show.pl?bid=4

37. Thanks!
Questions?