A Self Balancing Creature

I’ve been finishing up the last few courses for my MSc degree (next year I will fully devote to research). This means that I didn’t have time to create original content for this blog. But for one of the courses (Master Level Game Physics) by Nicolas Pronost I created a self balancing creature. This was quite a cool project so I’d like to share the video with you guys.

The creature keeps balance by trying to keep the center of mass above the center of support. This is done by feeding the joint errors into PID-Controllers. The gains for the PID-Controllers were found using a self-learning algorithm (based on Simulated Annealing). The simulation also contained a muscle-actuated version (not shown in the video).

We used the Bullet Physics Engine to make this all work, however I cannot really recommend it. The documentation is horrid and we encountered a few very nasty bugs regarding joint constraints and motors.

29
Jun 2013
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Farseer physics 3.3.1 and XNA: Joints

In the previous tutorial, which you can find here, we created static and dynamic bodies. In this second tutorial we are going to take a look at joints. Simply said a joint is used to make a connection between two bodies or a body and the background.  The place where a joint is attached is called the anchor. Usually a joint has two anchors. One placed on the first body and one placed on a second body or on the world itself. Farseer uses the following naming convention for this. If a joint is ‘fixed’ it provides a joint between a body and the background/world. If a joint is not fixed it is used to connect two separate bodies to each other. So a RevoluteJoint connects two bodies and a FixedRevoluteJoint connects a body and the background/world.

Overview of joints

There are quite a few types of joints in Farseer:

Standard joints

Angle joints

Keeps a body at a fixed angle from another body or the world.

Distance joints

Keeps a body at a fixed distance from another body or from an anchor in the world.

Friction joints

Applies translational and angular friction to a body.

Line joints

A line joint can best be seen as a spring. It linearly connects to bodies (or a body and the background) together and tries to keep them at a fixed distance. It does not limit relative rotations but it does limit translation over one axis. They can be a bit tricky to place since the mass of the bodies themselves already affects the distance between the two bodies so setting the ‘at rest’ state is a bit of trial and error. Note that Line Joints are a new addition to Farseer and are not available in Box2D.

Prismatic joints

Enforces that two bodies can only slide in a linear motion (one degree of freedom) with respect to each other. See http://en.wikipedia.org/wiki/Prismatic_joint

Revolute joints

The most standard joints. Allows a body to rotate around an implicit axis coming from the screen (Z-Axis). Can be used to connect a body to the background or to another body.  Can be motorized.

Slider joints

A sort of prismatic joint that doesn’t limit the bodies to just linear motion. Best described as a combination of a revolute joint and a prismatic joint. Note that Slider Joints are also not available in Box2D.

Weld joints

Connects two bodies together disallowing any form of relative rotation/translation.

More complex joints

Gear joints

Connects two revolute joints or a revolute joint and a prismatic joint. Simulates that they are connected by gears so if the body on the first revolute joint rotates the body on the second revolute joint will rotate in the opposite direction.

Pulley joints

Used to create a pulley system. (Two bodies both connected to a rope that runs over a pulley). Can be used to create sophisticated elevators.

If you would like to know more about the joints Farseer offers you can check the Box2D documentation here (Farseer is a C# implementation of Box2D, and although it’s growing slowly to become more than that the best place for documentation is still the Box2D website).

Refactoring last week’s example

After doing some coding I found that I need access to the world-to-screen and screen-to-world conversion methods for the joints and they are currently members of the DrawablePhysicsObject class. So before we start creating or own joints we must first refactor last week’s example.

Create this new helper class:

    public static class CoordinateHelper
    {
        // Because Farseer uses 1 unit = 1 meter we need to convert
        // between pixel coordinates and physics coordinates.
        // I've chosen to use the rule that 100 pixels is one meter.
        // We have to take care to convert between these two
        // coordinate-sets wherever we mix them!

        public const float unitToPixel = 100.0f;
        public const float pixelToUnit = 1 / unitToPixel;

        public static Vector2 ToScreen(Vector2 worldCoordinates)
        {
            return worldCoordinates * unitToPixel;
        }

        public static Vector2 ToWorld(Vector2 screenCoordinates)
        {
            return screenCoordinates * pixelToUnit;
        }
    }

And then update the DrawablePhysicsObject class to use the helper class.

