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PA199
Advanced Game Design
Lecture 4
Game Engine Architectures
Dr. Fotis Liarokapis
06th March 2020
Game Engine Programming
• Game engine
programming supports the
execution of the game
• Game Engine
development is one of the
basic jobs within the
computer games industry
What is a Game Engine?
• A game engine is an open,
extendable software system
on which a computer game
can be built
• A game engine is free from
any function, parameter,
variable, class or data
structure that could be
considered part of an actual
game [Zerbst et al. 2003]
Game Engines
• Developed to abstract away underlying
aspects of a game
• Enables reuse of code
• Facilitates porting code to other hardware
platforms
Game Engine Purpose
• Provision of a generic infrastructure for game
creation
– i.e. I/O and resource/asset management facilities
• A game engine does not provide data or functions
that could be associated with any game or other
application of the game engine
• Provision of a glue layer that connects the engine’s
component parts, which sets a game engine apart
from an API (a set of reusable components that can
be transferred between different games), making it
more than the sum of its components and sub-
systems
Engine Development History
• Game Engines started simple:
– Sound
– File I/O
– Simulation
– Graphics
– Misc
[Blow2004]
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Engine Development History .
• Game Engines
complexity
quickly
increased:
– Sound
– File I/O
– Simulation
– Graphics
– Rendering
– Collision
detection
– Misc [Blow2004]
Engine Development History ..
• Modern engines are complex modular systems
• Engines now also include content creation
tools
[Blow2004] [Blow2004]
Game Engine Modules
• Contains a number of software modules integrated
together to provide the gaming experience:
– Graphics
– Physics
– Sound
– Scripting
– Animation
– Artificial Intelligence
– Networking
– User Interface
Graphics Module
• Provides:
– Abstraction of the GPU in the hardware
– Ability to render the geometric content created by
artists at the highest level of fidelity possible
• Incorporates advanced real-time mathematical
models of how light interacts with the surface of
an object
• Modern GPUs are programmable allowing for
very sophisticated effects
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Graphics Module .
• Higher level interface, tuned to a particular graphics
and game type
– Sprite-based, isometric, full 3D, etc
• Can deal with higher level modelling concepts
– Sprites, solids, characters (articulated), etc
• Handles more complicated display aspects
– Mini map, multiple views, overlays, special effects, etc
• Some engines are for sale or available on the web
• Often remade or heavily tuned for each game
– Too much time and money is spent on this
Physics Module
• Implements Newtonian model of physics for the
game environment objects
• Gives the game object’s mass, force and velocity
and uses collision detection methods to provide
animation
• Often uses simplifications of physics models to
simulate the physics in order to provide realtime
responses
Physics Module .
• Limited or non-existent in simple games
• Some commercial/open source engines:
– ODE, Havok, Tokamak, etc
• Physics hardware
– NVidia/Ageia PhysX
• Physics is more and more integrated into the
gameplay and game subsystems
– Physics-based animation
– Interaction with objects using physics
Sound Module
• Manages the components of the soundtrack
content developed by musicians and sound
designers
• Executes sounds based on triggers in the
environment from explicit execution, time
events or interactions between game objects
– i.e. collisions
Sound Module .
• Function of sound
– Effects to enhance reality
– Ambience
– Clues about what to do
– Clues about what is about to happen (but be careful)
• Sound formats
– Wave (high quality, lots of memory, fast)
– MP3 (high quality, compressed, slower)
– Midi (lower quality, very low storage, limited,
adaptable)
– CD (Very high quality, fast, limited to background music)
Sound Module ..
• Simultaneous sounds
– Mixers (hidden in the HAL)
– Buffer management
– Streaming sound
• Special features
– Positional 3D sound (possibly with Dolby
surround)
• Important for clues
– Adaptive music (DirectMusic)
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Scripting Module
• A language add-on to the engine that exposes
internal engine architecture
• Allows designers to glue the game together by
implementing the gameplay mechanics
– i.e. events, AI, rewards, etc.
Scripting Module .
• What belongs in a script and what belongs in
the engine?
Scripting Module ..
• Advantages:
– Easy control of many (or all) features in the game
engine
– Scripting language often provides full OO control
• i.e. Lua
– Promotes data-driven design
• Disadvantages:
– Performance
– Development support tools
– Learning curve
Scripting Module …
• Common languages used for scripting:
– Python
• http://www.python.org
– Lua
• http://www.lua.org
– GameMonkey
• http://www.somedude.net/gamemonkey
– AngelScript
• http://www.angelcode.com/angelscript
Animation
• Data structures are provided to:
– Manage and support hierarchical rigging
– Animate the scripts stored from mocap and
animations hard coded by the animators
• The animation system needs to blend between
the various canned animations and the physics
interactions
AI Module
• Provides finite state automata and a behaviour
model for the agents in the game to behave
• Also provides facilities for learned behaviours –
Neural Networks
• Group behaviours (swarming), predator (seek),
prey (flee) behaviours
• Path finding is another included component for
Non-Player Character (NPCs) to find their way
around a game environment
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Game AI
• Different from conventional ‘academic’ AI
• Behaviour more important than intelligence
• Illusion of intelligence – not real intelligence
AI Game AI
Game AI .
• Usually restricted to:
– Decision making
– Path finding (planning)
– Steering (motion control)
Crowd Simulation
• Population of virtual environments
• Add realism to scenes that might otherwise
look desolate
British & Colonial armies in “Empire Total War” (Creative Assembly)
Crowd Simulation Example
• A battle in Empire Total War with over 30,000
units:
– https://www.youtube.com/watch?v=v-xL_ITURB4
Networking
• Software to support network connections to
other clients on a network or to servers
• Used to maintain verified game state on any
machine connected to the server
• Sophisticated caching software is used to
make sure every node is made aware of the
present state of the game
Networking Example
http://bigworldtech.com/en/technology/server_en.php/
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Scene Graph
• Tree data structure used to store the content
• Is operated upon by the Artificial Intelligence
and Physics modules to update its state
• Fed to Graphics Module to be rendered into
the framebuffer (screen memory)
User Interface
• Rather simple
– But becoming more complex nowadays
• Monitors input devices and buffers any data
received
• Displays menus and online help
– Can nowadays be pretty complex
• Should be reusable, especially as a part of a
game engine
Game Loop
• A game is a real-time interactive application
• Three tasks that run concurrently:
– Re-compute the state of the world
– The player interacts with the world
– The resulting state must be presented to the user
• Graphics, sound, etc.
