Intro to the Unity Physics Engine - 2019.3

The Rigidbody Component (Figure 04), allows GameObjects to be affected by Physics properties, such as Gravity. It also includes properties for Mass, Velocity, and Drag (air resistance). Objects of larger mass are less affected by objects with lower mass and vice versa. Drag affects the dampening of velocity over time. Angular Drag affects angular velocity.

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Figure 04: The Rigidbody component’s Physics properties

The Is Kinematic checkbox allows the Rigidbody to affect other objects via the Unity Physics Engine, but will not be affected themselves. For example, a Hand Avatar in a VR game can interact with objects via Physics, but we don’t want Physics to act on the hand.

The Is Kinematic checkbox also affects objects controlled by the Animation Engine. If the Is Kinematic checkbox is selected (on), the Animation Engine affects objects. If deselected (off), the Physics Engine retains control.

The Interpolate setting detects how collisions are checked. From the dropdown, select either Interpolate or Extrapolate.

• Interpolate: Smooth movements of objects are based on information from the previous frame in an animation’s timeline.
• Extrapolate: Smooth movements of objects are based on a guess of the next frame.

• Discrete: Default.
• Continuous: Fast objects that interact with static objects.
• Continuous Dynamic: Fast objects that interact with other fast objects.
• Continuous Speculative: Predictive collision checking.

The Constraints checkboxes for the X, Y, and Z axes, defines which axis objects should not move.

Additionally, we can redefine and change elements of the Unity Physics Engine values by selecting from the drop-down menu: Edit > Project Settings > Physics (Figure 05).

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Figure 05: The default settings for the Unity Physics Engine.

OnCollisionEnter (Collision)

void OnCollisionEnter(Collision collision)
{
     if(collision.gameObject.CompareTag("Enemy")
     {
          //Hit the enemy
     }
}

OnCollisionStay (Collision)

OnCollisionExit (Collision)

bool rigidBody.useGravity

*rigidBody.AddForce(Vector3)

rigidBody.AddTorque(Vector3, Force Mode)

• Acceleration: Applies a force that increases at a constant rate.
• Force: Default, gradually applies a force accounting for its mass.
• Impulse: Applies an instant force instead of one that gradually builds up over time.
• VelocityChange: Applies instant forces in different directions. Disregards mass.

• Update: Called once per frame.
• FixedUpdate: Called multiple times per frame. Most physics calculations will be called in FixedUpdate. The time between calculations is fixed, as the name implies.

Physics.Raycast (Vector3 Origin, Vector3 Direction)
Physics.Raycast (Vector3 Origin, Vector3 Direction, RaycastHit Info, float Distance, int Layermask)

Physics.Raycast (Ray RayName, RaycastHit Info, float Distance, int Layermask)

• origin: The starting point of the ray in world space.
• direction: The direction of the ray being cast in object space.
• maxDistance: The maximum distance the ray should check for collisions.
• layerMask: A layer mask that is used to selectively ignore Colliders when casting a ray.
• queryTriggerInteraction: Specifies whether this query should hit Triggers.

The colliding GameObject can be assigned a new or existing tag so a RaycastHit will be easier to identify (Figure 08).

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Figure 08: RaycastHit scripting example.

In this example, a Raycast is casting a ray downward from the origin of whatever gameObject this script is attached to.

If the distance between the transform position and the floor (GameObject with the environment tag) is at deploymentHeight, call the function: DeployParachute().

It is a best practice when working with Raycasts to:

• Reduce the number of Raycasts in a scene.
• Not to call Raycasts inside of the FixedUpdate() or Update() functions.
• MeshColliders should be avoided.