Physics, Rigidbody Simulation in Unity3D

Syed Haroon Alam

Pre Requisite:Basic understanding of game development, unity3d and scripting language

Introduction• Physics in Game Development: Involves the laws of physics into

a game engine for the purpose of making effects more real to the observer

• Game Physics in Unity3D: Built in physics library and components to produce results in real-time that replicate real world physics

Physics engines used in unity:

• NVIDIA PhysX Physics engine - 3D game development• Box2D physics system - 2D game development

Components

Built in physics components include:

• Colliders

• RigidBodies

• Joints • Physics materials

Colliders

• Colliders are used to define the collision shape of objects in your scene

• Unity primitive colliders: Box, Sphere and Capsule

• Mesh Collider can be used for complex meshes

• Wheel Collider component for vehicles

• Terrain Collider component for terrains.

• A concave polygon always have an interior angle with a measure that is greater than 180 degrees

• A convex polygon has internal angle is less than or equal to 180 degrees. Every line segment between two vertices remains inside or on the boundary of the polygon.

Trigger Colliders

• Colliders can be used as a Trigger by setting the Is Trigger property to true

• Not participate in collision simulations

• Fire OnTriggerEnter, OnTriggerStay, and OnTriggerExit events

Rigidbodies

• Used For objects to act under the influence of Physics.

• Forces and torques to the Rigidbody component can be applied from scripts

• Requires a Collider component to be present on the GameObject for correct Physics simulation

• Can be Physics controlled or Kinematic controlled

RigidbodiesCollision Detection

• Discrete:- Uses Discreet collision detection against all other colliders in the scene.- Skips detection when an object is on one side of a collider in one frame, and

already passed the collider in the next frame

• Continuous:- Uses Continuous Collision Detection against colliders with no rigidbody- Uses CCD against rigidbodies set to Continuous Dynamic- User Discrete collision detection for all other colliders

• Continuous Dynamic:- Uses CCD on rigidbodies that are either set to Continuous or Continuous

Dynamic and colliders with no rigidbody- Use only for fast moving objects- User Discrete collision detection for all other colliders

Continuous vs Discrete collisionsFrame - 1 Frame - 2 Frame - 3

Using continuous and continuous dynamic

Moving rigidbodies

• If isKinematic is enabled, the object will not be driven by the physics engine, and can only be manipulated by its Transform

• FixedUpdate() should be used instead of Update() when dealing with Rigidbody

• rigidbody.MovePosition

• rigidbody.AddForce

• rigidbody.AddTorque

Rigidbody Colliders

• Colliders that also have a Rigidbody will react to collisions

• Contains information about contact points, impact velocity, rigidbody and collider

• Fire OnCollisionEnter(), OnCollisionStay(), and OnCollisionExit() events

Physics Material

• Reaction of physically controlled objects on each other.

• Used to get desired physical behavior on collisions

• Unity provides a few default Physic Materials

Directional Friction

Character Controller• Used on characters i.e for third-person or first-person player

control

• Does not require a Rigidbody

• Does not react to forces on its own and it does not automatically push Rigidbodies away

• Character Controller colliding with non-static Rigidbodies must be done via script

• Events: OnControllerColliderHit()

Joints

• Joint is a constraint that is defined between two Rigidbodies

• Hinge Joint• Spring Joint• Fixed Joint• Configurable Joint

Joints

• To constrain one Rigidbody to another along a particular axis

• Spring Joint can be used to create an invisible spring that tries to keep two rigid bodies together.

• Fixed Joint can be used to keep two Rigidbody objects together at a constant distance and orientation.

• Configurable Joint allows you to create complex joint configuration

Raycasting

• Shooting ray from specific direction• Use to check if collision is detected

Example Project

Upcoming Lectures

• Wheel Colliders and Car Racing example game• Particle Systems• Rag dolls and Cloth Simulation

Assignment for next lecture – Make simple pool game

Famous Physics based mobile gamesPaper Toss Angry Birds

Cut the Rope

Thank You

Please feel free to drop me note at:syedharoonalam@gmail.com