• Limitations of real-world technology
– 1-2 processors with limited memory and speed
Game Loop .
• While user input not exit:
– Update Scene-Graph with network cache
– Update Scene-Graph via AI
– Update Scene-Graph via Physics and Animation
– Render Scene-Graph to Screen via Graphics System
• Endwhile
Game Loop: 1st Try
• Design update/render process in a single loop
(coupled approach):
Advantages and Disadvantages
• Advantages of the coupled approach:
– Both routines are given equal importance
– Logic and presentation are fully coupled
• Disadvantages:
– Variation in complexity in one of the two routines
influences the other one
– No control over how often a routine is updated
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Game Loop: 2nd Try
• Design update process using two threads:
Advantages and Disadvantages
• Advantages of the multi-threaded approach:
– Both update and render loops run at their own
frame rate
• Disadvantages:
– Not all machines are that good at handling threads
• Precise timing problems
– Synchronization issues
• Two threads accessing the same data
Game Engine is Real-time
• All of this has to be updated at ~30 frames per
second for a game
• Thus entire loop must run in 1/30th second or
less for high quality animation rates
• Thus a very tight real-time system
• Thus programmer skills MUST be at a high level
Game Engine Developer
• Programs the core of the game engine
– Programs one or multiple modules in the engine
• Tends to be a specialist in one area
– i.e. Graphics component
• Usually the best programmers around
– Needs a lot of experience
Typical Roles
• Engine Development
• Engine Research and Development
• Cross Platform Development
• Bug fixing with Quality Assurance (QA)
• Work with a lead developer in a development
team
• Work with technical director who oversees a
number of projects
• Work with animators and designers to negotiate
technical requirements for the game
Skills Required
• High level Software Engineering skills –
specification, testing, documentation skills
• Deep understanding of basic computer science
algorithms – hashing, data structures, pointer
arithmetic, code optimisation (necessary)
• C++ a necessity
• Usual analytical skills required from general ICT
programming for specification/understanding of
program design
• Problem solving skills – creative solutions and or
bug fixing
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Skills Required .
• Mathematics, Physics and AI skills very desirable
• Multiprocessor programming – concurrant
programming of multiple threads (mini
programs) - for the new multicore Xbox, PC and
PS3s
• Do lots of development of your own mods -
good
• Development of deep technical skills via
creation of your own engine components
• People skills – yep, that again
Development Tools
• Interactive Development Environments (IDE)
– Visual Studio, Eclipse, etc
• Sophisticated environments for the
development of software:
– Intelligent Editors
• Think word processor for source code
– Debuggers
• Allows you to watch the code execute to find problems
(bugs)
– Profilers
• Enables analysis of the efficiency of developed code
Development Tools .
• Application Programming Interfaces (APIs)
– Libraries of useful code to use within into your source
code
– DirectX – graphics/audio
– OpenGL – graphics
– Open Audio – audio
• Middleware – useful software APIs that facilitate
various smaller tasks in games (goes between
other components)
– Physics, Data Processing, Networking, AI, User
Interfaces
Console/Handheld Development Tools
• Console/Handheld Development Kits
• To develop for a console or handheld device
– i.e. X360, PS3, DS, PSP, etc
• Specially manufactured hardware is used to
communicate via a network link
• Remotely debug software on device
• Consoles only run signed code, so need dev. kit
hardware for unsigned code
• Code is then given signature which runs on normal
consoles
Game Engine Organisation
• “game code” – accesses core
• core – delegates tasks to sub-systems
Engine Classification
• Game engines can be classified by:
– Architecture (structure of system design)
• Mode of access to features, ranging from indirect access
through different abstractions, to direct access through
individual function calls
– Primary use (type/genre of game)
• Available engines are often targeted at specific game
genres, such as real-time-strategy (RTS) or first-personshooter
(FPS) games
– Completeness (features and toolset)
• The extent to which a game engine provides a one-stop
solution to the development of computer games
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Engine Architecture Categories
• Monolithic Engines
• Compact Engines
• Modular Framework Engines
• Modular Component Engines
Monolithic Engines
(Data-Driven, High Level of Abstraction)
Modular Framework
Engines
(Glue-Layer for Subsystems)
Compact Engines
(Lightweight, Bare-Bones)
Modular Component
Engines (Game API)
Monolithic Game Engines
• A “Monolithic Game Engine” [Bishop et al.
1998] uses a high level of abstraction
– All of a game’s components that are unique to the
game are stored in external game assets
• These game engines are purely data-driven
systems that are a ‘player’ for games, similar in
the way that ‘media player’ programs are used
for playing back audio or video data [BinSubaih
et al. 2007]
Compact Game Engines
• Compact Game Engines are relatively
lightweight engines with limited features and an
architecture of low complexity
– Due to their limitations they usually require the
addition of additional middleware components to
provide a more complete feature set
– Compact engines are simpler than modular engines
and most of the early game engines fall into this
category
Modular Framework Engines
• Modular Framework Engine are engines whose
design has been separated out into a number of
distinct modules that provide sub-systems to
the engine
– Usually provide different layers of access, including a
high-level abstraction layer that will allow game
construction without the need to explicitly access
the engine’s sub-systems (black-box)
– Allow low-level access to sub-systems to be
requested from the high-level layer to provide
developers with additional control of the behaviour
of the engine
Example: Modular Framework Engine id
Tech 2
Described by [Arvesen 2003]
Modular Component Engines
• Modular Component Engines provide little more
than a complex game development API
– Contains all of the components that are necessary to
build games, while all of the game logic, including
the game loop, is kept separate from the engine
[Franke 2005]
• i.e. the simulation aspects are separated out and the
engine is purely a means of providing input and output to
the player
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Engine Subsystems
• Components (outside the core)
– That make up a game engine
• Common components:
– Graphical output (rendering) component
– A sound output component
– A user input component
Subsystem Integration
• Creating control & data-flow connections
between different engine subsystems
– May require data to be transformed / customised
for different subsystems
• Achievable through different methods (can be
mixed)
– Statically linked objects/libraries
– Shared objects (loaded at startup)
– Shared objects (managed dynamically)
Game Middleware
• Different from middleware in software
engineering (integration software)
• Robust/reliable 3rd party software
components used in own product
• Ranges from complete game engine
solutions to parts of engine
subsystems
• (Usually) includes service
• Often includes tools to simplify
content creation for the software
Build Middleware
• Own technology = own IP
• Creates reusable core technology for later
projects
• No code integration problems
• Up-front costs are reduced
Buy Middleware
• Focus on game (content) creation
• Includes service
– i.e. developer can help with middleware
integration
• No need for “core technology group”
• Development time is reduced
Middleware Usage
• Effectively outsourcing of R&D
• Almost all games use some kind
of middleware
• In 2006 middleware accounted
for 26% of the game industry’s
spending
• One of the largest markets is
middleware for mobile (phone
etc.) game development
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Middleware Pros
• Provides a generic solution
– That may be genre specific
• Often cheaper than the cost of development
• Saves development time
• Generally well tested and stable
Middleware Cons
• Provides a generic solution
• Rarely works “out of the box”
– Must be adapted
• Requires time consuming integration with the
game
• Creates dependency on middleware developer
Good Middleware
• There is no good or bad middleware:
– Middleware fits your requirements or it does not
• Some useful pointers – good middleware:
– Allows the use of custom memory allocators
– Allows the use of custom I/O functions
– Is extensible (at a reasonably low level)
– Has no external dependencies (avoiding symbolic
conflicts)
– Is thread-safe (allowing concurrency) and stable
– Includes source code
[Wilson 2008]
Evaluating Middleware
• Evaluate performance
– Against real-world scenarios
• Evaluate the learning curve
– Hard to learn using middleware?
