Effects — fire, smoke and water

An effect in Lightbox is not a filter over your pixels. It is a small fluid simulation whose result is drawn: bands of colour with outlines around them, in your line style, written into the document as ordinary strokes. You can select them, recolour them, transform them, erase into them and export them, and nothing downstream knows they came from a simulation.

That is the whole design. A drawn effect can sit inside a hand-drawn scene without looking pasted on, and it survives everything a drawing survives.

A document that never adds an effect writes nothing about them and shows no effects controls.

Opening the window

Effects ▸ Fluid effects…, or Ctrl+Shift+E. It is a window rather than a panel because tuning an effect is a look-and-adjust loop, and the preview has to be big enough to judge a silhouette in.

The window has three columns:

• Left — the document's elements, and the emitters inside the selected one.
• Middle — everything about the selected element.
• Right — the preview, a frame scrubber, and the two buttons that cost something.

Effects: several elements that are one thing

A real explosion is not one simulation. It is a flash, a fireball, a roll of smoke and some sparks, each with its own timing, its own grid and its own line treatment — and each baking to its own layer, so they composite, blend and z-order like any drawing.

An effect is the name for that set. Select an element and press Group to start one; select another and press Add to put it in. Out takes an element back out and leaves it exactly where it is.

Once elements are in an effect you get three numbers that act on all of them:

• Place X / Y — where the effect's left and top edges sit. Moving it moves every member by the same amount.
• Starts on — the frame the effect begins. Shifting it shifts every member equally, so the smoke stays four frames behind the flash. That spacing is the effect's timing, and lining members up on a common frame would destroy it.

There is no group transform hiding anywhere — those numbers read the members and write the difference back. An element's origin is always honestly its origin, and Ungroup is lossless because there is nothing to un-apply.

• Copy duplicates the whole effect, elements and all, so the copy can be retuned without touching the original.
• Delete removes the effect and everything in it, drawings included.
• Bake the whole effect bakes every member and puts their layers in one folder named after the effect.

Because each element keeps its own grid, layering is cheaper here than in a system with one shared simulation: a small hot fireball can run at four document pixels per cell beside a slow smoke at ten, and each pays only for its own resolution.

Elements do not interact. Each runs its own solver on its own grid, so a fireball does not push its own smoke. If you want the blast to shove the smoke, give the smoke element matching wind or burst — that is a deliberate trade, and it is what buys the per-element grid sizes above.

The one thing to understand: Simulate and Bake

They are separate buttons because they cost different things, by about a factor of forty.

Simulate	Runs the fluid again. Seconds. Needed after anything under The fluid, the emitters, or placement and timing.
Bake	Draws the strokes into the document, replacing whatever this element drew before. Fast, and one undo step.

Everything under Line treatment and Bands re-draws from the simulation already in hand, so it previews as you move the slider and costs nothing. That is where the art direction happens, and it is meant to be played with freely.

When a change does need a new simulation, the preview says "the picture is out of date — press Simulate" rather than quietly running for two seconds. Nothing is lost by leaving it stale while you change five more things.

Clear takes an element's drawings back out and leaves the element itself, so you can keep tuning without a stack of undos.

Making an element

Fire and Smoke on the left both make a new element with one emitter already in it, burning or smoking on the floor of its own grid — an element with no emitter simulates still air and draws nothing, which is not a useful place to start from. It arrives torch-sized, about 192 × 176 document pixels, and already alight rather than starting from cold. Anything wanting a bonfire raises the grid, which is one slider.

The difference between them is what the bands read from, what colours they start with, and two things worth knowing about because they are the difference between smoke that works and smoke that does not:

• A smoke emitter is warm. Smoke rises because it is hot, and the solver means that literally — buoyancy reads heat while weight reads density, so an emitter at zero heat gets pushed down by its own mass and spreads on the floor as a pancake. Smoke's emitter starts at a third of fire's.
• Smoke arrives lit and fire does not. A flame is the light source, so nothing lights it: its bands stay concentric and the ramp from dull red to white is the drawing. Smoke is lit from outside, and unshaded it reads as an onion — three rings round one centre, which is a cross-section rather than a volume. See Shading below.

Everything else is the same machinery, and you can turn one into the other with the Bands read control.

