a simple 2D game engine

1
a simple 2D game engine

• input handling and collision detection

2
Getting input from the user

• We want to keep track of
• Where the mouse is
• Whether a mouse button has been pressed
• What keys have been pressed
• Well do this by storing the information in
  global variables
• For mouse position and state
• And a list of states (up/down) for each key

• define mouse-location point 0 0
• define mouse-in-window? false
• define mouse-pressed? false
• define key-states
• This just makes a list of 256 elements, all
  set to false
• make-list 256 false
• define key-pressed? Keyvalue
• get key-states keyvalue

3
Event handling

• The operating system tells you what the user is
  doing by signaling events
• KeyDown/KeyUp
• A key went down/up on the keyboard
• MouseDown/MouseUp
• A button went up/down on the mouse
• MouseMove
• The mouse moved inside the window
• MouseEnter/MouseLeave
• The user dragged the mouse into/out of the window
• So we need to set up handlers for them

4
Setting up input handling

• First, we initialize all those global variables
• Set all the elements of the list of key states to
  false (i.e. not pressed)
• Assume the mouse buttons are up
• Assume the mouse is out of the window

• define initialize-input form
• up-to 256
• i ? item
  key-states i ? false
• mouse-location ? point 0 0
• mouse-down? ? false
• mouse-in-window? ? false

5
Setting up event handlers

• When the MouseEnter event happens
• Set mouse-in-window? to be true
• When MouseLeave happens
• Set it to be false

• define initialize-input form
• form.MouseEnter ? ignore ignore ?
• mouse-in-window? ? true
• form.MouseLeave ?
• form.MouseMove ? ignore e ?
• update-mouse e
• form.MouseDown ? ignore e ?
• mouse-down? ? true
• update-mouse e
• form.MouseUp ?

6
Setting up event handlers

• When MouseMove happens
• The OS passes a MouseEventArgs object
• Called e here
• It contains the coordinates of the mouse
• Call update-mouse to update the mouse-location
  variable using the MouseEventArgs object

• define initialize-input form
• form.MouseEnter ? ignore ignore ?
• mouse-in-window? ? true
• form.MouseLeave ?
• form.MouseMove ? ignore e ?
• update-mouse e
• form.MouseDown ? ignore e ?
• mouse-down? ? true
• update-mouse e
• form.MouseUp ?

7
update-mouse

• The .X and .Y fields of the MouseEventArgs object
  give the location of the mouse
• Make a point out of them and update mouse-location

• define update-mouse emouse-location ?
  point e.X e.Y

8
Setting up event handlers

• MouseDown/MouseUp events are the same way
• The MouseEventArgs object also has a field called
  Button that tells which button was pressed
• For simplicity, well ignore that
• Update the mouse-down? Variable
• Call update-mouse to update mouse-location

• define initialize-input form
• form.MouseDown ? ignore e ?
• mouse-down? ? true
• update-mouse e
• form.MouseUp ?

9
Setting up the keyboard handlers

• The OS passes a KeyEventArgs object to keyboard
  handlers
• Its KeyValue field has a number corresponding to
  the key that was pressed

• define initialize-input form
• form.KeyDown ? ignore e ?
  set-key-state e.KeyValue
  true
• form.KeyUp ? ignore e ?
  set-key-state e.KeyValue
  false

10
Implementing set-key-state

• Basically, we
• Take the key number (keyvalue)
• And set the corresponding element of key-states
  to state (true or false)
• However, we need to do the bitwise-and thingy to
  fix the number
• Well explain this later
• For now, trust me

• define set-key-state keyvalue state
• set key-states
• bitwise-and keyvalue 255
• state

11
Getting the magic key numbers

• Theres an object called Keys
• It has fields with all the key numbers
• Except theyre represented as a funny kind of
  object
• So you need to do use int (short for integer)
  to make them real numbers
• You can get any keyboard key you want this way
  (e.g. using Keys.A for the A key)
• To get a list of all the magic key codes, google
• .NET Keys class members

• define up-arrow int Keys.Up
• define down-arrow int Keys.Down
• define left-arrow int Keys.Left
• define right-arrow int Keys.Right

12
Example controlling a car

• A simple car that you can steer with the arrow
  keys
• Up go forward
• Down go back
• Left, right - steer

13
Example controlling a car tank

• A simple car that you can steer with the arrow
  keys
• Unfortunately, its hard to tell which way the
  car is facing
• So well use a tank

14
The Tank class

• Basically like the Obstacle class, but a slightly
  different draw method

• define Tank
• class Tank position
• GameObject
• define-method draw Tank t g
• g.DrawRectangle black-pen
  -12 -10 24 20
• g.DrawRectangle black-pen 12 -2 10 4

15
Controlling speed

• Simple version
• If they hold down the up-arrow key
• Move right at 10 pixels per second

• define-method tick Tank t ?t
• t.velocity ?
• if key-pressed? up-arrow
• point 10 0
• point 0 0