• Evaluate extensibility
• Evaluate technology
• Source code included?
• Evaluate support service
[Macris 2003]
Useful Components: STL
• STL can be very fast …
– If used properly i.e. choice of correct containers, etc.
• Works well for the general case
– Possible to code faster routines for special/specific cases if on
is an expert programmer
– Advice: Use it rather than trying to code your own basic
classes
– Afterwards, find time critical areas of code and see if they
can be optimised separately
• Supposed to be standard, but differing implementations
available…
– STLport, Microsoft STL, SGI STL, etc.
– So be careful!
Useful Components: SDL
• Simple Directmedia Layer
• Cross-platform multimedia library provides low-level
access to:
– Audio, Keyboard, Mouse, Joystick, 3D hardware via OpenGL,
2D video frame buffer, sounds, CD-ROM audio
– Threads, timers
• Written in C, works in C++, bindings to many languages
– Including C#, Python, Java, Lua, Perl, PHP…
• Free to use, even in commercial programs, as long as
dynamic library version is linked to (at least up to v1.2)
– LGPL license
http://www.libsdl.org/
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Useful Components: AssImp
• Open Asset Import Library
• Cross-platform scene/model loading library
provides functionality for:
– Loading objects (meshes)
– Processing/optimising meshes
– Loading animation data
– Exporting scenes/models (latest SVN version)
• Written in C++, bindings to many languages
• Free to use, even in commercial programs
– BSD-style license
http://assimp.sourceforge.net/
Useful Components: OGRE
• Object-oriented Graphics Rendering Engine
– OGRE is primarily a graphics engine
– Does not concentrate on sound, AI, networking, collision,
physics, although these are often available as add-ons
– Can be made into a game engine with lots of work
• Lots of features
• Ambient occlusion, parallax mapping, soft shadows, etc
http://www.ogre3d.org/
Useful Components: FMOD
• Music and sound effects system
• Library and toolkit for creation and playback of interactive
audio
• Cross-platform, supporting many ‘next gen’ consoles
• Regular releases
• FMOD Ex provides low-level API and data-driven API
– Includes a suite of effects, such as echo, chorus, reverb, etc.
– Supports numerous file formats: wav, mp3, midi, XMA, mod
– Can play audio files with up to 16 channels
• Virtual voices enables thousands of sounds to be played at
once on limited hardware
– Voice management using priorities and 3D distance metrics
http://www.fmod.org
Useful Components: ODE
• Open Dynamics Engine
• High performance library for simulating rigid
body dynamics
• Relatively easy to use C/C++ API
• Advanced joint types and integrated collision
detection
• Appropriate for simulating vehicles, creatures
and interactions between objects
• BSD license: free for use in commercial products
http://www.ode.org
Useful Components: Lua
• Lua “extensible extension language”
• Embeddable scripting language compiler &
virtual machine with a very small footprint.
• Very easy to use C/C++ API
• Very easy to use syntax
• Used widely in game development
• MIT license: free for use in commercial products
http://www.lua.org
Software Engineering
• Definition from Collins Dictionary of Personal
Computing:
– The discipline of providing software of a high standard
– This includes analysis of the problem which the software
is designed to solve, searching for methods that provide
the minimum use of memory and the lowest running
time at minimum expense to the user, programming and
marketing
– This is engineering in the sense that it involves the
traditional engineering problems of reconciling
conflicting objectives and working with a view to
acceptance by the ultimate user
• Game development greatly benefits from adopting
software engineering methods & best practices!!!
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Software Prototyping
• Prototyping is an iterative software design
method, useful for game development
– Prototyping employs a top-down approach of
stepwise refinement
– Working parts of the application are created for
testing of ideas and concepts
– In successive iterations the functionality of early
prototypes may be improved or replaced by rewrites
from scratch
Revision Control
• Tools that allow versioning of software in
development
– Several developers can work simultaneously
– Old versions are stored, allowing development to
“roll back” if problems arise
• Popular examples:
– CVS (Concurrent Versions System)
– SVN (Subversion)
– GIT
Reuse & Refactoring
• Never design code for reuse [Norneby and Olsson
2009]
– Design code that you need now, not code that you might
need in the future
– A good system will automatically have many reusable
features, many of which will not have been anticipated
• Factor out common code elements (reused from a
different project) into an external library
• Control change – ensure that any code
restructuring that happens does not change the
functionality of existing code (unless desired)
Subsystem Integration
• Creating control & data-flow connections
between different engine subsystems
– May require data to be transformed / customised
for different subsystems
• Achievable through different methods (can be
mixed)
– Statically linked objects/libraries
– Shared objects (loaded at startup)
– Shared objects (managed dynamically)
Middleware Integration Issues
• Language clash – middleware may require Cstyle
callbacks, whereas the engine uses C++
– No “pretty” solutions – can be solved using bad C++
programming style
• i.e. wrapper functions, global variables
• Middleware may require existing data in a
different format
– Data replication in correct formats – synchronisation
issues & excessive memory usage
– Conversion functions – execution overheads
– No ideal solution
Callbacks
• Programming interface for event-driven input
• Define a callback function for each type of
event the graphics system recognizes
• This user-supplied function is executed when
the event occurs
• GLUT example:
– glutMouseFunc(mymouse)
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Rendering
• Goal:
– Transform computer models into images
– May or may not be photo-realistic
• Methods:
– Interactive rendering
– Offline rendering
Interactive vs Offline Rendering
• Interactive rendering
– Fast, but limited quality
– Roughly follows a fixed patterns of operations
• Rendering pipeline
• Offline rendering
– Ray tracing
– Radiosity
– Global illumination
Rendering Tasks
• Tasks that must be performed (no particular
order):
– Project all 3D geometry onto the image plane
• Geometric transformations
– Determine which primitives of primitives are visible
• Hidden surface removal
– Determine which pixels a geometric primitive covers
• Scan conversion
– Compute the color of every visible surface point
• Lighting, shading, texture mapping
What is the Rendering Pipeline?
