Index

1. PGE Input and Timers

• PGE is a Predictive physics Game Engine
  • it operates by predicting the time of next collision rather than using a frame based approach

2. PGE Screen and world coordinates 1m x 1m

3. PGE input

• PyGame keyboard and mouse events can be utilised in PGE
• you can define a call back and register it within the game

• examples/breakout/breakout.py

      ...
      pge.register_handler (myquit, [QUIT])
      pge.register_handler (key_pressed, [KEYDOWN])
      pge.register_handler (mouse_hit, [MOUSEBUTTONDOWN])
      ...

4. Breakout input handler functions

• examples/breakout/breakout.py

  def finish_game (event = None, unused = None):
      sys.exit (0)
  
  def myquit (e):
      print ‘‘goodbye''
      finish_game ()
  
  
  def key_pressed (e):
      if e.key == K_ESCAPE:
          myquit (e)

• notice that the PyGame Event object e is passed into myquit
  

5. Breakout mouse input

• examples/breakout/breakout.py

  def mouse_hit (e):
      global gb
      mouse = pge.pyg_to_unit_coord (e.pos)
      if e.button == 1:
          # left button
          gb.put_xvel (gb.get_xvel ()-0.3)
      elif e.button == 3:
          # right button
          gb.put_xvel (gb.get_xvel ()+0.3)
      elif gb.moving_towards (mouse[0], mouse[1]):
          # middle button
          pos = gb.get_unit_coord ()
          gb.apply_impulse (pge.sub_coord (mouse, pos), 0.4)
      else:
          # middle button
          gb.put_yvel (gb.get_yvel ()+0.4)

• notice a series of helper function/methods exist
• mouse = pge.pyg_to_unit_coord (e.pos)
  • mouse is a unit vector [x,y] containing the current mouse position
  • x and y are in the PGE world range 0.0 to 1.0
• pos = gb.get_unit_coord ()
  • pos is a unit vector containing the gold ball position
• we can test whether an object is moving towards a position using

• mouse = pge.pyg_to_unit_coord (e.pos)
  if gb.moving_towards (mouse[0], mouse[1]):
    ...

6. Newton's laws of motion

• he stated three physical laws that, together, laid the foundation for classical mechanics
• describe the relationship between a body and the forces acting upon it
• describe its motion in response to those forces

7. Summary of the First law

• in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a net force

8. Summary of the Second law

• in an inertial reference frame, the vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration a of the object:

• 

9. Summary of the Third law

• when one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body
• these three laws of motion were first compiled by Isaac Newton in his Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), first published in 1687
• Newton used them to explain and investigate the motion of many physical objects and systems

10. Adding energy into the PGE world

• one of the considerations in building a game engine, is how to introduce new energy into the simulation
• you need to be careful, too much and objects become chaotic
  • too little and the objects are starved of motion
• the amount of energy depends whether the object collisions are elastic or inelastic
• elastic object collisions, energy is never lost
• inelastic object collisions, energy is lost (modelling friction, heat, noise energy)

11. Applying an impulse to an object

• examples/breakout/breakout.py

      gb.apply_impulse (pge.sub_coord (mouse, ball), 0.4)

• applies an impulse to an object
• an impulse is a non-physics term and in the game engine it means
• a force applied instantaneously to an object
• notice that in Newtons 2 nd law we see the equation

  

• the acceleration, implies time

  

• we don't necessarily have a change in velocity over some time
  • we simply want to introduce energy into the engine
• we will see this same problem when handling collisions
  • it is not always necessary to model the world exactly to get the desired effect in the game engine
• we can also instantaneously change an objects velocity
  • violating Newtons 2 nd law
• 
  examples/breakout/breakout.py

          gb.put_yvel (gb.get_yvel ()+0.4)

• obviously we need to be careful with these hacks, or the game will feel unnatural
  

12. Timers

• PGE allows users to introduce timer callbacks
• here is how a simple second count down might be implemented

• examples/breakout/breakout.py

  def timer (event = None, unused = None):
      global seconds_left, previous
      if seconds_left >= 0:
          pge.at_time (1.0, timer)
          s = ‘‘%d'' % seconds_left
          if previous != None:
              previous.rm ()
          previous = pge.text (0.8, 0.9, s, white, 100, 1)
          seconds_left -= 1

• which when called from main() will display the current number of seconds left and register itself to be called 1.0 seconds in the future
• at_time returns an integer id representing the timer created
• this timer can be cancelled using at_cancel (id)
• if it is cancelled, the callback still occurs, your program could check cancellation by:

• def timer (event = None, unused = None):
      global seconds_left, previous
      if seconds_left >= 0:
          if event != None and event.was_cancelled ():
             print ‘‘event was cancelled''
          pge.at_time (1.0, timer)
          s = ‘‘%d'' % seconds_left
          if previous != None:
              previous.rm ()
          previous = pge.text (0.8, 0.9, s, white, 100, 1)
          seconds_left -= 1

13. Conclusion

• we have seen how energy can be added into PGE
• we have also explored some of the API calls surrounding
  • timer events
  • mouse movement
  • object movement and how to detect if two objects are moving towards each other
• next week we will examine how the an application can interact with collisions

Index

This document was produced using groff-1.22.