Real world catapult prototype using Box2D – Adding wheels

April 9, 2010 • Actionscript 3, Box2D, Flash by Emanuele Feronato • 4 Comments

Did you enjoy the catapult prototype?

Here I am with the second part, adding wheels controlled by left and right arrows.

Have a look:

Now you can move the catapult with left and arrow keys, and shoot with a mouse click

If you feel the catapult is running on an icy surface, that’s because I did not set any friction. I’ll do it next time, when I’ll also come with a decent code, meanwhile check this:

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package {
	import flash.display.Sprite;
	import flash.events.Event;
	import flash.events.MouseEvent;
	import flash.events.KeyboardEvent;
	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 front_wheel_joint:b2RevoluteJointDef = new b2RevoluteJointDef();
		var rear_wheel_joint:b2RevoluteJointDef = new b2RevoluteJointDef();
		var arm_revolute_joint_def:b2RevoluteJoint;
		var front_wheel_joint_def:b2RevoluteJoint;
		var rear_wheel_joint_def:b2RevoluteJoint;
		var catapult_body:b2BodyDef = new b2BodyDef();
		var catapult_itself:b2Body;
		public function main():void {
			debug_draw();
			draw_box(250,400,2000,30,false,"ground");
			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);
			catapult_itself=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);
			arm_revolute_joint_def=world.CreateJoint(arm_joint) as b2RevoluteJoint;
			arm_revolute_joint_def.SetMotorSpeed(1000);
			arm_revolute_joint_def.SetLimits(-Math.PI,Math.PI/3);
			arm_revolute_joint_def.SetMaxMotorTorque(1);
			//
			var rear_wheel:b2BodyDef= new b2BodyDef();
			rear_wheel.position.Set(250/world_scale, 200/world_scale);
			rear_wheel.type=b2Body.b2_dynamicBody;
			var rear_wheel_shape:b2CircleShape=new b2CircleShape(40/world_scale);
			var the_rear_wheel_itself:b2Body=world.CreateBody(rear_wheel);
			the_rear_wheel_itself.CreateFixture2(rear_wheel_shape,20);
			var front_wheel:b2BodyDef= new b2BodyDef();
			front_wheel.position.Set(450/world_scale, 200/world_scale);
			front_wheel.type=b2Body.b2_dynamicBody;
			var front_wheel_shape:b2CircleShape=new b2CircleShape(40/world_scale);
			var the_front_wheel_itself:b2Body=world.CreateBody(front_wheel);
			the_front_wheel_itself.CreateFixture2(front_wheel_shape,20);
			//
			front_wheel_joint.enableMotor=true;
			front_wheel_joint.Initialize(catapult_itself, the_front_wheel_itself,new b2Vec2(0,0));
			front_wheel_joint.localAnchorA=new b2Vec2(80/world_scale,0);
			front_wheel_joint.localAnchorB=new b2Vec2(0,0);
			front_wheel_joint_def=world.CreateJoint(front_wheel_joint) as b2RevoluteJoint;
			front_wheel_joint_def.SetMaxMotorTorque(1000000);
			//
			rear_wheel_joint.enableMotor=true;
			rear_wheel_joint.Initialize(catapult_itself, the_rear_wheel_itself,new b2Vec2(0,0));
			rear_wheel_joint.localAnchorA=new b2Vec2(-80/world_scale,0);
			rear_wheel_joint.localAnchorB=new b2Vec2(0,0);
			rear_wheel_joint_def=world.CreateJoint(rear_wheel_joint) as b2RevoluteJoint;
			rear_wheel_joint_def.SetMaxMotorTorque(1000000);
			//
			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);
			stage.addEventListener(KeyboardEvent.KEY_DOWN, key_down);
		}
		public function key_down(event:KeyboardEvent):void {
			switch (event.keyCode) {
				case 39 :
					rear_wheel_joint_def.SetMotorSpeed(10);
					front_wheel_joint_def.SetMotorSpeed(10);
					break;
				case 37 :
					rear_wheel_joint_def.SetMotorSpeed(-10);
					front_wheel_joint_def.SetMotorSpeed(-10);
					break;
			}
		}
 
		public function fire(e:MouseEvent):void {
			arm_revolute_joint_def.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();
		}
	}
}

No need to download anything, just paste the new code in the catapult prototype file.

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This post has 4 comments

  1. audas

    on April 9, 2010 at 2:05 pm

    dude,
    whats with all that code ? Time to download quickbox2d – do that in two lines.

  2. Gabriel Bianconi

    on April 9, 2010 at 4:00 pm

    Great prototype, but the wheels don’t seem very real, they’re just too fast!

    Great articles in general too!

  3. Labici Danut

    on April 12, 2010 at 5:33 am

    DUDE! This is just too cool!
    I love it.

  4. T_N_D

    on December 15, 2011 at 8:57 pm

    How to move catapult whiout easing?? I already try put friction =0…. :(

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