• What is the pipeline?
– Abstract model for sequence of operations to transform
geometric model into digital image
– Abstraction of the way graphics hardware works
– Underlying model for application programming interfaces
(APIs) that allow programming of graphics hardware
• OpenGL
• Direct 3D
• Actual implementation details of rendering pipeline
will vary
The Rending Pipeline
Geometry
Database
Model/View
Transform.
Lighting
Perspective
Transform.
Clipping
Scan
Conversion
Depth
Test
Texturing Blending
Frame-
buffer
Pipeline Advantages
• Modularity: logical separation of different
components
• Easy to parallelize
– Earlier stages can already work on new data while
later stages still work with previous data
– Similar to pipelining in modern CPUs
– But much more aggressive parallelization possible
(special purpose hardware!)
– Important for hardware implementations
• Only local knowledge of the scene is necessary
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Pipeline Disadvantages
• Limited flexibility
• Some algorithms would require different
ordering of pipeline stages
– Hard to achieve while still preserving compatibility
• Only local knowledge of scene is available
– Shadows, global illumination difficult
OpenGL
• Started in 1989 by Kurt Akeley
– Based on IRIS_GL by SGI
• API to graphics hardware
• Designed to exploit hardware optimized for
display and manipulation of 3D graphics
• Implemented on many different platforms
• Low level, powerful flexible
• Pipeline processing
– Set state as needed
Developer-Driven Advantages
• Industry standard
– Vendor-neutral, multiplatform graphics standard
• Stable
• Reliable and portable
• Evolving
• Scalable
– Consumer electronics to PCs, workstations, and
supercomputers
• Very easy to use
• Well-documented
OpenGL Graphics State
• Set the state once, remains until overwritten
• glColor3f(1.0, 1.0, 0.0)
– Set color to yellow
• glSetClearColor(0.0, 0.0, 0.2)
– Dark blue background
• glEnable(LIGHT0)
– Turn on a light
• glEnable(GL_DEPTH_TEST)
– Hidden surface
Geometry Pipeline
• Tell it how to interpret geometry
– glBegin(<mode of geometric primitives>)
– mode = GL_TRIANGLE, GL_POLYGON, etc.
• Feed it vertices
– glVertex3f(-1.0, 0.0, -1.0)
– glVertex3f(1.0, 0.0, -1.0)
– glVertex3f(0.0, 1.0, -1.0)
• Tell it you’re done
– glEnd()
OpenGL Geometric Primitives
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Simple Tasks for OpenGL
• The basic structure of a useful OpenGL
program can be very simple
• The main tasks include:
– Initialisation of certain states that control how
OpenGL rendering is done
– Specification of the objects/scenes to be rendered
OpenGL Includes
• For all OpenGL applications always include:
– #include <GL/gl.h>
– #include <GL/glu.h>
• When using GLUT (see next slides) include:
– #include <GL/glut.h>
A Code Sample
void display()
{
glClearColor(0.0, 0.0, 0.0, 0.0);
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(1.0, 1.0, 1.0);
glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0);
glBegin(GL_POLYGON);
glVertex3f(0.25, 0.25, 0.0);
glVertex3f(0.75, 0.25, 0.0);
glVertex3f(0.75, 0.75, 0.0);
glVertex3f(0.25, 0.75, 0.0);
glEnd();
}
Lighting in OpenGL
• Steps required to add lighting to your scene
– Define normal vectors for each vertex of all the
objects
• These normals determine the orientation of the object
relative to the light sources
– Create, select, and position one or more light sources
– Create and select a lighting model, which defines the
level of global ambient light and the effective location
of the viewpoint
• For the purposes of lighting calculations
– Define material properties for the objects in the
scene
Create, Position, and Enable One or
More Light Sources
• A white light source is specified by the glLightfv()
call
– If you want a differently colored light, use glLight*() to
indicate this
• You can include at least eight different light sources
in your scene of various colors
• The default color of these other lights is black
• Remember that each light source adds significantly
to the calculations needed to render the scene
– Performanceis affected by the number of lights in the
scene
Select a Lighting Model
• The glLightModel*() command describes the
parameters of a lighting model
• The lighting model also defines whether:
– The viewer of the scene should be considered to
be an infinite distance away or local to the scene
• Lighting calculations should be performed
differently for the front and back surfaces of
objects in the scene
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Define Material Properties for the
Objects in the Scene
• An object's material properties determine
how it reflects light
• Specify material properties so that an object
has a certain desired appearance is an art
• You can specify a material's ambient, diffuse,
and specular colors and how shiny it is
Defining Colors and Position for a Light
Source Example
GLfloat light_ambient[] = { 0.0, 0.0, 0.0, 1.0 };
GLfloat light_diffuse[] = { 1.0, 1.0, 1.0, 1.0 };
GLfloat light_specular[] = { 1.0, 1.0, 1.0, 1.0 };
GLfloat light_position[] = { 1.0, 1.0, 1.0, 0.0 };
glLightfv(GL_LIGHT0, GL_AMBIENT, light_ambient);
glLightfv(GL_LIGHT0, GL_DIFFUSE, light_diffuse);
glLightfv(GL_LIGHT0, GL_SPECULAR, light_specular);
glLightfv(GL_LIGHT0, GL_POSITION, light_position);
Position and Attenuation Examples
GLfloat light_position[] = { 1.0, 1.0, 1.0, 0.0 };
glLightfv(GL_LIGHT0, GL_POSITION, light_position);
glLightf(GL_LIGHT0, GL_CONSTANT_ATTENUATION,
2.0);
glLightf(GL_LIGHT0, GL_LINEAR_ATTENUATION, 1.0);
glLightf(GL_LIGHT0, GL_QUADRATIC_ATTENUATION,
0.5);
Two-sided Lighting
• Lighting calculations are performed for all polygons
– Whether they're front-facing or back-facing
– Usually set up lighting conditions with front-facing polygons
– However, the back-facing ones typically aren't correctly
illuminated
• When you turn on two-sided lighting with
– glLightModeli(GL_LIGHT_MODEL_TWO_SIDE, GL_TRUE);
• OpenGL reverses the surface normals for back-facing
polygons
– Typically,this means that the surface normals of visible backand
front-facing polygons face the viewer, rather than pointing
away
• As a result, all polygons are illuminated correctly
Emission
• By specifying an RGBA color for GL_EMISSION, can make
an object appear to be giving off light of that color
• Since most real-world objects (except lights) don't emit
light, use this feature mostly to simulate lamps and other
light sources in a scene
GLfloat mat_emission[] = {0.3, 0.2, 0.2, 0.0};
glMaterialfv(GL_FRONT, GL_EMISSION, mat_emission);
• Need to create a light source and position it at the same
location as the sphere to create that effect
OpenGL Lighting Hints
• Lighting in OpenGL is not so easy!
