Easy Popforge Setup
I will start by showing you what turns out to be an easy way to get started with the popforge library.
Step 1. Get a SVN Client. On the mac I suggest you check out Versions and on the PC there is the old faithful Tortoise.
Step 2. Check out the popforge source.
Step 3. Create a new AS Project in Flex (download here if you don’t have it yet). Click Next after entering a project name. In the source path window select add folder. Navigate to “pathToPopforge/flash/PopforgeLibrary/src” and click ok.
Step 4. Click finish.
Step 4. Rejoice because you are done!

Creating Motors
The motor features of a revolute joint provide a lot of functionality. Just look at the Crank/Gear/Pulley and Theo Jansen Walker test bed examples for a hint of their potential. When creating a motor the maxMotorTorque is the most import property. Measured in newton meters (N•m), when using the normal units (not converted to pixels as in my example), it determines the amount of mass the motor can rotate. If your motors is not rotating or rotating poorly you likely need to increase its maxMotorTorque. Simple Motor Example:

//Create and setup a revolute joint as a simple motor
pivot.position.Set(80,250);
var pivotBody1:b2Body = m_world.CreateBody(pivot);
pivotBody1.CreateShape(pivotPoly)
pivotBody1.SetMassFromShapes();

var motorPoly:b2PolygonDef = new b2PolygonDef();
motorPoly.SetAsOrientedBox(30, 10, new b2Vec2(0, 0), 0);
motorPoly.friction = .3;
motorPoly.density = .01;
motorPoly.restitution = .1;

var motorDef:b2BodyDef = new b2BodyDef();
motorDef.position.Set(110, 250);
var motorBody:b2Body = m_world.CreateBody(motorDef);
motorBody.CreateShape(motorPoly);
motorBody.SetMassFromShapes();

//Setup the revolute joint definition for later instantiation. rjd is a predefined b2RevoluteJointDef object.
rjd.Initialize(pivotBody1, motorBody, new b2Vec2(80,250));
rjd.motorSpeed = 3;
rjd.maxMotorTorque = 10000000;
rjd.enableMotor = true;
rjd.enableLimit = false;
//Create the motor joint and cast it as a b2RevoluteJoint
var joint1:b2RevoluteJoint = m_world.CreateJoint(rjd) as b2RevoluteJoint;

Limits
Revolute joints are not limited to continuous rotation. You can set upper and lower limits to the joint (in radians). When used with the enableMotor property set to false, this joint is ideal for simulating limbs. See the TestRagdoll example for a great demo of this.

Combining joint limits, motors and conditionals allows for creation of some interesting systems. I could see this used as valves, platforms in games, fins on fish, etc. In this example I have setup a motor to oscillate between its limits.

//Setup a limited revolute joint that oscillates between its limits
pivot.position.Set(80,100);
var pivotBody2:b2Body = m_world.CreateBody(pivot);
pivotBody2.CreateShape(pivotPoly)
pivotBody2.SetMassFromShapes();

var limitMotorPoly:b2PolygonDef = new b2PolygonDef();
limitMotorPoly.SetAsOrientedBox(30, 10, new b2Vec2(0, 0), 0);
limitMotorPoly.friction = .3;
limitMotorPoly.density = .01;
limitMotorPoly.restitution = .1;

var limitedMotorDef:b2BodyDef = new b2BodyDef();
limitedMotorDef.position.Set(110, 100);
var limitedMotorBody:b2Body = m_world.CreateBody(limitedMotorDef);
limitedMotorBody.CreateShape(limitMotorPoly);
limitedMotorBody.SetMassFromShapes();

//Setup the revolute joint definition for later instantiation
rjd.Initialize(pivotBody2, limitedMotorBody, new b2Vec2(80,100));
rjd.motorSpeed = 3;
rjd.lowerAngle = 0;
//Convert the angle in degrees to radians
rjd.upperAngle = 270*(Math.PI/180);
rjd.enableLimit = true;
rjd.maxMotorTorque = 10000000;
rjd.referenceAngle = 0;
rjd.enableMotor = true;
//Create the motor joint and cast it as a b2RevoluteJoint. Store the joint object in a class var for use later in the update method.
limitMotorJoint = m_world.CreateJoint(rjd) as b2RevoluteJoint;

Friction Joints
Friction joints are revolute joints with the motor enabled, a non-zero maxMotorTorque and the motor speed set to 0. This will cause the joint to resist rotation but give when the maxMotorTorque is exceeded, giving the effect of a joint with friction at the pivot. Once the force to the joint is removed it stops moving. Example:

