Talking about Actionscript 3, Box2D and Flash.
I was thinking about a game concept which features physics and a rotating room, so I asked to myself how to make a rotating room with Box2D, and what is the best way to do it.
These are the three ways I tried.
1 – “Manually” rotating a static room
In my first attempt I built a static room with a compound object (see the theory of compound objects in this post), then I rotated the object at every frame.
This is the source code:
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 Main extends Sprite {
private var world:b2World=new b2World(new b2Vec2(0,10),true);
private var worldScale:Number=30;
private var container:b2Body;
private var containerRotation:Number=0;
private var containerStep:Number=Math.PI/1000;
public function Main() {
debugDraw();
var polygonShape:b2PolygonShape = new b2PolygonShape();
var fixtureDef:b2FixtureDef = new b2FixtureDef();
fixtureDef.restitution=0.5;
fixtureDef.density=1
var bodyDef:b2BodyDef= new b2BodyDef();
bodyDef.position.Set(240/worldScale,240/worldScale);
container=world.CreateBody(bodyDef);
polygonShape.SetAsOrientedBox(480/worldScale/2,12/worldScale/2,new b2Vec2(0/worldScale,-234/worldScale),0);
fixtureDef.shape=polygonShape;
container.CreateFixture(fixtureDef);
polygonShape.SetAsOrientedBox(480/worldScale/2,12/worldScale/2,new b2Vec2(0/worldScale,234/worldScale),0);
fixtureDef.shape=polygonShape;
container.CreateFixture(fixtureDef);
polygonShape.SetAsOrientedBox(12/worldScale/2,456/worldScale/2,new b2Vec2(-234/worldScale,0/worldScale),0);
fixtureDef.shape=polygonShape;
container.CreateFixture(fixtureDef);
polygonShape.SetAsOrientedBox(12/worldScale/2,456/worldScale/2,new b2Vec2(234/worldScale,0/worldScale),0);
fixtureDef.shape=polygonShape;
container.CreateFixture(fixtureDef);
for (var i:Number=0; i<12; i++) {
addBox(Math.random()*300+50,Math.random()*300+50,38,38,true);
}
addEventListener(Event.ENTER_FRAME,update);
}
private function addBox(pX:Number,pY:Number,w:Number,h:Number,d:Boolean):void {
var polygonShape:b2PolygonShape = new b2PolygonShape();
polygonShape.SetAsBox(w/worldScale/2,h/worldScale/2);
var fixtureDef:b2FixtureDef = new b2FixtureDef();
fixtureDef.density=1;
fixtureDef.friction=1;
fixtureDef.restitution=0.5;
fixtureDef.shape=polygonShape;
var bodyDef:b2BodyDef = new b2BodyDef();
if (d) {
bodyDef.type=b2Body.b2_dynamicBody;
}
bodyDef.position.Set(pX/worldScale,pY/worldScale);
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 {
containerRotation-=containerStep
var mat22:b2Mat22=new b2Mat22();
mat22.Set(containerRotation)
var boxTransform:b2Transform=new b2Transform(container.GetPosition(),mat22);
container.SetTransform(boxTransform);
world.Step(1/30, 10, 10);
world.ClearForces();
world.DrawDebugData();
}
}
}
Lines 18-36: create the static room object
Lines 37-39: add some stuff
Lines 69-73: rotate the room object using SetTransform method
This is the result:
I did not like it because sometimes the stuff inside the room sleeps and messes up everything. Also, too much hassle with radians, b2Mat22 types, and positions.
2 - Pinning the room to a static body with a revolute joint
In this example the room is a dynamic object pinned to a static body (a sphere) with a revolute joint, which also has a motor
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 Main extends Sprite {
private var world:b2World=new b2World(new b2Vec2(0,10),true);
private var worldScale:Number=30;
public function Main() {
debugDraw();
var polygonShape:b2PolygonShape = new b2PolygonShape();
var fixtureDef:b2FixtureDef = new b2FixtureDef();
fixtureDef.restitution=0.5;
fixtureDef.density=1
var bodyDef:b2BodyDef= new b2BodyDef();
bodyDef.position.Set(240/worldScale,240/worldScale);
bodyDef.type=b2Body.b2_dynamicBody;
var container:b2Body;
container=world.CreateBody(bodyDef);
polygonShape.SetAsOrientedBox(480/worldScale/2,12/worldScale/2,new b2Vec2(0/worldScale,-234/worldScale),0);
fixtureDef.shape=polygonShape;
container.CreateFixture(fixtureDef);
polygonShape.SetAsOrientedBox(480/worldScale/2,12/worldScale/2,new b2Vec2(0/worldScale,234/worldScale),0);
fixtureDef.shape=polygonShape;
container.CreateFixture(fixtureDef);
polygonShape.SetAsOrientedBox(12/worldScale/2,456/worldScale/2,new b2Vec2(-234/worldScale,0/worldScale),0);
fixtureDef.shape=polygonShape;
container.CreateFixture(fixtureDef);
polygonShape.SetAsOrientedBox(12/worldScale/2,456/worldScale/2,new b2Vec2(234/worldScale,0/worldScale),0);
fixtureDef.shape=polygonShape;
container.CreateFixture(fixtureDef);
var circleShape:b2CircleShape=new b2CircleShape(0.5);
fixtureDef.shape=circleShape;
fixtureDef.isSensor=true;
bodyDef.type=b2Body.b2_staticBody;