• Must get light sources, materials, and surface
normals all correct
• If any one of them is wrong, it doesn't work at all
• As always, the key to successful learning is to keep
things simple and change only one value at a time
• Don't go crazy with lights
– Most scenes can be rendered quite comfortably with
just one
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OpenGL Lighting Hints .
• The light values for ambient, diffuse, etc are RGB
colors
– Use only white lights (R=G=B)
– The fourth (alpha) value should always be 1
• Ambient light can be either specified globally or on
your primary light source only
– Intensity should be in the 0.1 to 0.3 range
– Never have multiple ambient lights
• Diffuse lighting is what photographers call 'soft'
lighting
– It should always be present, intensity 1
OpenGL Lighting Hints ..
• Specular light is 'hard' lighting that creates highlights,
sparkles, etc
– Your primary light source always has specular intensity 1
– If you need secondary lights as the equivalent of a
photographers backlighting or extra flash, set the specular to
0 on those
• Lights have a position (x, y, z,1 ) or a direction (x, y, z, 0)
– Directional lights are easier to work with
• For a directional light the coordinates are a vector (the
direction from which the light comes)
– Try to use only the values 0 or 1 at each of (x, y, z)
– The vectors (0, 1, 0) and (0, 0, 1) are good values for
directional lighting
OpenGL Materials Hints
• Can make something appear yellow by either
coloring it yellow and shining a white light on it, or
coloring it white and shining a yellow light on it
– The first is easier, so lights are set up once and rarely
altered
• Materials are where most of your design will be
done
• Ambient and diffuse material values should be
identical
– OpenGL allows you to specify both at the same time for
this reason
OpenGL Materials Hints .
• Very roughly, all materials are either:
– Matte: with high diffuse color and low specular
– Plastic: with high diffuse color and white specular
– Metal: with low diffuse and high specular color
• When something is described as being a
particular color, that means the diffuse color for
matte and plastic surfaces, the specular for
metals
OpenGL Materials Hints ..
• OpenGL has the capability to set material
values from the current vertex color values
through use of glColorMaterial
– While this is very useful to experienced
programmers, it is sometimes tricky
• Using glColorMaterial does not mean you don't
have to set up materials for your objects
– It just adds one more thing that can go wrong
OpenGL Surface Normals Hints
• Surface normals are essential for lighting
– Always irritating to calculate!
• Where possible, use the glu_ and glut_
primitives such as spheres and cylinders
– They calculate the surface normals for you
– Good for testing your own lights and materials
• Can't get the normals wrong
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OpenGL Surface Normals Hints .
• A surface normal is a vector perpendicular to
the polygon (triangle, etc) being lit
– It is in local coordinates before any
transformations
– If you rotate or translate say, a cylinder, the
surface normals get transformed with it
• When learning to calculate normals, start with
a cube that you construct yourself
– The top face has surface normal (0, 1, 0), straight
up, and all the others are equally easy
OpenGL Surface Normals Hints ..
• The surface normal has to be a normal or unit
length vector
– Otherwise the lighting calculations won't work
• If you use glScale, surface normals may no
longer be correct because the scaling will
shorten or lengthen the vector
– At startup, write:
• glEnable(GL_NORMALIZE);
– OpenGL will re-normalise all your surface normals
GLUT: OpenGL Utility Toolkit
• Developed by Mark Kilgard (also from SGI)
• Simple, portable window manager
– Opening windows
• Handling graphics contexts
– Handling input with callbacks
• Keyboard, mouse, window reshape events
– Timing
• Idle processing, idle events
– Designed for small-medium size applications
– Distributed as binaries
Create a Window in GLUT
int main(int argc, char **argv)
{
glutInit( &argc, argv );
glutInitDisplayMode( GLUT_RGB | GLUT_DOUBLE
| GLUT_DEPTH);
glutInitWindowSize( 640, 480 );
glutCreateWindow( "openGLDemo" );
glutDisplayFunc( DrawWorld );
glutIdleFunc(Idle);
glClearColor( 1,1,1 );
glutMainLoop();
return 0;
}
Event-Driven Programming
• Main loop not under your control
– vs. batch mode where you control the flow
• Control flow through event callbacks
– Redraw the window now
– Key was pressed
– Mouse moved
• Callback functions called from main loop when
events occur
– Mouse/keyboard state setting vs. redrawing
GLUT Callback Functions
// you supply these kind of functions
void reshape(int w, int h);
void keyboard(unsigned char key, int x, int y);
void mouse(int but, int state, int x, int y);
void idle();
void display();
// and register them with glut
glutReshapeFunc(reshape);
glutKeyboardFunc(keyboard);
glutMouseFunc(mouse);
glutIdleFunc(idle);
glutDisplayFunc(display);
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More GLUT Callback Functions
• void glutDisplayFunc (void (*func)(void));
• void glutKeyboardFunc
(void (*func)(unsigned char key, int x, int y));
• void glutIdleFunc (void (*func)());
• void glutReshapeFunc
(void (*func)(int width, int height));
GLUT Callbacks
• GLUT recognizes a subset of the events
recognized by any particular window system
(Windows, X, Macintosh)
– glutDisplayFunc
– glutMouseFunc
– glutReshapeFunc
– glutKeyboardFunc
– glutIdleFunc
– glutMotionFunc
– glutPassiveMotionFunc
GLUT Event Loop
• Recall that the last line in main.c for a program using
GLUT must be:
– glutMainLoop();
– Which puts the program in an infinite event loop
• In each pass through the event loop, GLUT
– Looks at the events in the queue
– For each event in the queue, GLUT executes the
appropriate callback function if one is defined
– If no callback is defined for the event, the event is ignored
The display Callback
• The display callback is executed whenever
GLUT determines that the window should be
refreshed, for example
– When the window is first opened
– When the window is reshaped
– When a window is exposed
– When the user program decides it wants to
change the display
The display Callback .
