/// <summary>
/// PickRandomDirection is used by AddParticle to decide which direction
/// particles will move.
/// </summary>
private Vector2 PickRandomDirection()
{
float angle = ParticleHelpers.RandomBetween(settings.MinDirectionAngle, settings.MaxDirectionAngle);
// our settings angles are in degrees, so we must convert to radians
angle = MathHelper.ToRadians(angle);
return(new Vector2((float)Math.Cos(angle), (float)Math.Sin(angle)));
}
// initialize is called by ParticleSystem to set up the particle, and prepares
// the particle for use.
public void Initialize(Vector2 position, Vector2 velocity, Vector2 acceleration,
float lifetime, float scale, float rotationSpeed)
{
// set the values to the requested values
this.Position = position;
this.Velocity = velocity;
this.Acceleration = acceleration;
this.Lifetime = lifetime;
this.Scale = scale;
this.RotationSpeed = rotationSpeed;
// reset TimeSinceStart - we have to do this because particles will be
// reused.
this.TimeSinceStart = 0.0f;
// set rotation to some random value between 0 and 360 degrees.
this.Rotation = ParticleHelpers.RandomBetween(0, MathHelper.TwoPi);
}
// this function is called when we want to demo the explosion effect. it
// updates the timeTillExplosion timer, and starts another explosion effect
// when the timer reaches zero.
private void UpdateExplosions(float dt)
{
timeTillExplosion -= dt;
if (timeTillExplosion < 0)
{
Vector2 where = Vector2.Zero;
// create the explosion at some random point on the screen.
where.X = ParticleHelpers.RandomBetween(0, graphics.GraphicsDevice.Viewport.Width);
where.Y = ParticleHelpers.RandomBetween(0, graphics.GraphicsDevice.Viewport.Height);
// the overall explosion effect is actually comprised of two particle
// systems: the fiery bit, and the smoke behind it. add particles to
// both of those systems.
explosion.AddParticles(where, Vector2.Zero);
smoke.AddParticles(where, Vector2.Zero);
// reset the timer.
timeTillExplosion = TimeBetweenExplosions;
}
}
/// <summary>
/// InitializeParticle randomizes some properties for a particle, then
/// calls initialize on it. It can be overriden by subclasses if they
/// want to modify the way particles are created. For example,
/// SmokePlumeParticleSystem overrides this function make all particles
/// accelerate to the right, simulating wind.
/// </summary>
/// <param name="p">the particle to initialize</param>
/// <param name="where">the position on the screen that the particle should be
/// </param>
/// <param name="velocity">The base velocity that the particle should have</param>
private void InitializeParticle(Particle p, Vector2 where, Vector2 velocity)
{
// Adjust the input velocity based on how much
// this particle system wants to be affected by it.
velocity *= settings.EmitterVelocitySensitivity;
// Adjust the velocity based on our random values
Vector2 direction = PickRandomDirection();
float speed = ParticleHelpers.RandomBetween(settings.MinInitialSpeed, settings.MaxInitialSpeed);
velocity += direction * speed;
// pick some random values for our particle
float lifetime =
ParticleHelpers.RandomBetween(settings.MinLifetime, settings.MaxLifetime);
float scale =
ParticleHelpers.RandomBetween(settings.MinSize, settings.MaxSize);
float rotationSpeed =
ParticleHelpers.RandomBetween(settings.MinRotationSpeed, settings.MaxRotationSpeed);
// our settings angles are in degrees, so we must convert to radians
rotationSpeed = MathHelper.ToRadians(rotationSpeed);
// figure out our acceleration base on our AccelerationMode
Vector2 acceleration = Vector2.Zero;
switch (settings.AccelerationMode)
{
case AccelerationMode.Scalar:
// randomly pick our acceleration using our direction and
// the MinAcceleration/MaxAcceleration values
float accelerationScale = ParticleHelpers.RandomBetween(
settings.MinAccelerationScale, settings.MaxAccelerationScale);
acceleration = direction * accelerationScale;
break;
case AccelerationMode.EndVelocity:
// Compute our acceleration based on our ending velocity from the settings.
// We'll use the equation vt = v0 + (a0 * t). (If you're not familar with
// this, it's one of the basic kinematics equations for constant
// acceleration, and basically says:
// velocity at time t = initial velocity + acceleration * t)
// We're solving for a0 by substituting t for our lifetime, v0 for our
// velocity, and vt as velocity * settings.EndVelocity.
acceleration = (velocity * (settings.EndVelocity - 1)) / lifetime;
break;
case AccelerationMode.Vector:
acceleration = new Vector2(
ParticleHelpers.RandomBetween(settings.MinAccelerationVector.X, settings.MaxAccelerationVector.X),
ParticleHelpers.RandomBetween(settings.MinAccelerationVector.Y, settings.MaxAccelerationVector.Y));
break;
default:
break;
}
// then initialize it with those random values. initialize will save those,
// and make sure it is marked as active.
p.Initialize(
where,
velocity,
acceleration,
lifetime,
scale,
rotationSpeed);
}