Swing Setup

Let’s set up the GameObjects of our swing first so they’re all ready to be connected by joints. The swing will consist of a hovering cube that remains stationary and cannot be pulled or pushed. Beneath this cube, three identical “chain links” will hang below. Each chain link is a sphere with a long, slender cube hanging beneath it. Each sphere will connect to the cube of the chain link above it. The finished product is shown in Figure 39-1.

Start by creating an empty GameObject named “Swing” with no parent. We’ll use this as a root for the Swing. We’ll be able to select it to move the whole Swing and all its pieces wherever we want.

After we set this one Chain Link up, we’ll be copy-pasting it to create the others. But first, let’s make sure it has the components we need so we aren’t adding them and setting them up three times.

We’ll need each Chain Link root GameObject (not the Sphere or Cube within) to have a Rigidbody and a Configurable Joint added. Go ahead and add each of those now and give the Rigidbody a Mass value of 1.5.

You might think we should add a Rigidbody not to our Chain Link, but to both the Sphere and the Cube within it, but this isn’t necessary.

The Rigidbody of the Chain Link will detect the Box Collider and Sphere Collider of its child GameObjects and will consider them fused together to form a single body, acting as if both of those colliders are part of the same object. We can effectively think of them as two pieces of metal welded together. If the Cube is struck by something, then it moves and the Sphere pivots with it – and vice versa.

These are known as compound colliders and can be used to represent a more complex object by shaping it out of “primitive” collider types. Primitive colliders are the colliders for basic built-in shapes: Box Collider, Sphere Collider, and Capsule Collider. By creating a parent GameObject with a Rigidbody attached, then adding children with primitive colliders, we create one whole object with a more complex shape than just a cube, sphere, or capsule.

You can do this to “summarize” the shape of a more complex mesh with a combination of primitive shapes.

Moving on to our Configurable Joint, we have a few values to set.

The Configurable Joint component is attached to the GameObject that you want to connect another GameObject to, and the Rigidbody of the other GameObject is referenced in the Configurable Joint. So when making a chain out of these Chain Link objects, this means that the Configurable Joint will be attached to the upper (higher) link in the chain, and the link beneath that chain will be referenced as the “Connected Body” member (the first member listed in the Inspector).

The Anchor value of the Configurable Joint is a Vector3 depicting from where the Connected Body pivots. The location is local to the Transform of the GameObject with the Configurable Joint component. We want our Anchor to be at a value of (0, –6, 0).

The position of the anchor is shown in the Scene as a dark-gray set of position handles. With our Anchor setting, this will place it at the bottom of the cube, shown in Figure 39-2.

This will ensure that the Chain Link we position below this one will be pivoting around the bottom of this link, not the center of the Sphere (which would be quite awkward).

Moving on, the six “Motion” and “Angular Motion” dropdown fields in the Configurable Joint are all that’s left to set.

For our purposes, we’ll set all three of the Motion fields to Locked because we don’t want the joints to cause movement, just rotation – they pivot around each other.

The swing will be more controlled if you only allow it to pivot on the X or Z axis and lock the other two. The Y axis isn’t all too important, simply depicting whether we’ll see the cubes twisting and turning from the forces applied to them.

For our tests, let’s set the X angular motion to Free and set both the Y and Z angular motions to Locked. This will allow it to swing forward and back only, something like a pendulum. Since this is all local to the Transform, if you want it to swing left or right instead, you can just rotate the entire Swing on the Y axis instead of changing the Configurable Joint settings.

The rest of the fields beneath the Motion fields won’t need any tampering. At this point, Figure 39-3 shows how your Rigidbody and Configurable Joint components should look in the Inspector.

We now have three Chain Links, all touching at their tips to create – you guessed it – a chain. They stick out the bottom of our Hovering Cube, as shown in Figure 39-4.

With that, our setup of the bits and pieces of our swing is complete, which means your swing should at last look like it does in Figure 39-1 from before.

