Tutorials · Particle Effects · Chapter 8

Wandering

20 minutes Uses: ParticleEmitter

By the end

Fireflies, fireflies, born over the well and drifting left through the trees to the second lamp
Both chimneys’ smoke curling, the second a copy of the first with a Seed of its own
Noise: Strength, Frequency, Scroll and Seed, each pictured on the smoke

On this page

A field that pushes
Smoke that curls
What a field costs
The fireflies and the chimney smoke
Check it works

Reference

ParticleEmitter

A field that pushes

Everything so far has flown a path its numbers set: a line, an arc, a plume that fans out. Nothing in the real night goes so straight. Noise lays an invisible field of currents over the town, and every particle is pushed a little by whichever current it is in at that moment. The currents are smooth, so nearby particles are pushed alike, and they wander together.

The first thing to wander is a handful of fireflies, near the second lamp. They are born above the well, and they head off to the left, towards the lamp.

Select Town, press + Add node and choose Particles. Rename it fireflies, and set its Position to 528 and 232, above the well.
Set Rate to 4, Lifetime to 7, Speed to 80, Direction to -1 and 0, Spread to 1, Gravity to 0 and Drag to 0.5. They head left, and slow as they go.
Set Start Color to 0.85, 1, 0.35, alpha 1, and End Color to 0.6, 0.9, 0.2, alpha 0: a yellow-green that fades.
Set Start Size to 9, End Size to 6, Capacity to 32, Sort Layer to 5 and Blend to Add. Set Texture to soft-dot.png.
Open Emission Shape, set Shape to Circle, and Radius to 28. Leave Edge Only unticked, so they are born anywhere in the circle.
Open the Noise group and tick Noise. Set Strength to 14, Frequency to 0.04, Scroll to 3 and 0, and Seed to 7.
Press Ctrl+S, then F5 to see the fireflies, and F5 again to stop.

fireflies selected, its Inspector narrowed with the search box to the rows that mention noise: under Noise, Noise ticked, with Strength 14.00 u/s, Frequency 0.040, Scroll x 3.0 and y 0.0, and Seed 7; in the Viewport the node sits above the well on the right, with its move arrows and two small blue handles to its left

Noise: Switches the field of currents on. Off, particles fly only the paths their other numbers give them. The four controls under it set what the field is like.
Strength: How hard the field pushes, in world units a second: the most it adds to a particle’s travel along each axis. It is added to each step’s movement, not to the speed, so it bends a path without piling up into a drift that never ends. Here 14, a gentle push beside the fireflies’ speed of 80. 10 to 20 suits smoke and fireflies; 0 switches the field off and keeps the other three.
Frequency: How fine the currents are, in cells per world unit: the reciprocal of how wide one swirl is. 0.04 is one swirl every 25 units, wide and slow. A low number is a wide, lazy swirl, and a high one is jitter. 0.01 to 0.05 looks like air. Here 0.04 for the fireflies and 0.025, a swirl every 40 units, for the smoke.
Scroll: How fast the field itself drifts, in world units a second, across and down. A still field pushes a particle in one place the same way for ever, which reads as a bent path, not as air. A scrolling one keeps the currents changing under a particle. Here 3 and 0 for the fireflies, a slow drift to the right, and 0 and -10 for the smoke, upward with it.
Seed: Which field, of all the fields that frequency could make: a whole number, with no bigger or smaller meaning. Two emitters with the same Seed, Frequency and Scroll are pushed by the same field wherever they stand. Here 7.

A patch of the night town from the second lamp to the well on the right, in rain, with the trees behind: a dozen or more small glowing yellow-green dots scattered over the dark between the brick wall by the lamp and the well, some low near the lamp’s glow and some higher among the trees, and a ring of blue sparks at the well — Eight seconds in, the fireflies’ **Lifetime** of `7` passed once: fireflies scattered between the well and the second lamp.

Smoke that curls

Smoke leaves its path behind it, so the currents show. The same field goes on both chimneys. The second is a copy of the first, set a little to the right.

