Create REAL explosions with Box2D – adding textures

Ok, you asked it, now you have it. Now the Box2D explosion prototype features bitmap textures and a commented source code.

I have to say a big thank you to Antoan Angelov for his version of the slicing engine as he allowed me to manage textures with no hassle.

So now basically an explosion is a number of cuts at a random angle, all passing for the same point, which is the origin of the explosion. Then, every resulting slice will have a linear velocity according to the distance between the center of the slice mass and the origin of the explosion.

So this is what you’ll get:

Click on a crate to make it explode.

And this is the full, commented source code:

package {
	import flash.display.Sprite;
	import flash.events.Event;
	import flash.events.MouseEvent;
	import flash.geom.Matrix;
	import flash.display.BitmapData;
	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(0,10),true);
		private var enterPointsVec:Vector. = new Vector.();
		private var numEnterPoints:int=0;
		private var worldScale:Number=30;
		// variables used in the explosion process:
		// the vector of exploding bodies
		private var explodingBodies:Vector.;
		// the number of cuts for every explosion
		private var explosionCuts:Number=5;
		// explosion x and y center
		private var explosionX:Number;
		private var explosionY:Number;
		// explosion radius, useful to determine the velocity of debris
		private var explosionRadius:Number=50;
		public function Main() {
			// calling the debug draw. This is used to show you the bitmaps are correctly applied,
			// and because I did not want to draw the walls :)
			debugDraw();
			// this is the BitmapData representation of my 100x100 pixels crate image
			// check the library to see both the raw image and CrateImage Sprite
			var crateBitmap:BitmapData=new BitmapData(100,100);
			crateBitmap.draw(new CrateImage());
			// adding the four static, undestroyable walls
			addWall(320,480,640,20);
			addWall(320,0,640,20);
			addWall(0,240,20,480);
			addWall(640,240,20,480);
			// this vector stores the clockwise local coordinates of the 100x100 pixels crate
			var crateCoordVector:Vector.=new [new b2Vec2(-50,-50),new b2Vec2(50,-50),new b2Vec2(50,50),new b2Vec2(-50,50)];
			// then createBody builds the final body and applies the bitmap.
			// the first two arguments are the X and Y position of the center of the crate, in pixels
			createBody(95,420,crateCoordVector,crateBitmap);
			createBody(245,420,crateCoordVector,crateBitmap);
			createBody(395,420,crateCoordVector,crateBitmap);
			createBody(545,420,crateCoordVector,crateBitmap);
			createBody(170,320,crateCoordVector,crateBitmap);
			createBody(320,320,crateCoordVector,crateBitmap);
			createBody(470,320,crateCoordVector,crateBitmap);
			createBody(245,220,crateCoordVector,crateBitmap);
			createBody(395,220,crateCoordVector,crateBitmap);
			createBody(320,120,crateCoordVector,crateBitmap);
			// You can see the reason for creating the enterPointsVec in the coments in the intersection() method.
			enterPointsVec=new Vector.(numEnterPoints);
			// listeners
			stage.addEventListener(MouseEvent.MOUSE_DOWN, boom);
			addEventListener(Event.ENTER_FRAME, update);
		}
		// my old friend debugDraw function
		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);
		}
		// this function returns the body at a X,Y coordinate without using a temp body like the one in
		// the original Box2D distribution. It uses QueryPoint method.
		// returns the body ad X,Y coordinate or null
		private function GetBodyAtXY(coordinate:b2Vec2):b2Body {
			var touchedBody:b2Body=null;
			world.QueryPoint(GetBodyCallback,coordinate);
			function GetBodyCallback(fixture:b2Fixture):Boolean {
				var shape:b2Shape=fixture.GetShape();
				var inside:Boolean=shape.TestPoint(fixture.GetBody().GetTransform(),coordinate);
				if (inside) {
					touchedBody=fixture.GetBody();
					return false;
				}
				return true;
			}
			return touchedBody;
		}
		// simple function to add a static wall
		private function addWall(pX:Number,pY:Number,w:Number,h:Number):void {
			var wallShape:b2PolygonShape = new b2PolygonShape();
			wallShape.SetAsBox(w/worldScale/2,h/worldScale/2);
			var wallFixture:b2FixtureDef = new b2FixtureDef();
			wallFixture.density=0;
			wallFixture.friction=1;
			wallFixture.restitution=0.5;
			wallFixture.shape=wallShape;
			var wallBodyDef:b2BodyDef = new b2BodyDef();
			wallBodyDef.position.Set(pX/worldScale,pY/worldScale);
			var wall:b2Body=world.CreateBody(wallBodyDef);
			wall.CreateFixture(wallFixture);
			numEnterPoints++;
		}
		// function to create and texture a dynamic body
		private function createBody(xPos:Number, yPos:Number, verticesArr:Vector., texture:BitmapData) {
			// I need this temp vector to convert pixels coordinates to Box2D meters coordinates
			var vec:Vector.=new Vector.();
			for (var i:Number=0; i();
				explodingBodies.push(clickedBody);
				// the explosion begins!