Some setup

Last week we added a list to store all the boxes that we randomly spawn. This week we’re going to create ‘paddles’ connected by joints. To store the paddles we’re going to need another list. So open Game1.cs and add the following field:

List paddles;

Then add the following three lines to draw the paddles:

foreach (DrawablePhysicsObject paddle in paddles)
{
    paddle.Draw(spriteBatch);
}

I’ve also added another image to the content project called ‘Paddle.png’.

Adding some bodies and joints

Now to demonstrate how to use joints I’m going to create three paddles. The first paddle will just be a simple body connected with revolute joint to the background. This means that it can spin freely. The second paddle is almost the same, but it will be motorized, adding some interaction to the scene. Finally we combine two line joints to create a trampoline. After this you should have a basic understanding on how joints work in Farseer.

A simple revolute joint

Add the end of the LoadContent method initialize the list we use to store the paddles. Then after that add the following code to create the first paddle:

// Create a simple paddle which center is anchored
// in the background. It can rotate freely
DrawablePhysicsObject simplePaddle = new DrawablePhysicsObject
(
    world,
    Content.Load("Paddle"),
    new Vector2(128, 16),
    10
);

JointFactory.CreateFixedRevoluteJoint
(
    world,
    simplePaddle.body,
    CoordinateHelper.ToWorld(new Vector2(0, 0)),
    CoordinateHelper.ToWorld(new Vector2(GraphicsDevice.Viewport.Width / 2.0f - 150,
                                         GraphicsDevice.Viewport.Height - 300))
);

paddles.Add(simplePaddle);

As you can see we first create a simple body, just as in the previous tutorial. The body is 128 pixels wide and 16 pixels high (note that the constructor of the DrawablePhysicsObject converts these units to world coordinates). It has a mass of 10KG. Note that we don’t have to set the position of the body. This is implicitly done by the joint, which fixes the body on the background at the anchor positions.

Next we create the joint. The first argument is our physics simulation and is used to register our joint to the simulation. We then pass the body this joint should be applied to. Since this is a Fixed joint we don’t need to pass a second body. The joint will connect the first body to the world. We then have to give some coordinates. We pass the center of the body (0,0) and somewhere in our world. Note that we use the conversion methods to go from pixel to world coordinates. Also don’t forget the last line, where we add the paddle to our list of paddles, else it won’t show up for drawing.

You can now run the simulation. Again press space to drop crate, you will see that we’ve create a small paddle that rotates freely when hit by a crate.

A motorized joint

The motorized joint is very similar:

// Creates a motorized paddle which left side is anchored in the background
// it will rotate slowly but the motor is not set soo strong that
// it can push everything away.
DrawablePhysicsObject motorPaddle = new DrawablePhysicsObject
(
    world,
    Content.Load("Paddle"),
    new Vector2(128, 16),
    10
);

var j = JointFactory.CreateFixedRevoluteJoint
(
    world,
    motorPaddle.body,
    CoordinateHelper.ToWorld(new Vector2(-48, 0)),
    CoordinateHelper.ToWorld(new Vector2(GraphicsDevice.Viewport.Width / 2.0f,
                                         GraphicsDevice.Viewport.Height - 280))
);

// rotate 1/4 of a circle per second
j.MotorSpeed = MathHelper.PiOver2;
// have little torque (power) so it can push away a few blocks
j.MotorTorque = 3;
j.MotorEnabled = true;
j.MaxMotorTorque = 10;

paddles.Add(motorPaddle);

Again we create a body and a Fixed Revolute Joint. But this time we store the joint created by the joint factory in the variable j so that we can access the properties of the joint. The revolute joint exposes a few interesting properties. The most interesting one is to motorize it. We’ve set a motor speed of pi/2. This means that the every second the joint will rotate the connected body by pi/2 radians, or a 1/4 of a circle. We also have to set the torque to give the motor enough power to rotate the body and to keep rotating even when a few crates are blocking the paddle. We also set the max motor torque and enable the motor. Again in the last line we add the paddle to our list of paddles so that it will be drawn.

