Simulating mud/slime with Box2D, bitmaps and filters

This is an attempt to simulate something like mud/slime with Box2D for a game concept I am trying do develop.
I won’t go much in detail with the tutorial because at the moment I am still fine-tuning the effect, but I am showing you a step by step process to achieve a slime-looking effect.
First, let’s create some static objects:
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package {
 import flash.display.Sprite;
 import flash.events.Event;
 import Box2D.Dynamics.*;
 import Box2D.Collision.*;
 import Box2D.Collision.Shapes.*;
 import Box2D.Common.Math.*;
 public class Main extends Sprite {
  private var world:b2World=new b2World(new b2Vec2(0,5),true);
  private var worldScale:Number=30;
  public function Main() {
   debugDraw();
   var polygonShape:b2PolygonShape = new b2PolygonShape();
   var fixtureDef:b2FixtureDef = new b2FixtureDef();
   fixtureDef.restitution=0;
   fixtureDef.density=1;
   fixtureDef.friction=0.4;
   var bodyDef:b2BodyDef= new b2BodyDef();
   bodyDef.position.Set(320/worldScale,240/worldScale);
   var container:b2Body;
   container=world.CreateBody(bodyDef);
   polygonShape.SetAsOrientedBox(132/worldScale/2,12/worldScale/2,new b2Vec2(0/worldScale,144/worldScale),0);
   fixtureDef.shape=polygonShape;
   container.CreateFixture(fixtureDef);
   polygonShape.SetAsOrientedBox(12/worldScale/2,200/worldScale/2,new b2Vec2(-60/worldScale,0/worldScale),0);
   fixtureDef.shape=polygonShape;
   container.CreateFixture(fixtureDef);
   polygonShape.SetAsOrientedBox(12/worldScale/2,276/worldScale/2,new b2Vec2(60/worldScale,0/worldScale),0);
   fixtureDef.shape=polygonShape;
   container.CreateFixture(fixtureDef);
   addEventListener(Event.ENTER_FRAME,update);
  }
  private function debugDraw():void {
   var debugDraw:b2DebugDraw = new b2DebugDraw();
   var debugSprite:Sprite = new Sprite();
   addChild(debugSprite);
   debugDraw.SetSprite(debugSprite);
   debugDraw.SetDrawScale(worldScale);
   debugDraw.SetFlags(b2DebugDraw.e_shapeBit|b2DebugDraw.e_jointBit);
   debugDraw.SetFillAlpha(0.5);
   world.SetDebugDraw(debugDraw);
  }
  private function update(e:Event):void {
   world.Step(1/30,6,2);
   world.ClearForces();
   world.DrawDebugData();
  }
 }
}
I am just creating some kind of container with a hole in the bottom.
Then, I skin the container with a transparent image and create one little ball at every frame, someone would call them particle. I remove them when they leave the stage to the bottom, and I don’t place more than 200 balls at the same time, although I was able to place about 300 of them without stressing that much my CPU.
