Question

A Question is a condition that, when passed, triggers the items nested beneath it.

Add a Question from the + button on the nxQuestion Questions list, then nest an Action (or other items) under it. The question is asked every frame, and its children run only when the condition passes.

A Question item in the nxQuestion panel

A Question selected in the Questions list, here testing each particle’s Age. The Category, Data and Condition menus and the value fields below them define what the question asks.

Question

The first setting chooses the kind of question: If (the default), Else If or Else.

If

The starting question of any hierarchy must always be an If.

For example, to set a particle’s color once it reaches a certain age, use an If question on the particle’s Age with a Set Color action nested under it. The question is asked every frame and the action fires only when the condition passes.

Else If

Else If can only be used following an If question. Think of it as a follow-up that is asked only when the preceding If answered “no”. Add one to test a second condition when the first fails, for example a different color change at another age.

Else

Else can only be used following an If or Else If question. It is asked only when every preceding question answered “no”, so it acts as the fallback.

Consider a case with bad logic: an Age > 2s question placed above an Age > 3s question. Because the lower-age question comes first, once a particle passes 2s the first question always passes and the second is never reached. The list is evaluated top to bottom, so put the narrower, higher-threshold conditions first.

Operator

Operators layer questions together so that conditions across multiple data types must be met to trigger an action. To test for age AND radius, add a second question as a child of the first and set its Operator.

The options are AND, AND NOT, OR and OR NOT. The default is - (none).

AND / AND NOT

Both questions must pass for the action to trigger. AND requires the second condition to be true. AND NOT requires it to be false.

OR / OR NOT

Only one of the questions needs to pass. OR passes when the second condition is true. OR NOT passes when it is false.

Data

The data available from the chosen Category. Each category has its own set of Data options.

Document category

Tests a scene-level value rather than a per-particle one.

Frame

Triggers at a specified scene frame. Opens the Condition, Value and Check Once parameters.

Camera Distance

The distance from the particle to a camera. Opens a Camera field, where you assign a scene camera, plus the Condition, Value and Check Once parameters.

Camera FOV

Tests whether a particle is within the assigned camera’s field of view. Opens a Camera field, a Widen FOV angle that widens the searched cone and Check Once.

Object Distance

Works like Camera Distance, but measures the distance to one or more objects in a list, and adds a Mode menu.

Mode

How the object distance is measured. Set as Position, by default.

Points: distance to the nearest vertex.
Position: distance to the object’s origin.
Polygons: distance to the nearest polygon.
Volume: distance to the nearest polygon, including a volume test, where a value below 0 is inside the object.

Time

The scene time. Whether this reads in seconds or frames depends on the frames/seconds toggle on the Time value. As well as Condition and Check Once, this adds the Time parameter.

Math category

Generates a value to test against.

Value

A constant value set in the panel, which can be driven by a driver or keyframes.

Random

A random value between the Min and Max settings.

Enable Particle ID to give each particle its own random number. Time Variation controls whether the random number is constant or re-rolled: set to None it stays fixed for the whole scene; in Document mode it changes every frame globally; in Particle mode it changes every frame per particle. A Seed sets the random sequence.

Spline

A value read from a curve, where the x axis is time (from the particle or the document).

Variable

A value read from a named Variable, or a variable defined in a script. Enter the variable name in the Variable Name field, then the question tests that variable’s value against the condition.

Wave

A sine-based wave in the range -100 to 100, whose value varies with the Frequency amount and the Time Variation setting (Document or Particle). Useful for oscillating effects such as regularly flashing particles. Enable Particle ID to offset the wave per particle.

Particle category

Tests a per-particle property. This is the default category.

Age

The particle’s age, in seconds.

Color

The particle’s color, tested by Vector Component: R, G, B or Brightness.

Count

The total number of particles. Tested with Within or Not Within a range.

Fluid Density

The calculated fluid density around a particle.

Traveled

The total distance the particle has traveled so far.

Emitter

Tests whether the particle’s emitter is (or is not) in the Emitters list shown. Use it to limit an action to particles from specific emitters. The condition is Within or Not Within.

Field

The falloff value for the particle. This requires an active nxFalloff in the nxQuestion object’s Falloff tab, so contact with the falloff region can trigger the action.

Flags

Tests whether any of the shown flags are set for the particle: Hit Object, Hit Particle, Changed Group, Stuck, Frozen, Born and Escaped. Enable the flags you want to test for. Escaped is set when a particle leaves an nxFlowField domain that has Tag On Escape enabled.

A Question on the Hit Object flag with a Kill Action nested under it. The falling particles disappear the instant they strike Suzanne, so none pile up or pass through.

Fuel

The particle’s fuel value.

Group

The nxGroup the particle belongs to (0 means no group assigned).

ID

The particle’s unique ID. Tested with Within or Not Within.

Life

The particle’s lifespan, in seconds.

Mass

The particle’s mass.

Neighbors

How many particles are within a Distance around the particle. Set the search Distance (defaults to 0.1m), then test the count against the condition.

Position

The world position of the particle, tested by Vector Component: X, Y or Z.

Radius

The particle’s radius.

Rotation

The particle’s rotation, tested by Vector Component: H (heading), P (pitch) or B (bank).

Smoke

The particle’s smoke value.

Speed

The particle’s speed.

Temperature

The particle’s temperature value.

UVW

The particle’s UVW coordinates, tested by Vector Component: X, Y or Z. UVW data is only available when the emitter is set up to emit from an object’s texture coordinates.

Velocity

The particle’s velocity, tested by Vector Component: X, Y or Z.

Vertex Weight

The particle’s assigned vertex weight, usually set on emission. This data is generated when particles are emitted from the vertices of an object that carries a vertex group, with vertex emission enabled on the emitter.

Shared parameters

Condition

How the data is compared to the Value. The options are Less, Less-Equal, Equal, Not Equal, Greater-Equal and Greater. Defaults to Greater.

Within and Not Within test whether the value falls inside a range, which is useful for floating-point data that is rarely exact. They add a To value for the top of the range.

For the Count, Emitter and ID data the only conditions are Within and Not Within.

Value

The constant value the data is measured against. Integer data, including Frame, uses a whole-number Value. Time-based data (Age, Life and Time) uses a Time value with a frames/seconds toggle.

Variation

A random amount added to the Value, so the threshold is not identical for every particle.

To

The upper end of the range, shown when the Condition is Within or Not Within.

Weight

The weight given to this question when it passes, as a percentage. Defaults to 100%.

Check Once

When enabled, once the question passes for a particle it is not checked again for that particle.