16
Controlling speed

• Simple version
• If they hold down the up-arrow key
• Move right at 10 pixels per second
• Slightly fancier
• Let them move backward

• define-method tick Tank t ?t
• t.velocity ?
• if key-pressed? up-arrow
• point 10 0
• if key-pressed? down-arrow
  point -10 0
• point 0 0

17
Controlling speed

• Simple version
• If they hold down the up-arrow key
• Move right at 10 pixels per second
• Slightly fancier
• Let them move backward
• More readable
• Turn the two ifs into one cond

• define-method tick Tank t ?t
• t.velocity ?
• cond key-pressed? up-arrow
• point 10 0
• key-pressed? down-arrow
  point -10 0
• else point 0
  0

18
Controlling speed

• Simple version
• If they hold down the up-arrow key
• Move right at 10 pixels per second
• Slightly fancier
• Let them move backward
• Oh, yea. Let them steer.

• define-method tick Tank t ?t
• t.velocity ? rotate-vector-degrees
• cond key-pressed? up-arrow
• point 10 0
• key-pressed? down-arrow
  point -10 0
  else
• point 0 0
  t.orientation

19
Controlling speed

• Writing it this way makes it easier to change the
  speed if we want to
• Only one number to change

• define-method tick Tank t ?t
• t.velocity ? 10
• cond key-pressed? up-arrow
  1 key-pressed?
  down-arrow
• -1 else
  0
• rotate-vector-degrees
  point 1 0
• t.orientation

20
Steering

• To steer
• Just do the same trick with the angular velocity
• Only, remember its a number, not a vector

• define-method tick Tank t ?t
• t.velocity ? ...
• t.angular-velocity ? 30
• cond key-pressed? left-arrow
  -1
• key-pressed?
  right-arrow 1
• else
  0

21
Sticking to the mouse

• We can force an object to follow the mouse by
  just setting its position to the mouse position

• define dragger
• class dragger position
• GameObject
• define-method draw dragger o g
• g.DrawEllipse black-pen -20 -20 40 40
• define-method tick dragger o ?t
• o.position ? mouse-location

22
Following the mouse

• We can make the object follow the mouse
• by setting its speed to always be some fraction
  of the vector between the object and the mouse

• define Mouser
• class Mouser position
• GameObject
• define-method draw Mouser o g
• g.DrawEllipse black-pen -20 -20 40 40
• define-method tick Mouser o ?t
• o.velocity ? - mouse-location
• o.position
• 0.5

23
Making it more interesting

• We can complicate the dynamics of the ball by
• Accelerating it toward the mouse

• define SpringMouser
• class SpringMouser position
• GameObject
• define-method tick SpringMouser o ?t
• o.velocity ? o.velocity
• -
  mouse-location
  o.position
• 0.1
• -0.001
  o.velocity

24
Making it more interesting

• We can complicate the dynamics of the ball by
• Accelerating it toward the mouse
• Rather than moving straight toward it

• define SpringMouser
• class SpringMouser position
• GameObject
• define-method tick SpringMouser o ?t
• o.velocity ? o.velocity
• -
  mouse-location
  o.position
• 0.1
• -0.001
  o.velocity

25
Making it more interesting

• We can complicate the dynamics of the ball by
• Accelerating it toward the mouse
• Rather than moving straight toward it
• And adding some damping to slow it down

• define SpringMouser
• class SpringMouser position
• GameObject
• define-method tick SpringMouser o ?t
• o.velocity ? o.velocity
• -
  mouse-location
• 
  o.position
• 0.1
• -0.001
  o.velocity

26
Changing the appearance slightly

• We can make a filled circle rather than a normal
  circle
• By substituting
• FillEllipse
• For DrawEllipse
• And a Brush object
• For a Pen

• define green-brushnew SolidBrush color
  green
• define-method draw SpringMouser o g
• g.FillEllipse green-brush
• -20 -20 40 40

27
Collision handling

• Many games need to notice when two objects
  collide
• This is an expensive operation
• Its an on-going area of research
• Well just talk about the absolute dumbest
  version of it

• define update-objects form
• with current-time DateTime.Now
• ?t current-time.Subtract
  last-time.TotalSeconds
• for-each-game-object o ? tick o ?t
• for-each-game-object o ?
• o.position ? o.position
• 
  o.velocity ?t
• o.orientation ? o.orientation
• 
  o.angular-velocity
  ?t
• check-collisions
• last-time ? current-time
• form.Invalidate

28
Naïve algorithm

• Compare every object to every object
• Check if theyve collided
• Deal with it if they have
• Notes
• We havent said how to check whether two objects
  have collided
• Or what to do about it

• define check-collisions
• for-each-game-object
• o1 ? for-each-game-object
• o2 ? if collided? o1 o2
  handle-collision o1 o2

29
Computational complexity

• Suppose there are n objects in the game

• define check-collisions
• for-each-game-object
• o1 ? for-each-game-object
• o2 ? if collided? o1 o2
  handle-collision o1 o2

30
Computational complexity

• Suppose there are n objects in the game
• Then this procedure runs n times

• define check-collisions
• for-each-game-object
• o1 ? for-each-game-object
• o2 ? if collided? o1 o2
  handle-collision o1 o2

31
Computational complexity

• Suppose there are n objects in the game
• Then this procedure runs n times
• But each time it runs, this procedure runs n
  times!