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package {
 import flash.display.Sprite;
 import flash.events.Event;
 import Box2D.Dynamics.*;
 import Box2D.Collision.*;
 import Box2D.Collision.Shapes.*;
 import Box2D.Common.Math.*;
 public class Main extends Sprite {
  private var world:b2World=new b2World(new b2Vec2(0,5),true);
  private var worldScale:Number=30;
  public function Main() {
   debugDraw();
   var backgroundImage:BackgroundImage=new BackgroundImage();
   addChild(backgroundImage);
   var polygonShape:b2PolygonShape = new b2PolygonShape();
   var fixtureDef:b2FixtureDef = new b2FixtureDef();
   fixtureDef.restitution=0;
   fixtureDef.density=1;
   fixtureDef.friction=0.4;
   var bodyDef:b2BodyDef= new b2BodyDef();
   bodyDef.position.Set(320/worldScale,240/worldScale);
   var container:b2Body;
   container=world.CreateBody(bodyDef);
   polygonShape.SetAsOrientedBox(132/worldScale/2,12/worldScale/2,new b2Vec2(0/worldScale,144/worldScale),0);
   fixtureDef.shape=polygonShape;
   container.CreateFixture(fixtureDef);
   polygonShape.SetAsOrientedBox(12/worldScale/2,200/worldScale/2,new b2Vec2(-60/worldScale,0/worldScale),0);
   fixtureDef.shape=polygonShape;
   container.CreateFixture(fixtureDef);
   polygonShape.SetAsOrientedBox(12/worldScale/2,276/worldScale/2,new b2Vec2(60/worldScale,0/worldScale),0);
   fixtureDef.shape=polygonShape;
   container.CreateFixture(fixtureDef);
   addEventListener(Event.ENTER_FRAME,update);
  }
  private function addCircle(pX:Number,pY:Number,h:Number):void {
   var circleShape:b2CircleShape = new b2CircleShape(h/worldScale);
   var fixtureDef:b2FixtureDef = new b2FixtureDef();
   fixtureDef.density=3;
   fixtureDef.friction=0.4;
   fixtureDef.restitution=0;
   fixtureDef.shape=circleShape;
   var bodyDef:b2BodyDef = new b2BodyDef();
   bodyDef.type=b2Body.b2_dynamicBody;
   bodyDef.position.Set(pX/worldScale,pY/worldScale);
   bodyDef.userData=new WaterCircle();
   var box:b2Body=world.CreateBody(bodyDef);
   box.CreateFixture(fixtureDef);
  }
  private function debugDraw():void {
   var debugDraw:b2DebugDraw = new b2DebugDraw();
   var debugSprite:Sprite = new Sprite();
   addChild(debugSprite);
   debugDraw.SetSprite(debugSprite);
   debugDraw.SetDrawScale(worldScale);
   debugDraw.SetFlags(b2DebugDraw.e_shapeBit|b2DebugDraw.e_jointBit);
   debugDraw.SetFillAlpha(0.5);
   world.SetDebugDraw(debugDraw);
  }
  private function update(e:Event):void {
   var waterParticles:Number=0;
   world.Step(1/30,6,2);
   world.ClearForces();
   for (var b:b2Body = world.GetBodyList(); b; b = b.GetNext()) {
    if (b.GetUserData()!=null) {
     waterParticles++;
     if (b.GetPosition().y*worldScale>480) {
      world.DestroyBody(b);
     }
    }
   }
   if (waterParticles<300) {
    addCircle(320+1-Math.random()*2,-10,5);
   }
   world.DrawDebugData();
  }
 }
}
At this time I have a continuous flux of balls falling down the container.
Next steps consists in rendering balls on a separate DisplayObject using a Bitmap
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package {
 import flash.display.Sprite;
 import flash.events.Event;
 import flash.display.BitmapData;
 import flash.display.Bitmap;
 import flash.geom.Matrix;
 import Box2D.Dynamics.*;
 import Box2D.Collision.*;
 import Box2D.Collision.Shapes.*;
 import Box2D.Common.Math.*;
 public class Main extends Sprite {
  private var world:b2World=new b2World(new b2Vec2(0,5),true);
  private var worldScale:Number=30;
  private var waterBitmapData:BitmapData=new BitmapData(640,480,false,0xFF333333);
  private var waterBitmap:Bitmap=new Bitmap(waterBitmapData);
