Exercise 2: Sparks that spread and fall

Purpose. To turn the single file of particles from Exercise 1 into a shower of sparks.

The emitter from Exercise 1 throws every particle up the same line, for the same length of time. Sparks do not behave that way. Each leaves at its own angle, gravity bends its path, and each burns out at its own moment. Four settings produce that.

Before starting

• The emitter from Exercise 1 is in the workspace, with Speed 20 and Drag 2.
• The emitter is selected, so the HUD and the Properties panel are showing.

Procedure

Step	Control	Action	Result
1	Properties ▸ Emission ▸ Enabled	Switch on	The stream from Exercise 1 returns: one file of particles, straight up.
2	Properties ▸ Motion ▸ SpreadAngle	Enter 25	Each particle leaves at its own angle, leaning up to 25 degrees to either side of the Part’s up axis. The file becomes a fan.
3	Properties ▸ Motion ▸ Acceleration	Enter 0, -40, 0	Particles rise, slow, and fall. A particle that leaves straight up peaks about 2.7 studs above the Part after 0.3 seconds. It ends between 11 and 12 studs below the Part.
4	Properties ▸ Emission ▸ Lifetime	Enter 0.6, 1.4	Each particle lasts its own length of time, between 0.6 and 1.4 seconds. They no longer vanish at the same height.
5	Properties ▸ Emission ▸ Rate	Enter 40	Forty particles a second. The shower thickens at once.
6	Properties ▸ Emission ▸ Enabled	Switch off	No new particles appear. Those in flight finish.

NOTE

Drag slows only the travel that comes from Speed. It does not slow the fall that comes from Acceleration. A particle with Drag still falls faster and faster.

NOTE

An edit applies to particles made after it. Particles already in flight keep the settings they were made with. Rate and Enabled are the exceptions: both act at once.

What the numbers did

SpreadAngle holds two angles in degrees, one for each of the two ways a particle can lean away from its line. For every particle, each lean is picked at random between minus the value and plus the value. One number typed alone fills both. At 0, 0 every particle takes exactly the same line.

Acceleration is a steady push, written as three numbers for the X, Y and Z directions of the world, in studs per second per second. A push of 40 downward moves a particle 20 studs in its first second and 80 studs in two.

Lifetime takes two numbers, the shortest life and the longest. Each particle picks one value between them when it is made. One number typed alone gives every particle the same life.

Rate multiplied by Lifetime gives the number of particles in the air at once. Here that is at most 40 × 1.4, or 56.

CAUTION

The plugin stops making new particles when the number alive reaches the Particle Cap in Settings, which starts at 1000. A Rate of 500 with a Lifetime of 4 asks for 2000 at once and will not get them. The Output window reports when the cap is reached.

If the result differs

Symptom	Cause	Action
The fan is flat, like a sheet	Two numbers were typed and one is 0.	Enter 25 alone, or 25, 25.
Particles drift sideways as they fall	Acceleration has a value in X or Z.	Enter 0, -40, 0.
Particles already in flight ignore an edit	They keep the settings they were made with.	Wait one Lifetime.

Variations

Change	Result
SpreadAngle 180	Particles leave in every direction.
SpreadAngle 40, 0	A flat fan, spread one way only.
Acceleration 0, 15, 0	Embers: the push is upward. Particles slow at first, then gather speed again in the second half of their life.
Acceleration 10, -40, 0	A side wind bends the shower along the world’s X direction.
Motion ▸ Direction set to Front	The fan leaves the Part’s front face instead of its top.
Lifetime 1	Every particle lasts exactly one second again.

Log

New in this exercise	SpreadAngle. Acceleration. A Lifetime with two numbers. Rate.
Done before	Selecting the emitter. Enabled on and off (Exercise 1). Typing a value into a Motion row (Exercise 1).
Carried forward	Keep this emitter. Exercise 3 fires it as a single burst.