• define check-collisions
• for-each-game-object
• o1 ? for-each-game-object
• o2 ? if collided? o1 o2
  handle-collision o1 o2

32
Computational complexity

• Suppose there are n objects in the game
• Then this procedure runs n times
• But each time it runs, this procedure runs n
  times!
• So that means we do n2 collision checks

• define check-collisions
• for-each-game-object
• o1 ? for-each-game-object
• o2 ? if collided? o1 o2
  handle-collision o1 o2

33
Computational complexity

• Suppose there are n objects in the game
• Then this procedure runs n times
• But each time it runs, this procedure runs n
  times!
• So that means we do n2 collision checks
• Very expensive

• define check-collisions
• for-each-game-object
• o1 ? for-each-game-object
• o2 ? if collided? o1 o2
  handle-collision o1 o2

34
Computational complexity

• The time complexity of a program is
• The rate at which the running time increases
• With the size of its input
• A linear algorithm increases about as fast as n
• We then say the algorithm is O(n)
• A quadratic algorithm increases as fast as n2
• We then say its O(n2)
• An exponential time algorithm increases
  exponentially with n
• Exponential algorithms are useless unless n is
  very small

35
Problems with the algorithm

• This actually has some bugs

• define check-collisions
• for-each-game-object
• o1 ? for-each-game-object
• o2 ? if collided? o1 o2
  handle-collision o1 o2

36
Problem 1

• This will check whether each object has collided
  with itself
• It probably has

• define check-collisions
• for-each-game-object
• o1 ? for-each-game-object
• o2 ? if collided? o1 o2
  handle-collision o1 o2

37
Problem 2

• For any two objects, x and y, it will call both
• collided? x y, and
• collided? y x

• define check-collisions
• for-each-game-object
• o1 ? for-each-game-object
• o2 ? if collided? o1 o2
  handle-collision o1 o2

38
Fancy version

• This version only compares objects
• Against objects that are later in the list of
  game objects
• So we only compare each pair once

• define check-collisions
• if collision-detection-enabled?
• with i 0
• while lt i length all-game-objects
• with o1 get all-game-objects i
• j i 1
• while lt j length
  all-game-objects
• with o2 get
  all-game-objects j
• if collided? o1 o2
• handle-collision o1
  o2
• j ? j 1
• i ? i 1

39
Adding collision handling to the SpringBall
example

• Two balls have collided if

• define collided? o1 o2lt magnitude -
  o1.position
• o2.position
• 10
• define-method handle-collision
  SpringBall o1
  SpringBall o2
• destroy o1
• destroy o2

40
Adding collision handling to the SpringBall
example

• Two balls have collided if
• The length

• define collided? o1 o2lt magnitude -
  o1.position
• o2.position
• 10
• define-method handle-collision
  SpringBall o1
  SpringBall o2
• destroy o1
• destroy o2

41
Adding collision handling to the SpringBall
example

• Two balls have collided if
• The length
• Of the space between them

• define collided? o1 o2lt magnitude -
  o1.position
• o2.position
• 10
• define-method handle-collision
  SpringBall o1
  SpringBall o2
• destroy o1
• destroy o2

42
Adding collision handling to the SpringBall
example

• Two balls have collided if
• The length
• Of the space between them
• Is less than 10 pixels

• define collided? o1 o2lt magnitude -
  o1.position
• o2.position
• 10
• define-method handle-collision
  SpringBall o1
  SpringBall o2
• destroy o1
• destroy o2

43
Adding collision handling to the SpringBall
example

• When two balls collide

• define collided? o1 o2lt magnitude -
  o1.position
• o2.position
• 10
• define-method handle-collision
  SpringBall o1
  SpringBall o2
• destroy o1
• destroy o2

44
Adding collision handling to the SpringBall
example

• When two balls collide
• Blow the little buggers away

• define collided? o1 o2lt magnitude -
  o1.position
• o2.position
• 10
• define-method handle-collision
  SpringBall o1
  SpringBall o2
• destroy o1
• destroy o2

45
Final project

• On blackboard
• Work in teams of 1-3 people
• Make a 2D game of some sort
• Must include
• User input
• At least 2 classes
• At least 6 methods total for tick, draw,
  collided?, handle-collision

• Should also include at least one of (or 2 or 3 if
  youre working in a group)
• Collision handling
• Simple physics
• Sound
• In-game object creation
• Object destruction
• Something else you get me to agree to in advance
• Rough cut due a week from Thursday
• Final version due the Saturday before finals

46
Movie today V for Vendetta

• 4pm, Animate Arts studio, Kresge 1-370
• Feel welcome to attend if youre still in town
  and youre bored