				for (var i:Number=1; i<=explosionCuts; i++) {
					// choosing a random angle
					cutAngle=Math.random()*Math.PI*2;
					// creating the two points to be used for the raycast, according to the random angle and mouse position
					// also notice how I need to add a little offset (i/10) or Box2D will crash. Probably it's not able to 
					// determine raycast on objects whose area is very very close to zero (or zero)
					var p1:b2Vec2=new b2Vec2((explosionX+i/10-2000*Math.cos(cutAngle))/worldScale,(explosionY-2000*Math.sin(cutAngle))/worldScale);
					var p2:b2Vec2=new b2Vec2((explosionX+2000*Math.cos(cutAngle))/worldScale,(explosionY+2000*Math.sin(cutAngle))/worldScale);
					world.RayCast(intersection, p1, p2);
					world.RayCast(intersection, p2, p1);
					enterPointsVec=new Vector.(numEnterPoints);
				}
			}
		}
		// update function to simulate and render the world
		public function update(e:Event):void {
			world.Step(1/30, 10, 10);
			world.ClearForces();
			var spr:Sprite;
			for (var b:b2Body = world.GetBodyList(); b; b = b.GetNext()) {
				spr=b.GetUserData();
				if (spr) {
					spr.x=b.GetPosition().x*worldScale;
					spr.y=b.GetPosition().y*worldScale;
					spr.rotation=b.GetAngle()*180/Math.PI;
				}
			}
			world.DrawDebugData();
		}
		private function intersection(fixture:b2Fixture, point:b2Vec2, normal:b2Vec2, fraction:Number):Number {
			if (explodingBodies.indexOf(fixture.GetBody())!=-1) {
				var spr:Sprite=fixture.GetBody().GetUserData();
				// Throughout this whole code I use only one global vector, and that is enterPointsVec. Why do I need it you ask? 
				// Well, the problem is that the world.RayCast() method calls this function only when it sees that a given line gets into the body - it doesnt see when the line gets out of it.
				// I must have 2 intersection points with a body so that it can be sliced, thats why I use world.RayCast() again, but this time from B to A - that way the point, at which BA enters the body is the point at which AB leaves it!
				// For that reason, I use a vector enterPointsVec, where I store the points, at which AB enters the body. And later on, if I see that BA enters a body, which has been entered already by AB, I fire the splitObj() function!
				// I need a unique ID for each body, in order to know where its corresponding enter point is - I store that id in the userData of each body.
				if (spr is userData) {
					var userD:userData=spr as userData;
					if (enterPointsVec[userD.id]) {
						// If this body has already had an intersection point, then it now has two intersection points, thus it must be split in two - thats where the splitObj() method comes in.
						splitObj(fixture.GetBody(), enterPointsVec[userD.id], point.Copy());
					}
					else {
						enterPointsVec[userD.id]=point;
					}
				}
			}
			return 1;
		}
		// function to get the area of a shape. I will remove tiny shape to increase performance
		private function getArea(vs:Vector., count:uint):Number {
			var area:Number=0.0;
			var p1X:Number=0.0;
			var p1Y:Number=0.0;
			var inv3:Number=1.0/3.0;
			for (var i:int = 0; i < count; ++i) {
				var p2:b2Vec2=vs[i];
				var p3:b2Vec2=i+1=poly.GetVertices(),numVertices:int=poly.GetVertexCount();
			var shape1Vertices:Vector. = new Vector.(), shape2Vertices:Vector. = new Vector.();
			var origUserData:userData=sliceBody.GetUserData(),origUserDataId:int=origUserData.id,d:Number;
			var polyShape:b2PolygonShape=new b2PolygonShape();
			var body:b2Body;
			// First, I destroy the original body and remove its Sprite representation from the childlist.
			world.DestroyBody(sliceBody);
			removeChild(origUserData);
			// The world.RayCast() method returns points in world coordinates, so I use the b2Body.GetLocalPoint() to convert them to local coordinates.;
			A=sliceBody.GetLocalPoint(A);
			B=sliceBody.GetLocalPoint(B);
			// I use shape1Vertices and shape2Vertices to store the vertices of the two new shapes that are about to be created. 
			// Since both point A and B are vertices of the two new shapes, I add them to both vectors.
			shape1Vertices.push(A, B);
			shape2Vertices.push(A, B);
			// I iterate over all vertices of the original body. ;
			// I use the function det() ("det" stands for "determinant") to see on which side of AB each point is standing on. The parameters it needs are the coordinates of 3 points:
			// - if it returns a value >0, then the three points are in clockwise order (the point is under AB)
			// - if it returns a value =0, then the three points lie on the same line (the point is on AB)
			// - if it returns a value <0, then the three points are in counter-clockwise order (the point is above AB). 
			for (var i:Number=0; i0) {
					shape1Vertices.push(verticesVec[i]);
				}
				else {
					shape2Vertices.push(verticesVec[i]);
				}
			}
			// In order to be able to create the two new shapes, I need to have the vertices arranged in clockwise order.
