Emitter shapes

since 0.2.0 Physics and particles

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Where a particle starts and which way it leaves is the emitter's job, and this package ships four of them. Nothing about a burst's colour or lifetime changes what you see here; only the emitter does.

Step 1: Four shapes, four spots #

A sphere throws particles outwards in every direction, which is what an explosion looks like. A cone narrows that spread around one axis, for a muzzle flash or sparks off a wall. A box starts particles anywhere inside a volume rather than at a single point, which is the only way to get rain over an area rather than rain from a point. A drift barely spreads at all: it is what smoke and rising dust use.

// A sphere throws particles outwards in every direction: an explosion.
// A cone narrows that spread around one axis: a muzzle flash, sparks off
// a wall. A box fills a volume rather than starting at a point: rain over
// an area. A drift barely moves outward at all: smoke.
_shapes = <(Vector3, ParticleEmitter)>[
  (Vector3(-3.0, 0.0, 0.0), const SphereEmitter(speed: Range(1.0, 2.5))),
  (
    Vector3(-1.0, 0.0, 0.0),
    const ConeEmitter(speed: Range(2.0, 4.0), halfAngleDegrees: 18.0),
  ),
  (
    Vector3(1.0, 0.0, 0.0),
    BoxEmitter(
      halfExtents: Vector3(0.6, 0.05, 0.6),
      speed: const Range(0.3, 0.6),
      along: Vector3(0, 1, 0),
    ),
  ),
  (Vector3(3.0, 0.0, 0.0), const DriftEmitter()),
];

Step 2: One burst each #

Every shape is burst from its own position, so the four read as four separate small explosions standing in a row rather than one blur.

_burstAll();
_contributor = context.renderer.addContributor(
  ParticleContributor(_particles),
);

Step 3: What each one is for #

A sphere is the right choice whenever nothing about the burst should favour one direction. A cone is for anything that comes out of a barrel or off a surface: it takes a direction and narrows around it. A drift ignores the direction it is given and always heads up, slowly, which is the one shape built for something that should look weightless.

A box is different: the point of emitting over an area is that every particle should go the same way, rain falling down rather than outwards, so it takes a fixed along direction instead of using the burst's own.

BoxEmitter(
  halfExtents: Vector3(0.6, 0.05, 0.6),
  speed: const Range(0.3, 0.6),
  along: Vector3(0, 1, 0),
),

Tip. All four take a speed range rather than one fixed number. Identical particles read as a texture rather than as an effect, and speed is the cheapest place to add the variation that makes a burst look alive.

Step 4: What the page checks #

Four bursts of a known count should add up to a known total, and the contributor should have actually drawn them.

if (_particles.aliveCount != _perBurst * 4) {
  throw StateError(
    'expected ${_perBurst * 4} particles from four emitters, '
    'got ${_particles.aliveCount}',
  );
}
if (!_contributor.isActive || frame.drawCalls < 1) {
  throw StateError('the emitters did not reach the frame');
}