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 }); }