// PickRandomDirection is used by InitializeParticles to decide which direction
// particles will move. The default implementation is a random vector on a circle.
protected virtual Vector2 PickRandomDirection()
{
float angle = Utils.RandomBetween(0, (float)Math.PI * 2);
return(new Vector2((float)Math.Cos(angle),
(float)Math.Sin(angle)));
}
// 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.
void CreateExplosions(float elapsedTime)
{
timeTillExplosion -= elapsedTime;
while (timeTillExplosion < 0)
{
Vector2 where = canvas.Size.ToVector2();
// Create the explosion at some random point on the screen.
if (ThumbnailGenerator.IsDrawingThumbnail)
{
where /= 2;
}
else
{
where.X *= Utils.RandomBetween(0.25f, 0.75f);
where.Y *= Utils.RandomBetween(0.25f, 0.75f);
}
// 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);
smoke.AddParticles(where);
// Reset the timer.
timeTillExplosion += TimeBetweenExplosions;
}
}
// InitializeParticle is overridden to add the appearance of wind.
protected override void InitializeParticle(Particle particle, Vector2 where)
{
base.InitializeParticle(particle, where);
// The base is mostly good, but we want to simulate a little bit of wind heading to the right.
particle.Acceleration.X += Utils.RandomBetween(10, 50);
}
// PickRandomDirection is overriden so we can make the particles always have an initial velocity pointing up.
protected override Vector2 PickRandomDirection()
{
// Point the particles somewhere between 80 and 100 degrees.
// Tweak this to make the smoke have more or less spread.
float angle = Utils.RandomBetween(Utils.DegreesToRadians(80), Utils.DegreesToRadians(100));
// From the unit circle, cosine is the x coordinate and sine is the y coordinate.
// We're negating y because on the screen increasing y moves down the display.
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)
{
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 = Utils.RandomBetween(0, (float)Math.PI * 2);
}
// Randomizes some properties for a particle, then calls Initialize on it.
// This 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.
protected virtual void InitializeParticle(Particle particle, Vector2 where)
{
// First, call PickRandomDirection to figure out which way the particle
// will be moving. Velocity and acceleration values will come from this.
Vector2 direction = PickRandomDirection();
// Pick some random values for our particle.
float velocity = Utils.RandomBetween(minInitialSpeed, maxInitialSpeed);
float acceleration = Utils.RandomBetween(minAcceleration, maxAcceleration);
float lifetime = Utils.RandomBetween(minLifetime, maxLifetime);
float scale = Utils.RandomBetween(minScale, maxScale);
float rotationSpeed = Utils.RandomBetween(minRotationSpeed, maxRotationSpeed);
// Then initialize the particle with these random values.
particle.Initialize(where, velocity * direction, acceleration * direction, lifetime, scale, rotationSpeed);
}