Placement and timing

• First frame / Frames — where the element sits on the timeline.
• Expose on — bake one drawing every N frames and hold it. Two is animating on 2s, and it does more than halve the work: tracing every frame makes the outline crawl, and holding kills it.
• Pre-roll — frames simulated before the first drawn one, so the element opens on an established plume rather than on still air. It does not make a cycle seamless; that is a different problem and not solved yet.
• Grid width / height / Cell size — the simulation's own grid, in cells, and how many document pixels a cell is. The grid is deliberately coarser than your document: cell size is the main lever on cost, and a flame does not get better by being simulated at drawing resolution.
• Origin X / Y — where cell (0,0) sits in the document.
• Substeps — solver steps per frame. The other main lever on cost. More is more accurate, not more detailed.

The fluid

These are what the simulation does, and every one of them means a re-simulate.

• Buoyancy — how hard heat lifts. A flame with none of it stalls.
• Weight — how hard the fluid's own mass pulls down. Smoke has some; fire has almost none.
• Vorticity — puts back the curl a coarse grid loses. This is what makes the tongues and the curls, and it is usually the first thing to reach for when an effect looks limp.
• Drag — air resistance. Without it a closed element circulates forever and reads as a lava lamp rather than as fire.
• Turbulence, scale and drift — a standing eddy field. Scale is the size of the eddies in cells; set it larger than the element and it stops being turbulence and becomes a steady wind, which is a mistake that looks like the fluid being broken.
• Dissipation — how fast the stuff thins out. Smoke wants a little, steam a lot.
• Cooling — how fast heat leaves. This is what gives a flame its length.
• Burning — lets the fluid's own density burn as fuel. See below; it is on for a new fire element and off for everything else.
• Wind X / Y — ambient flow across the element, in cells per step.

Burning — why a flame can shed its tip

Watch a real flame and pieces come off the top and rise on their own for a moment before they go. Without Burning ours could not do that, and the reason was structural rather than a setting being wrong: heat is put in at the emitter and from then on only leaves. A piece still inside the column is refilled from below every frame; a piece that has detached has nothing, so it drops out of the outermost band within one frame. Measured on a plain torch — six pieces came off over forty frames and every one of them lasted a single frame, which reads as sparkle rather than as fire.

Burning gives a detached piece something to live on. The fluid's density is the fuel: an emitter already puts density in beside heat, the flow already carries it, so a piece that breaks away is already carrying a fuel supply — it only needed permission to spend it.

• Burn rate — what fraction of the fuel present is spent per step. Low is a piece that stays lit for half a second; high is a flash that is over before it has risen.
• Ignition — how hot fuel has to be before it burns at all, in the emitter's own heat units (its centre is 1 by default). Raising it stops the cool tail burning, which shortens the flame without shortening the pieces that come off it. Set it above the emitter's heat and nothing ignites, which is how warm smoke stays smoke.
• Yield — heat released per unit of fuel: how fiercely a piece burns, as against how long it lasts.

It puts itself out, and that is deliberate. Burning consumes the fuel, so the heat stops coming while cooling goes on taking its cut, and what is left is cool density — which is to say smoke. A fireball that burns and then becomes smoke is that arc rather than anything you have to author, so an explosion gets it for free.

Burning makes a fire hotter, and a hotter fire climbs. On a grid with room overhead, switching it on lengthens the flame; Vorticity is what takes that back, by spending the rise on curl instead of on height. On the small grid a new element starts with there is no room to climb and it barely shows. Measured, a new fire element:

	flame height	longest-lived piece
burning off	50% of the grid	1 frame
as it ships	47%	4 frames
Vorticity raised to 0.7	43%	7 frames

So if you want the pieces to hang in the air longer, raise Vorticity and expect to pay for it in height.

Wind, and the number that surprises everybody

Wind is in the same units as the flow it acts on, and a plume rises at roughly 0.15 cells per step. So:

• 0.15 bends a flame by about half a right angle.
• 0.5 lays it flat and horizontal.

The useful range for a figure in motion is well under a tenth of what the field's own top speed would suggest. If wind seems to do nothing, you are probably two decimal places out.

A character running right is wind blowing left. A run cycle runs on the spot, so nothing about her motion can be derived automatically — you are choosing the reference frame, and this is the one control artists get backwards.

Emitters

An emitter is where the stuff comes from. An element can have several.

