Motion

The Motion section sets how a particle travels and turns after it is made. Part, Attachment and Model emitters share it. The Accel toward rows are on Part emitters only.

What it makes possible:

Sparks that rise, slow and fall. Speed 20, Drag 2, Acceleration 0, -40, 0.
A path that curls into a loop. Dir. Mode Local, Rotation Mode Speed, and a level Rotation Speed.
Particles that gather on the emitter, as for a charge-up. Reverse Motion on.
Smoke that drifts. A Turbulence graph that rises over life, with a low Turb. Frequency.
Sparks that home in on a target. Accel toward on, with a Target and a Strength.
Particles pushed away from a target. A negative Strength.

The rows

Row	Kind	Starts at	Unit	What it does
Direction	List	`Top`		The face that particles leave from.
SpreadAngle	Two numbers	`0, 0`	Degrees	How far a particle can lean away from the launch direction.
Dir. Mode	List	`RigidLocal`		Whose faces Direction means: the emitter’s, the particle’s or the world’s.
Speed	Graph	`0`	Studs per second	The speed along the launch direction, over the particle’s life.
Acceleration	Three numbers	`0, 0, 0`	Studs per second per second	A steady push, written as X, Y and Z.
Drag	Number	`0`	Per second	Slows the travel that comes from Speed.
Reverse Motion	Switch	Off		Plays the particle’s path backwards.
Displacement Mode	List	`Global`		Chooses the axes the Displacement graphs are measured along.
Displacement	Graph for each of X, Y, Z	`0`	Studs	An offset from where the particle would otherwise be, over its life.
Turbulence	Graph	`0`	Studs	The size of a smooth random wander, over the particle’s life.
Turb. Frequency	Number	`1`	Per second	How fast the wander changes.
Rotation Mode	List	`OverLife`		Whether Rotation Speed gives an angle or a rate of turn.
Rotation Speed	Graph for each of X, Y, Z	`0`	Degrees, or degrees per second	Turns the particle during its life.
Accel toward	Switch	Off		Part only. Pulls particles toward a target. Shows the two rows below.
Target	Instance, picked with `*`	None		Part only. The object particles are pulled toward.
Strength	Graph	`0`	Studs per second per second	Part only. The strength of the pull, over the particle’s life.

Exercise 1 enters Speed and Drag for the first time. Exercise 2 adds SpreadAngle and Acceleration.

How the terms add up

At every step, a particle moves by the sum of five terms.

Term	Set by	Slowed by Drag
Travel along the launch direction	Direction, SpreadAngle, Dir. Mode, Speed	Yes
A steady push	Acceleration	No
A pull toward a target	Accel toward, on a Part emitter	No
A drawn offset	Displacement	No
A random wander	Turbulence	No

Drag acts on the first term only. Under a downward Acceleration, a particle with Drag still falls faster and faster. It never reaches a top speed.

For an emitter nested in a Model emitter with Scale Motion on, the whole sum is multiplied by the Model’s scale.

What is ignored when

Condition	Effect
Speed is 0	Direction, SpreadAngle and Dir. Mode do not move the particle. Dir. Mode `Global` still removes the emitter’s rotation from the particle.
Use shape is on, on a Part emitter	Direction is replaced by the direction out of the shape.
Drag is not 0	Only the travel from Speed is slowed.
Reverse Motion is on	Accel toward is off. An Anim. Steps above 500 counts as 500.
Dir. Mode is `Local`	Orientation `VelocityParallel` is not applied. Rotation and Rotation Speed steer the path.
Target is empty, has been removed, or is of a kind the row does not accept	Accel toward adds nothing.
The emitter is a Model or an Attachment	There are no Accel toward rows.

Direction

Direction names the face that particles leave from.

Option	Particles leave along
`Top`, `Bottom`	The up axis, upward or downward
`Front`, `Back`	The front axis, forward or backward. `Front` is the part’s Front face.
`Right`, `Left`	The sideways axis, to the right or to the left

Whose axes these are depends on Dir. Mode. With Use shape on, a Part emitter does not use Direction. See Spawn.

SpreadAngle

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. The pick is made once, when the particle is made.

SpreadAngle	Result
`0, 0`	Every particle takes exactly the same line.
`30, 0`	A flat fan, 30 degrees to either side.
`30, 30`	A pyramid of directions with a squarish base. The two leans are picked separately, so it is not a round cone.
`180, 180`	Every direction.

One number typed alone fills both angles.

Dir. Mode

Dir. Mode decides whose faces Direction means.

