Tag: Box2D

[SWF(width = 800, height = 600, frameRate = 60)]
import com.actionsnippet.qbox.*;
stage.frameRate = 60;

var sim:QuickBox2D = new QuickBox2D(this);

sim.createStageWalls();

sim.start();

var output:TextField = new TextField();
output.text = "Click anywhere to add points to a polygon. Hit any key to test.nn";
output.x = output.y = 50;
with(output) width = 300, height = 400, border = true, selectable = true, wordWrap = true, multiline = true;
addChild(output);

function display(str:*):void{
	output.appendText(str.toString() + "n");
}
								
var points:Array = [];
var poly:Shape = new Shape();
addChild(poly);

stage.addEventListener(MouseEvent.CLICK, onClick);
stage.addEventListener(KeyboardEvent.KEY_DOWN, onKeyPressed);

function onClick(evt:MouseEvent):void {
	if (points.length == 0){
		poly.graphics.beginFill(0xCCCCCC);
		poly.graphics.lineStyle(1, 0xFF0000);
		poly.graphics.moveTo(mouseX, mouseY);
	}else{
		poly.graphics.lineTo(mouseX, mouseY);
	}
	poly.graphics.drawCircle(mouseX, mouseY, 2);
	 
	points.push(mouseX / 30.0, mouseY / 30.0);
}

function onKeyPressed(evt:KeyboardEvent):void {
	 // average all points
	 var avgX:Number=0
	 var avgY:Number = 0;
	 
	 for (var i:int = 0; i<points.length; i+=2){
	     avgX += points[i];
		 avgY += points[i + 1];
	 }
	
	 avgX /= points.length/2;
	 avgY /=  points.length/2;
	 avgX = avgX;
	 avgY = avgY;
	 
	 // subtract averages and fix decimal place
	  for (i = 0; i<points.length; i+=2){
		  var yp:int = i + 1;
	      points[i] -= avgX;
		  points[yp] -= avgY;
		  points[i] = Number(points[i].toFixed(2));
		  points[yp] = Number(points[yp].toFixed(2));
	 }
	 
	 display("points array:");
	 display(points);
	 
	 try{
		 var p:QuickObject = sim.addPoly({x:avgX, y:avgY, points:points});
		 p.userData.graphics.beginFill(0xFF0000);
		 p.userData.graphics.drawCircle(0,0,5);
	 }catch(e:*){
		display("Invalid polygon data!"); 
	 }
	 
	 poly.graphics.clear();
	 points = [];
}

This snippet shows the basic concepts needed to go about creating a polygon editor for QuickBox2D. I have an unreleased editor that I use for my QuickBox2D projects, at some point I may release it… but for now I figured I’d post this extremely simplified version for people to expand on.

Have a look at the swf here…

Since this is a rather large snippet check out this demo first:

View Demo…

and here is the source:

import com.actionsnippet.qbox.*
import Box2D.Collision.Shapes.*;
import Box2D.Common.Math.*;

[SWF(width = 800, height = 600, backgroundColor = 0x000000, frameRate = 60)]

var sim:QuickBox2D = new QuickBox2D(this, {debug:true});
 
var sw:Number = stage.stageWidth / 30;
var sh:Number = stage.stageHeight / 30;
			
sim.addBox({x:sw / 2, y:sh, width:sw - 3, height:1,  density:.0})
sim.addBox({x:sw / 2, y:0, width:sw - 3, height:1,  density:.0 })
sim.addBox({x:0, y:sh / 2, width:1, height:sh ,  density:.0})  
sim.addBox({x:sw, y:sh / 2, width:1, height:sh,  density:.0})  

var slider:QuickObject = sim.addBox({x:3, y:6, width:6, height:0.25, density:0, angle:0.05});
var pusher:QuickObject = sim.addBox({x:15, y:18, width:4, height:4, density:0});

sim.setDefault({collideConnected:false, frequencyHz:10});
var anchor:QuickObject = sim.addCircle({x:3, y:2, radius:0.2, density:0});

var m0:QuickObject = sim.addBox({x:3, y:5, width:1, height:0.25, fixedRotation:true});

var j0:QuickObject = sim.addJoint({type:"distance", a:anchor.body, b:m0.body});

