Games Development Practices The Game Loop and Timing

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Games Development PracticesThe Game Loop and
Timing

• CO2301 Games Development 1
• Week 3

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Basic Game Program Structure

• Basic program structure used in TL-Engine
• include ltgame engine librariesgt
• Initialise game engine
• ...
• Load game media
• Initialise game scene
• while (game is playing)
• Render the game scene
• Move/Animate/Update game scene
• ...
• Shut down game engine
• End program

Game Setup
Game Loop
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Limitations of this Structure

• Too simplistic, assumes
• Game is set-up only once, i.e. only one level
• No front-end - nothing before game set-up
• Program ends when game-loop ends
• Cant go to next level, or back to the front-end
• We need a more complex structure
• The exact structure needed depends on the style
  of the game progression

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Better Game Structure

• Better generic game structure
• Initialise engine
• Front-end setup
• Front-end loop (until Start Game or Quit)
• Front-end shutdown
• if (Quit) Shut-down engine and end program
• Set starting level
• Level setup
• Game loop (until Next Level or Game Over)
• Level shutdown
• while Next Level, loop to Level setup
• Loop to Front-end setup

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Basic Game Loop

• Game is displayed like a film or animation
• Sequence of static images (frames), displayed
  very quickly
• Objects change position slightly from frame to
  frame
• We dont perceive separate images, instead we see
  an animated scene

• Basic game loop performs in this manner
• Render static image of the scene
• Move the objects a small amount
• Repeat process as long as the game is running

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Limitations of Basic Game Loop

• No timing in a basic loop
• Render and update scene as fast as possible
• Loop frequency dependent on computer speed
• Scene updates will occur more frequently on a
  fast machine, less so on a slow one
• Using statements like MoveX(0.1) will cause
  models to move at different speeds on different
  computers.
• Rendering frequency will also vary - lesser
  problem
• How can we make our scene updates constant?

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Game Loop Timing

• Need to time the duration of each game loop
• Get accurate measure of the time on each
  iteration of the game loop (each frame)
• Subtract the previous time from the current time
• ..to give us the duration since the last frame
• This is the frame time
• Use the frame time to determine how much to
  move/rotate models in the current scene update
• N.B. The frame rate 1 / frame time
• And frame time 1 / frame rate

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Update Ticks

• Decide on a fixed rate at which we would ideally
  like to update the scene
• E.g. we would like the game scene updated at
  50fps
• This is a scene update every 1/50 0.02s
• Call this the scene update tick
• We will use timing algorithms based on this tick
• Can have different ticks for other game aspects
• E.g. an AI component with a tick of 2fps, a
  slower tick as it uses a complex algorithm

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Using Ticks 1 Variable Style

• Each time we reach the scene update
• Divide the latest frame time by the update tick
  length
• To get the number of updates needed (floating
  pt)
• Frame Time (time since last update) 0.09s
• If tick 0.02s, then NumUpdates 4.5
• Use result to scale values used in scene update
• E.g. MoveX( CarSpeed NumUpdates )
• The constant CarSpeed will now be a speed in
  world units per tick - regardless of machine speed

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Using Ticks 1 Variable Style

• Some drawbacks to this method
• Cannot scale matrices in this way
• Will see importance of matrices shortly
• Can scale angles and quaternions though
• On a network game, different speed machines will
  perform different calculations
• With potentially different results

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Using Ticks 2 Fixed Style

• Divide the latest frame time by update tick
  length
• But get the whole number of updates needed
• Frame Time 0.09s
• If tick 0.02s, then WholeUpdates 4
• Run scene update several times in a loop once
  for each whole update needed
• Scene update code does not need to be altered
• E.g. MoveX(CarSpeed), speed is still measured in
  world units per tick
• Carry remaining time to add to next frame time
• Here carry 0.01s to add to next frame time

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Using Ticks 2 Fixed Style

• Drawbacks of this method
• Not as smooth as a variable update method
• Cant run game in slow motion(!)
• Many ticks on slow machine slows it down more
• E.g. a machine takes 0.2s per frame
• it will call scene update 0.2 / 0.02 10 times a
  frame
• Sometimes no ticks at all between scene updates
  on a fast machine
• E.g. a machine takes 0.01s per frame
• Doesnt need to update scene so what to do?

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Choosing a Tick Method

• Variable style is more flexible where practical
• Some game aspects cant be scaled easily
• AI, physics engines etc. can cause difficulties
• Or scaling may add an unacceptable overhead
• Drawbacks of fixed method dont always occur
• Particularly on consoles - machine speed is
  constant
• Many games use both methods for different game
  aspects