			// I call my custom method, arrangeClockwise(), which takes as a parameter a vector, representing the coordinates of the shape's vertices and returns a new vector, with the same points arranged clockwise.
			shape1Vertices=arrangeClockwise(shape1Vertices);
			shape2Vertices=arrangeClockwise(shape2Vertices);
			// setting the properties of the two newly created shapes
			var bodyDef:b2BodyDef = new b2BodyDef();
			bodyDef.type=b2Body.b2_dynamicBody;
			bodyDef.position=sliceBody.GetPosition();
			var fixtureDef:b2FixtureDef = new b2FixtureDef();
			fixtureDef.density=origFixture.GetDensity();
			fixtureDef.friction=origFixture.GetFriction();
			fixtureDef.restitution=origFixture.GetRestitution();
			// creating the first shape, if big enough
			if (getArea(shape1Vertices,shape1Vertices.length)>=0.05) {
				polyShape.SetAsVector(shape1Vertices);
				fixtureDef.shape=polyShape;
				bodyDef.userData=new userData(origUserDataId,shape1Vertices,origUserData.texture);
				addChild(bodyDef.userData);
				enterPointsVec[origUserDataId]=null;
				body=world.CreateBody(bodyDef);
				body.SetAngle(sliceBody.GetAngle());
				body.CreateFixture(fixtureDef);
				// setting a velocity for the debris
				body.SetLinearVelocity(setExplosionVelocity(body));
				// the shape will be also part of the explosion and can explode too
				explodingBodies.push(body);
			}
			// creating the second shape, if big enough
			if (getArea(shape2Vertices,shape2Vertices.length)>=0.05) {
				polyShape.SetAsVector(shape2Vertices);
				fixtureDef.shape=polyShape;
				bodyDef.userData=new userData(numEnterPoints,shape2Vertices,origUserData.texture);
				addChild(bodyDef.userData);
				enterPointsVec.push(null);
				numEnterPoints++;
				body=world.CreateBody(bodyDef);
				body.SetAngle(sliceBody.GetAngle());
				body.CreateFixture(fixtureDef);
				// setting a velocity for the debris
				body.SetLinearVelocity(setExplosionVelocity(body));
				// the shape will be also part of the explosion and can explode too
				explodingBodies.push(body);
			}
		}
		// this function will determine the velocity of the debris according
		// to the center of mass of the body and the distance from the explosion point
		private function setExplosionVelocity(b:b2Body):b2Vec2 {
			var distX:Number=b.GetWorldCenter().x*worldScale-explosionX;
			if (distX<0) {
				if (distX<-explosionRadius) {
					distX=0;
				}
				else {
					distX=- explosionRadius-distX;
				}
			}
			else {
				if (distX>explosionRadius) {
					distX=0;
				}
				else {
					distX=explosionRadius-distX;
				}
			}
			var distY:Number=b.GetWorldCenter().y*worldScale-explosionY;
			if (distY<0) {
				if (distY<-explosionRadius) {
					distY=0;
				}
				else {
					distY=- explosionRadius-distY;
				}
			}
			else {
				if (distY>explosionRadius) {
					distY=0;
				}
				else {
					distY=explosionRadius-distY;
				}
			}
			distX*=0.25;
			distY*=0.25;
			return new b2Vec2(distX,distY);
		}
		private function arrangeClockwise(vec:Vector.):Vector. {
			// The algorithm is simple: 
			// First, it arranges all given points in ascending order, according to their x-coordinate.
			// Secondly, it takes the leftmost and rightmost points (lets call them C and D), and creates tempVec, where the points arranged in clockwise order will be stored.
			// Then, it iterates over the vertices vector, and uses the det() method I talked about earlier. It starts putting the points above CD from the beginning of the vector, and the points below CD from the end of the vector. 
			// That was it!
			var n:int=vec.length,d:Number,i1:int=1,i2:int=n-1;
			var tempVec:Vector.=new Vector.(n),C:b2Vec2,D:b2Vec2;
			vec.sort(comp1);
			tempVec[0]=vec[0];
			C=vec[0];
			D=vec[n-1];
			for (var i:Number=1; ib.x) {
				return 1;
			}
			else if (a.x

and this is userData class:

package {
	import Box2D.Common.Math.b2Vec2;
	import flash.display.Sprite;
	import flash.display.BitmapData;
	import flash.geom.Matrix;
	public class userData extends Sprite {
		var id:int,texture:BitmapData;
		public function userData(id:int, verticesVec:Vector., texture:BitmapData) {
			this.id=id;
			this.texture=texture;
			// I use the matrix so that I can have the center of the shape I'm drawing match the center of the BitmapData image - I "move" the BitmapData projection left by half its width and up by half its height.
			var m:Matrix = new Matrix();
			m.tx=- texture.width*0.5;
			m.ty=- texture.height*0.5;
			// I then draw lines from each vertex to the next, in clockwise order and use the beginBitmapFill() method to add the texture.
			this.graphics.lineStyle(1);
			this.graphics.beginBitmapFill(texture, m, true, true);
			this.graphics.moveTo(verticesVec[0].x*30, verticesVec[0].y*30);
			for (var i:int=1; i

And now let me see what kind of games can you make using this concept.

Download the source code.

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// Stringy
// Sudoku
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// Survival Horror
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