Option	Launch direction	Also
`RigidLocal`	The chosen face of the emitter, at the moment the particle is made. The picked Rotation does not change the direction.	The path is a straight line, bent only by the other terms.
`Local`	The chosen face of the particle itself, after its Rotation has been picked. It is read again at every step from the particle’s current rotation.	Rotation Speed bends the path. A steady rate of turn gives a circular arc.
`Global`	The chosen axis of the world. `Top` is the world’s up.	The rotation of the emitter and of the link is removed from the particle. It keeps only its picked Rotation.

With a link and RigidLocal, the Link Mode decides whose rotation gives the face.

With Local, the bend is traced one step at a time. Anim. Steps sets how fine the steps are.

Uses:

Local with a Rotation range and a SpreadAngle of 0: a fan of paths, one for each picked Rotation.
Local with Rotation Mode Speed: spirals and loops.
Global with Direction Bottom: particles that always leave straight down, however the emitter is turned.

Speed

Speed is measured in studs per second along the launch direction. It is read from the graph at the particle’s age. A negative value moves the particle backwards along the same line.

Without Drag, the distance travelled is the Lifetime multiplied by the average height of the graph.

Speed	Lifetime	Distance
A level line at 10	2 seconds	20 studs
A straight line from 20 down to 0	1 second	10 studs

Speed starts at 0, so the particles of a new emitter do not travel. See Speed of 0.

Two numbers typed into the box, as 8, 20, give each particle one speed of its own between the two. It keeps that speed for life.

Acceleration

Acceleration is a steady push, written as three numbers for X, Y and Z. It is measured in studs per second per second. The speed it adds grows every second, so the distance grows with the square of the time.

Acceleration	After 1 second	After 2 seconds
`0, -50, 0`	25 studs down	100 studs down
`0, -40, 0`	20 studs down	80 studs down

On a Part or a Model emitter, X, Y and Z are the axes of the world.

Fewer than three numbers typed into the box repeat the last one. 5 becomes 5, 5, 5, and 0, -50 becomes 0, -50, -50.

Drag

Drag slows the travel that comes from Speed. The larger the number, the sooner that travel stops.

Drag	Speed left after 1 second
0	All of it
1	36.8 percent
2	13.5 percent
5	0.7 percent
-1	272 percent. A negative Drag makes the speed grow.

Take a level Speed of 10 with a Drag of 2. After 1 second the particle moves at 1.35 studs per second. However long it lives, it never travels farther than Speed divided by Drag, here 5 studs.

Drag does not slow Acceleration, Accel toward, Displacement or Turbulence.

Reverse Motion

With Reverse Motion on, a particle starts where its path would have ended. It then travels back along the path to where it would have started.

Position and rotation play backwards.
Size, Color, Transparency, Brightness and flipbooks still play forwards.
The reversed path includes Speed, Drag, Acceleration, Displacement, Turbulence and rotation.
Accel toward is off.
An Anim. Steps above 500 counts as 500.
With a link, the reversed path is carried by the linked object. Under Link Mode Weld the particle still rides the object.

Uses:

Particles that gather on the emitter, for a charge-up or an absorbing effect.
A falling arc played backwards, which lands exactly on the emitter.

Displacement

Displacement is three graphs, one for each of X, Y and Z. Each graph is an offset in studs from where the particle would otherwise be, at each age. It is added on top of the travel from Speed and Acceleration.

Displacement Y	Result
A line from 0 to 5	The particle ends 5 studs higher than it would have.
0, then 3 in the middle, then 0	The particle is lifted 3 studs in mid-life and returned.
A graph that does not start at 0	The place the particle is made is shifted by that first value.

Displacement Mode chooses the axes the three graphs are measured along.

Displacement Mode	The X, Y and Z of Displacement run along
`Global`	The axes of the world.
`Local`	The particle’s own axes as it was made, after its Rotation was picked. Every particle’s path is turned differently.
`RigidLocal`	The emitter’s axes at the moment the particle was made, before its Rotation was picked. All particles share them.

Under Link Mode Weld, a Local Displacement turns with the linked object.

Displacement is an axis row. The X, Y and Z buttons choose which of the three graphs the box and the ⋯ button act on.

The first button clicked is outlined. The box shows its graph.
A value typed into the box is written to every lit axis.
With no button lit, typed text flashes red and ⋯ does nothing.
The * button in the label links axes. See Axis linking.

Uses:

A wobble on one axis. A graph that rises and falls several times.
An orbit. A cosine-shaped graph on X and a sine-shaped graph on Z.
An arc that ends at a known offset, whatever the Lifetime.
A fan of drawn paths. Displacement Mode Local with a Rotation range.

Turbulence

Turbulence makes each particle wander smoothly and at random around its path, on all three axes. The Turbulence graph sets the size of the wander over the particle’s life. A larger value gives a wider wander.