bodyDef.position.Set(240/worldScale,240/worldScale);
var jointPin:b2Body=world.CreateBody(bodyDef);
jointPin.CreateFixture(fixtureDef);
var containerJoint:b2RevoluteJointDef=new b2RevoluteJointDef();
var anchorA:b2Vec2=new b2Vec2(0,0)
var anchorB:b2Vec2=new b2Vec2(0,0)
containerJoint.localAnchorA.Set(anchorA.x,anchorA.y);
containerJoint.localAnchorB.Set(anchorB.x,anchorB.y);
containerJoint.bodyA=container;
containerJoint.bodyB=jointPin;
containerJoint.enableMotor=true;
containerJoint.maxMotorTorque=50000;
containerJoint.motorSpeed=0.1;
world.CreateJoint(containerJoint);
for (var i:Number=0; i<12; i++) {
addBox(Math.random()*300+50,Math.random()*300+50,38,38,true);
}
addEventListener(Event.ENTER_FRAME,update);
}
private function addBox(pX:Number,pY:Number,w:Number,h:Number,d:Boolean):void {
var polygonShape:b2PolygonShape = new b2PolygonShape();
polygonShape.SetAsBox(w/worldScale/2,h/worldScale/2);
var fixtureDef:b2FixtureDef = new b2FixtureDef();
fixtureDef.density=1;
fixtureDef.friction=1;
fixtureDef.restitution=0.5;
fixtureDef.shape=polygonShape;
var bodyDef:b2BodyDef = new b2BodyDef();
if (d) {
bodyDef.type=b2Body.b2_dynamicBody;
}
bodyDef.position.Set(pX/worldScale,pY/worldScale);
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 {
world.Step(1/30, 10, 10);
world.ClearForces();
world.DrawDebugData();
}
}
}
Line 21: Makes the room a dynamic body
Lines 36-40: Create the static sphere, also set as a sensor to avoid collisions with the stuff inside the room
Lines 41-53: Pin the room to the sphere with a revolute joint and set a motor to make it rotate.
All transform-related code in update function should also be removed.
This is the result:
Now it works well but I wanted to remove the unnecessary sphere, so I decided to move onto step 3
3 - Pinning the room on the "ground" with a revolute joint
Box2D world also have a "ground" object (in this case I would call it "wall") which can be used to pin the dynamic body and apply the same revolute joint:
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 Main extends Sprite {
private var world:b2World=new b2World(new b2Vec2(0,10),true);
private var worldScale:Number=30;
public function Main() {
debugDraw();
var polygonShape:b2PolygonShape = new b2PolygonShape();
var fixtureDef:b2FixtureDef = new b2FixtureDef();
fixtureDef.restitution=0.5;
fixtureDef.density=1
var bodyDef:b2BodyDef= new b2BodyDef();
bodyDef.position.Set(240/worldScale,240/worldScale);
bodyDef.type=b2Body.b2_dynamicBody;
var container:b2Body;
container=world.CreateBody(bodyDef);
polygonShape.SetAsOrientedBox(480/worldScale/2,12/worldScale/2,new b2Vec2(0/worldScale,-234/worldScale),0);
fixtureDef.shape=polygonShape;
container.CreateFixture(fixtureDef);
polygonShape.SetAsOrientedBox(480/worldScale/2,12/worldScale/2,new b2Vec2(0/worldScale,234/worldScale),0);
fixtureDef.shape=polygonShape;
container.CreateFixture(fixtureDef);
polygonShape.SetAsOrientedBox(12/worldScale/2,456/worldScale/2,new b2Vec2(-234/worldScale,0/worldScale),0);
fixtureDef.shape=polygonShape;
container.CreateFixture(fixtureDef);
polygonShape.SetAsOrientedBox(12/worldScale/2,456/worldScale/2,new b2Vec2(234/worldScale,0/worldScale),0);
fixtureDef.shape=polygonShape;
container.CreateFixture(fixtureDef);
var containerJoint:b2RevoluteJointDef=new b2RevoluteJointDef();
var anchorA:b2Vec2=new b2Vec2(0,0)
var anchorB:b2Vec2=new b2Vec2(240/worldScale,240/worldScale)
containerJoint.localAnchorA.Set(anchorA.x,anchorA.y);
containerJoint.localAnchorB.Set(anchorB.x,anchorB.y);
containerJoint.bodyA=container;
containerJoint.bodyB=world.GetGroundBody();
containerJoint.enableMotor=true;
containerJoint.maxMotorTorque=50000;
containerJoint.motorSpeed=0.1;
world.CreateJoint(containerJoint);
for (var i:Number=0; i<12; i++) {
addBox(Math.random()*300+50,Math.random()*300+50,38,38,true);
}
addEventListener(Event.ENTER_FRAME,update);
}
private function addBox(pX:Number,pY:Number,w:Number,h:Number,d:Boolean):void {
var polygonShape:b2PolygonShape = new b2PolygonShape();
polygonShape.SetAsBox(w/worldScale/2,h/worldScale/2);
var fixtureDef:b2FixtureDef = new b2FixtureDef();
fixtureDef.density=1;
fixtureDef.friction=1;
fixtureDef.restitution=0.5;
fixtureDef.shape=polygonShape;
var bodyDef:b2BodyDef = new b2BodyDef();
if (d) {
bodyDef.type=b2Body.b2_dynamicBody;
}
bodyDef.position.Set(pX/worldScale,pY/worldScale);
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 {
world.Step(1/30, 10, 10);
world.ClearForces();
world.DrawDebugData();
}
}
}
On line 42 you can see how to get the ground body.
And this is the final result:
I hope it can inspire you some new game design ideas.
Download the source code of the last example.
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