//Setup a friction joint
pivot.position.Set(190,100);
var pivotBody3:b2Body = m_world.CreateBody(pivot);
pivotBody3.CreateShape(pivotPoly)
pivotBody3.SetMassFromShapes();

var armPoly:b2PolygonDef = new b2PolygonDef();
armPoly.SetAsOrientedBox(90, 10, new b2Vec2(0, 0), 0);
armPoly.friction = .5;
armPoly.density = .01;
armPoly.restitution=0;

var armBodyDef:b2BodyDef = new b2BodyDef();
armBodyDef.position.Set(280,100);
var armBody = m_world.CreateBody(armBodyDef);
armBody.CreateShape(armPoly)
armBody.SetMassFromShapes();

rjd.Initialize(pivotBody3, armBody, new b2Vec2(190,100));
rjd.motorSpeed = 0;
rjd.maxMotorTorque = 10000000;
rjd.enableMotor = true;
rjd.enableLimit = false;
var joint2:b2RevoluteJoint = m_world.CreateJoint(rjd) as b2RevoluteJoint;

Example:

Full Source not including Box2D

Creating Bodies
Previously when creating bodies you had to specify if the body was static or dynamic, this is no longer the case. The engine itself will determine the body type based on its density setting. If you set the density of the shapes to 0 the body will be static and any non-zero value will make it dynamic. Example:

//Create a static body
var staticPoly:b2PolygonDef = new b2PolygonDef();
staticPoly.SetAsOrientedBox(10, 10, new b2Vec2(0, 0), 0);
staticPoly.friction = .5;
//Setting the density to 0 will make the body that contains this poly static
staticPoly.density = 0;
staticPoly.restitution=0;

var staticBodyDef:b2BodyDef = new b2BodyDef();
staticBodyDef.position.Set(0,0);
var staticBody = m_world.CreateBody(staticBodyDef);
//New shape instantiation does not need to define static or dynamic
staticBody.CreateShape(staticPoly)
staticBody.SetMassFromShapes();

//Create a dynamic body
var dynamicPoly:b2PolygonDef = new b2PolygonDef();
dynamicPoly.SetAsOrientedBox(10, 10, new b2Vec2(0, 0), 0);
dynamicPoly.friction = .5;
//Setting the density to any value greater then 0 will make the body that contains this poly dynamic
dynamicPoly.density = 1;
dynamicPoly.restitution=0;

var dynamicBodyDef:b2BodyDef = new b2BodyDef();
dynamicBodyDef.position.Set(0,0);
var dynamicBody = m_world.CreateBody(dynamicBodyDef);
//New shape instantiation does not need to define static or dynamic
dynamicBody.CreateShape(dynamicPoly)
dynamicBody.SetMassFromShapes();

Distance Joints
A couple of minor changes have been made to the distance joint type. It now includes a dampening factor and a frequency. I can’t really seem to figure out what they do though. I am probably wrong but I don’t think they are working properly in flash yet.

Before You Start
I have switched over to pixels as my primary unit for working with Box2D. This was done by changing the scale factor and it impacts defining joints because they use N•m as their units. So now the values used in joints will need to be 30x bigger. Nothing major but if you use the same values as the test bed examples your joint won’t work properly. This will be more important with other joint types.

Distance Joints
A distance joint is the simplest of the joints to use. As the name implies it links two bodies together with a rigid connection that maintains the distance between the bodies. Each body is free to rotate and the collideConnected property of the inherited b2JointDef controls whether the bodies collide. The joint itself will not collide with other objects. As with most things in Box2D the vectors that specify the connection points are relative to the world and not the bodies. Example:

//Create a new distance joint definition
var djd:b2DistanceJointDef = new b2DistanceJointDef();
//Initialize the joint between two bodies. The b2Vec2() objects are used to define the location of the joint ends. Values are sample only.
djd.Initialize(body1, body2, new b2Vec2(0,0),new b2Vec2(0,30));
//Define whether the two objects will be allowed to collide with each other.
djd.collideConnected = true;
var distJoint:b2DistanceJoint = m_world.CreateJoint(djd);

Example:

Before we start
When I started to experiment with box2d I discounted the importance/utility of the debug draw mode. It doesn’t really do anything other then show you what is in the box2d world, which when your understanding of positioning is still bad helps a ton. So turn it on, it’s magic. If you change the m_drawScale to 1 you can use pixels as your units. Include the following after you create the b2World.

