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KIPA Game Engine Seminars

Jonathan Blow

Ajou University, Suwon

November 28, 2002

Day 3/25

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Thatcher Ulrich’sChunked 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

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Parabolic sin/cosapproximation

• How to handle phase limits

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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

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Vectors vs. “Axial Vectors”References

• “Computer Graphics: Principles and Practice” by Foley, van Dam, Feiner, Hughes

• “Jim Blinn’s Corner: Notation, Notation, Notation” by Jim Blinn

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Batching Geometry by Material

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

enough!

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Short Triangle Strips

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

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SLERP

• “Spherical Linear Interpolation”

• Popularized in graphics by Ken Shoemake

• Search for his name on the Web to find a lot of related papers

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Deriving ‘SLERP’

• For arbitrary-dimension vectors

• Useful for interpolation of rotations, vertex normals, etc.

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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

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Source Control:Visual Source Safe

• Non-Atomic Transactions

• Manual Check-Out -- unacceptable

• Bad Integration (doesn’t even work in vc7!)

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Source Control:CVS

• 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

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Source Control:Perforce, AlienBrain, etc

• Cost money

• Better interfaces than SourceSafe and CVS

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Forward Declaration:Macros and typedefs

• … are bad because they can’t be forward-declared!

• (FILE *), (size_t)

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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)

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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

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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

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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

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Counter-Strike / Half-LifeNetworking Update

• Yahn Bernier’s lecture from GDC 2000, “Half-Life and Team Fortress Networking”

• Susceptible to internal and external hacking– Executable modifications– Clock speed hacks / DLL replacement

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Client/Server architecturesare the safest against cheating

• When they have a “don’t 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)

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Governing Systems

• Idea: Can we let the client be authoritative, without trust?

• Let the client say what it’s doing, but only believe the client within limits

• Difficult

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TCP vs UDP

• TCP provides reliability

• But it also serializes, which is usually bad– “reliable, ordered delivery”

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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

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Types of Messages

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

• Some things we don’t– 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, won’t 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

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Traditional Reliable Delivery

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

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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

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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 won’t be perfectly updated about everything, always a little bit wrong

• Approach network communications as an error minimization problem

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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!

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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)