Dr. Matt Boggus

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Advanced Game Design

• Dr. Matt Boggus
• Slides based on work by Prof. Roger Crawfis
• Computer Science Engineering
• The Ohio State University

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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.

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Course Structure

• We will have some lecture topics for the first
  1-2 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.

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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.

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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

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Requirements of a gaming engine

• Stunning Visuals
• Artificial Intelligence
• Immersive sound stage
• Simulation
• Animation
• Networking

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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

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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

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Artificial Intelligence

• Games need specialized AI
• Strategy
• Path finding
• Modeling behavior
• Learning

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Immersive sound stage

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

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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

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Scalability

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

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Scalability

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

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Animation

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

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Networking

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

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Scripting

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

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Artificial Intelligence

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

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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

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Interactive Programs

• Games are interactive systems - they must respond
  to the user
• How?

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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
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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

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Polling for Events
while ( true ) if ( e checkEvent() ) switch
( e.type ) do more work

• 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 wouldnt you always use it?

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Waiting for Events
e nextEvent() switch ( e.type )

• 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?

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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?

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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

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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
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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

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Computing Lag

• 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?

Process input
Event time
Frame time
Update state
Render
Lag
Process input
Update state
Render
Process input
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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?

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Frame Rate Answers (I)

• Twitch games demand at least 30fs, but the higher
  the better (lower lag)
• Users see enemys 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

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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

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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

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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

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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

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