A trampoline

To create a trampoline we will use two Line Joints (a sort of springs) to connect a paddle to the ground. This way we create some sort of trampoline. Add the following code:

// Use two line joints (a sort of springs) to create a trampoline
DrawablePhysicsObject trampolinePaddle = new DrawablePhysicsObject
(
    world,
    Content.Load("Paddle"),
    new Vector2(128, 16),
    10
);

trampolinePaddle.Position = new Vector2(600, floor.Position.Y - 175);

var l = JointFactory.CreateLineJoint
(
    floor.body,
    trampolinePaddle.body,
    CoordinateHelper.ToWorld(trampolinePaddle.Position - new Vector2(64, 0)),
    Vector2.UnitY
);

l.CollideConnected = true;
l.Frequency = 2.0f;
l.DampingRatio = 0.05f;

var r = JointFactory.CreateLineJoint
(
    floor.body,
    trampolinePaddle.body,
    CoordinateHelper.ToWorld(trampolinePaddle.Position + new Vector2(64, 0)),
    Vector2.UnitY
);

r.CollideConnected = true;
r.Frequency = 2.0f;
r.DampingRatio = 0.05f;

world.AddJoint(l);
world.AddJoint(r);

paddles.Add(trampolinePaddle);

Now the creation of the body should look really familiar now. Note that this time we do have to set the position of the body. The line joint will try to keep the paddle and the ground at roughly the same position as the starting position from each other so it needs some initial position.

We create two joints, they are exactly the same, except for where they connect to the paddle. One is anchored on the left side of the paddle and one on the right side. So I will only explain the first line joint.

We start by passing the body of the first body the line joint should connect to, the floor. We then pass the body of the paddle. We tell the line joint to connect to the paddle at the given relative coordinates.  We then give the axis of freedom the line joint should offer. In our case the Y-Axis.

We also set some properties, CollideConnected  means that the paddle and the floor can collide with each other. Handy now but for a wheel inside a car you might want to keep this at the default, turned off, state. We also set a nice frequency and damping ratio. Finally we add the joints to the world. Don’t forget this step! Only Fixed joints are automatically added to the world. We also add the paddle to our list of paddles to draw.

You can now run the simulation again. Try to drop a few crates on the trampoline, you will see that it behaves quite nicely.

Conclusion

Joints in Farseer are fairly easy to use and can add a dynamicity lot to your scene. Try playing around with all different joints to get a feel from them. All joints in Farseer are created using a way similar as shown in this tutorial. In the next tutorial we will add a controllable character and create a small platformer.

06
Sep 2012
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Farseer physics 3.3.1 and XNA

Part 2
Part 3

A physics engine, like Farseer, is a piece of technology that allows you to simulate real-world-physics inside your game. It enables you to incorporate elements like gravity, weight, collision detection/response and much more into your game without having to rediscover and implement the laws of physics yourself.

Farseer is one of the most used physics engine for 2D games in C# and XNA, it’s tries to mimic the functionality of the popular Box2D physics engine which was built with C++ in mind. Note that you can also use Farseer if you’re not using XNA, it also has bindings for Silverlight and plain C#/.Net.

A while ago I wrote a tutorial for Farseer 2.X on how to create and manipulate a character and how to setup a few platforms and a seesaw. That tutorial still remains popular but since then a lot of things have changed in the newer versions of Farseer so I think it is useful to revisit this topic.
This tutorial is the first in a series. The end result will be the same as the old platformer tutorial but we’ll get there in byte sized pieces this time and I’ll try to go a bit more in depth. Anyway, let’s get started!

Setting up Farseer and XNA

Ok so first things first, start visual studio and create a new XNA 4 Windows Game Project. You can then download the Farseer Physics Engine from here. At the time of writing the latest version was v3.3.1. Extract the archive and open the Visual Studio solution named ‘Samples XNA’. Switch to Release mode and build the project named ‘Farseer Physics XNA’. Now in the output directory (bin/x86/release) find the file ‘FarseerPhysicsXNA.dll’ and copy it to your game project. Switch back to the your own game project (you can close the other Visual Studio window). Locate the references in the solution explorer, right click it and click ‘Add Reference…’ browse to your project folder and select ‘FarseerPhysicsXNA.dll’. You can now use Farseer in your XNA project!

Setting up a simple physics simulation

Now let’s get some physics going. Open Game1.cs and add the following using statements at the top of the file.

using FarseerPhysics.Dynamics;
using FarseerPhysics.Factories;

Also add the following member to the Game1 class.