Connecting the Joints

In order to connect the joints to each other, we’ll first want to connect the upmost Chain Link to the Hovering Cube itself.

To add a Configurable Joint component, you will have to add a Rigidbody as well. Trying to add one without a Rigidbody already attached will result in Unity automatically adding one.

Since we don’t want our Hovering Cube to be affected by any collisions or forces, we’ll constrain its position and rotation through the Rigidbody. Check all six boxes under the Constraints field of the Rigidbody in the Inspector. You can also uncheck the Use Gravity field, since it wouldn’t make much sense to apply gravity when we’ve already frozen our Rigidbody in place.

The Rigidbody settings should look like Figure 39-5 when you’re done.

Now we have the Hovering Cube anchored in place above the ground, so if we attach our first Chain Link to it by a Configurable Joint (which we’ll do in a second), the link will be bound to the Hovering Cube by that joint, keeping it from falling freely through the air. With the other links attached to that one, they’ll all hang off the Hovering Cube.

With a Configurable Joint added to your Hovering Cube, don’t forget to set the Anchor to the bottom of the cube with a value of (0, –.5, 0). Remember, the Anchor is local to the Transform, so each unit in the Anchor is multiplied by the Transform scale. For our Chain Links, the scale was (1, 1, 1) so the Anchor value was effectively in world units, but since our Hovering Cube is scaled to (10, 10, 10), that means each unit in the Anchor setting is worth 10 world space units. That’s why it’s –.5 to put it at the bottom of the Cube, not –5. Think of it as “50% of the height of the cube,” not “.5 units”.

We also need to lock our Motion values in the Configurable Joint of the Hovering Cube. Following the same settings you used on the Chain Links, shown before in Figure 39-3, apply them again to the Hovering Cube: set all six fields to Locked except for the Angular X Motion, which should remain set to Free.

So first, select the Hovering Cube and drag Chain Link (the topmost one) from the Hierarchy onto the “Connected Body” field of the Configurable Joint. This binds the first Chain Link to the Hovering Cube.

Finishing Touches

You should now be able to play the game, run over to your Swing with the Player, and try pulling and pushing the Platform or the Chain Links with the Telekinesis feature. Remember, the swing has been set to only swing on the X axis, which is forward and backward. This means it’s like a pendulum, swaying back and forth in only one direction. Trying to pull and push it from the wrong side won’t generate much of a reaction.

With our current gravity settings, the swinging may seem a bit “off.” Unity’s default gravity settings are tweaked to appear realistic when the unit of measurement for your game is 1 meter. As you’ll recall from our earliest chapters, what a single unit resembles is totally relative. Since our player is 6 units tall, we’re really using a unit measurement of about 1 foot. The gravity setting thus thinks we’re a bit larger than we really are: the player would be 6 meters tall by that standard, not 6 feet! This can account for the swing moving in a somewhat “slow-motion” way.

As with many things, you can change the force that gravity applies in the Edit ➤ Project Settings window. The gravity is the first field shown when the Physics tab is selected, as depicted in Figure 39-6.

This shows a Vector3 for the gravity force applied to Rigidbodies constantly (per second). Of course, the X and Z axes are set to 0 since gravity does not standardly pull you in such directions. The Y axis is what we want.

The default setting of –9.81 mimics the pull of gravity on Earth but using meters as a measurement. Since a meter is about 3.28 feet, we can multiply this default setting of 9.81 by 3.28, which is roughly 32.2. Of course, it should be –32.2 since we want it to be downward force.

Once you’ve set the gravity this way, your swing movement should look more natural.

One final touch is to add a Platform script to our Platform cube GameObject. Once you’ve done that, try setting the swing in motion with Telekinesis and then hopping onto it. The Player should stick to the swing and move with it.

As expected, since everything involved in the motion of the Swing is driven by the physics system, other Rigidbodies will stay on the swing and move with it as well. Try stacking some Rigidbody cubes on top of the swing and then set it in motion with Telekinesis.

Summary