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Game System

Game System. Simplify Your Thinking into Modula (1/2). Control system User input Mouse or keyboard Keyboard layout Camera control View angle View range Interaction with player A “walk through” system Combat system Controls Motion management and blending NPC AI FX Visual Audio.

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Game System

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  1. Game System

  2. Simplify Your Thinking into Modula (1/2) • Control system • User input • Mouse or keyboard • Keyboard layout • Camera control • View angle • View range • Interaction with player • A “walk through” system • Combat system • Controls • Motion management and blending • NPC AI • FX • Visual • Audio

  3. Simplify Your Thinking into Modula (2/2) • Reward system • Number system • Levels • Damage • User Interface • Menu • Mini-map • Messages • Blood bar • Caption • The main program • Main loop • Level management • Configuration • Save/Load

  4. Game AI Steering Behavior

  5. Motion Behavior A Hierarchy of Motion Behavior • Action selection • Steering • Locomotion

  6. Action Selection • Game AI engine • State machine • Discussed in “Finite State Machine” section • Goals • Planning • Strategy • Scripting • Assigned by players • Players’ input

  7. Steering • Path determination • Path finding or path planning • Discussed in “Path Finding” • Behaviors • Seek & flee • Pursuit & evasion • Obstacle avoidance • Wander • Path following • Unaligned collision avoidance • Group steering

  8. Locomotion • Character physically-based models • Movement • Turn right, move forward, … • Animation • By artists • Implemented / managed by game engine

  9. A Simple Vehicle Model (1/2) • A point mass • Linear momentum • No rotational momentum • Parameters • Mass • Position • Velocity • Modified by applied forces • Max speed • Top speed of a vehicle • Max steering force • Self-applied • Orientation • Car • Aircraft

  10. A Simple Vehicle Model (2/2) • Local space • Origin • Forward • Up • Side • Steering forces • Asymmetrical • Thrust • Braking • Steering • Velocity alignment • No slide, spin, … • Turn

  11. Euler Integration • The approach : • Steer_force = Truncate(streer_direction, Max_force) • Acceleration = Steer_force / mass • Velocity = Truncate(Velocity + Acceleration, Max_speed) • Position = Position + Velocity

  12. Seek & Flee Behaviors • Pursuit to a static target • Steer a character toward to a target position • “A moth buzzing a light bulb” • Flee • Inverse of seek • Variants • Arrival • Pursuit to a moving target • Seek Steering force • desired_velocity = normalize(target - position)*max_speed • steering = desired_velocity – velocity

  13. Arrival Behavior • One of the idea : a stopping radius • Outside the radius, arrival is identical to seek • Inside the radius, the speed is ramped down to zero • target_offset = target – position • distance = length(target_offset) • ramped_speed = max_speed*(distance/slowing_distance) • clipped_speed = minimum(ramped_speed, max_speed) • desired_velocity = (clipped_speed/distance)*target_offset • steering = desired_velocity – velocity

  14. Pursuit & Evasion Behaviors • Estimate the prediction interval T • T = Dc • D = distance(pursur, quarry) • c = turning parameter • Target is moving • Apply seek or flee to the target’s predicted position • Variants • Offset pursuit • “Fly by”

  15. Obstacle Avoidance Behavior steering force • Use bounding sphere • Not collision detection • Probe • A cylinder lying along forward axis • Diameter = character’s bounding sphere • Length = speed (means Alert range) • Find the most threaten obstacle • Nearest intersected obstacle • Steering

  16. Wander Behavior • Random steering • One solution : • Retain steering direction state • Constrain steering force to the sphere surface located slightly ahead of the character • Make small random displacements to it each frame • A small sphere on sphere surface to indicate and constrain the displacement • Another one : • Perlin noise • Variants • Explore

  17. Path Following Behavior • The path • Spine • A spline or poly-line to define the path • Pipe • The tube or generated cylinder by a defined “radius” • Following • A velocity-based prediction position • Inside the tube • Do nothing about steering • Outside the tube • “Seek” to the on-path projection • Variants • Wall following • Containment