• In main.c
– glutDisplayFunc(mydisplay) identifies the function
to be executed
– Every GLUT program must have a display callback
Posting Redisplays
• Many events may invoke the display callback
function
– Can lead to multiple executions of the display
callback on a single pass through the event loop
• Can avoid this problem by instead using
– glutPostRedisplay();
• Which sets a flag
• GLUT checks to see if the flag is set at the end of
the event loop
– If set then the display callback function is executed
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21
Animating a Display
• When we redraw the display through the
display callback, we usually start by clearing
the window
– glClear()
– Then draw the altered display
• Problem: the drawing of information in the
frame buffer is decoupled from the display of
its contents
– Graphics systems use dual ported memory
• Hence we can see partially drawn display
Double Buffering
• Instead of one color buffer, we use two
– Front Buffer: one that is displayed but not written to
– Back Buffer: one that is written to but not displayed
• Program then requests a double buffer in main.c
– glutInitDisplayMode(GL_RGB | GL_DOUBLE)
– At the end of the display callback buffers are swapped
void mydisplay()
{
glClear(GL_COLOR_BUFFER_BIT|….)
/* draw graphics here */
glutSwapBuffers()
}
Using the idle Callback
• The idle callback is executed whenever there are no events in
the event queue
– glutIdleFunc(myidle)
– Useful for animations
void myidle() {
/* change something */
t += dt
glutPostRedisplay();
}
void mydisplay() {
glClear();
/* draw something that depends on t */
glutSwapBuffers();
}
Using Globals
• The form of all GLUT callbacks is fixed
– void mydisplay()
– void mymouse(GLint button, GLint state, GLint x, GLint y)
• Must use globals to pass information to callbacks
float t; // global variable
void mydisplay()
{
// draw something that depends on t
}
The mouse callback
• glutMouseFunc(mymouse)
– void mymouse(GLint button, GLint state, GLint x,
GLint y)
• Returns
– Which button (GLUT_LEFT_BUTTON,
GLUT_MIDDLE_BUTTON, GLUT_RIGHT_BUTTON)
caused event
– State of that button (GLUT_UP, GLUT_DOWN)
– Position in window
Positioning
• The position in the screen window is usually measured
in pixels with the origin at the top-left corner
– Consequence of refresh done from top to bottom
• OpenGL uses a world coordinate system with origin at
the bottom left
– Must invert y coordinate returned by callback by height of
window
– y = h – y;
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Obtaining the Window Size
• To invert the y position we need the window
height
– Height can change during program execution
– Track with a global variable
– New height returned to reshape callback that we
will look at in detail soon
• Can also use query functions
– glGetIntv
– glGetFloatv
• To obtain any value that is part of the state
Using the Mouse Position
• To draw a small square at the location of the
mouse each time the left mouse button is
clicked
– This example does not use the display callback but
one is required by GLUT
– We can use the empty display callback function
– mydisplay(){}
Drawing Squares at Cursor
Location
void mymouse(int btn, int state, int x, int y)
{
if(btn==GLUT_RIGHT_BUTTON &&
state==GLUT_DOWN)
exit(0); // exits the
application
if(btn==GLUT_LEFT_BUTTON &&
state==GLUT_DOWN)
drawSquare(x, y); // draw a square
}
Drawing Squares at Cursor
Location .
void drawSquare(int x, int y){
y=w-y; // invert y position
glColor3ub( (char) rand()%256, (char) rand
)%256, (char) rand()%256); // random color
glBegin(GL_POLYGON);
glVertex2f(x+size, y+size);
glVertex2f(x-size, y+size);
glVertex2f(x-size, y-size);
glVertex2f(x+size, y-size);
glEnd();
}
Using the motion callback
• We can draw squares (or anything else)
continuously as long as a mouse button is
depressed by using the motion callback
– glutMotionFunc(drawSquare)
• We can draw squares without depressing a
button using the passive motion callback
– glutPassiveMotionFunc(drawSquare)
Using the Keyboard
glutKeyboardFunc(mykey)
void mykey(unsigned char key, int x, int y)
• Returns ASCII code of key depressed and mouse
location
void mykey()
{
if(key == ‘Q’ | key == ‘q’)
exit(0);
}
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23
Special and Modifier Keys
• GLUT defines the special keys in glut.h
– Function key 1: GLUT_KEY_F1
– Up arrow key: GLUT_KEY_UP
• if(key == ‘GLUT_KEY_F1’ ……
• glutGetModifiers() allows emulation of three-button
mouse with one- or two-button mice by checking if
one of the modifiers is depressed
– GLUT_ACTIVE_SHIFT
– GLUT_ACTIVE_CTRL
– GLUT_ACTIVE_ALT
Reshaping the Window
• We can reshape and resize the OpenGL display
window by pulling the corner of the window
• What happens to the display?
