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2 . 1. The Game Loop

2 . 1. The Game Loop. Central game update and render processes. Key Components/Actions in a (2d) Game. Identifying commonality between different game types. Exercise: Distilling Key Components/Actions. Racing game. Adventure game. Puzzle game. Hard: . Easy: . Platform game.

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2 . 1. The Game Loop

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  1. 2.1.The Game Loop Central game update and render processes

  2. Key Components/Actions in a (2d) Game Identifying commonality between different game types

  3. Exercise: Distilling Key Components/Actions Racing game Adventure game Puzzle game Hard: Easy: Platform game Rhythm game Select one of the shown genre of game (assuming a 2D game). Think about the game in terms of how it looks, the features that it has, the things that it does. Try to identify common aspects that most games in the genre will share in terms of features and behaviour. Start 10 mins 9 mins 8 mins Finished 30 sec 5 mins 2 mins 7 mins 4 mins 6 mins 1 min 3 mins

  4. Key (2d) Game Components/Actions Things most 2D game have... Front-end (titles, menus) Assets Graphical assets (animations, backgrounds) Sound assets (sfx, background music) Objects In-game objects (sprites, platforms, etc.) HUD objects (score, lives, time, etc.) Object Containers Levels, Areas, Maps Input Events {other things as needed} Things most 2D games do... Once per game/per level Load assets Construct objects Populate containers Lots of times / second Consider input events Update objects Draw graphics Consider sounds {other things as needed} Note: There is no correct answer. Each model will impose a certain set of assumptions, suiting some types of game but not others.

  5. There may be separate update/draw management for game containers Input events may be ‘consumed’ by the main game loop, layers and/or objects Input Events Game Engine Game Container construct { Load assets() Build containers() } construct { Build objects() } update { Update objects() } draw { Draw objects() } object object object object Game Object run { loop { Update active layers() Draw visible layers() } } object object object object Game consists of objects (player, collectables, score, etc.). Objects reside within a container (front-end, game level, config screen, etc.) Assets are loaded at runtime and can be managed via an asset manager Asset Manager

  6. The Update/Draw Loop The central mechanism to evolve and render game objects

  7. The importance of timing… • Timers • ● Most games require a timer to manage the game loop • ● Early games used the machine as the timer • ○ the update/render loop would run as fast as possible (tied to processor speed) • ○ used as typically no spare CPU resource and common platform hardware • ○ Approach does not scale to varied platforms or increased hardware capabilities

  8. set up fixed rate ticker ( 30/s ) when( tick event ) { update() render() } Tying things together… Early Approaches ● Early solution was to use a fixed frame rate as the timer (with one update/render tick every frame) while( running ) { record tStart update() render() record tEnd if( tEnd-tStart < tickPeriod ) wait( tickPeriod – (tEnd-tStart)) } Why is this problematic? ● UPS reduced if update and draw take too long ● game runs slower, not just displayed with lower FPS

  9. Breaking things apart… Assume you have access to the following methods ● update() – update all game objects ● render() – draw all game objects ● time() – get the current time (ms) ● sleep(ms) – sleep for specified number of ms And the following constraint and target: ● ups –number of update/s (constraint) ● fps – equal to ups (target) Develop an algorithm that adapts to different update and render loads. Hint: The fps should be reduced to ensure the ups remain on target. Optional: Produce an algorithm that compensates for timing/sleep inaccuracies (see SleepTimerTest in the Java Code Repository) Start 10 mins 9 mins 8 mins 30 sec 7 mins 4 mins 6 mins 3 mins 2 mins 1 min 5 mins Finished

  10. Loop 1 Loop 2 u r Period s o Loop n Loop m Period while( running ) { tBefore = time() update() render() tAfter = time() sleepTime = updatePeriod - (tAfter - tBefore) - overTime if( sleepTime > 0 ) { sleep( sleepTime) overTime= time() - tAfter - sleepTime } else { timeBeyondPeriod= timeBeyondPeriod - sleepTime overTime= 0 while( timeBeyondPeriod > updatePeriod ) { update() timeBeyondPeriod = - updatePeriod } } } updatePeriod = 1000 / UPS timeBeyondPeriod = 0 overTime = 0

  11. Generalised approaches (Semi-decoupling) ● Previous slide can be viewed as a form of semi-decoupling ● The AI (update) runs at a fixed rate as before, but the frame (draw) rate just runs as fast as it possibly can (however, no real point running faster than the update loop).

  12. Generalised approaches (Full-decoupling) ● “Full” decoupling runs the update loop as fast as possible • ● A variable interval duration and delta (change) value are used: • ○ on faster machines more AI ticks/s with a smaller delta value • ○ on slower machines ticks take longer and the delta value will be larger

  13. Practical Advice Issues and aspects you should consider as part of your game loop

  14. Decoupling behaviour When designing game objects decouple the update and render behaviours. Ask two separate questions: ‘how will I update this’ and ‘how will I draw this’ e.g. for an animation, select the animation to be display in the update phase and render it in the draw phase The draw phase should not change the game state (i.e. properties, etc. of objects, it should simply visualise the game state)

  15. XNA update/draw loop control XNA provides good control over how the update/draw loop performs GraphicsDeviceManager class SynchronizeWithVerticalRetrace property – get/set if the maximum draw rate will be limited by the screen refresh rate Gameclass IsFixedTimeStep property – controls if a fixed update time should be used TargetElapsedTime property – get/set the target fixed time step duration GameTime class Has methods to determine the actual ellapsed time (needed if not using a fixed time step) IsRunningSlowly property – true if the target fixed update time step cannot be obtained For Java: explore game.engine.GameEngine.run()

  16. Concurrency (multi-threading) The update/draw game loop increasingly involves aspects of concurrency In the most basic sense, the CPU/GPU run in parallel, i.e. CPU tells GPU what to render and lets it get on with it Aside: The graphics pipeline can be stalled (forcing the CPU to wait for the GPU to ‘catch-up’) in various situations (sometimes unavoidable, e.g. getting render target data) Multi-core CPUs, e.g. current gen consoles, PCs, etc. can have numerous concurrent update threads. This will likely require synchronization.

  17. Summary Today we explored: • Core game components and actions • The update / draw loop and the different ways of managing time • Things to keep in mind about the update / draw loop To do: • Finalise initial game idea, team/no-team, development language • Read section to be completed in Project Development Report • Think about game idea in terms of game objects, layers, updating, drawing

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