🔗LBJT: Weld Joint7️⃣.

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Weld Joint

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This article is about the Weld Joint, its settings, and how to make it work.

To make it easier to understand what is happening and how to perform practical tasks, start with the article: **🔗LBJT: Сommon in Joints 1️⃣.**

And so the… Weld Joint

Weld joint — a joint intended for connecting 2 bodies together with the possibility of adjusting the elasticity and the rate of disappearance of oscillations. (usually used as welding 2 bodies together)

First, we need to set up the joint. For this, each joint has its own implementation of JointDef. You can read about this in the article: **🔗LBJT: Common in Joints 1️⃣. And now let’s talk about the WeldJointDef**.

WeldJointDef settings:

(Mandatory)

• bodyA — first joint body.
• bodyB — second joint body.
• collideConnected — <boolean> Specifies whether bodyA should collide with bodyB.

(Optional)

• localAnchorA — the local anchor point relative to bodyA origin.
• localAnchorB — the local anchor point relative to bodyB origin.
• referenceAngle — initial angle between bodyA and bodyB, (default 0 | measured in radians).
• frequencyHz — affects the elasticity of the joint between bodyA and bodyB.

Value < 1: With values less than 1, such as 0.5 or 0.7, the joint will be strongly stretched, resembling a soft spring.

Value > 1: With values greater than 1, such as 2, 5, or 10, the joint will stretch very little or not at all, similar to a stiff spring.

• dampingRatio — suppresses vibrations and takes values from 0 to 1.

0: does not suppress.

1: almost absolute suppression.

WeldJointDef settings

Practice. How to create a Weld Joint?

In order for you to be able to test Joints, you need to interact with bodies, for this you need to implement MouseJoint. How to implement it is described in article* 🔗LBJT: Mouse Joint 2️⃣.*

Our example will consist of a static circle (blue) and a dynamic circle (green) connected by WeldJoint.

Weld Joint

1️⃣Create 2 bodies: a static circle and a dynamic circle:

val staticCircle  = StaticBody
val dynamicCircle = DynamicBody

2️⃣Create WeldJoint:

world.createJoint(WeldJointDef().apply {
      bodyA = staticCircle
      bodyB = dynamicCircle
      collideConnected = false
}

Note that collideConnected = false, this is done so that the connected bodies do not touch each other.

That’s it, you can already run the program and the bodies will be connected, but there is one but, they will be connected at their anchor points, where the center of mass is specified, to fix this, you need to configure localAnchorA, localAnchorB about how I explained how to calculate anchor points in the article: **🔗LBJT: Сommon in Joints 1️⃣.**

3️⃣Configure: localAnchorA, localAnchorB:

world.createJoint(WeldJointDef().apply {
      bodyA = staticCircle
      bodyB = dynamicCircle
      collideConnected = false

      localAnchorA.set(Vector2(2.5f, 3.0f))
      localAnchorB.set(Vector2(2.5f, 0.0f))
}

*Do not forget that the values are indicated in meters*. About the units of measurement of meters, kilograms, seconds (MKS) is written in the article:* **🔗LBJT: Сommon in Joints 1️⃣.*

4️⃣Configure: referenceAngle:

referenceAngle — just changes the angle between bodyA and bodyB.

Do not forget that all angles in Box2d are measured in radians.

Clockwise negative angles. Counter-clockwise are positive angles.

WeldJoint referenceAngle

world.createJoint(WeldJointDef().apply {
      ...
      referenceAngle = 45f * DEGTORAD
}

5️⃣Configure: frequencyHz (elasticity):

By default, WeldJoint will connect 2 bodies and they will be like one body, if you want it to be like a spring between them and there are oscillations, then you need to adjust the frequencyHz.

world.createJoint(WeldJointDef().apply {
      ...
      frequencyHz = 0.5f
}

WeldJoint frequenceHz

6️⃣Configure: dampingRatio:

dampingRatio — damps oscillations to the sides, so it can be said that it works together with the frequencyHz.

world.createJoint(WeldJointDef().apply {
      ...
      frequencyHz  = 0.5f
      dampingRatio = 0.8f
}

WeldJoint dampingRatio

The body returns to its original position very smoothly and slowly, a good effect, the larger the value of dampingRatio, the slower the body will return to its original position (remember that this only works together with the frequencyHz).

It works and it’s great 😎.

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GitHub: https://github.com/Vladislav-Shesternin/LBJT 😺.

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PS. Vel_daN: Love what You DO 💚.