User:Wmonroe4/Motion controllers

Creating a motion controller

At its most basic, setting up a motion controller is as simple as:

system.require('std/movement/motion.em');
// ...
new motion.SomeTypeOfMotion(presence, extra arguments);

As a simple example, with no extra arguments:

new motion.Gravity(presence);

This will set presence falling at about 9.81 meters per second squared downward, until the end of time.

suspend and reset

If this is all you want, then that one line is all you need. If, however, you may want to release the presence from the hold of gravity, or change the direction or acceleration of gravity, you're going to need to save the controller object for later:

var gravity = new motion.Gravity(presence);

Then when you need to, you can stop gravity:

gravity.suspend();

and restart it later:

gravity.reset();

Every motion controller has suspend and reset methods. suspend makes the controller stop acting on the presence, and reset makes it start again. (These methods are not well tested, so let me know if you think you've found a bug in one.)

Extra arguments and fields

Every motion controller also has fields that you can modify at any time to change the behavior of the controller. Gravity has two: presence and accel. By reassigning presence, you can change which presence the gravitational acceleration applies to, and by changing accel (a vector), you can change how fast the presence falls:

gravity.accel = <0, -5, 0>;

or even make the presence fall sideways:

gravity.accel = <4, 0, 0>;

These fields are initialized from arguments to the constructor, so if you simply wanted gravity to be in a bizarre direction to begin with, you could just create it like this:

var gravity = new motion.Gravity(presence, <4, 2, -9>);

The constructor arguments are much more versatile than the dynamic fields, so it can be to your advantage to set up the properties you want in the constructor. Above, you saw that the accel argument to the gravity constructor is optional; if you leave it out, it will assume a reasonable default (down, at standard Earth acceleration). If instead you just want really weak gravity, the constructor can simply take a number:

var gravity = new motion.Gravity(presence, 0.8);

Down is still implied. The dynamic accel field isn't as smart: unlike the constructor argument, the accel field has to be a vector, so assigning 0.8 to gravity.accel won't work—you have to explicitly assign <0, -0.8, 0>.