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1
KIPA Game Engine Seminars
Day 3/25

• Jonathan Blow
• Ajou University, Suwon
• November 28, 2002

2
Thatcher UlrichsChunked LOD Demo

• Shown at SIGGRAPH 2002
• He has put new data sets on his web page since
  then

http//www.tulrich.com/geekstuff/chunklod.html
3
Parabolic sin/cosapproximation

• How to handle phase limits

4
Vectors vs. Axial Vectors

• or, covariant vs. contravariant forms
• or, physical quantities vs. relationships
  between physical quantities
• (simple demo on whiteboard)
• Clifford Algebra
• Differential Forms

5
Vectors vs. Axial VectorsReferences

• Computer Graphics Principles and Practice by
  Foley, van Dam, Feiner, Hughes
• Jim Blinns Corner Notation, Notation,
  Notation by Jim Blinn

6
Batching Geometry by Material

• Still helps even for software rendering, due to
  cache effects
• Provided the rest of your renderer is tight
  enough!

7
Short Triangle Strips

• Strips that are 3 triangles long or less can
  always be made without swaps

8
SLERP

• Spherical Linear Interpolation
• Popularized in graphics by Ken Shoemake
• Search for his name on the Web to find a lot of
  related papers

9
Deriving SLERP

• For arbitrary-dimension vectors
• Useful for interpolation of rotations, vertex
  normals, etc.

10
Source Control Systems

• Editorial regarding the flaws in current source
  control systems
• They should be a valuable tool but instead they
  are often a nuisance, and at best only alleviate
  some problems of multi-programmer development

11
Source ControlVisual Source Safe

• Non-Atomic Transactions
• Manual Check-Out -- unacceptable
• Bad Integration (doesnt even work in vc7!)

12
Source ControlCVS

• Bad at binary files
• Bad at directory versioning
• Leaves annoying CVS/ directories everywhere in
  your source tree
• Too difficult for non-programmers to use
• Emphasize the need for a general asset management
  system
• Maybe subversion will fix these problems
• subversion.tigris.org

13
Source ControlPerforce, AlienBrain, etc

• Cost money
• Better interfaces than SourceSafe and CVS

14
Forward DeclarationMacros and typedefs

• are bad because they cant be forward-declared!
• (FILE ), (size_t)

15
Network Architecture

• Introduction
• Peer-to-Peer, Client/Server, Hybrid
• Lockstep versus Updated
• Lockstep is supposed to simplify life but it can
  cause software engineering problems! (Shadow
  Warrior)

16
Client/Server

• Big bandwidth responsibility
• Everquest bandwidth bill
• But part of this is the Everquest guys fault for
  writing poor low-level communications
• You control the server
• Unless someone hacks you

17
Peer-to-Peer, Hybrid

• Peer-to-peer does not scale
• Hybrid requires very difficult software
  engineering (handling dropouts, lag, etc)
• Hybrid still cannot solve the malicious user
  problem

18
Cheating

• Cheating has ruined several games
• Diablo 1 and 2
• It has hurt others tremendously
• Counter-Strike
• Sometimes games can recover by fixing the cheat.
• Ultima Online

19
Counter-Strike / Half-LifeNetworking Update

• Yahn Berniers lecture from GDC 2000, Half-Life
  and Team Fortress Networking
• Susceptible to internal and external hacking
• Executable modifications
• Clock speed hacks / DLL replacement

20
Client/Server architecturesare the safest
against cheating

• When they have a dont trust the client
  philosophy
• But this is actually difficult to do, and get
  good performance
• Example of player motion in a shooter
• Example of my first 3D game (bad-feeling motion)

21
Governing Systems

• Idea Can we let the client be authoritative,
  without trust?
• Let the client say what its doing, but only
  believe the client within limits
• Difficult

22
TCP vs UDP

• TCP provides reliability
• But it also serializes, which is usually bad
• reliable, ordered delivery

23
TCP vs UDP

• UDP is not reliable
• Unfortunately, UDP packets have a big header size
• TCP Header Compression is very useful, but
  generally does not apply to UDP

24
Types of Messages

• Some things we want reliably delivered, and
  sometimes even ordered
• Object creation/destruction
• Chat messages
• Some things we dont
• Position updates

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A Common Solution

• Use TCP for reliable messages, UDP for unreliable
  messages
• This has drawbacks
• Example of multiple entity creation/destruction,
  fireworks
• Old bug in windows 95/98

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How to Solve This?

• Many TCP channels per client?
• Inelegant, and may not solve the problem
• If organized by semantics, wont solve the entity
  deletes problem!

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The Best Solution

• Implement your own reliable delivery in UDP
• Requires some work, but your code becomes more
  unified
• Unification is important!
• You have the option of implementing more
  sophisticated reliability than ACK/retransmit

28
Traditional Reliable Delivery

• Buffer the packet bytes in the network layer, and
  re-send that exact message

29
Application-Centric Reliable Delivery

• Can modify the contents of the packet to reflect
  the most recent information
• Examples
• Health bar update
• Object creation
• Toggle variable
• Latency is effectively reduced in the case of
  retransmits

30
Dealing with NATand lobby servers

• NAT network address translation
• Example of people on a LAN connecting to a lobby

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Deciding What To Transmit

• Limited bandwidth to fit all entity updates into
• Apportion this out into slices somehow?
• Can we do this without adding latency? A hard
  problem!
• Need to accept the fact that the client wont be
  perfectly updated about everything, always a
  little bit wrong
• Approach network communications as an error
  minimization problem

32
Deciding What To Transmit

• Need a metric for the amount of error between
  object on client and server
• Position, orientation, state variables
• Probably attenuated by distance to deal with
  viewpoint issues!
• What about a rocket with contrail attached?
• Record the world state that you sent to the
  client, and diff it against current state
• A lot of memory!
• A lot of CPU!

33
There is a lot of coherence between clients
world views

• We ought to be able to exploit that
• Example of objects that move a lot
• They will have high error in all world views
• How do we detect a lot of motion?
• Should not use average distance traveled per
  frame
• Localize within a neighborhood box?
• Measure vector-valued variance!

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Variance of a vector

• Also Covariance of the vector components
• Variance/Covariance Matrix of components
• (demo)
• Can be filtered, like scalars, to approximate
  where something has been over different periods
  of time

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Summary of Variance Methods

• Characterized by ellipsoid
• Find ellipsoid by eigenvalues/eigenvectors of
  outer product matrix
• These variances can be treated intuitively like
  mass (tensor of inertia, in physics)