Question

The Question layer is used to set up conditions which, if passed, will trigger actions.

The first step is to assign what kind of question will be asked using the options: If (the default), Else If or Else.

The starting question of any hierarchy must always use the If option.

If you want to set the particle color when it gets to a specific age (older than 90 frames, for example), you would use an If question of the particle’s age, along with a Set Color action as its child.

The question is asked every frame and the Set Color action is only triggered when the condition of the question is passed.

Else If questions can only be used when following an If question.

You can think of it as a follow-up question, which is only asked if the answer to the preceding If question is ‘no’.

An Else If question can be added to the previous example to trigger more particle color changes at another specified particle age.

Else questions can only be used when following an If or Else If question.

This question is only asked when any preceding questions are answered ‘no’.

An Else question can be added to the previous example to create a final layer of particle color change.

There are now 3 color changes in this scene. Prior to frame 60, the first If question is asked and not passed. Therefore the following Else If question is asked, but this too is not passed. So the final Else condition is asked, which triggers the green Set Color action. After frame 60, the first If question is not passed, so the following Else If question is asked. This time the condition is passed and the pink Set Color action is triggered. After frame 90, the first If question is passed, so the blue Set Color action is triggered.

Note: The order of Question layers is very important.

In the above example, you may ask why the last color change in the viewport (blue) is first on the questions list.

To understand this you could consider an example that demonstrates bad logic.

Here, the question order has been switched, so the AGE > 2s is 1st and the AGE > 3s is 2nd. The problem here is that the lower age question comes first in the list. This means that if the answer is true for the 2nd question, it will always be true for the 1st question as well. As a result, as soon as the particles get to 60 frames old, the first question will pass and the second question will be ignored. When the particles get to 90 frames old, the first question will still pass and the second will continue to be ignored. The logic does not work with this question order and the 2nd question will never be asked.

Operators can be used to layer up questions so that conditions of multiple data types must be met to trigger actions.

In the preceding examples, the questions have driven a color change based on one data type; particle age.

But what if you wanted to test for age AND radius?

To do that, you would add a new question as a child of the first and choose one of the Operator options.

These are: AND, AND NOT, OR and OR NOT.

These operator modes require both questions to be passed for the action to be triggered.

The difference is demonstrated in the examples below.

These operators require one of the questions to be passed for the action to be triggered.

The difference is demonstrated in the examples below.

Set as Particle, by default, this is what you are questioning for the condition.

The above examples have all used the Particle category, which give options like Age, Color and Radius.

The alternatives are: Document and Math.

Most types share the same parameter options (such as Condition, Value and Check Once); these are explained at the bottom of this page.

Parameters relating to individual Data options are explained below.

The Data option is the available data that can be used from the chosen Category.

Each Category has a different set of Data options available to it.

Use this option to trigger actions at a specified scene Frame number.

This setting opens parameters for Condition, Value and Check Once.

The distance to the camera.

This setting opens a Camera field, where you can drop a scene camera, and the Condition, Value and Check Once parameters.

This asks if a particle is within the specified camera’s FOV (Field of View).

This will also open up 3 additional parameters: Camera field, Widen FOV, which allows the degree searched to be widened, and the Check Once option.

This works like the Camera Distance mode, explained above.

The difference is, as well as the usual parameters, this includes an additional Mode menu with more options.

This has the following settings: Points - distance to the nearest vertex. Position - distance to the object’s world location. Polygon - distance to the nearest polygon. Volume - distance to the nearest polygon, including volume test, where <0 is inside the object.

Here the Object Distance data is set to Volume mode. This tests whether particles are inside the volume of the sphere and, if they are, their color is set to blue.

The document scene time.

Whether this is in seconds or frames will depend upon your Cinema 4D unit preferences.

By default this is set to frames.

As well as Condition and Check Once, this has the Time parameter.

A constant value, set in the UI (User Interface), which can be set from expressions or animation.

A random value generated from within the Min and Max values.

This can be made random for each particle, by enabling the Particle ID option.

Use the Time Variation setting to control whether the random number is constant or if it should change every frame.

With Time Variation set to none, the random number will remain the same throughout the scene.

In Document mode, the random number will change every frame, globally, for all particles.

In Particle mode the random number will change every frame, per particle.

A value based on a spline curve setting, where the x axis is time, based on the particle or the document time.

This is a value taken from a user variable, or a variable that is defined within a script (if local to the question, e.g. from an earlier or parent script).

The variable name must be entered correctly in the Name field.

A sine-based wave, with the range -100 to 100, where the value varies based on the Frequency amount and the Document or Particle Time Variation option.

This mode is useful for creating oscillating effects, like regularly flashing particles.

The particle’s age, in seconds.

The particle’s color; a Vector setting of R, G, B or Brightness.

The total number of particles.

The calculated fluid density around a particle.

Note: This data is only generated when using a fluid solver, such as nxFluid.

The total distance currently travelled by the particle.

A true/false for if the particle’s emitter is (or is not) in the Emitter list shown.

Use this to isolate the affects of an action to specific emitters.

The field (falloff) value for the particle.

This mode requires an active field to be present within the nxQuestion object Fields tab.

If any of the shown flags (Hit Object, Hit Particle, Changed Group, Stuck and Frozen) are enabled for the particle.

The group the particle is in (0 - zero - means no group assigned, default).

The particle’s unique ID.

In this scene, the particle IDs have been made visible, via the xpEmitter Display tab options. The question is set to Particle ID with value set to 4. This means the Set Radius action only affects the particle with this ID.

The life span of the particle.

The mass of the particle.

How many particles are within a Distance setting around the particle.

The world position of the particle per Vector: X, Y or Z.

The radius of the particles.

The rotation of the particles, in the heading (H), pitch (P) or bank (B).

The particle’s speed.

The particle’s UVW components (picked from X, Y and Z).

To activate UVW data, the xpEmitter must be setup to emit from an object texture.

In addition, the UV Data must be checked in the Extended Data - General Data tab.

The particle’s velocity components (picked from the Vector setting of X, Y and Z).

The particle’s assigned vertex weight, usually set on emission.

This data is not generated by default.

Setting this up requires the following steps.

1: The particles must be emitted from the points of a scene object that has a vertex weight tag; to do this, go to the emitter Object tab, and set the Emitter Shape to Object.

2: Drag the scene object into the Object link field

3: Drag the vertex map into the Selection link field.

4: Set Emit From to Points.

5: In the emitter Extended Data - General Data tab, activate Emission Vertex.

Now, vertex data is being generated per particle, this can be used in an nxQuestion setup.

Less, Less-Equal, Equal, Not Equal, Greater-Equal and Greater are all conditions for how to use the data and compare it to a Value setting to give if the question is successful (true).

Within, Not Within can be used as some data may not be exact (floating point), this gives the option for a value to be within a range of values.

The constant value set and measured against.

The scene frame set and measured against.

If, enabled, once the question is successful, it won't be checked again.