Turb. Frequency sets how fast the wander changes. A high number gives a jitter. A low number gives a slow drift.

Every particle wanders its own way.
The wander is measured from the place the particle is made. It does not move that place.
A level graph at 0 switches Turbulence off.
The axes of the wander follow Displacement Mode.
The wander is tied to the particle’s age. Under Reverse Motion or a negative Timescale, the particle retraces the same wander.
At a Turb. Frequency of 0 the particle does not wander. It is pushed in one random direction, by the amount the graph gives.

Effect	Turbulence	Turb. Frequency
Smoke that drifts	A graph that rises over life	Low
Nervous sparks	A small level value	High
Embers that settle	A graph that falls to 0	Any

Rotation Speed

Rotation Speed is three graphs, one for each of X, Y and Z. The three values are combined by Rot Order. They turn the particle about its own axes, on top of the rotation it was made with.

Rotation Mode decides what a graph value means.

Rotation Mode	A graph value is	Worked figure
`OverLife`	The angle itself, in degrees, at that age.	A line from 0 to 360 gives exactly one turn in a life, whatever the Lifetime. A level line gives a fixed tilt and no spin.
`Speed`	A rate of turn, in degrees per second.	A level line at 180 gives half a turn every second. A particle that lives 0.5 seconds makes a quarter turn.

One mode applies to all three axes. A graph with a random spread gives each particle its own offset, picked once for each axis.

With Dir. Mode Local, Rotation Speed also bends the path the particle travels.

For an emitter nested in a Model emitter with Scale Rotation on, the values are multiplied by the Model’s scale.

Rotation Speed is an axis row. Its X, Y and Z buttons and its * button work as on the Displacement row.

Accel toward

Accel toward is on Part emitters only. Switched on, it shows the Target and Strength rows at the end of the section.

Target is picked with the * button in the row’s label. It accepts a Part, an Attachment, a Model, a Bone or the camera. For a Model, the pull is toward its pivot. For a Bone, the pull is toward its animated pose.

Strength is the pull toward the Target, in studs per second per second. It is read from the graph at the particle’s age. A negative Strength pushes the particle away.

The Target’s position is read as it moves. The pull is aimed again at every step.
The speed from the pull keeps building. There is no top speed and no arrival, so particles overshoot and can orbit.
If the Target is removed, the speed built up by the pull is no longer applied. The particle carries on with Speed and Acceleration only.
Accel toward is off while Reverse Motion is on.

Take a Strength of 40 and a Target 20 studs away, with nothing else moving the particle. The particle reaches the Target after 1 second, travelling at 40 studs per second, and overshoots.

Uses:

A vortex round a moving character. A pull toward the character, with a Speed to one side.
A field that pushes particles away. A negative Strength.
A pull toward the camera.

The Target is kept in an ObjectValue named AccelTarget inside the emitter. See EmitParent, Link, AccelTarget, ShapePart.

Graphs on these rows

Speed, Displacement, Turbulence, Rotation Speed and Strength are graph rows. Every graph row in the plugin follows the same rules. Exercise 4 draws a graph for the first time.

Topic	Rule
The left-to-right scale	0 is the moment the particle is made. 1 is the moment it ends. The scale is a share of that particle’s Lifetime, not a time in seconds.
Between keypoints	The line is straight.
Bends	A bend drawn with handles in the graph editor is saved as up to 20 straight pieces. The editor remembers the handles.
Keypoints	At most 20 in one graph. The limit is Roblox’s.
Random spread	Each particle picks once, when it is made, how far above or below the line it runs. It keeps that place for life and does not waver. Each graph has its own pick. A colour graph has no spread.
One number typed into the box	A level line at that number.
Two numbers typed into the box	A level line half-way between them, with a spread that reaches both. Each particle gets one value between the two and keeps it.
Any typed value	It replaces the whole graph, handles included.
What the box shows	A number for a level line. `<NumberSequence>` for a drawn graph. `~` when the selected emitters differ.
Limits on values	None on these rows. Negative values are accepted.
Linked axes	A linked axis uses the primary’s graph and the primary’s spread.
Animate mode with Loop on	Graphs are read again, and spreads picked again, at the start of every cycle.

Timescale changes how fast a particle moves along the scale. The scale itself is explained in The 0 to 1 scale.

Every control for drawing a graph is described in The graph editor.

Limits

Drag slows only the travel that comes from Speed.
Accel toward has no top speed. Particles do not stop at the Target.
Reverse Motion switches Accel toward off and counts an Anim. Steps above 500 as 500.
Accel toward is on Part emitters only.