– Must redraw from application
• Two possibilities
– Display part of world
– Display whole world but force to fit in new
window
• Can alter aspect ratio
The reshape Callback
glutReshapeFunc(myreshape)
void myreshape( int w, int h)
• Returns width and height of new window (in
pixels)
• A redisplay is posted automatically at end of
execution of the callback
• GLUT has a default reshape callback but you
probably want to define your own
• The reshape callback is a good place to put
viewing functions because it is invoked when the
window is first opened
Example Reshape
• This reshape preserves shapes by making the
viewport and world window have the same
aspect ratio
void myReshape(int w, int h){
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION); // switch matrix mode
glLoadIdentity();
if (w <= h)
gluOrtho2D(-2.0, 2.0, -2.0 * (GLfloat) h / (GLfloat) w, 2.0 *
(GLfloat) h / (GLfloat) w);
else gluOrtho2D(-2.0 * (GLfloat) w / (GLfloat) h, 2.0 *
(GLfloat) w / (GLfloat) h, -2.0, 2.0);
glMatrixMode(GL_MODELVIEW); /* return to modelview
mode */
}
Toolkits and Widgets
• Most window systems provide a toolkit or library of
functions for building user interfaces that use special
types of windows called widgets
• Widget sets include tools such as:
– Menus
– Slidebars
– Dials
– Input boxes
• But toolkits tend to be platform dependent
• GLUT provides a few widgets including menus
GLUT Menus
• GLUT supports pop-up menus
– A menu can have submenus
• Three steps
– Define entries for the menu
– Define action for each menu item
• Action carried out if entry selected
– Attach menu to a mouse button
2/27/2020
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Defining A Simple GLUT Menu
menu_id = glutCreateMenu(mymenu);
glutAddmenuEntry(“clear Screen”, 1);
glutAddMenuEntry(“exit”, 2);
glutAttachMenu(GLUT_RIGHT_BUTTON);
GLUT Menu Actions
• Menu callback
void mymenu(int id)
{
if(id == 1) glClear();
if(id == 2) exit(0);
}
• Note each menu has an id that is returned when it is
created
• Add submenus by:
– glutAddSubMenu(char *submenu_name, submenu id)
Timers
• On modern graphics processors may need to
slow down rendering or get a blur
• Options
– Use OS timers
– Lock buffer swap on graphics card to refresh rate
– Use GLUT timer
• glutTimerFunc(int delay,
void(*timer_func)(int), int value);
– Delay the event loop for delay seconds
What is Sound?
• Sound is caused when a vibrating
object creates waves of pressure
• These waves travel through the air
(or other medium)
– Speed of sound in air ~340 m/s
– Water 1480m/s, Steel 4000m/s
• The waves radiate outwards from
their source in a spherical manner
• The pressure waves diminish with
distance
Sound waves
• Sound waves are often simplified to a
description in terms of sinusoidal plane waves
Properties of sound wave
• Frequency, or its inverse, the period
• Wavelength
• Wavenumber
• Amplitude
• Sound pressure
• Sound intensity
• Speed of sound
• Direction
2/27/2020
25
How do we Perceive Sound?
• Our ears are very sensitive to
such pressure waves
• The volume of a sound is
determined by the amount of
pressure variation
– Amplitude of the waves
• The pitch of a sound (how high
or low it is) is driven by how
quickly the pressure varies
– Frequency of the waves
Quieter
Louder
Higher Pitched
Waveforms
• Draw waveforms to represent
the pressure variation created
at the source
• Simple waveforms make very
simple sounds
• Real-world sounds have much
more complex waveforms
– And waveform is densely
packed
• Computers cannot store
these analogue waveforms
– Use a digital representation
instead
Acoustic Guitar
Guitar Clip
Electric Guitar
Sampling Sounds
• A computer stores a sound by sampling the
waveform
• A sample is just the measurement of a
waveform at a given time
Sampling Sounds .
• Samples taken at a fixed sample frequency
(rate)
– e.g. CDs sampled at 44100Hz : 44100 samples /
second
• Each sample is a value representing the
amplitude (pressure) of the waveform at that
point
• Samples are usually integers
– Hence they have a max/min value
• Not realistic
Digital Sound Quality
• Higher sample rates result in a better approximation to
the original sound and hence a better quality sound
– Nyquist criterion: sample rate must be greater than 2 x
required max frequency in the sound
– Typical sample frequencies: 11,000->48,000Hz
• The bit-depth of the samples is the number of bits used
for each sample and determines the accuracy of each
sample
– E.g. An 8-bit, 16-bit or even floating point value for each
sample
• A higher bit-depth will also improve digital sound quality
– But improvement is less than the effect of increasing sample
rate
Stereo Sound
• Each ear receives a different waveform from a
single sound
• If the sound is off-centre then the sound waves will
reach one ear sooner (and louder)
• Also, our head is an obstacle for the pressure waves
and they distort when passing by
– So the position of the head relative to the sound affects
the received waveforms at each ear
• We can model this difference by recording stereo
sound: 2 waveforms, one for each ear
– Greatly improves sound quality
– But pre-recorded sound is statically positioned
2/27/2020
26
3D Sound
• Can dynamically model the different sounds
received by our ears:
– We first consider the 3D position of the sound source
– Then calculate how the sound emitted will be perceived
at each ear of the observer
• Need to take account of:
– The 3D position of the sound relative to the observer
– The head position (HRTF – head related transfer functions
are used here)
– Any obstacles in the way (e.g. walls)
• These calculations can be performed in hardware
• Can then model dynamic sound in our 3D scenes
Doppler Effect
• We have considered the relative 3D
position of a sound source
• But the relative velocity also has an
effect on the perceived sound:
– Sounds moving towards an observer
are higher in pitch
– Those moving away are lower in pitch
• The waves are compressed /
stretched by the movement:
• This should also be modelled to
create a compelling 3D sound
• Also typically supported in hardware
Animation by Dr. Dan Russell, Kettering University
Other Special Effects
• There is other special processing
we may perform on a waveform:
– Echo, reverberation, distortion, etc.
– Involve numeric processing of the
samples
• Reverberation is most interesting
for games
– The reflection and damping of a
sound against the walls and furniture
of a environment
• We often apply an environmental
reverberation to all sounds
depending on the current scene
Basic Sound
Church
Bathroom
OpenAL
• OpenAL is a simple, effective audio API
– Free, open source project
– Supported by Creative Labs
– Cross-platform, supports Windows, Macs, Linux
• Appropriate for games
– Particularly good for 3D sound
– Used in several titles
• Fairly easy to use
– Similar principles to OpenGL
OpenAL Libraries
• OpenAL
– #include <al\al.h>
– #include <al\alu.h>
– #include <al\alc.h>
• ALUT
– #include <al\alut.h>
Init
• Similar to OpenGL
• In main()
//initialize OpenAL
alutInit(&argc, argv) ;
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OpenAL Concepts
• OpenAL introduces three basic concepts:
– Buffers:
• A buffer holds sound data in memory
• Creating a buffer doesn't play a sound
– Sources:
• An actual sound in the world.
