Advanced Game Design

1. Advanced Game Design Prof. Roger Crawfis Computer Science & Engineering The Ohio State University

2. Course Overview • Project-based / Team-based • Little lecturing • Focus on programming for games • Systems integration – graphics, sound, AI, networking, user-interfaces, physics, scripting • Utilize higher-level toolkits, allowing for more advanced progress while still developing programming skills.

3. Course Structure • We will lecture for about the first two weeks. • Student game project groups will provide several presentations on their game ideas and progress. • Student technology teams will provide an intermediate and an advanced lecture on their findings and analysis about their area.

4. Project Goals • Large-scale software development • Team-based (synergistic development) • Toolkit-based (fast-start development) • Learn and utilize the many non-graphical elements needed for games. • Leverage and extend your graphics and AI expertise.

5. Elements • Gaming Engine • Responsible for providing primitives • Hardware abstraction • Handle different areas of the game • Physics, AI, etc. • Game • Defined by a genre • Defines the gameplay

6. Requirements of a gaming engine • Stunning Visuals • Artificial Intelligence • Immersive sound stage • Simulation • Animation • Networking

7. Requirements of a game • Scripting • Varied input/output devices • Supporting Tools • Optimizing game content • Developing game content • Extending game content • Debugging / Tuning of game performance

8. Stunning Visuals • Adding realism • Smarter Models • Clutter • Use hardware • Bump-mapping • Dynamic water or other liquids • Rich textures (Billboards, gloss-maps, light-maps, etc.) • Shadows • Particle systems

9. Artificial Intelligence • Games need specialized AI • Strategy • Path finding • Modeling behavior • Learning

10. Immersive sound stage • Multi-track sound support • Positional sound effects (3D immersion) • Dynamic sounds / movement (doppler effects)

11. Input devices • Commonly available devices are • Keyboard, mouse, gamepads and joysticks • Force feedback (haptic) devices are gaining popularity • Steering wheels • Joysticks • Motion tracking • Output devices • Multiple monitors • Head mounted displays

12. Scalability • Multiple hardware capabilities • Multi-resolution models • Multi-user support • LOD • Multiple model definitions • Multi-res models • Subdivision surfaces

13. Scalability • Multiple hardware capabilities • Multi-resolution models • Multi-user support • LOD (Level-Of-Detail) • Control – when to switch • Construction – what levels to have

14. Animation • Linear transformations • Modeled animations • Articulated motion • Lip syncing • Facial Expressions • Blending animations

15. Networking • Multi-player support essential • Common problems • Latency • Synchronization • Scalability • Consistent game state • Security

16. Scripting • Strict coding is tedious • Support for scripting is essential for RAD • Scripting has added a whole new fun factor for many games.

17. Artificial Intelligence • Games need specialized AI • Strategy • Path finding • Modeling behavior • Learning • Non-perfect! • Fast!

18. Tools • Creating varied content • models, video, images, sound • Integrating content • Common file format support • Supporting existing popular tools via plug-ins • 3DS Max, Lightwave, Maya etc. • Adobe premier, Adobe Photoshop

19. Interactive Programs • Games are interactive systems - they must respond to the user • How?

20. Interactive Program Structure • Event driven programming • Everything happens in response to an event • Events come from two sources: • The user • The system • Events are also called messages • An event causes a message to be sent… Initialize User Does Something or Timer Goes Off System Updates

21. User Events • The OS manages user input • Interrupts at the hardware level … • Get converted into events in queues at the windowing level … • Are made available to your program • It is generally up to the application to make use of the event stream • Windowing system / Game Framework may abstract the events for you

22. Polling for Events • Most windowing systems provide a non-blocking event function • Does not wait for an event, just returns NULL if one is not ready • What type of games might use this structure? • Why wouldn’t you always use it? while ( true ) if ( e = checkEvent() ) switch ( e.type ) … do more work

23. Waiting for Events • Most windowing systems provide a blocking event function • Waits (blocks) until an event is available • Usually used with timer events. Why? • On what systems is this better than the previous method? • What types of games is it useful for? e = nextEvent(); switch ( e.type ) …

24. The Callback Abstraction • A common event abstraction is the callback mechanism • Applications register functions they wish to have called in response to particular events • Translation table says which callbacks go with which events • Generally found in GUI (graphical user interface) toolkits • “When the button is pressed, invoke the callback” • Many systems mix methods, or have a catch-all callback for unclaimed events • Why are callbacks good? Why are they bad?

