On the Impact of Delay on Real-Time Multiplayer Games

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On the Impact of Delay on Real-Time Multiplayer
Games

• Authors Lothar Pantel, Lars C. Wolf
• Presented by Bryan Wong

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Outline

• Introduction
• Multiplayer Games
• Experimental Evaluation
• Related Work
• Conclusions

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Introduction

• Major problem of networked multiplayer games is
  caused by transmission delay
• A primary goal is to provide a global consistent
  state of the game
• One approach is to introduce a local presentation
  delay

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Introduction

• Simulator-like games such as flight simulators,
  racing and sports games are most critical because
  they require smooth movements of the objects

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Multiplayer Games

• Architectures
• Server based with a dynamically based or central
  server
• Distributed scheme

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Influence of Basic System Parameters

• Designer must decide which messages are
  transmitted reliably/unreliably

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Delay

• The time between the generation of an event and
  the resulting update of the game state
• Important issue of how the system handles old
  state information and how large the difference
  between the real and expected positions of the
  players in a game may be due to the delay of
  messages

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Experiments with Commercial Games

• Studied the delay-handling approaches used by two
  commercial games in two player mode
• Re-Volt
• Need-for-Speed
• Examine how commercial game developers handle the
  delay problem

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Start Release

• Direct play assigns the role of coordinator to
  one of the two players
• This contains the release of the start
• Use the start countdown and start sound to detect
  correct synchronization

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Start Process

• As soon as the vehicles begin to move, the first
  deviations in the positions of the cars can occur
• Identical vehicles, motors, etc. are selected to
  avoid acceleration differences
• Expected that both cars remain head to head

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Simulated Start

• Cars are stopped in the course of the race and
  are aligned at the beginning of a straight path
• Both cars are accelerated simultaneously by one
  person using both keyboards
• Cars should remain side by side

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Driving with Constant Speed

• Vehicles drive with same constant speed side by
  side
• An identical screen situation should be presented
• Occurs only if position consistency is maintained
  during the game

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Collision Treatment

• Collision of game objects is detected through a
  polygon-collision
• Only a position difference of 0 of opposing
  vehicles will cause an identical, local
  evaluation of the polygon collision
• If position consistency is not sufficient,
  contradictory situations can occur

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Observed Results

• Games provided very limited handling of delay
• Both provided a synchronized start release
• Start process was not handled correctly the
  local car was always in leadership
• Both games failed tests 3,4
• Neither game provided satisfying treatment of
  collisions

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Experimental Evaluation of the Impact of Delay

• Inconsistent state brings several problems and
  can be confusing to the players
• One approach to enhance consistency is to delay
  the presentation of the actual game state
• How much delay is acceptable for users in real
  time games?
• Delay includes all communication, processing and
  buffering costs

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Principle Setup

• Performed steps

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Implementation

• Implemented in an RC-Car simulation
• Driver does not use a cockpit view but a higher
  view allowing the user to view the whole area
• Virtual RC Racing implements the static delay
  scheme
• The feeling how the car reacts is influenced by
  the delay between steering and reaction

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Experiments

• Purpose was to determine acceptable presentation
  delay
• 12 candidates performed experiments
• For each of the following graphs, three players
  were selected who were representatives for the
  following categories Beginner, Average and
  Excellent driver
• Static presentation delay was increased in 50ms
  steps up to 500ms

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Experiments

• Avg time per round in dependence on delay
• It becomes more difficult to control the car,
  requiring the driver to slow down
• Best time per round in dependence on delay
• Reflects the best possible value for the driver,
  under the given conditions
• Frequency of Leaving the Course

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Results
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Results
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Results
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Results
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Results
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View Dependance

• Delay value that can be tolerated by the
  participant depends on the camera perspective
• A cockpit view allows at least an additional 50ms
  of delay to be tolerated

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Related Work

• MiMaze distributed 3D Pacman game
• Consistency of the is improved by delaying the
  presentation of the events using buckets
• The whole game is 100ms in the past
• Players with gt100ms delay cannot participate

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Conclusions

• Delay of 50ms is uncritical for a racing game
• Delay of gt100ms should be avoided
• Consider the racing game as a worst-case system
• For other games (first person shooter,) a delay
  of 100ms may be acceptable
• For comparison, lip synchronization requirements
  for audio and video streams are -80ms,80ms
• Static delay schemes are not well suited for
  real-time multiplayer games