• Must be associated with a buffer
– The Listener:
• OpenAL always assumes there is a listener
OpenAL Basics
• Buffers, sources & listener have properties:
– Buffers:
• Sample rate, bit depth, and other source data related
properties
– Sources:
• Pitch, gain (volume), looping, position, velocity, etc.
– The Listener
• Position, orientation, velocity, master gain (volume)
– Relative position of source / listener used to
determine how to play sound out of speakers
– Velocity is used to generate Doppler effect
OpenAL Architecture
Buffer0
Buffer1
Buffer2
Buffer3
Source0
Source1
Source2
Listener
Fundamental OpenAL Objects
OpenAL Programmer's Guide
Listener
• For every context, there is automatically one
Listener object
alListenerfv(AL_POSITION, listenerPos);
alListenerfv(AL_VELOCITY, listenerVel);
alListenerfv(AL_ORIENTATION, listenerOri);
Buffer
• Each buffer generated by alGenBuffers has
properties which can be retrieved
2/27/2020
28
Buffer Example
const int NUM_BUFFERS = 3;
ALuint buffer[NUM_BUFFERS];
ALboolean al_bool;
ALsizei size,freq;
ALenum format;
ALvoid *data = NULL;
int ch;
// Generate buffers, or no sound will be produced
alGenBuffers(NUM_BUFFERS, buffer);
if(alGetError() != AL_NO_ERROR) {
printf("- Error creating buffers !!\n");
exit(1);
}
alutLoadWAVFile("c.wav", &format ,&data, &size, &freq, &al_bool);
alBufferData(buffer[0], format, data, size, freq);
alutUnloadWAV(format, data, size, freq);
Source
• A source in OpenAL is exactly what it sounds
like, a source of a sound in the world
Source Example
const int NUM_SOURCES = 3;
ALuint source[NUM_SOURCES];
alGetError(); /* clear error */
alGenSources(NUM_SOURCES, source);
if(alGetError() != AL_NO_ERROR) {
printf("- Error creating sources !!\n");
exit(2);
}
alSourcef(source[0], AL_PITCH, 1.0f);
alSourcef(source[0], AL_GAIN, 1.0f);
alSourcefv(source[0], AL_POSITION, source0Pos);
alSourcefv(source[0], AL_VELOCITY, source0Vel);
alSourcei(source[0], AL_BUFFER, buffer[0]); //attach buffer
alSourcei(source[0], AL_LOOPING, AL_TRUE);
Play and Stop
• Combine with keyboardfunc(), or some other
way
– alSourcePlay(source[0]);
– alSourceStop(source[0]);
– alSourcePause(source[0]);
Exit
In main()
// Setup an exit procedure.
atexit(KillALData);
void KillALData()
{
alDeleteSources(NUM_SOURCES, source);
alDeleteBuffers(NUM_BUFFERS, buffers);
alutExit();
}
Other APIs
• DirectSound is part of the DirectX SDK
– Deprecated now, may see in older projects
• XAudio2 is DirectSound’s replacement
– Cross-platform API for Windows and 360
– Fully featured and effective, many tools/APIs
Proprietary, not open source
– Rather more complex than OpenAL
• FMOD Ex is a free API, widely used for games
– Supports all platforms (Windows, Linux, PS3, Xbox)
– Low-level and a little more complex than OpenAL
2/27/2020
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References
• Anderson, E.F., Engel, S., McLoughlin, L. and Comninos, P. (2008). The
case for research in game engine architecture. In Proceedings of the
ACM FuturePlay 2008 International Academic Conference on the
Future of Game Design and Technology, pp. 228–231
• Arvesen, R. (2003). Quake II.NET. Whitepaper, Vertigo Software -
http://www.vertigosoftware.com
• Binsubaih, A., Maddock, S. and Romano, D. (2007). A survey of ‘game’
portability. Tech. Rep. CS-07-05, University of Sheffield. Department of
Computer Science.
• Bishop, L., Eberly, D., Whitted, T., Finch, M. and Shantz, M. (1998).
Designing a PC game engine. IEEE Comput. Graph. Appl. 18(1), pp. 46–
53
• Blow, J. (2004). Game Development: Harder than you think, ACM
Queue 1(10), pp. 28–37
• Franke, S. (2005). Game Development Architecture. Tech. Rep. TR-CS-
2005-01, Fachhochschule Augsburg, pp. 172-182
References .
• Franke, S. (2005). Game Development Architecture. Tech. Rep. TR-CS-
2005-01, Fachhochschule Augsburg, pp. 172-182
• Zerbst, St, Düvel, O. and Anderson, E. (2003). 3DSpieleprogrammierung.
Markt + Technik
• Macris, A. (2003). Effective Middleware Evaluation. Game Developer
10(5)
• Wilson, K. (2008). Gamasutra - http://www.gamasutra.com/php-
bin/news_index.php?story=20406
• Norneby, J. and Olsson, T. (2009). A New Attitude To Game
Engineering: Embrace Change, Re-Use, Fun. Gamasutra.com
• Blow, J. (2004). Game Development: Harder than you think, ACM
Queue 1(10), pp. 28–37.
• El Rhalibi, A., England, D. and Costa, S. (2005). %T Game Engineering
for a Multiprocessor Architecture. In Changing Views: Worlds in Play Proceedings
of the 2005 Digital Games Research Association
Conference.
Useful Links
• GLUT Libraries
– http://www.xmission.com/~nate/glut.html
• GLUT Tutorials
– http://www.lighthouse3d.com/opengl/glut/
– http://www.opengl.org/code/category/C19
– http://www.nullterminator.net/glut.html
– http://www.zeuscmd.com/tutorials/glut/
Useful Links .
• http://devmaster.net/devdb/engines
• http://www.gamedev.net/page/resources/_/t
echnical/game-programming/a-guide-to-
starting-with-openal-r2008
• http://www.openal.org/creative-installers/
• http://www.openal.org/documentation/open
al-1.1-specification.pdf
Questions