Creating a sling with Box2D using joints
Talking about Actionscript 3, Box2D, Flash and Game development.
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This is an example about the creation of a sling using joints and motors.
Click the mouse to break the joint and release the bullet. Try to hit the stack of boxes.
I used a distance joint, a revolute joint and a motor. The code does not need to be commented because there is nothing new, just look how I destroy the joint to launch the bullet at lines 122-123
This is the source:
package {
import flash.display.Sprite;
import flash.events.Event;
import Box2D.Dynamics.*;
import Box2D.Collision.*;
import Box2D.Collision.Shapes.*;
import Box2D.Common.Math.*;
import Box2D.Dynamics.Joints.*;
import flash.events.MouseEvent;
public class sling extends Sprite {
var mouseJoint:b2MouseJoint;
var mousePVec:b2Vec2=new b2Vec2;
var bd:b2BodyDef;
var the_box:b2PolygonDef=new b2PolygonDef;
var the_pivot:b2CircleDef=new b2CircleDef;
var the_rev_joint:b2RevoluteJointDef=new b2RevoluteJointDef;
var the_dist_joint:b2DistanceJointDef = new b2DistanceJointDef();
var dest_joint:b2DistanceJoint;
public function sling() {
// world setup
var worldAABB:b2AABB=new b2AABB ;
worldAABB.lowerBound.Set(-100.0,-100.0);
worldAABB.upperBound.Set(100.0,100.0);
var gravity:b2Vec2=new b2Vec2(0.0,10.0);
var doSleep:Boolean=true;
m_world=new b2World(worldAABB,gravity,doSleep);
// debug draw
var m_sprite:Sprite;
m_sprite=new Sprite;
addChild(m_sprite);
var dbgDraw:b2DebugDraw=new b2DebugDraw;
var dbgSprite:Sprite=new Sprite;
m_sprite.addChild(dbgSprite);
dbgDraw.m_sprite=m_sprite;
dbgDraw.m_drawScale=30;
dbgDraw.m_alpha=1;
dbgDraw.m_fillAlpha=0.5;
dbgDraw.m_lineThickness=1;
dbgDraw.m_drawFlags=b2DebugDraw.e_shapeBit|b2DebugDraw.e_jointBit;
m_world.SetDebugDraw(dbgDraw);
// the ground
var final_body:b2Body;
var the_body:b2BodyDef;
var the_box:b2PolygonDef;
the_body = new b2BodyDef();
the_body.position.Set(8.5, 13);
the_box = new b2PolygonDef();
the_box.SetAsBox(8.5, 0.5);
the_box.friction=0.3;
the_box.density=0;
final_body=m_world.CreateBody(the_body);
final_body.CreateShape(the_box);
final_body.SetMassFromShapes();
// some boxes
for (var i:int=0; i<7; i++) {
the_body = new b2BodyDef();
the_body.position.Set(12, 12-i);
the_box = new b2PolygonDef();
the_box.SetAsBox(0.5,0.5);
the_box.friction=0.3;
the_box.density=0.5;
final_body=m_world.CreateBody(the_body);
final_body.CreateShape(the_box);
final_body.SetMassFromShapes();
}
// pivot for revolute joint
the_pivot.radius=0.2;
the_pivot.density=0;
bd=new b2BodyDef ;
bd.position.Set(3,6.5);
var rev_joint:b2Body=m_world.CreateBody(bd);
rev_joint.CreateShape(the_pivot);
rev_joint.SetMassFromShapes();
// box for the revolute joint
the_box.SetAsBox(2,0.1);
the_box.density=0.01;
the_box.friction=1;
the_box.restitution=0.1;
bd=new b2BodyDef ;
bd.position.Set(3,6.5);
var rev_box:b2Body=m_world.CreateBody(bd);
rev_box.CreateShape(the_box);
rev_box.SetMassFromShapes();
// revolute joint
the_rev_joint.Initialize(rev_joint,rev_box,new b2Vec2(3,6.5));
the_rev_joint.enableMotor=true;
the_rev_joint.motorSpeed=10;
the_rev_joint.maxMotorTorque=1000;
var joint_added:b2RevoluteJoint=m_world.CreateJoint(the_rev_joint) as b2RevoluteJoint;
// bullet
the_box.SetAsBox(0.2,0.2);
the_box.density=1.0;
the_box.friction=0.4;
the_box.restitution=0.1;
bd = new b2BodyDef();
bd.position.Set(2, 5);
var bullet:b2Body=m_world.CreateBody(bd);
bullet.CreateShape(the_box);
bullet.SetMassFromShapes();
// distance joint
the_dist_joint.Initialize(bullet, rev_box, new b2Vec2(2,5),new b2Vec2(1,6.5));
the_dist_joint.collideConnected=true;
dest_joint=m_world.CreateJoint(the_dist_joint) as b2DistanceJoint;
// listeners
stage.addEventListener(MouseEvent.MOUSE_DOWN,createMouse);
stage.addEventListener(MouseEvent.MOUSE_UP,destroyMouse);
addEventListener(Event.ENTER_FRAME,Update,false,0,true);
}
public function createMouse(evt:MouseEvent):void {
var body:b2Body=GetBodyAtMouse();
if (body) {
var mouseJointDef:b2MouseJointDef=new b2MouseJointDef ;
mouseJointDef.body1=m_world.GetGroundBody();
mouseJointDef.body2=body;
mouseJointDef.target.Set(mouseX / 30,mouseY / 30);
mouseJointDef.maxForce=30000;
mouseJointDef.timeStep=m_timeStep;
mouseJoint=m_world.CreateJoint(mouseJointDef) as b2MouseJoint;
}
}
public function destroyMouse(evt:MouseEvent):void {
m_world.DestroyJoint(dest_joint);
dest_joint=null;
if (mouseJoint) {
m_world.DestroyJoint(mouseJoint);
mouseJoint=null;
}
}
public function GetBodyAtMouse(includeStatic:Boolean=false):b2Body {
var mouseXWorldPhys=mouseX/30;
var mouseYWorldPhys=mouseY/30;
mousePVec.Set(mouseXWorldPhys,mouseYWorldPhys);
var aabb:b2AABB=new b2AABB ;
aabb.lowerBound.Set(mouseXWorldPhys - 0.001,mouseYWorldPhys - 0.001);
aabb.upperBound.Set(mouseXWorldPhys + 0.001,mouseYWorldPhys + 0.001);
var k_maxCount:int=10;
var shapes:Array=new Array ;
var count:int=m_world.Query(aabb,shapes,k_maxCount);
var body:b2Body=null;
for (var i:int=0; i < count; ++i) {
if (shapes[i].GetBody().IsStatic()==false||includeStatic) {
var tShape:b2Shape=shapes[i] as b2Shape;
var inside:Boolean=tShape.TestPoint(tShape.GetBody().GetXForm(),mousePVec);
if (inside) {
body=tShape.GetBody();
break;
}
}
}
return body;
}
public function Update(e:Event):void {
m_world.Step(m_timeStep,m_iterations);
if (mouseJoint) {
var mouseXWorldPhys=mouseX/30;
var mouseYWorldPhys=mouseY/30;
var p2:b2Vec2=new b2Vec2(mouseXWorldPhys,mouseYWorldPhys);
mouseJoint.SetTarget(p2);
}
}
public var m_world:b2World;
public var m_iterations:int=10;
public var m_timeStep:Number=1.0/30.0;
}
}
How would you turn it into a game?
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