• Shape — a disc, a line between (X,Y) and (X2,Y2), or a whole layer's ink.
• Density — how much stuff it puts in per step.
• Heat — how hot. Buoyancy reads this, so heat is what makes a flame rise rather than sit. A smoke emitter usually has none.
• Velocity X / Y — a push given to the fluid at the emitter, for a jet or a vent.
• Burst — push outward from the emitter's own centre rather than in one direction. This is the difference between an explosion and a plume: the velocities above move the whole stamp one way, as a lump, while a burst pushes every part of it a different way, so the front expands as a ring. The two combine — a burst that also travels is a muzzle flash.
• Emit from / Emit for — when the emitter feeds, and for how long. Normally it runs the whole element and these write nothing. A blast is one or two frames: an emitter that keeps feeding refuels its own fireball every frame, so it never cools into smoke and never disperses.
• Travel X / Y — where the emitter's origin goes. This is how an effect is carried along with something: the emitter moves, and the fluid it has already laid down does not, which is what makes a trail behave like a trail instead of like a rigidly translated picture.

Emitting from a drawing

Emit from points an emitter at a layer, and then it emits from wherever that layer has ink. This is how flames follow a drawn costume: paint the hem of a cloak on its own layer, point an emitter at it, and the fire is on the hem.

The mask is the emission — it is not intersected with anything at bake time, and only the emitter's own origin (and its Travel keys) moves it.

Scatter — flames on a surface, rather than a surface on fire

An emitter feeds every cell it covers, every frame. Over a wide area that means nothing can ever detach: whatever rises is replaced from below the same frame, so a painted hem reads as one continuous burning edge.

Scatter breaks it into separate flames standing over the same area.

• Coverage — what fraction of the surface burns. A fraction rather than a count, so painting a longer hem gives proportionally more flames with nothing else to change.
• Spacing — how far apart they stand, in cells, and how big they are: each flame is half a spacing across, so they just touch at full coverage. One number, because that is how you would describe a row of flames.
• Size varies — how much they differ in width at the base.
• Heat varies — how much they differ in fierceness, which for fire means which colour bands each one reaches: some running up into the pale core, others staying dull red.
• Lean — a sideways push that differs per flame, so they stop swaying in step with each other.

The flames come out at different heights on their own. You do not need to ask for it, and turning heat varies up will not give you more of it. What makes it is the fluid: a flame with neighbours either side is fed by their rising column and runs tall, one on the end of a run is not and stays short. Measured with every variation at zero, a scattered hem burns at heights of 10, 16, 24, 30, 38, 42 and 44 cells.

Scatter belongs to the emitter, not to fire and not to masks — a plain disc scatters into a handful of small flames inside its own outline, and smoke or steam off a surface scatters the same way.

Bands and colour

The drawing is a set of nested contours through the field — the bands — each filled, with outlines around them.

• Bands read — Temperature for fire, where the heat ramp is the drawing, or Density for smoke.
• Band low / high — where the bands sit in the field's range, as a fraction of the highest value the element reaches anywhere. They belong low. A plume's field is steeply peaked: only about 4% of cells hold more than a hundredth of the peak, so bands spread across the whole range all land inside the brightest core and draw scraps. The defaults put the outermost band around the visible edge, which is where a silhouette belongs.
• Colours — one hex per band, outermost first, separated by spaces.
• Outline — the colour the outlines are drawn in.

The range is measured over the whole element, never per frame. A range that followed each frame's own peak would rescale the bands every frame, and a band that moves because its scale moved is flicker.

Line treatment — the art style

This is the part that makes an effect look like your drawing rather than like a simulation. All of it is cheap, and all of it previews live.

• Bands — how many.

• Outlined — None (fills only), Silhouette (outline the outer edge only) or Every band.

• Band spacing — Even, or CoreBiased to crowd them towards the hot middle.

• Line weight — the base thickness, in stroke widths.

• Offset — push the outline in or out of the fill's edge.

• Simplify — how hard the contour is reduced. Higher is fewer, longer, more deliberate lines.

• Smoothing — how much the corners are rounded off.

• Break length / gap — a broken, sketched line instead of a continuous one.

• Light angle — where the light is, in degrees clockwise from straight up. One light serves both halves of lighting: the shading below, and the line weight if a driver reads it.

• Shading — slide the inner bands toward the light, in stroke widths, so a volume reads as lit instead of as an onion of concentric rings. The silhouette never moves — it is the silhouette — and the bands inside it move further the deeper they are, which puts the highlight on the lit side and crowds the rest into a crescent opposite. A flame wants none of this; smoke, steam and dust want most of it.