25. Upon Receiving an Event … • Event responses fall into two classes: • Task events: The event sparks a specific task or results in some change of state within the current mode • eg Load, Save, Pick up a weapon, turn on the lights, … • Call a function to do the job • Mode switches: The event causes the game to shift to some other mode of operation • eg Start game, quit, go to menu, … • Switch event loops, because events now have different meanings • Software structure reflects this - menu system is separate from run-time game system, for example

26. Real-Time Loop • At the core of interactive games is a real-time loop: • What else might you need to do? • The number of times this loop executes per second is the frame rate • # frames per second (fps) while ( true ) process events update animation / scene render

27. Lag • Lag is the time between when a user does something and when they see the result - also called latency • Too much lag and causality is distorted • With tight visual/motion coupling, too much lag makes people motion sick • Big problem with head-mounted displays for virtual reality • Too much lag makes it hard to target objects (and track them, and do all sorts of other perceptual tasks) • High variance in lag also makes interaction difficult • Users can adjust to constant lag, but not variable lag • From a psychological perspective, lag is the important variable

28. Computing Lag Process input Event time • Lag is NOT the time it takes to compute 1 frame! • What is the formula for maximum lag as a function of frame rate, fr? • What is the formula for average lag? Frame time Update state Render Lag Process input Update state Render Process input

29. Frame Rate Questions • What is an acceptable frame rate for twitch games? Why? • What is the maximum useful frame rate? Why? • What is the frame rate for NTSC television? • What is the minimum frame rate required for a sense of presence? How do we know? • How can we manipulate the frame rate?

30. Frame Rate Answers (I) • Twitch games demand at least 30fs, but the higher the better (lower lag) • Users see enemy’s motions sooner • Higher frame rates make targeting easier • The maximum useful frame rate is the monitor refresh rate • Time taken for the monitor to draw one screen • Synchronization issues • Buffer swap in graphics is timed with vertical sweep, so ideal frame rate is monitor refresh rate • Can turn of synchronization, but get nasty artifacts on screen

31. Frame Rate Answers (II) • NTSC television draws all the odd lines of the screen, then all the even ones (interlace format) • Full screen takes 1/30th of a second • Use 60fps to improve visuals, but only half of each frame actually gets drawn by the screen • Do consoles only render 1/2 screen each time? • It was once argued that 10fps was required for a sense of presence (being there) • Head mounted displays require 20fps or higher to avoid illness • Many factors influence the sense of presence • Perceptual studies indicate what frame rates are acceptable

32. Reducing Lag • Faster algorithms and hardware is the obvious answer • Designers choose a frame rate and put as much into the game as they can without going below the threshold • Part of design documents presented to the publisher • Threshold assumes fastest hardware and all game features turned on • Options given to players to reduce game features and improve their frame rate • There is a resource budget: How much of the loop is dedicated to each aspect of the game (graphics, AI, sound, …) • Some other techniques allow for more features and less lag

33. Decoupling Computation • It is most important to minimize lag between the user actions and their direct consequences • So the input/rendering loop must have low latency • Lag between actions and other consequences may be less severe • Time between input and the reaction of enemy can be greater • Time to switch animations can be greater • Technique: Update different parts of the game at different rates, which requires decoupling them • For example, run graphics at 60fps, AI at 10fps • Done in Unreal engine, for instance

34. Animation and Sound • Animation and sound need not be changed at high frequency, but they must be updated at high frequency • For example, switching from walk to run can happen at low frequency, but joint angles for walking must be updated at every frame • Solution is to package multiple frames of animation and submit them all at once to the renderer • Good idea anyway, makes animation independent of frame rate • Sound is offloaded to the sound card