World world;

The World class is the most important class in Farseer. It represents the entire physics simulation. Every object that influences the simulation should be registered with your instance of the world class. Of course to make it work we have to instantiate it and tell it what kind of gravity we want. To do so add the following to your LoadContent() method.

world = new World(new Vector2(0, 9.8f));

As you can see I’ve chosen a gravity of 0m*s^2 on the horizontal axis and 9.8m*s^2 on the vertical axis. Just as on earth!
The world object needs to do some work every frame to keep the simulation going so add this to your update method:

world.Step((float)gameTime.ElapsedGameTime.TotalSeconds);

Now we still wont see anything, there aren’t any objects, or bodies as they are called in Farseer, yet in our simulation. Lets remedy this situation by adding a crate. Add the following member to Game1.cs

Body body;
const float unitToPixel = 100.0f;
const float pixelToUnit = 1 / unitToPixel;

And add the following code at end of the LoadContent method

Vector2 size = new Vector2(50, 50);
body = BodyFactory.CreateRectangle(world, size.X * pixelToUnit, size.Y * pixelToUnit, 1);
body.BodyType = BodyType.Dynamic;
body.Position = new Vector2((GraphicsDevice.Viewport.Width / 2.0f) * pixelToUnit, 0);

Now as you can see we use a handy factory to create a rectangular body, we pass the world object so that it can be registered to it. However when passing the size of the body we first multiply this by the newly introduced constant pixelToUnit. Farseer uses Meters, Kilograms and Seconds as units while we are using pixels. So we need to convert all sizes and lengths to meters when we pass them to Farseer by multiplying them with pixelToUnit and, vice versa, when we get data back from Farseer, for example the position of a body, we need to convert back from meters to pixels. We use the same idea when setting the body’s position.

We also set the body type to Dynamic. This is a normal body, you also have Static, which means that it is immovable and Kinematic which means that it has no mass and some other properties (we wont use it).

Anyway add an image for the crate to your content project, I’ve named mine ‘Crate.png’. Then add the following member to Game1.cs.

Texture2D texture;

Now we can finally draw something to the screen. Add the following code to your Draw method

spriteBatch.Begin(SpriteSortMode.Immediate, BlendState.Opaque);
Vector2 position = body.Position * unitToPixel;
Vector2 scale = new Vector2(50 / (float)texture.Width, 50 / (float)texture.Height);
spriteBatch.Draw(texture, position, null, Color.White, body.Rotation, new Vector2(texture.Width / 2.0f, texture.Height / 2.0f), scale, SpriteEffects.None, 0);
spriteBatch.End();

When you run the game now you should briefly see a crate fall to its doom before it disappears from the screen.

A more interesting simulation

Now of course this is a bit of a boring simulation to see something interesting happen we would need at least two objects, and preferably much more. Now we can duplicate the code a couple of times but I believe that you should never have to write the same code twice. Delete the following lines.

  • The line that starts with spriteBatch.Draw(…) in your Draw method
  • The members unitToPixel and pixelToUnit
  • The body member and everything that involves the body in the LoadContent method.

Now create a new class called DrawablePhysicsObject. This class will be a wrapper around a body object. We will automatically convert between pixel coordinates and the coordinates used by Farseer and we will make drawing a bit easier. Since there are no new concepts introduced in this class I’ll just place the code right here:

public class DrawablePhysicsObject
    {
        // Because Farseer uses 1 unit = 1 meter we need to convert
        // between pixel coordinates and physics coordinates.
        // I've chosen to use the rule that 100 pixels is one meter.
        // We have to take care to convert between these two
        // coordinate-sets wherever we mix them!

        public const float unitToPixel = 100.0f;
        public const float pixelToUnit = 1 / unitToPixel;

        public Body body;
        public Vector2 Position
        {
            get { return body.Position * unitToPixel; }
            set { body.Position = value * pixelToUnit; }
        }

        public Texture2D texture;

        private Vector2 size;
        public Vector2 Size
        {
            get { return size * unitToPixel; }
            set { size = value * pixelToUnit; }
        }

        ///The farseer simulation this object should be part of
        ///The image that will be drawn at the place of the body
        ///The size in pixels
        ///The mass in kilograms
        public DrawablePhysicsObject(World world, Texture2D texture, Vector2 size, float mass)
        {
            body = BodyFactory.CreateRectangle(world, size.X * pixelToUnit, size.Y * pixelToUnit, 1);
            body.BodyType = BodyType.Dynamic;

            this.Size = size;
            this.texture = texture;
        }

        public void Draw(SpriteBatch spriteBatch)
        {
            Vector2 scale = new Vector2(Size.X / (float)texture.Width, Size.Y / (float)texture.Height);
            spriteBatch.Draw(texture, Position, null, Color.White, body.Rotation, new Vector2(texture.Width / 2.0f, texture.Height / 2.0f), scale, SpriteEffects.None, 0);
        }
    }
}

Now lets set this code to work!
Create the following members in Game1.cs

List<DrawablePhysicsObject> crateList;
DrawablePhysicsObject floor;
KeyboardState prevKeyboardState;
Random random;

