Emanuele Feronato

Real world catapult prototype using Box2D

Talking about Crush the Castle game, Actionscript 3, Box2D and Flash.

This is an early attempt to simulate a real world catapult using Box2D.

Raw and uncommented code, but a lot of useful information about compound objects, revolute joints and motors.

This is what you’ll get:

Click with the mouse to shoot the ball.

And this is the source code:

package {
	import flash.display.Sprite;
	import flash.events.Event;
	import flash.events.MouseEvent;
	import Box2D.Dynamics.*;
	import Box2D.Collision.*;
	import Box2D.Collision.Shapes.*;
	import Box2D.Common.Math.*;
	import Box2D.Dynamics.Joints.*;
	public class main extends Sprite {
		public var world:b2World=new b2World(new b2Vec2(0,10.0),true);
		public var world_scale:int=30;
		var arm_joint:b2RevoluteJointDef = new b2RevoluteJointDef();
		var the_joint_itself:b2RevoluteJoint;
		public function main():void {
			debug_draw();
			draw_box(250,400,500,30,false,"ground");
			var catapult_body:b2BodyDef = new b2BodyDef();
			catapult_body.position.Set(350/world_scale,200/world_scale);
			catapult_body.type=b2Body.b2_dynamicBody;
			var main_part:b2PolygonShape = new b2PolygonShape();
			main_part.SetAsOrientedBox(125/world_scale, 20/world_scale, new b2Vec2(0,0),0);
			var fixed_arm:b2PolygonShape = new b2PolygonShape();
			fixed_arm.SetAsOrientedBox(20/world_scale, 60/world_scale, new b2Vec2(-80/world_scale,-40/world_scale),0);
			var catapult_itself:b2Body=world.CreateBody(catapult_body);
			catapult_itself.CreateFixture2(main_part,200);
			catapult_itself.CreateFixture2(fixed_arm,200);
			var catapult_arm:b2BodyDef = new b2BodyDef();
			catapult_arm.allowSleep = false;
			catapult_arm.position.Set(210/world_scale,110/world_scale);
			catapult_arm.type=b2Body.b2_dynamicBody;
			var arm_part:b2PolygonShape = new b2PolygonShape();
			arm_part.SetAsOrientedBox(150/world_scale, 10/world_scale, new b2Vec2(0,0),0);
			var stopper:b2PolygonShape = new b2PolygonShape();
			stopper.SetAsOrientedBox(10/world_scale, 20/world_scale, new b2Vec2(-140/world_scale,-30/world_scale),0);
			var catapult_arm_itself:b2Body=world.CreateBody(catapult_arm);
			catapult_arm_itself.CreateFixture2(arm_part,1);
			catapult_arm_itself.CreateFixture2(stopper,1);
			arm_joint.enableMotor = true;
			arm_joint.enableLimit = true;
			arm_joint.Initialize(catapult_itself, catapult_arm_itself,new b2Vec2(0,0));
			arm_joint.localAnchorA=new b2Vec2(-80/world_scale,-90/world_scale);
			arm_joint.localAnchorB=new b2Vec2(60/world_scale,0);
			the_joint_itself=world.CreateJoint(arm_joint) as b2RevoluteJoint;
			the_joint_itself.SetMotorSpeed(1000);
			the_joint_itself.SetLimits(-Math.PI,Math.PI/3);
			the_joint_itself.SetMaxMotorTorque(1) 
			var cannonball:b2BodyDef= new b2BodyDef();
			cannonball.position.Set(90/world_scale, 90/world_scale);
			cannonball.type=b2Body.b2_dynamicBody;
			var ball:b2CircleShape=new b2CircleShape(10/world_scale);
			var the_cannonball_itself:b2Body=world.CreateBody(cannonball);
			the_cannonball_itself.CreateFixture2(ball,20);
			addEventListener(Event.ENTER_FRAME, update);
			stage.addEventListener(MouseEvent.CLICK,fire);
		}
		public function fire(e:MouseEvent):void {
			the_joint_itself.SetMaxMotorTorque(10000)
		}
		public function draw_box(px,py,w,h,d,ud):void {
			var ground:b2BodyDef= new b2BodyDef();
			ground.position.Set(px/world_scale, py/world_scale);
			if (d) {
				ground.type=b2Body.b2_dynamicBody;
			}
			var my_box:b2PolygonShape = new b2PolygonShape();
			my_box.SetAsBox(w/2/world_scale, h/2/world_scale);
			var my_fixture:b2FixtureDef = new b2FixtureDef();
			my_fixture.shape=my_box;
			var the_ground_itself:b2Body=world.CreateBody(ground);
			the_ground_itself.SetUserData(ud);
			the_ground_itself.CreateFixture(my_fixture);
		}
		public function debug_draw():void {
			var debug_draw:b2DebugDraw = new b2DebugDraw();
			var debug_sprite:Sprite = new Sprite();
			addChild(debug_sprite);
			debug_draw.SetSprite(debug_sprite);
			debug_draw.SetDrawScale(world_scale);
			debug_draw.SetFlags(b2DebugDraw.e_shapeBit|b2DebugDraw.e_jointBit);
			debug_draw.SetFillAlpha(0.5);
			world.SetDebugDraw(debug_draw);
		}
		public function update(e:Event):void {
			world.Step(1/30,10,10);
			world.ClearForces();
			world.DrawDebugData();
		}
	}
}

Download the source code.