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 = 1;
dbgDraw.m_fillAlpha = 0.3;
dbgDraw.m_lineThickness = 1.0;
dbgDraw.m_drawFlags = b2DebugDraw.e_shapeBit | b2DebugDraw.e_jointBit;
m_world.SetDebugDraw(dbgDraw);

Complex shapes need a body
Ok onto complex shapes. When creating any object in box2d you start by creating a b2BodyDef. The b2BodyDef defines the intial properties of a b2Body object. When you create a b2Body it will either be a static (not moving) or a dynamic (moving) object. It also provides the container for shape definitions and custom visuals . b2Body creation example:

//Create a new body definition without any shapes
var bodyDef:b2BodyDef = new b2BodyDef();
//Set its position in the world. Units in meters.
bodyDef.position.Set(150, 150);
//Create a dynamic body in the world from the bodyDef
var body:b2Body = m_world.CreateDynamicBody(bodyDef);

Creating shapes and adding them to the body
After creating the body, we need to define and add some shapes to it. There are 2 basic types of shapes, circles (b2CircleDef) and polygons (b2PolygonDef). Both of these inherit from the b2ShapeDef class which is where you will find most of the editable shape properties (friction, restitution, density, etc). When creating shapes everything is relative to a centered registration point. b2PolygonDef shapes can be defined either using SetAsBox, for squares, or via a vertex array. In the documentation it says to limit to 8 vertex points for any polygon and it has to be convex. Box2DFlash differs a little from the C++ version when creating square polygons. To position a square poly in the body you use SetAsOrientedBox instead of SetAsBox. SetAsOrientedBox takes two extra parameters for position (b2Vec2) and angle (in radians). If you create a shape using vertex points you don’t need to use SetAsBox or SetAsOrientedBox because you have defined all the points manually. Shape creation example:

//Create a single rectangular poylgon
var poly1:b2PolygonDef = new b2PolygonDef();
//Make a 120px by 50px poly that will be centered on the body centered and without any angle
poly1.SetAsOrientedBox(60, 30, new b2Vec2(0, 0), 0);
//Give it some friction. 0 being none 1 being a lot
poly1.friction = 0.3;
//Give it some density which will be used to calculate mass later
poly1.density = 1;
//Restitution is how elastic something is 0 being in elastic and 1 being totally elastic
poly1.restitution = .1;

Putting it all together
Combining the two objects is really easy. Call the CreateShape() method of the b2Body with the shape as an argument. If you are going to add more shapes to the body you can just call CreateShape() again. Example:

//Add the poly1 shape to the body
body.CreateShape(poly1);
//If you were going to add more its as simple as
//body.CreateShape(poly2)
//SetMassFromShapes is a great feature of Box2D. It will analyze your body and determine the mass and center of gravity from the shapes.
body.SetMassFromShapes();

Complete Example Source

/*
Original software provided by Erin Catto http://www.gphysics.com. Thanks Erin for all the hard work.
Also thanks to Matthew Bush for all the hard work in porting box2D to flash
Some of the code in this example is from the Box2D Test bed examples. Check them out they are really helpful.
*/
package {

	import flash.display.*;
	import flash.events.*;
	import flash.text.*;
	import flash.geom.*;

	import Box2D.Dynamics.*;
	import Box2D.Collision.*;
	import Box2D.Collision.Shapes.*;
	import Box2D.Dynamics.Joints.*;
	import Box2D.Dynamics.Contacts.*;
	import Box2D.Common.*;
	import Box2D.Common.Math.*;

	public class ComplexTest extends Sprite {
		private var m_world:b2World;
		private var m_iterations:int = 10;
		private var m_timeStep:Number = 1/30;
		private var m_sprite:Sprite;
		//Convert to rads from degrees
		private var dtor:Number = Math.PI/180;

		public function ComplexTest():void {
			this.addEventListener(Event.ENTER_FRAME, update, false, 0, true);
			// Creat world AABB
			var worldAABB:b2AABB = new b2AABB();
			worldAABB.lowerBound.Set(-3000, -3000);
			worldAABB.upperBound.Set(3000, 3000);

			// Define the gravity vector
			var gravity:b2Vec2 = new b2Vec2(0, 300);

			// Allow bodies to sleep
			var doSleep:Boolean = true;

			// Construct a world object
			m_world = new b2World(worldAABB, gravity, doSleep);
			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 = 1;
			dbgDraw.m_fillAlpha = .3;
			dbgDraw.m_lineThickness = 1;
			dbgDraw.m_drawFlags = b2DebugDraw.e_shapeBit | b2DebugDraw.e_jointBit;
			m_world.SetDebugDraw(dbgDraw);

			// Add ground body
			var ground:b2BodyDef = new b2BodyDef();
			ground.position.Set(0, stage.stageHeight);

			//Define a ground poly
			var groundPoly:b2PolygonDef = new b2PolygonDef();
			groundPoly.SetAsBox(1000, 90);
			groundPoly.friction = .3;

			var groundBody:b2Body = m_world.CreateStaticBody(ground);
			groundBody.CreateShape(groundPoly);
			groundBody.SetMassFromShapes();