  private var waterMatrix:Matrix=new Matrix();
  public function Main() {
   addChild(waterBitmap);
   var backgroundImage:BackgroundImage=new BackgroundImage();
   addChild(backgroundImage);
   var polygonShape:b2PolygonShape = new b2PolygonShape();
   var fixtureDef:b2FixtureDef = new b2FixtureDef();
   fixtureDef.restitution=0;
   fixtureDef.density=1;
   fixtureDef.friction=0.4;
   var bodyDef:b2BodyDef= new b2BodyDef();
   bodyDef.position.Set(320/worldScale,240/worldScale);
   var container:b2Body;
   container=world.CreateBody(bodyDef);
   polygonShape.SetAsOrientedBox(132/worldScale/2,12/worldScale/2,new b2Vec2(0/worldScale,144/worldScale),0);
   fixtureDef.shape=polygonShape;
   container.CreateFixture(fixtureDef);
   polygonShape.SetAsOrientedBox(12/worldScale/2,200/worldScale/2,new b2Vec2(-60/worldScale,0/worldScale),0);
   fixtureDef.shape=polygonShape;
   container.CreateFixture(fixtureDef);
   polygonShape.SetAsOrientedBox(12/worldScale/2,276/worldScale/2,new b2Vec2(60/worldScale,0/worldScale),0);
   fixtureDef.shape=polygonShape;
   container.CreateFixture(fixtureDef);
   addEventListener(Event.ENTER_FRAME,update);
  }
  private function addCircle(pX:Number,pY:Number,h:Number):void {
   var circleShape:b2CircleShape = new b2CircleShape(h/worldScale);
   var fixtureDef:b2FixtureDef = new b2FixtureDef();
   fixtureDef.density=3;
   fixtureDef.friction=0.4;
   fixtureDef.restitution=0;
   fixtureDef.shape=circleShape;
   var bodyDef:b2BodyDef = new b2BodyDef();
   bodyDef.type=b2Body.b2_dynamicBody;
   bodyDef.position.Set(pX/worldScale,pY/worldScale);
   bodyDef.userData=new WaterCircle();
   var box:b2Body=world.CreateBody(bodyDef);
   box.CreateFixture(fixtureDef);
  }
  private function debugDraw():void {
   var debugDraw:b2DebugDraw = new b2DebugDraw();
   var debugSprite:Sprite = new Sprite();
   addChild(debugSprite);
   debugDraw.SetSprite(debugSprite);
   debugDraw.SetDrawScale(worldScale);
   debugDraw.SetFlags(b2DebugDraw.e_shapeBit|b2DebugDraw.e_jointBit);
   debugDraw.SetFillAlpha(0.5);
   world.SetDebugDraw(debugDraw);
  }
  private function update(e:Event):void {
   var waterParticles:Number=0;
   waterBitmapData.fillRect(waterBitmapData.rect,0xFF333333);
   world.Step(1/30,6,2);
   world.ClearForces();
   for (var b:b2Body = world.GetBodyList(); b; b = b.GetNext()) {
    if (b.GetUserData()!=null) {
     waterParticles++;
     if (b.GetPosition().y*worldScale>480) {
      world.DestroyBody(b);
     }
     else {
      waterMatrix.tx=b.GetPosition().x*worldScale;
      waterMatrix.ty=b.GetPosition().y*worldScale;
      waterBitmapData.draw(b.GetUserData(),waterMatrix);
     }
    }
   }
   if (waterParticles<300) {
    addCircle(320+1-Math.random()*2,-10,5);
   }
   world.DrawDebugData();
  }
 }
}
Now I can remove the debug draw as all Box2D assets have their own DisplayObject representing them
And now, since I am using a Bitmap, I can add a blur filter to balls
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package {
 import flash.display.Sprite;
 import flash.events.Event;
 import flash.display.BitmapData;
 import flash.display.Bitmap;
 import flash.geom.Matrix;
 import flash.filters.BlurFilter;
 import flash.filters.BitmapFilterQuality;
 import flash.geom.Point;
 import Box2D.Dynamics.*;
 import Box2D.Collision.*;
 import Box2D.Collision.Shapes.*;
 import Box2D.Common.Math.*;
 public class Main extends Sprite {
  private var world:b2World=new b2World(new b2Vec2(0,5),true);
  private var worldScale:Number=30;
  private var waterBitmapData:BitmapData=new BitmapData(640,480,false,0xFF333333);
  private var waterBitmap:Bitmap=new Bitmap(waterBitmapData);
