Intro to Unity DOTS Physics

What you'll develop in this DOTS Physics section

Github branch link: https://github.com/moetsi/Unity-DOTS-Multiplayer-XR-Sample/tree/Updating-Bullets-to-Destroy

Functionalities included

• Unity DOTS Physics

  • Using FixedStepSimulationGroup for custom systems

• Physics Body

  • Adjusting PhysicsVelocity programmatically

• Physics Shape

  • Updating shape type to prefab

• Physics Categories

  • Custom categories for collisions and interactions

• Triggers on collisions

  • Changing stateless triggers to buffer of stateful triggers

• Acting on triggers to change materials and destroy entities

Unity DOTS Physics

Unity Resources

Unity's 2019 overview of physics in DOTS: https://www.youtube.com/watch?v=tI9QfqQ9ATA Recommended to watch.

Unity's 2020 update of physics: https://www.youtube.com/watch?v=n_5RGdF7Doo If interested in the latest and greatest.

Link to Unity's DOTS Physics samples: https://github.com/Unity-Technologies/EntityComponentSystemSamples/blob/master/UnityPhysicsSamples/Documentation/samples.md Good place to see the contents of the Physics Samples without opening Unity Editor (ReadMe is missing some samples that are contained in the repo so a little misleading)

Unity's DOTS Physics v0.50.0-preview.43 documentation overview: https://docs.unity3d.com/Packages/[email protected]/manual/getting_started.html. Key to get an understanding

Unity DOTS Physics forum: https://forum.unity.com/forums/dots-physics.422/ Helpful to find answers to questions.

Setting up for this gitbook section on DOTS Physics

• Go to https://github.com/Unity-Technologies/EntityComponentSystemSamples and download the repository for Unity's ECS Samples (if you haven't already)

  • Or you could use:

  git clone https://github.com/Unity-Technologies/EntityComponentSystemSamples.git

  • in terminal to download the repository

• Open the downloaded repository navigate to PhysicsSamples > Assets > Demos > 2. Setup > 2d. Events > 2d1. Events - Triggers.unity

  • This will warn you that our Unity version (2020 LTS) is different than the sample version

  • This is okay and will not cause issues so click Continue

• This is a good sample because it shows both triggers and collisions

  • Breaking the translucent red cubes triggers gravity to switch

  • The balls collide with each other

• Navigate into the "2d1. Triggers" folder and open "2d1a. Triggers - Change Material"

• We will base our Physics updates to our project on this sample

• The balls change colors when passing through the colored prisms and collide with the block objects

• Our bullets will pass through and change the material of asteroids and players (similar to how the balls pass through the prisms)

• Bullets passing through an asteroid or player will also trigger adding a DestroyTag to those entities which will cause them to be destroyed

• Select different prefab GameObjects in the Hierarchy (ex: "Floor" under Physics Scene Basic Elements), and notice the Physics Shape component in the Inspector

  • Under "Collision Filter" in the Material section (still in the Physics Shape component) check out the "Belongs To" and "Collides With" fields

    • If you can't find Collision Filter, expand the Physics Shape component section

  • Different GameObjects in the Hierarchy have different values set for "Belongs To" and "Collides With"

  • This is how we tell Unity Physics what entities interact with each other

• Scroll to the bottom of the "Collides With" drop down list and select "Edit Physics Category Names" to be taken to PhysicsCategoryNames in the main Assets folder

• Take a look at the different categories for all the samples in the repo

• We can see that in DOTS Physics, the Physics Shape component on our prefabs will decide what the prefab interacts with

• Also in the Physics Shape component under "Material" there is a field called "Collision Response" with 4 options:

  • Collide (bang together, which our players and asteroids will do)

  • Collide and Raise Collision Events (bang together and cause an event; we will not be using this in this gitbook only because we do not want to over-complicate things by doing too much)

  • Raise Trigger Events (not bang together but cause a trigger event, like our bullets will do to players and asteroids)

  • None (nothing happens from collisions)

To best prepare for the following DOTS Physics code-alongs, we recommend you complete the following check-list:

• Watch a Physics explanation video(s)

• Read through Unity's DOTS Physics overview documentation

• Download and open Unity's Physics samples in the repo you just downloaded above

• Navigate sample scene Hierarchies and check out the different Physics Body and Physics Shape settings to get a feel of different set ups