Introduction Fundamental requirement in many games is to move - PDF document

3/31/2016 Introduction • Fundamental requirement in many games is to move characters (player avatar and NPC’s) around realistically and pleasantly Autonomous Movement • For some games (e.g., FPS) realistic NPC movement is pretty much core (along with shooting) � there is no higher level decision making! IMGD 4000 • At other extreme (e.g., chess), no “movement” per se � pieces just placed With material from: Millington and Funge, Artificial Intelligence for Games , Morgan Kaufmann 2009 (Chapter 3), Note: as for pathfinding, we’re going to treat everything in 2D, Buckland, Programming Game AI by Example , Wordware 2005 (Chapter 3), http://opensteer.sourceforge.net and since most game motion in gravity on surface (i.e., 2 ½ D) http://gamedevelopment.tutsplus.com/series/understanding -steering-behaviors--gamedev-12732 2 Craig Reynolds Outline Website: http://www.red3d.com/cwr/ • The “giant” in this area – his influence cannot be • Introduction (done) overstated – 1987 : “Flocks, Herds and Schools: A Distributed • The “Steering” Model (next) Behavioral Model,” Computer Graphics • Steering Methods – 1998 : Winner of Academy Award in Scientific and Engineering category • Flocking • Recognition of “his pioneering contributions to the development of three-dimensional computer animation for • Combining Steering Forces motion picture production” – 1999 : “Steering Behaviors for Autonomous Characters,” Proc. Game Developers Conference – Left U.S. R&D group of Sony Computer Entertainment in April 2012 after 13 years – Now (2015) at SparX (eCommerce coding within Staples ) 3 The “Steering” Model The “Steering” Model – Example • Cowboys tend herd of cattle • Trail boss decision Choosing goals and plans, e.g. Action Selection represents action • Cow wanders away • “go here” – Observes world – cow is • Trail boss tells cowboy to missing • “do A, B, and then C” fetch stray – Setting goal – retrieve cow • Cowboy says “giddy-up” and Calculate trajectories to satisfy goals and • Steering done by cowboy Steering guides horse to cow, plans – Go faster, slower, turn right, avoiding obstacles left … Produce steering force that determines • Horse implements where and how fast character moves locomotion Mechanics (“how”) of motion Locomotion – With signal, go in indicated Note, depending upon the • Differs for characters, e.g., fish vs. direction game, player could control horse (e.g., compare animations) – Account for mass when boss or cowboy (or both)! accelerating/turning • Independent of steering – Provide animation 5 1

3/31/2016 Action Selection Locomotion Dynamics Steering Locomotion • Done through variety of means… class Body // Point mass of rigid body // Scale vector to appropriate size (max) – e.g., decision tree or FSM vector truncate(vector v, int max) { mass // scalar float f; position // vector f = max / v.getLength(); – (see earlier slide deck) velocity // vector if (f < 1.0) Action Selection f = 1.0 • Examples: // Orientation of body v.scaleBy(f); heading // vector return v; } – “Get health pack” // Dynamic properties of body maxForce // scalar – “Charge at enemy” maxSpeed // scalar • Player input def update (dt) { force = ...; // Combine forces from steering behaviors acceleration = force / mass; // Update acceleration w/Newton's 2nd law – “Return to base” velocity += truncate ( acceleration * dt, maxSpeed ); // Update speed position += velocity * dt; // Update position – “Fetch cow” if ( | velocity | > 0.000001 ) // If vehicle moving enough heading = normalize ( velocity ); // Update heading to velocity vector // render … } 8 Individual Steering “Behaviors” So “Steering” in this Context Means Compute forces Making objects move by: • Applying forces seek flee instead of arrive pursue Steering • Directly transforming their positions wander evade Why? interpose hide • ...because it looks much more natural avoid obstacles follow path i.e., “steering” does not mean just using, say, the arrow/WASD keys to Multiple behaviors combine forces (e.g., flocking ) move an avatar, but doing motion by applying forces 9 10 Adding Forces in UE4 Steering Methods Add force to a single rigid body class Body { def update (dt) { virtual void AddForce (Fvector Force, Fname BoneName) force = ... // combine forces from steering behaviors … – Force – force vector to apply. Magnitude is } strength of force • Forces returned by each def seek (target) { ... return force; } method are combined def flee (target) { ... return force; } – BoneName – name of body to apply it to (‘None’ (shown later) def arrive (target) { ... return force; } def pursue (body) { ... return force; } • Individual behaviors can to apply to root body) def evade (body) { ... return force; } be turned on/off (next def hide (body) { ... return force; } slide) def interpose (body1, body2) { return force: } void AMyCharacter::AddUpwardForce() { def wander () { ... return force; } def avoidObstacles () { ... return force; } const float ForceAmount = 20000.0f; ... Fvector force(0.0f, 0.0f, ForceAmount); }; Fname bone; // defaults to “NAME_None” this->AddForce(force, bone); } Distance units are centimeters C++ Blueprints Note: max velocity property of object 12 i.e., earth gravity 981 cm/s 2 2

3/31/2016 Turning Steering Methods On & Off Reference Code in C++ • Complete example code for this unit from • Action Selection controls which steering Buckland’s book can be downloaded from: behaviors on/off http://samples.jbpub.com/9781556220784/Buckland_SourceCode.zip – Folder for Chapter 3 class Body { • See also learning guide’s “Understanding private: Steering Behaviors”: vector Body::calcForce() { bool seek_on; if ( doSeek() ) { http://gamedevelopment.tutsplus.com/series/understanding-steering- force += seek(); public: … behaviors--gamedev-12732 void setSeek(bool on=true); } bool doSeek(); – Similar concepts, slightly different code … implementation } 13 14 Seek: Steering Force Outline Note: treat position as a target vector (direction is ignored) • Introduction (done) velocity • The “Steering” Model (done) desired velocity • Steering Methods (next) • Flocking • Combining Steering Forces steering force def seek (target) { // vector from here to target scaled by maxSpeed Will result in smooth path! desired = truncate ( target - position, maxSpeed ); // return steering force DEMO return desired - velocity; // vector difference } (Force: INS/DEL, Speed: HOME/END) 16 Problem with Seek? Problem with Seek • What happens when reaches target? • What happens when reaches target? – Overshoots target • How bad is it? • How bad is it? – Amount of overshoot determined by ratio of maxSpeed to maximum force applied • Intuitively, should decelerate as gets closer to target � Arrive 17 18 3