Animation in games

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Animation in games
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MVC

• MVC stands for Model-View-Controller
• The Model is the actual internal representation
• The View (or a View) is a way of looking at or
  displaying the model
• The Controller provides for user input and
  modification
• The Model should always be kept separate
• The View and Controller can sometimes be combined

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Applets and applications

• An Applet is only part of a program
• The browser, or appletviewer, is the other part
• An Applet cannot access your files
• It can't keep a high-scores page (except on the
  server)
• An application can't appear on a web page
• Painting and animation is the same for both
• You can write an "appletcation" that works either
  as an Applet or as an application

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The class Applet

• class Applet extends Panel public void
  init ( ) public void start ( )
  public void stop ( ) public void destroy
  ( ) public void paint (Graphics g)
  . . .

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Applet flow of control
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Painting is easy

• A Graphics is a painting area
• If g is a Graphics, you can
• g.setColor(Color.red)
• g.fillRect(10, 20, 300, 200)
• g.drawString(50, 50, "Hello, world!")
• When you write an Applet, you get a Graphics as a
  parameter to paint() and update()
• For an application, call component.getGraphics()

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Animation is repeated painting

• To animate something
• Update the Model, so you know what the thing is
  currently doing
• The View then paints it on the screen by calling
  repaint( ) -- not paint( )
• Call Thread.sleep(ms) to wait until it's time to
  paint again

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Animation with less flicker

• To animate something
• Update the Model, so you know what the thing is
  currently doing
• The View then paints it on an offscreen Image
• Then View copies the offscreen to the screen
• Call Thread.sleep(ms) to wait until it's time to
  paint again

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update( ), paint( ), and repaint( )

• When you need to change the image on the screen,
  call repaint( )
• repaint( ) tells Java to do it "as soon as you
  can"
• Painting immediately isn't always possible
• Java should eventually call update(Graphics g)
• update clears ("erases") g, then calls paint(g)
• Your paint does the actual painting
• So write paint(Graphics g), but call repaint( )

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Threads

• A thread is a single flow of control
• If you have animation and user input, they must
  be in separate threads
• Your animation is an infinite loop update model,
  paint, sleep, update model, paint, sleep, ...
• Nowhere in that infinite loop does it say "get
  user input"!
• Solution put the animation in a new thread

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Implementing a Thread

• Every Thread must have a run( ) method
• This tells it what to do
• When run( ) finishes, the thread dies
• Hence, run( ) should be an (almost) infinite loop
• There are two ways to create a new thread
• Extend Thread
• Implement Runnable
• Either way, you must supply a run( ) method

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Extending Thread

• class Animation extends Thread
• Animation anim new Animation ( )
• A newly created thread is ready to run, but not
  actually running yet
• anim.start ( )
• start( ) is a request to the scheduler to run the
  Thread--it may not happen right away
• The thread should eventually start running

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Implementing Runnable

• class Animation implements Runnable
• Animation anim new Animation ( )
• We still need an actual Thread
• Thread myThread new Thread(anim)
• myThread.start( )
• As always, start( ) is a request to the scheduler
  to run the Thread--it may not happen right away

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Controlling the animation thread

• With two or more threads, you never know what
  order things are going to happen in
• This leads to some real headaches
• In my examples I used two variables
• alive -- to exit the animation when it become
  false
• okToRun -- when this is false, the animation does
  nothing except sleep
• These are initialized before the Thread is
  started
• You should look at my example code!

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run( )
public void run() while(alive)
if(okToRun) model.makeOneStep()
repaint() try
Thread.sleep(delay) catch(InterruptedExc
eption e)
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Using MediaTracker for images

• When you load an image from a file, Java only
  starts loading it
• You need a MediaTracker to tell when it's done
• The steps are
• Create a MediaTracker
• For each image,
• Start loading the image
• Tell your MediaTracker about the image
• Tell the MediaTracker to wait for loading to
  finish

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The End