Understanding steering behavior: HTML5 example using Phaser

Talking about Boids game, Game development, HTML5, Javascript and Phaser.

In a previous post I showed you how to simulate flocking behavior with Boids using HTML5 and Phaser. There are a lot of things you can do with boids, and one of these nice things is the creation of a steering behavior. When a moving body has to change its direction to go towards a point, in real world it never stop moving in its old direction to suddenly move towards target point, but it gently steers until current direction matches to preferred direction, which is usually following the straight line from the boid to the target. Look at this example:

Here the boids steer to reach the target point – the red dot – making the movement more realistic than just following a straight line. As you can see in the source code, everything is just a matter of vector operations. Phaser does not features a vector class but we can rely on Point class which has a lot of methods we can use to handle Points as if they were vectors. Look at the commented source code:

var game;

var boidsAmount = 5;
var boids = [];
var target;

window.onload = function() {	
	game = new Phaser.Game(640, 640, Phaser.AUTO, "");
     game.state.add("Simulate", simulate);
     game.state.start("Simulate");
}

var simulate = function(game){};
simulate.prototype = {
     preload: function(){
          game.load.image("boid", "boid.png");
          game.load.image("target", "target.png");     
     },
     create: function(){
          target = game.add.sprite(game.width / 2, game.height / 2, "target");
          target.anchor.set(0.5);
          for(var i = 0; i < boidsAmount; i++){
               var randomPoint = new Phaser.Point(game.rnd.between(0, game.width - 1), game.rnd.between(0, game.height - 1));
               boids[i] = game.add.sprite(randomPoint.x, randomPoint.y, "boid")
               boids[i].anchor.set(0.5);
               boids[i].speed = game.rnd.between(50, 150);
               boids[i].force = game.rnd.between(5, 25);
               game.physics.enable(boids[i], Phaser.Physics.ARCADE);
               boids[i].body.allowRotation = false;    
          }
     },
     update: function(){
          for(var i = 0; i < boidsAmount; i++){
               // direction vector is the straight direction from the boid to the target
               var direction = new Phaser.Point(target.x, target.y);
               // now we subtract the current boid position
               direction.subtract(boids[i].x, boids[i].y);
               // then we normalize it. A normalized vector has its length is 1, but it retains the same direction
               direction.normalize();
               // time to set magnitude (length) to boid speed
               direction.setMagnitude(boids[i].speed);
               // now we subtract the current boid velocity
               direction.subtract(boids[i].body.velocity.x, boids[i].body.velocity.y);
               // normalizing again
               direction.normalize();
               // finally we set the magnitude to boid force, which should be WAY lower than its velocity
               direction.setMagnitude(boids[i].force); 
               // Now we add boid direction to current boid velocity
               boids[i].body.velocity.add(direction.x, direction.y);
               // we normalize the velocity
               boids[i].body.velocity.normalize();
               // we set the magnitue to boid speed
               boids[i].body.velocity.setMagnitude(boids[i].speed);
               boids[i].angle = 180 + Phaser.Math.radToDeg(Phaser.Point.angle(boids[i].position, new Phaser.Point(boids[i].x + boids[i].body.velocity.x, boids[i].y + boids[i].body.velocity.y)));
               if(boids[i].position.distance(target.position) < 2){
                    target.x = game.rnd.between(10, game.width - 10);
                    target.y = game.rnd.between(10, game.height - 10);
               }
          }                    
     }     
}

This was just another demonstration of what you can do with boids. More will come, meanwhile download the source code.

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