PhysX is the Unity Engine's built-in physics engine. Because it is the default option, most controllers in TDW use PhysX.
In PhysX, objects can optionally be physics-enabled. In a video game, if you want to add a UI effect like a blinking arrow pointing to a location, you don't actually want the arrow to interact with the game's physics state.
In TDW, all objects are physics-enabled, meaning that they have Rigidbodies. The rigidbody has mass, velocity, momentum, friction, and so on.
Objects also have colliders. These are invisible 3D meshes defining the physical shape of the object. All objects in TDW have colliders:
When an object moves or collides with another object, the physics engine automatically updates its state.
- It is possible to add multiple objects to TDW such that their bodies intersect each other. This is technically possible in PhysX but not recommended! The objects will have very glitchy and unpredictable behavior.
- Objects in TDW initially have a mass of 1 kg and default friction values, meaning that you might apply a force to an object and it will respond in unexpected ways. The reason for this and how to set reasonable physics parameters will be covered in the next page of this tutorial.
- You should adjust the state of an object using non-physics commands such as
teleport_objectonly when an object is first created. After that, you should only use commands such asapply_force_to_object. If you don't, PhysX will interpret the command as a very fast motion. And because the motion is technically being handled outside of the physics state, it can result in glitchy behavior.
- PhysX has been optimized for video games and is therefore built to interpolate between physics states--the objective is to yield a reasonable result and keep the game running at a steady framerate. As such, it's optimized for precision. Most notably, PhysX is not a deterministic physics engine.
- The collider meshes usually don't exactly match the render meshes of an object. This is because collider meshes must be convex, and therefore many objects are composed of multiple convex collider hull sub-meshes. The collider meshes in TDW actually match the render meshes very accurately for a Unity Engine simulation! But it's still possible that there will be some noticeable occasional discrepancies; most commonly, objects will sometimes seem to float slightly above the floor.
- PhysX can only simulate rigidbody physics. It can't simulate soft body deformation, cloth, or fluids.
- PhysX is closed source and it's therefore impossible to make low-level adjustments or optimizations to the physics engine.
Next: Object physics parameters
Command API: