Monthly Archives: October 2013

Simulating Fixed and Line Joints in Farseer Physics Engine 3.5

Recently there was a new release of the excellent Farseer Physics Engine. Since my game is still in very early stage and I am still mucking around with samples and tweaking the settings I decided to upgrade since performance improvements is one of the things changed. Not that the old version was slow, but since I am inexperienced in game programming and quite likely doing some things in a non optimal way I though I could use those extra CPU cycles.

However version 3.5 is not backwards compatible and among other things it no longer includes fixed joints like FixedRevoluteJoint etc. and I have been using those in my proof of concept. As it turns out it is quite easy to achieve the things I used to use fixed joints for.

Firstly, credit where credit is due – a lot of my code is based on Farseer Samples and this post by Roy Triesscheijn.

Anyway I was testing various paddles as explained in the code that can be downloaded from the blog post mentioned above.

Here is how everything looks in action:

In Farseer 3.3, the code for the motorised paddle would look like this:

Body motorPaddle = CreateMotorPaddle();

var j = JointFactory.CreateFixedRevoluteJoint
        (
                World,
                motorPaddle,
                new Vector2(0.0f, 0.0f),
                new Vector2(-14.0f, 10.0f)
                );

// set motor speed
j.MotorSpeed = MathHelper.Pi;

// set torque and enable motor
j.MotorTorque = 100; 
j.MotorEnabled = true;
j.MaxMotorTorque = 100;

In version 3.5 you can achieve the same thing by the code below. Note the use of another static body to act as an axle:

Body motorPaddle = CreateMotorPaddle();
Body motorPaddleAxle = BodyFactory.CreateCircle(World, 0.1f, 1f);

var j = JointFactory.CreateRevoluteJoint
    (
        World,
        motorPaddle,
        motorPaddleAxle,
        new Vector2(0.0f, 0.0f),
        new Vector2(-14.0f, 10.0f)
        );

// set speed and torque
j.MotorSpeed = MathHelper.Pi;
j.MotorImpulse = 100; 
j.MotorEnabled = true;
j.MaxMotorTorque = 100;

The code for free swinging paddle is more or less the same, except that the motor is not enabled and would look like the code below:

Body simplePaddle = CreateSimplePaddle();
Body simplePaddleAxle = BodyFactory.CreateCircle(World, 0.1f, 1f);

JointFactory.CreateRevoluteJoint
    (
        World,
        simplePaddle,
        simplePaddleAxle,
        new Vector2(0.0f, -1.4f),
        new Vector2(25.0f, 10.0f)
        );

The trampoline was implemented using LineJoint in Farseer 3.3. That code looked like this:

Body trampolineAnchor1 = CreateTrampolineAnchor1();
Body trampolineAnchor2 = CreateTrampolineAnchor2();
Body trampolinePaddle = CreateTrampolinePaddle();

var spring1 = JointFactory.CreateLineJoint
        (
                trampolineAnchor1,
                trampolinePaddle,
                new Vector2(12.0f, 11.0f),
                Vector2.UnitY
                );

spring1.CollideConnected = true;
spring1.Frequency = 3.0f;
spring1.DampingRatio = 0.05f;

World.AddJoint(spring1);

var spring2 = JointFactory.CreateLineJoint
        (
                trampolineAnchor2,
                trampolinePaddle,
                new Vector2(16.0f, 11.0f),
                Vector2.UnitY
                );

spring2.CollideConnected = true;
spring2.Frequency = 3.0f;
spring2.DampingRatio = 0.05f;

World.AddJoint(spring2);

LineJoint is gone in Farseer 3.5, but there we can use WheelJoint instead, like in the code below. A wheel join allows rotation and suspension like movement along a specified axis.

Body trampolineAnchor1 = CreateTrampolineAnchor1();
Body trampolineAnchor2 = CreateTrampolineAnchor2();
Body trampolinePaddle = CreateTrampolinePaddle();

var spring1 = JointFactory.CreateWheelJoint
    (
        World,
        trampolinePaddle,
        trampolineAnchor1,
        Vector2.UnitY
        );

spring1.CollideConnected = false;
spring1.Frequency = 3.0f;
spring1.DampingRatio = 0.05f;

World.AddJoint(spring1);

var spring2 = JointFactory.CreateWheelJoint
    (
        World,
        trampolinePaddle,
        trampolineAnchor2,
        Vector2.UnitY
        );

spring2.CollideConnected = false;
spring2.Frequency = 3.0f;
spring2.DampingRatio = 0.05f;

World.AddJoint(spring2);

That is it!