Basic Box2D rope
2009-10-18 00:00:00 来源:WEB开发网It’s time to create a rope with Box2d.
To start defining a rope, we’ll start from something similar: a chain. A chain is a series of two or more connected links. The smaller and closer the links, the more flexible the chain. So we can define a rope as a “perfect chain”, with infinite links.
In a Box2D simulation, ropes as we know them aren’t possible. But we can construct chains, and according to the number of links they can approximate a rope quite well.
Obviously, the higher the number of links, the more accurate the simulation, the slowest and CPU intensive the simulation.
I found 10 links are a good compromise between simulation and performance.
Every link is (guess what?) linked to the previous one with a revolute joint. Refer to Box2D joints: Revolute Joint if you need more information about revolute joints.
The core of the whole script is the link variable. I simply “save” the reference of the last chain placed on the stage and create the joint between the last chain placed and the one I am going to place.
So I’m having the first link with a joint with the ceiling, the second one with a joint on the first one, the third with a joint on the second and so on.
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 rope extends Sprite {
var body:b2Body;
public var m_world:b2World;
public var m_iterations:int=10;
public var m_timeStep:Number=1.0/30.0;
var mouseJoint:b2MouseJoint;
var mousePVec:b2Vec2 = new b2Vec2();
var bodyDef:b2BodyDef;
var boxDef:b2PolygonDef;
var circleDef:b2CircleDef;
var revolute_joint:b2RevoluteJointDef=new b2RevoluteJointDef();
var link:b2Body;
public function rope() {
addEventListener(Event.ENTER_FRAME, Update, false, 0, true);
stage.addEventListener(MouseEvent.MOUSE_DOWN, createMouse);
stage.addEventListener(MouseEvent.MOUSE_UP, destroyMouse);
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 start
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;
m_world.SetDebugDraw(dbgDraw);
// ceiling
bodyDef = new b2BodyDef();
bodyDef.position.x=8.5;
bodyDef.position.y=0;
boxDef = new b2PolygonDef();
boxDef.SetAsBox(2, 0.5);
boxDef.density=0;
boxDef.friction=0.5;
boxDef.restitution=0.2;
body=m_world.CreateBody(bodyDef);
body.CreateShape(boxDef);
link=body;
// rope
for (var i:int = 1; i <= 10; i++) {
// rope segment
bodyDef = new b2BodyDef();
bodyDef.position.x=8.5;
bodyDef.position.y=i;
boxDef = new b2PolygonDef();
boxDef.SetAsBox(0.1, 0.5);
boxDef.density=100;
boxDef.friction=0.5;
boxDef.restitution=0.2;
body=m_world.CreateBody(bodyDef);
body.CreateShape(boxDef);
// joint
revolute_joint.Initialize(link, body, new b2Vec2(8.5, i-0.5));
m_world.CreateJoint(revolute_joint);
body.SetMassFromShapes();
// saving the reference of the last placed link
link=body;
}
// final body
bodyDef.position.x=8.5;
bodyDef.position.y=11;
boxDef = new b2PolygonDef();
boxDef.SetAsBox(0.5,0.5);
boxDef.density=2;
boxDef.friction=0.5;
boxDef.restitution=0.2;
body=m_world.CreateBody(bodyDef);
body.CreateShape(boxDef);
revolute_joint.Initialize(link, body, new b2Vec2(8.5, 10.5));
m_world.CreateJoint(revolute_joint);
body.SetMassFromShapes();
}
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 {
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);
}
}
}
}
更多精彩
赞助商链接