private void ActivateGravityForce()
{
//
// Setup the gravity+bounce inertia modifier.
//
var target = ExpressionValues.Target.CreateInteractionTrackerTarget();
var posY = target.Position.Y;
var velY = target.PositionVelocityInPixelsPerSecond.Y;
//
// Gravity Force
//
// Adding a factor of 100 since -9.8 pixels / second ^2 is not very fast.
var gravity = -9.8f * 100;
//
// Floor Force
//
// This is the force that resists gravity and causes the bouncing. It's defined as:
// 1. Zero if above the floor,
// 2. Equal and opposite to gravity if "on" the floor (1 pixel above floor or below),
// 3. The effects of 2., plus a reflective force if the direction of motion is still downward and the tracker is below the floor.
// This force is at its strongest (-1.8 * 100 * V) if the tracker is 5 or more
// pixels below floor, and weakest (-1.0 * 100 * V) if the tracker is "at" the
// floor.
//
// The amount the tracker is below the floor, capped to at most 5 below.
var belowFloor = EF.Clamp(0, 0 - posY, 5);
// The time slice our force engine uses.
float dt = .01f;
//
// Defining bounce constants.
// -2 would cause perfectly inellastic reflection, bouncing as high as it fell from.
// -1 would cause perfectly ellastic reflection, freezing motion.
// We want some bounce, but we also want the bouncing to decay, so choose
// bounce factors between -1 and -2.
//
// Also, divide by the time slice width to make this reflective force apply entirely
// all at once.
//
var weakestBounce = -1.1f / dt;
var strongestBounce = -1.8f / dt;
var floorForceExpression = EF.Conditional(posY < 1,
-gravity,
0) +
EF.Conditional(EF.And(velY < 0f, posY < 0f),
EF.Lerp(weakestBounce, strongestBounce, belowFloor / 5) * velY,
0);
//
// Apply the forces to the modifier
//
var modifier = InteractionTrackerInertiaMotion.Create(_compositor);
modifier.SetCondition((BooleanNode)true);
modifier.SetMotion(gravity + floorForceExpression);
_tracker.ConfigurePositionYInertiaModifiers(new InteractionTrackerInertiaModifier[] { modifier });
}