  It is clamped so a highlight can never leave the silhouette, so past a point the slider stops doing anything. That is the shape telling you it has run out of room, not the control breaking.

A treatment can be shared between elements. When an element states a field of its own, that row shows a ↺ button: the value is this element's override, and one press puts it back to what the shared treatment says. Revert all does the lot.

The pen

Take brush gives the element the brush the toolbar is currently holding, and its size becomes the width every treatment distance is measured in. It is copied on the press, not read when you bake — so a re-bake always draws the same line, and changing your brush afterwards does not silently restyle every effect in the document. Default pen takes it back off.

Making an explosion

Everything it needs is above; what is not obvious is which handful to reach for.

1. Emit for 1 or 2 frames. Everything else follows from the blast being over before the smoke starts.
2. Burst around 0.5–1. Enough that the front rolls outward rather than drifting.
3. Density high, heat modest. Heat is what makes it rise, and a blast that rises too fast reads as a plume with a bang at the bottom. Add heat back once the shape is right.
4. Vorticity up. This is what turns an expanding disc into something with rolls and tongues in it.
5. Band low down. The band range is measured against the highest value the element reaches anywhere, and for a blast that is frame one — so everything after it is being compared to a peak it will never see again, and the effect thins out of the bands sooner than you expect. Lowering Band low brings the dispersing smoke back.

Point 5 is the one that surprises people, and it is the cost of a deliberate choice: a band range that followed each frame's own peak would rescale the bands every frame, and a band that moves because its scale moved is flicker. Steady plumes get the better end of that trade and blasts get the worse one.

Keeping an effect — the library

Keep puts the selected effect in your library under its name; the box beside Effects lists what is on the shelf, and Use makes the chosen one again here. It arrives at the current effect's place and frame, so using one beside something you are already working on does not drop it at the corner of the canvas.

A preset stores the parameters, not the drawings. Using one re-simulates — which costs a moment, and in exchange the new one is a real effect you can then retune: forty frames instead of twenty-four, twice the size, blowing left. It draws exactly what the original drew, moved to wherever you put it.

Two consequences worth knowing:

• The library is somewhere to choose from, not something your document depends on. Using a preset copies it in, so the document keeps working with the library gone, and editing the preset afterwards does not reach back into effects already made from it.
• Layer references travel by name. An effect that emits from a painted layer cannot carry that layer's identity into another document — nothing there would match it, and an emitter pointed at a layer that is not present emits nothing at all. So the preset remembers the layer's name and reconnects to a layer called the same thing. If there is none, the reference is dropped and you are told which name was missing, rather than finding out when you bake.

(Planned: a project-scoped shelf, so a show can carry its own fire alongside each artist's own library.)

Baking, editing, and re-baking

Bake writes the drawings into a layer of the element's own, as one undoable step. They are ordinary strokes from that point on.

One thing to know about undo here. Moving a slider in this window is not its own undo step — a hundred of them while tuning would bury the history you actually want. The document is still marked as changed, so nothing is lost silently. But an undo taken after tuning and before baking puts the parameters back along with everything else. Bake when you like what you see.

Re-baking replaces what the element drew last time and leaves everything else alone. A stroke you have edited by hand stops belonging to the element, so a re-bake will not take your work back out — but it also will not update it.

What is planned

• Emission flicker, for shimmer. It used to be the planned answer to "a painted area reads as a burning edge" — Scatter is that answer now, and a better one, because its gaps are in the same place every frame so what rises off a flame actually leaves.
• Obstacles — pointing an element at a layer whose ink the fluid cannot pass through, so smoke goes around a figure rather than through her. There is no control for it yet, on purpose: the solver has walls only at the edge of its grid, and a picker that stored a layer and changed nothing would just look broken.
• Attaching an element to a drawing's anchor, so it follows a rigged figure without keying Travel by hand.
• Saving an effect as a symbol, so a baked explosion can be dropped in three times at different frames, one of them mirrored — symbols already carry animation frames and per-placement time offset, so most of this exists.
• A project-scoped effects shelf, so a show carries its own fire beside each artist's own library.
• Water and goo. The solver is the same; a free surface and a metaball source are not built.
• Style inference — a reference drawing in, a line treatment out.