  18. Flow Field Following Behavior • A flow field environment is defined. • Virtual reality • Not common in games

  19. Unaligned Collision Avoidance Behavior • Turn away from possible collision • Predict the potential collision • Use bounding spheres • If possibly collide, • Apply the steering on both characters • Steering direction is possible collision result • Use “future” possible position • The connected line between two sphere centers

  20. Steering Behaviors for Groups of Characters • Steering behaviors determining how the character reacts to the other characters within his/her local neighborhood • The behaviors including : • Separation • Cohesion • Alignment

  21. The Local Neighborhood of a Character The Neighborhood • The local neighborhood is defined as : • A distance • The field-of-view • Angle

  22. Separation Behavior • Make a character to maintain a distance from others nearby. • Compute the repulsive forces within local neighborhood • Calculate the position vector for each nearby • Normalize it • Weight the magnitude with distance • 1/distance • Sum the result forces • Negate it

  23. Cohesion Behavior • Make a character to cohere with the others nearby • Compute the cohesive forces within local neighborhood • Compute the average position of the others nearby • Gravity center • Apply “Seek” to the position

  24. Alignment Behavior • Make a character to align with the others nearby • Compute the steering force • Average the together velocity of all other characters nearby • The result is the desired velocity • Correct the current velocity to the desired one with the steering force

  25. Flocking Behavior • “Boids Model of Flocks” • [Reynolds 87] • Combination of : • Separation steering • Cohesion steering • Alignment steering • For each combination including : • A weight for combing • A distance • An Angle

  26. Leader Following Behavior • Follow a leader • Stay with the leader • “Pursuit” behavior (Arrival style) • Stay out of the leader’s way • Defined as “next position” with an extension • “Evasion” behavior when inside the above area • “Separation” behavior for the followers

  27. Behavior Conclusion • Wall following • Containment • Flow field following • Unaligned collision avoidance • Separation • Cohesion • Alignment • Flocking • Leader following • A simple vehicle model with local neighborhood • Common steering behaviors including : • Seek • Flee • Pursuit • Evasion • Offset pursuit • Arrival • Obstacle avoidance • Wander • Path following • Combining the above behaviors in your application

  28. Game AI Finite State Machine

  29. Introduction (1/2) • Finite State Machine (FSM) is the most commonly used game AI technology today. • Simple • Efficient • Easily extensible • Powerful enough to handle a wide variety of situations • Theory (simplified) • A set states, S • An input vocabulary, I • Transition function, T(s, i) • Map a state and an input to another state

  30. Introduction (2/2) • Practical use • State • Behavior • Transition • Across states • Conditions • It’s all about driving behavior • Flow-chart diagram • UML State chart • Arrow • Transition • Rectangle • State

  31. An Example of FSM As a Diagram Monster in sight Gather Treasure Flee No monster Monster dead Cornered Fight

  32. FSM for Games • Character AI • “Decision-Action” model • Behavior • Mental state • Transition • Players’ action • The other characters’ actions • Some features in the game world

  33. Implement FSM • Code-based FSM • Simple Code One Up • Straightforward • Most common • Macro-assisted FSM Language • Data-Driven FSM • FSM Script Language

  34. Coding an FSM – Code Example 1 void RunLogic(int *state) { switch(*state) { case 0: // Wander Wander(); if (SeeEnemy()) *state = 1; if (Dead()) *state = 2; break; case 1: // Attack Attack(); *state = 0; if (Dead()) *state = 2; break; case 2: // Dead SlowlyRot(); break; } }

  35. Coding an FSM – Code Example 2 void RunLogic(FSM *fsm) { // Do action based on the state and determine next input input = 0; switch(fsm->GetStateID()) { case 0: // Wander Wander(); if (SeeEnemy()) input = SEE_ENEMY; if (Dead()) input = DEAD; break; case 1: // Attack Attack(); input = WANDER; if (Dead()) input = DEAD; break; case 2: // Dead SlowlyRot(); break; } // DO state transition based on computed input fsm->StateTransition(input); }

