Java Reference
In-Depth Information
body.adjustVelocity(new Vector2f(Math.cos(theta)*startingSpeed,
Math.sin(theta)*startingSpeed));
insert body into bodies;
}
public override function update():Void{
stepsLeft--;
if (stepsLeft <= 0){
Main.removeWorldNode(this)
}
var ratio = stepsLeft/totalSteps;
if (ratio < 0){
ratio = 0;
}
opacity = fadeInterpolator.interpolate(0.0, 1.0, ratio) as Number;
translateX = bodies[0].getPosition().getX();
translateY = bodies[0].getPosition().getY();
rotate = Math.toDegrees(bodies[0].getRotation());
}
}
Listing 10-4 shows the class
PhysicsParticle
, which extends
Group
,
WorldNode
, and
Particle
. The
init
function of
PhysicsParticle
adds an
ImageView
to the
content
. A
Body
is also created with the same
location and rotation as the
PhysicsParticle
, and it's given a velocity based on the
startingDirection
and
startingSpeed
.
The
update
function is called after each step of the
world
object, and the location and rotation of the
PhysicsParticle
is adjusted to match the
Body
. The lifecycle of the
PhysicsParticle
is also handled by
the function
update
. The value of
stepsLeft
is reduced by one, and if it reduces to zero, the entity is
removed from the scene and the world.
If
fadeInterpolator
is set, the
opacity
of the
PhysicsParticle
is adjusted. The actual
opacity
is
calculated by figuring out what ratio of the
PhysicsParticle's
steps have expired. This ratio is then fed
into the
fadeInterpolator
to get the final result. The spark particles in this example use a spline
interpolator with its value set so that most of the fading is put off until the end of the lifecycle.
Emitters as Bodies
The next example uses only a few bodies, but each body specifies the location of a more traditional
particle emitter. As the emitters move about the screen, the particles they create basically stay in place.
The advantage of this technique is that the number of bodies in the simulation is greatly reduced, so
there are performance gains.
Figure 10-3 shows a number of fireballs bouncing down a field of pegs. As the fireball moves, a
particle is created that drifts slightly upward, giving each fireball a trail effect—something required by
fireballs.