var m1:QuickObject = sim.addBox({x:2.5, y:9.5, width:0.25, height:6, fixedRotation:true});
var m2:QuickObject = sim.addBox({x:3.5, y:9.5, width:0.25, height:6, fixedRotation:true});

var j1:QuickObject = sim.addJoint({type:"distance", a:m0.body, b:m1.body});
var j2:QuickObject = sim.addJoint({type:"distance", a:m0.body, b:m2.body});
var j3:QuickObject = sim.addJoint({type:"distance", a:m2.body, b:m1.body});

var boxes:Array = [];
var boxNum:int = 21;
var index:int = boxNum;
var filter:b2FilterData;
var lookupQuickObject:Dictionary = new Dictionary();
for (var i:int = 0; i<boxNum; i++){
	boxes[i] = sim.addBox({x:3, y:6.3 + i * 0.3, width:0.6, height:0.3,  friction:0.0, allowSleep:false});
	lookupQuickObject[boxes[i].shape] = boxes[i];
}

var m3:QuickObject = sim.addBox({x:3, y:12.25 , width:1, height:0.25, allowSleep:false,fixedRotation:true, groupIndex:-1});

var j4:QuickObject = sim.addJoint({type:"revolute", a:m1.body, b:m3.body});
var j5:QuickObject = sim.addJoint({type:"revolute", a:m2.body, b:m3.body});

var t:Number = Math.PI;
var dropBox:int = 0;
var prevBox:QuickObject;
var resetVec:b2Vec2 = new b2Vec2(0,0);

function setGroupIndex(b:QuickObject, index:int):void{
	filter = b.shape.GetFilterData();
	filter.groupIndex = index;
	b.shape.SetFilterData(filter);
}

sim.start();
sim.addEventListener(QuickBox2D.STEP, onStep);
function onStep(evt:Event):void{
	dropBox++;
	
	if (dropBox % 80 == 0){
		index--;
		if (index > -1){
			if (prevBox){
				setGroupIndex(prevBox, 1);
			}
			prevBox = boxes[index]
			setGroupIndex(boxes[index], -1);
			m3.y += 0.05;
		}
	}
	
	if (index < 0){
		if (pusher.x > 3.5){
			pusher.x -= 0.05;
		}
	}
	for (var i:int = 0; i<boxNum; i++){
		if (boxes[i].y > 22){
			boxes[i].angle = 0;
			boxes[i].y = -2;
			boxes[i].x = 1;
			boxes[i].body.SetLinearVelocity(resetVec);
		}
	}
	
	if (index > 0){
	  anchor.x = 6 + 3 * Math.cos(t);
	  t += 0.01;
	}
}

import com.actionsnippet.qbox.*;
import Box2D.Dynamics.*;
import Box2D.Collision.Shapes.*;
 
[SWF(width = 800, height = 600, backgroundColor = 0x222222, frameRate=60)]

var sim:QuickBox2D = new QuickBox2D(this, {debug:false});

var box:QuickObject = sim.addBox({x:12, y:16, width:3, height:3, density:0 , groupIndex:-1});
var circle:QuickObject = sim.addCircle({x:5, y:3, radius:0.5, groupIndex:-1});

// add another box to show that Box2D is still working
var littleBox:QuickObject = sim.addBox({x:12, y:3, width:1, height:1});

sim.createStageWalls();

sim.start();
sim.mouseDrag();

var filter:b2ContactFilter = new QuickContactFilter();
QuickContactFilter(filter).addGroupIndexCallback(onGroupNeg1, -1);

function onGroupNeg1(a:b2Shape, b:b2Shape):void{
	//trace("group index -1 had a collision");
	box.userData.alpha = 0.5;
	setTimeout(function():void{ box.userData.alpha = 1 }, 200);
}

sim.w.SetContactFilter(filter);

This snippet answers a question from a recent comment. The question was basically: “Can you detect a collision between two rigid bodies that have the same negative groupIndex?”. When you give QuickObjects a negative groupIndex it means that they will not collide with one another. So by default Box2D contacts will not be triggered when these two rigid bodies overlap. In order to determine if these rigid bodies overlap you need to implement a custom b2ContactFilter class. This may sound confusing but its actually very easy. You can take a look at the custom b2ContactFilter class used in this snippet here: QuickContactFilter.as – It’s important to note that QuickContactFilter.as is not yet part of QuickBox2D, so if you want to run this snippet you’ll need to download the .as file.

You can check out the swf here…