  36. Mealy & Moore Machines • Mealy machine • A Mealy machine is an FSM whose actions are performed on transitions • Moore machine • A Moore machine’s actions reside in states • More intuitive for game developers

  37. FSM Language Use Macros • Coding a state machine directly causes lack of structure • Going complex when FSM at their largest • Use macros • Beneficial properties • Structure • Readability • Debugging • Simplicity

  38. FSM Language Use Macros – An Example #define BeginStateMachine … #define State(a) … … bool MyStateMachine::States(StateMachineEvent event, int state) { BeginStateMachine State(0) OnUpdate Wander(); if (SeeEnemy()) SetState(1); if (Dead()) SetState(2); State(1) OnUpdate Attack(); SetState(0); if (Dead()) SetState(2); State(2); OnUpdate RotSlowly(); EndStateMachine }

  39. Data-Driven FSM • Scripting language • Text-based script file • Transformed into • C++ • Integrated into source code • Bytecode • Interpreted by the game • Authoring • Compiler • AI editing tool • Game • FSM script engine • FSM interface

  40. Data-Driven FSM Diagram Authoring Games FSMs FSM Script Engine bytecode Compiler Artist, Designers, & Developers AI Editing Tool FSM Interface Condition & Action Code Game Engine Condition & Action Vocabulary

  41. AI Editing Tool for FSM • Pure text • Syntax ? • Visual graph with text • Used by Designers, Artists, or Developers • Non-programmers • Conditions & action vocabulary • SeeEnemy • CloseToEnemy • Attack • …

  42. FSM Interface • Facilitating the binding between vocabulary and game world • Glue layer that implements the condition & action vocabulary in the game world • Native conditions • SeeEnemy(), CloseToEnemy() • Action library • Attack(…)

  43. FSM Script Language Benefits • Accelerated productivity • Contributions from artists & designers • Ease of use • Extensibility

  44. Processing Models for FSMs • Processing the FSMs • Evaluate the transition conditions for current state • Perform any associated actions • When and how ? • Depend on the exact need of games • Three common FSM processing models • Polling • Event-driven • Multithread

  45. Polling Processing Model • Processing each FSM at regular time intervals • Tied to game frame rate • Or some desired FSM update frequency • Limit one state transition in a cycle • Give a FSM a time-bound • Pros • Straightforward • Easy to implement • Easy to debug • Cons • Inefficiency • Some transition are not necessary to check every frame • Careful design to your FSM

  46. Event-driven Processing Model • Designed to prevent from wasted FSM processing • An FSM is only processed when it’s relevant • Implementation • A Publish-subscribe messaging system (Observer pattern) • Allows the engine to send events to individual FSMs • An FSM subscribes only to the events that have the potential to change the current state • When an event is generated, the FSMs subscribed to that events are all processed • “As-needed” approach • Should be much more efficient than polling ? • Tricky balance for fine-grained or coarse-grained events

  47. Multithread Processing Model • Both polling & event-driven are serially processed • Multithread processing model • Each FSM is assigned to its own thread for processing • Game engine is running in another separate thread • All FSM processing is effectively concurrent and continuous • Communication between threads must be thread-safe • Using standard locking & synchronization mechanisms • Pros • FSM as an autonomous agent who can constantly and independently examine and react to his environment • Cons • Overhead when many simultaneous characters active • Multithreaded programming is difficult

  48. Interfacing with Game Engine (1/2) • FSMs encapsulate complex behavior logic • Decision, condition, action, … • Game engine does corresponding • Character animation, movements, sounds, … • The interface : • Code each action as a function • Need recompile if any code is changed • ie., FleeWolf() • Callbacks • Function pointers • ie., actionFunction[fleeWolf]() • Container method • actionFunctions->FleeWolf(); • DLL

  49. Interfacing with Game Engine (2/2) class AArmyUnit : public FnCharacter { … void DoAttack(…); } AArmyUnit *army; army->Object(…); army->MoveForward(dist, …); … army->DoAttack(…); • Take TheFly3D as example:

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