And add this line to the end of the LoadContent method.

random = new Random();

floor = new DrawablePhysicsObject(world, Content.Load("Floor"), new Vector2(GraphicsDevice.Viewport.Width, 100.0f), 1000);
            floor.Position = new Vector2(GraphicsDevice.Viewport.Width / 2.0f, GraphicsDevice.Viewport.Height - 50);
floor.body.BodyType = BodyType.Static;
crateList = new List<DrawablePhysicsObject>();
prevKeyboardState = Keyboard.GetState();

You see that we create one DrawablePhysicsObject with a Static BodyType. That’s going to be our floor. We also create a list of crates and do some keyboard logic. Lets create a method to fill that list of crates. Add this method to Game1.cs

private void SpawnCrate()
{
	DrawablePhysicsObject crate;
	crate = new DrawablePhysicsObject(world, Content.Load<Texture2D>("Crate"), new Vector2(50.0f, 50.0f), 0.1f);
	crate.Position = new Vector2(random.Next(50, GraphicsDevice.Viewport.Width - 50), 1);

	crateList.Add(crate);
}

This method will spawn a random crate somewhere at the top of the screen. We will trigger this method by a pressing the spacebar. To do so add this to the update method before base.Update().

KeyboardState keyboardState = Keyboard.GetState();
if (keyboardState.IsKeyDown(Keys.Space) && !prevKeyboardState.IsKeyDown(Keys.Space))
{
    SpawnCrate();
}

prevKeyboardState = keyboardState;

And finally add these last few lines in between spriteBatch.Begin() and spriteBatch.End() in your draw method so that we can see what’s going on.

foreach (DrawablePhysicsObject crate in crateList)
{
    crate.Draw(spriteBatch);
}

floor.Draw(spriteBatch);

Run the simulation and start hammering on the spacebar you should see something like this:

This concludes this tutorial. In the next tutorial we will talk about springs and joints and ways to make compound bodies so that you can have more interesting simulations!

You can download the source code for this tutorial here: Farseer v3.3.1 XNA Tutorial 1

Thanks to Ryan Foy for pointing out some errors in the code that somehow made it onto this blog

19
Aug 2012
COMMENTS 13 Comments

XNA Farseer platformer physics tutorial

My XNA Farseer platformer physics tutorial is up and online at MadGameDev.com

Below is a sneak preview, but follow the above link for the complete article.

Today I’m going to introduce you to a neat 2D physics engine for XNA called Farseer. With the help of this 2D physics engine we are going to create a small platform ‘game’ while introducing the important concepts of Farseer like bodies, geometry, joints and springs.

By the end of the tutorial, you’ll be able to build something like:

Update 11-09-2012: links fixed

10
Sep 2010
COMMENTS 2 Comments

Hollandia and Farseer physics

I don’t know why I didn’t break this to everyone before. But I’ve been the lead developer of Hollandia for a few weeks already. Click the link to find out more :).

For Hollandia I’m doing a rewrite of the engine. Since it’s a platformer the characters need to be able to jump onto things. So I had a choice to either write my own mini-physics-engine for this or to look for a valid alternative that I could easily plug in into the code that I had already written. Luckily I chose the last option and came across Farseer physics and I’m completely hysterical about it. After only one day of using it I’ve donated $10,- to show my support. Ok the manual pages could use a bit of work, but to aid in that I’m writing a Farseer physics platformer tutorial for the Sgt. Conker XNA Article Contest in which I will put everything that I’ve found out about Farseer so far (see it as a gentle introduction, since once you ‘get it’ Farseer is very easy).

Anyway some who follow my youtube channel might’ve seen this already, but I made a small video where you can see that I’ve implemented Farseer physics into the engine. Things are really starting to come together (Screen managers, Scene Graphs, Layered system etc… unfortunately you can’t see any of that in the video). Especially the artwork is really nice, but since I didn’t make the art work, I’ve turned it off for the below video. (I’m just showing the physics bodies).

And no, this is not the 4×4 all terrain infinite horsepower brutal Segway of doom ©. This is actually our main character walking around. (It doesn’t look so silly if you imagine a texture of a Dutch girl moving around instead of these funny boxes :). The idea to do it this way came from this blog article by Bryan Dismas which in turn is based on this blog article by Robert Dodd

20
Aug 2010
CATEGORY

Blog, XNA

COMMENTS 3 Comments