			//Create a new body definition without any shapes
			var bodyDef:b2BodyDef = new b2BodyDef();
			//Set its position in the world. Units in meters.
			bodyDef.position.Set(150, 150);
			//Give a bit of an angle to show the object colliding
			bodyDef.angle = 15 * dtor;
			//Create a dynamic body in the world from the bodyDef
			var body:b2Body = m_world.CreateDynamicBody(bodyDef);

			//Create a single rectangular poylgon
			var poly1:b2PolygonDef = new b2PolygonDef();
			//Make a 4m by 2m poly that will be centered on the body centered and without any angle
			poly1.SetAsOrientedBox(60, 30, new b2Vec2(0, 0), 0);
			//Give it some friction. 0 being none 1 being a lot
			poly1.friction = .3;
			//Give it some density which will be used to calculate mass later
			poly1.density = 1;
			//Restitution is how elastic something is 0 being in elastic and 1 being totally elastic
			poly1.restitution = .1;

			//Create a second poly
			var poly2:b2PolygonDef = new b2PolygonDef();
			poly2.SetAsOrientedBox(30, 60, new b2Vec2(30, 0), 0);
			poly2.friction = .3;
			poly2.density = 1;
			poly2.restitution = .1;

			body.CreateShape(poly1);
			body.CreateShape(poly2);
			body.SetMassFromShapes();
		}

		private function update(e:Event):void {
			m_world.Step(m_timeStep, m_iterations);
			// Go through body list and update sprite positions/rotations
			var bb:b2Body = m_world.m_bodyList;
			for (bb; bb; bb = bb.m_next) {

				if (bb.m_userData is Sprite) {
					bb.m_userData.x = bb.GetPosition().x;
					bb.m_userData.y = bb.GetPosition().y;
					bb.m_userData.rotation = bb.GetAngle() * (180/Math.PI);
				}
			}
		}
	}
}

Units
Units in the Box2D examples seem strange at first until you find out the secret. They are use the units from the C++ package which measured things in meters. They are making a meter = 30px in flash. If you just change the m_drawScale of the debug draw to 1 everything will be in pixels

SetAsBox()
Everything in Box2D is setup with a centered registration point. SetAsBox() through me off at first because I was expecting it to take the full width and height of the box as parameters but it actually takes half the width and height. See diagram below.

That’s it for now. Next up will be setting center of gravity, friction, restitution, etc.

//Create a request for the weather page
var myURLRequest:URLRequest = new URLRequest("http://www.weatheroffice.gc.ca/marine/marine_e.html?46146");
//Load the weather page into a urlloader object.
var myURLLoader:URLLoader = new URLLoader(myURLRequest);
//Listen for the load complete event for the URLLoader object
myURLLoader.addEventListener(Event.COMPLETE,weatherLoadComplete);

Now that the HTML file is in flash we can write a regexp to find specific data. Creating named groups within your regexp was new to me and an easy way to access specific data later on.

//Create a regexp literal that will pull out the just the air temperature from the web page
var weatherPattern:RegExp = /Temperature<BR>.*<strong>(?P<airtemp>.*)°C<\/strong>/;

The next step is to run that regexp against the text document. If a match is found you can access the whole string of the match or just the named group.

//Load complete event handler that traces out the regular expression data
function weatherLoadComplete(e:Event):void{
    //Execute the pattern against the text document returning an object containing a match
    var match:Object = weatherPattern.exec(e.target.data);
    //Since exec returns an object this will can use the full match
    trace(match);
    //Or just the group
    trace(match.airtemp);
}

Complete source.

//Create a request for the weather page
var myURLRequest:URLRequest = new URLRequest("http://www.weatheroffice.gc.ca/marine/marine_e.html?46146");
//Load the weather page into a urlloader object.
var myURLLoader:URLLoader = new URLLoader(myURLRequest);
//Listen for the load complete event for the URLLoader object
myURLLoader.addEventListener(Event.COMPLETE,weatherLoadComplete);
//Create a regexp literal that will pull out the just the air temperature from the web page
var weatherPattern:RegExp = /Temperature<BR>.*<strong>(?P<airtemp>.*)°C<\/strong>/;
//Load complete event handler that traces out the regular expression data
function weatherLoadComplete(e:Event):void{
    //Execute the pattern against the text document returning an object containing a match.
    var match:Object = weatherPattern.exec(e.target.data);
    //Since exec returns an object this will can use the full match
    trace(match);
    //Or just the group
    trace(match.airtemp);
}