Squirmin’ Vermin

Core physics loop

The core physics loop using the PhysX library is very simple. The only complicated thing was setting everything up correctly so that the engine and PhysX sent each other all the necessary information. Once that was up and running the core loop was developed consisting of the three steps below.

Update

This first step is where all the information and potential changes that were made to engine objects are sent to the internal PhysX actors. This is done to then call PhysX’s “simulate” function to simulate one frame of physics in the internal PhysX scene. Lastly the new simulated PhysX information is sent to the engine’s internal entities.

Callback

The second step actually happens during the PhysX “simulate” function where the custom defined callbacks are called if PhysX actors collide with each other. These callbacks create contact data for every collision which is then put in a list and used in the next and last step.

Contacts

The last step uses the list of all contacts added during the previous PhysX simulation to resolve and notify all entities about any collisions. This is done by the entities who wants to know about collisions having a collision callback component. This component only has one member; a callback function to be called on collisions.

Character controller

In PhysX the character controllers are used as physics actors for the typical properties a character in the game want to have. Although it works as intended and is easy to use (most of the time) it can still be improved upon. This is why I made a wrapper for it where the biggest improvements i made is Gravity, Terrain movement and Slope movement. These features are all based on if the character is grounded, that is if the character reacts as if grounded even if it might not technically be on the ground in the PhysX system. This is done by setting it to false before the controller is moved by gravity and then having the collision callback set the controller to grounded during the gravity movement as shown in the code just below.

Gravity

The gravity is controlled by a gravitational force that the user can ajust to what the game needs. This force is scaled with the time between frames and then added to a compunding gravitational velocity. The controller is then moved with this velocity each frame thus creating it’s acceleration. The variable mySetGrounded is set to true so that the previously shown collision callback grounds the controller when landing on the ground because of gravity.

Terrain Movement

Terrain movement is where the controller stands on terrain that is currently moving. This created strange behavior with the default PhysX controller. The function below fixes this behavior so that the controller smoothly follows the terrain movement in any direction. It also keeps a user defined percentage of the terrain velocity when having jumped or fallen of the moving terrain. This velocity also has a user defined fall off time for extra ajustments.

Slopes

The slope movement is to have the controller not stutter when moving horizontally down a slope. This was an issue with PhysX’s default controller and is fixed with the simple yet effective function below. It checks if a raycast downwards with a user defined distance hits the ground. If it does the controller is moved this distance downwards and grounded is set to true. This way the user can define how close the controller at any give point has be from the ground to snap to it.