  private var waterMatrix:Matrix=new Matrix();
  private var waterBlur:BlurFilter=new BlurFilter(15,15,flash.filters.BitmapFilterQuality.HIGH);
  public function Main() {
   addChild(waterBitmap);
   var backgroundImage:BackgroundImage=new BackgroundImage();
   addChild(backgroundImage);
   var polygonShape:b2PolygonShape = new b2PolygonShape();
   var fixtureDef:b2FixtureDef = new b2FixtureDef();
   fixtureDef.restitution=0;
   fixtureDef.density=1;
   fixtureDef.friction=0.4;
   var bodyDef:b2BodyDef= new b2BodyDef();
   bodyDef.position.Set(320/worldScale,240/worldScale);
   var container:b2Body;
   container=world.CreateBody(bodyDef);
   polygonShape.SetAsOrientedBox(132/worldScale/2,12/worldScale/2,new b2Vec2(0/worldScale,144/worldScale),0);
   fixtureDef.shape=polygonShape;
   container.CreateFixture(fixtureDef);
   polygonShape.SetAsOrientedBox(12/worldScale/2,200/worldScale/2,new b2Vec2(-60/worldScale,0/worldScale),0);
   fixtureDef.shape=polygonShape;
   container.CreateFixture(fixtureDef);
   polygonShape.SetAsOrientedBox(12/worldScale/2,276/worldScale/2,new b2Vec2(60/worldScale,0/worldScale),0);
   fixtureDef.shape=polygonShape;
   container.CreateFixture(fixtureDef);
   addEventListener(Event.ENTER_FRAME,update);
  }
  private function addCircle(pX:Number,pY:Number,h:Number):void {
   var circleShape:b2CircleShape = new b2CircleShape(h/worldScale);
   var fixtureDef:b2FixtureDef = new b2FixtureDef();
   fixtureDef.density=3;
   fixtureDef.friction=0.4;
   fixtureDef.restitution=0;
   fixtureDef.shape=circleShape;
   var bodyDef:b2BodyDef = new b2BodyDef();
   bodyDef.type=b2Body.b2_dynamicBody;
   bodyDef.position.Set(pX/worldScale,pY/worldScale);
   bodyDef.userData=new WaterCircle();
   var box:b2Body=world.CreateBody(bodyDef);
   box.CreateFixture(fixtureDef);
  }
  private function debugDraw():void {
   var debugDraw:b2DebugDraw = new b2DebugDraw();
   var debugSprite:Sprite = new Sprite();
   addChild(debugSprite);
   debugDraw.SetSprite(debugSprite);
   debugDraw.SetDrawScale(worldScale);
   debugDraw.SetFlags(b2DebugDraw.e_shapeBit|b2DebugDraw.e_jointBit);
   debugDraw.SetFillAlpha(0.5);
   world.SetDebugDraw(debugDraw);
  }
  private function update(e:Event):void {
   var waterParticles:Number=0;
   waterBitmapData.fillRect(waterBitmapData.rect,0xFF333333);
   world.Step(1/30,6,2);
   world.ClearForces();
   for (var b:b2Body = world.GetBodyList(); b; b = b.GetNext()) {
    if (b.GetUserData()!=null) {
     waterParticles++;
     if (b.GetPosition().y*worldScale>480) {
      world.DestroyBody(b);
     }
     else {
      waterMatrix.tx=b.GetPosition().x*worldScale;
      waterMatrix.ty=b.GetPosition().y*worldScale;
      waterBitmapData.draw(b.GetUserData(),waterMatrix);
     }
    }
   }
   if (waterParticles<300) {
    addCircle(320+1-Math.random()*2,-10,5);
   }
   waterBitmapData.applyFilter(waterBitmapData,waterBitmapData.rect,new Point(0,0),waterBlur);
  }
 }
}
At this time I’m getting a “smoke” effect, but this is not what I want
So the final step is to add a threshold to change some colors of my bitmap balls/smoke, play a bit with transparency and add a better background
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package {
 import flash.display.Sprite;
 import flash.events.Event;
 import flash.display.BitmapData;
 import flash.display.Bitmap;
 import flash.geom.Matrix;
 import flash.filters.BlurFilter;
 import flash.filters.BitmapFilterQuality;
 import flash.geom.Point;
 import Box2D.Dynamics.*;
 import Box2D.Collision.*;
 import Box2D.Collision.Shapes.*;
 import Box2D.Common.Math.*;
 public class Main extends Sprite {
  private var world:b2World=new b2World(new b2Vec2(0,5),true);
  private var worldScale:Number=30;
  private var waterBitmapData:BitmapData=new BitmapData(640,480,true,0x00000000);
  private var waterBitmap:Bitmap=new Bitmap(waterBitmapData);
  private var waterMatrix:Matrix=new Matrix();
  private var waterBlur:BlurFilter=new BlurFilter(15,15,flash.filters.BitmapFilterQuality.HIGH);
  public function Main() {
   var realBg:RealBg=new RealBg();
   addChild(realBg);
   addChild(waterBitmap);
   var backgroundImage:BackgroundImage=new BackgroundImage();
   addChild(backgroundImage);
   var polygonShape:b2PolygonShape = new b2PolygonShape();
   var fixtureDef:b2FixtureDef = new b2FixtureDef();
   fixtureDef.restitution=0;
   fixtureDef.density=1;
   fixtureDef.friction=0.4;
   var bodyDef:b2BodyDef= new b2BodyDef();
   bodyDef.position.Set(320/worldScale,240/worldScale);
   var container:b2Body;
   container=world.CreateBody(bodyDef);
   polygonShape.SetAsOrientedBox(132/worldScale/2,12/worldScale/2,new b2Vec2(0/worldScale,144/worldScale),0);
   fixtureDef.shape=polygonShape;
   container.CreateFixture(fixtureDef);
   polygonShape.SetAsOrientedBox(12/worldScale/2,200/worldScale/2,new b2Vec2(-60/worldScale,0/worldScale),0);
   fixtureDef.shape=polygonShape;
   container.CreateFixture(fixtureDef);
   polygonShape.SetAsOrientedBox(12/worldScale/2,276/worldScale/2,new b2Vec2(60/worldScale,0/worldScale),0);
   fixtureDef.shape=polygonShape;
   container.CreateFixture(fixtureDef);
   addEventListener(Event.ENTER_FRAME,update);
  }
  private function addCircle(pX:Number,pY:Number,h:Number):void {
   var circleShape:b2CircleShape = new b2CircleShape(h/worldScale);
   var fixtureDef:b2FixtureDef = new b2FixtureDef();
   fixtureDef.density=3;
   fixtureDef.friction=0.4;
   fixtureDef.restitution=0;
   fixtureDef.shape=circleShape;
   var bodyDef:b2BodyDef = new b2BodyDef();
   bodyDef.type=b2Body.b2_dynamicBody;
   bodyDef.position.Set(pX/worldScale,pY/worldScale);
   bodyDef.userData=new WaterCircle();
   var box:b2Body=world.CreateBody(bodyDef);
   box.CreateFixture(fixtureDef);
  }
  private function debugDraw():void {
   var debugDraw:b2DebugDraw = new b2DebugDraw();
   var debugSprite:Sprite = new Sprite();
   addChild(debugSprite);
   debugDraw.SetSprite(debugSprite);
   debugDraw.SetDrawScale(worldScale);
   debugDraw.SetFlags(b2DebugDraw.e_shapeBit|b2DebugDraw.e_jointBit);
   debugDraw.SetFillAlpha(0.5);
   world.SetDebugDraw(debugDraw);
  }
  private function update(e:Event):void {
   var waterParticles:Number=0;
   waterBitmapData.fillRect(waterBitmapData.rect,0x00000000);
   world.Step(1/30,6,2);
   world.ClearForces();
   for (var b:b2Body = world.GetBodyList(); b; b = b.GetNext()) {
    if (b.GetUserData()!=null) {
     waterParticles++;
     if (b.GetPosition().y*worldScale>480) {
      world.DestroyBody(b);
     }
     else {
      waterMatrix.tx=b.GetPosition().x*worldScale;
      waterMatrix.ty=b.GetPosition().y*worldScale;
      waterBitmapData.draw(b.GetUserData(),waterMatrix);
     }
    }
   }
   if (waterParticles<300) {
    addCircle(320+1-Math.random()*2,-10,5);
   }
   waterBitmapData.applyFilter(waterBitmapData,waterBitmapData.rect,new Point(0,0),waterBlur);
   waterBitmapData.threshold(waterBitmapData,waterBitmapData.rect,new Point(0,0),">",0X11444444,0x88FF00FF,0xFFFFFFFF, true);
  }
 }
}
And that’s it! Not the best slime ever seen, but it’s dynamically generated by Box2D, this means it has a real mass, which is quite awesome.
How would you improve it?
Download the source code of the last example.
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