Select chimney_a. In the Noise group tick Noise, and set Strength to 18, Frequency to 0.025, Scroll to 0 and -10, and Seed to 1.
With chimney_a still selected, press Ctrl+D to duplicate it. Rename the copy chimney_b, and set its Position to 274 and 201, the second chimney.
On chimney_b, set Direction to 0.25 and -1, and Seed to 2.
Press Ctrl+S.

chimney_b selected, its Inspector narrowed with the search box to the rows that mention noise: under Noise, Noise ticked, with Strength 18.00 u/s, Frequency 0.025, Scroll x 0.0 and y -10.0, and Seed 2; in the Viewport the node sits at the second chimney’s top, with its move arrows and two small blue handles above it

The first two chimneys against the sky and mountains, in rain, with Noise off on both: two soft grey plumes rising almost straight, each a narrow column that spreads a little as it climbs, the first nearly upright and the second leaning slightly to the right — For comparison, **Noise** off on both chimneys: two straight plumes.

The same chimneys with Noise as you set it: the plumes bent and broken into curls and loose puffs, the first hooking to the left at its chimney and then scattering upward in pieces, the second looping at its chimney and drifting away to the right in a thin line — Four seconds in: both plumes curling.

The next four pictures change one control at a time on both chimneys, so each can be set beside the one above.

The same chimneys with Strength 40: the smoke torn into separate puffs thrown far from their chimneys, one high above the first, a pair of bright round puffs in the middle of the sky and a small trail beside the second chimney, with little left of either plume near its chimney — For comparison, **Strength** `40`: the smoke flung about.

The same chimneys with Frequency 0.15: each plume a short chain of round puffs, close to its chimney and stacked in a jittery zigzag, with none of the wide curls — For comparison, **Frequency** `0.15`: a jitter, with no wide curls.

The same chimneys with the two Seeds swapped, 2 on the first and 1 on the second: plumes of the same kind of curl but on other paths, the first now a loose arc of puffs rising to the upper left and the second a drifting line of puffs bending away to the right — For comparison, the two **Seed**s swapped: the same kind of curl, on other paths.

The same chimneys with Scroll 0 and 0: the first plume a smooth, unbroken S-shaped curve and the second a smooth column leaning to the right, neither broken into pieces — For comparison, **Scroll** `0` and `0`: the currents hold still, and each plume is one smooth, unbroken curve.

What a field costs

The fireflies and the chimney smoke

The fireflies

Field	Value
Position	`528`, `232`
Rate	`4`
Lifetime	`7`
Speed	`80`
Direction	`-1`, `0`
Spread	`1`
Gravity	`0`
Start Color	`0.85`, `1`, `0.35`, alpha `1`
End Color	`0.6`, `0.9`, `0.2`, alpha `0`
Start Size	`9`
End Size	`6`
Capacity	`32`
Sort Layer	`5`
Blend	Add
Texture	`soft-dot.png`
Drag	`0.5`
Shape	Circle, Radius `28`, Edge Only unticked
Noise	ticked: Strength `14`, Frequency `0.04`, Scroll `3`, `0`, Seed `7`

The chimney smoke

Field	`chimney_a`	`chimney_b`
Position	`181`, `201`	`274`, `201`
Rate	`12`	`12`
Lifetime	`3.5`	`3.5`
Speed	`18`	`18`
Direction	`0.2`, `-1`	`0.25`, `-1`
Spread	`0.35`	`0.35`
Gravity	`-6`	`-6`
Start Color	`0.62`, `0.65`, `0.68`, alpha `0.7`	the same
End Color	`0.45`, `0.48`, `0.5`, alpha `0`	the same
Start Size	`8`	`8`
End Size	`26`	`26`
Capacity	`48`	`48`
Texture	`soft-dot.png`	`soft-dot.png`
Drag	`0.3`	`0.3`
Noise	ticked: Strength `18`, Frequency `0.025`, Scroll `0`, `-10`, Seed `1`	the same, with Seed `2`

Every other field is as a new Particles node has it. Chapter 9 gives all of these a head start.