Age

Color

Velocity

Radius

Distance Traversed

Lifetime

Alignment

Angular Velocity

Here you will find an option list of the most important properties of particles, such as their color, radius, speed or orientation. Only the properties activated here are actually calculated and evaluated within the simulation. This also means that, for example, conditions or modifiers that access or influence these properties may no longer have any effect if the corresponding option in this list has been deactivated. The advantage of properties that are no longer used is that no memory needs to be reserved for them, which can be an advantage for simulations with a large number of particles.
If it is unavoidable for the function of a simulation at some points to have to calculate with a property switched off here, these default values are used as a substitute:

• Properties that are described by floating point values, such as age, radius or distance traveled, are interpreted with the value 0.
• Properties that can only be described by vectors, such as speeds or colors, are interpreted with the zero vector or the 4D vector (0,0,0,0).
• Properties based on quaternions, such as the alignment of the particle axes, are set to the standard unit qu.
• Group links are automatically set to the root group. The particles are thus interpreted as groupless.

Note that there are also properties that are interdependent, such as speed and distance traveled. In this case, deactivating the speed automatically deactivates the calculation of the distance traveled.

The options can also be switched on and off while a simulation is running. When switching off, the corresponding memory area is released; when switching on, the memory area is reserved again and filled with the corresponding default values of the property.

In order to ensure that the particle system functions in every case, not all properties can be deactivated manually. The properties for the position, the unique ID of the particles and the group are therefore always active.

Add Custom Property

Particles can also be given their own additional values via this Add Custom property button, which can be read or written using these modifiers and conditions:

• Base emitter (write only)
• Mesh emitter (write only)
• Spline emitter (write only)
• Reproduce emitter (write only)
• Condition (readout only)
• Color Mapper modifier (read only)
• Data Mapper modifier (read and write)
• Math modifier (read and write)
• Switch Group modifier (write only)
• Align modifier (write only)
• Blend modifier (read and write)
• Flock modifier (write only)
• Hunter Prey modifier (write only)
• Collide modifier (write only)
• Spline Follow modifier (write only)
• Pyro Advect modifier (write only)
• Stick modifier (writing only)
• Surface Attract modifier (write only)
• Particle Node modifier (readout only)

It should be noted that custom properties also have a data type that should be compatible with the selected operation or function of the respective modifier or condition. A property defined as a particle group link can, for example, only be read within a Condition and compared there with another particle group, for example.
Other properties based on floating point values, on the other hand, can be used in a more versatile way and can be interpreted, for example, as radius, airspeed or age.
The same applies to vectors, which can be used to describe a color, the direction of flight or a position, for example.
However, it is also the case with Node circuits that attempts are made to convert different data types in a meaningful way when exchanging data. This means that integer values can also become floating point values or, for example, a vector. Depending on the type of conversion, however, this can also result in value changes, e.g., when a floating point value is converted to a bool value or a vector is converted to an integer number. It therefore generally makes sense to select the data type so that it corresponds to the values that you want to manage with this user property.

When creating a new particle property, various setting options appear, as shown in the following illustration:

In the left-hand column you will find an option to switch the respective user property on or off. Conditions or modifiers that access a deactivated property can then no longer function and are deactivated.

The Name text field to the right of the activation option contains the name of the property. You can enter this yourself here. Note, however, that some property names are already in use, such as color, radius or position. These names can therefore not be used again.

Use the Type menu to select the data type for your user property. The following data types are available and must be selected to match the values that you want to save and process with the corresponding property. The values transferred to the user properties from the particle system are not converted and therefore always expect the same data type with which this data is also used within the particle system.

• Integer: This can only be used to manage integer values, such as those used to count collisions on a Collide modifier.
• Index: Each particle can be identified by an integer number, the so-called ID or index. This data type is specifically intended for storing this information.
• Boole: This data type only knows two states, true(i.e., true) and false (i.e., false). This makes it possible, for example, to save whether a particle has just been born or should be deleted. For example, the Base emitter allows you to save the Born status of the particles in a user property that uses this Boole data type.
• Group linking: This can be used to save particle groups. For example, the Switch Group modifier offers an option to save the previous group of a particle in this data type before changing it. A Condition can be used to read a group link.
• Floating point: This is a floating point value without a unit. This can be used, for example, to save a color component or whenever no value is to be saved with a unit.
• Floating point - meters: This floating point number can represent a radius or a distance covered, for example, and is automatically displayed with a distance unit such as meters, centimeters or inches, depending on your units defined from the Preferences settings. Because a distance unit is included, this value is automatically subject to automatic adjustment by the project scaling, for example.
• Floating point - degrees: This floating point number is intended for storing individual angles.
• Floating point - time: This floating point number is intended for storing times and time intervals, such as the Age or Lifetime properties.
• Floating point percentage: This floating point value is intended for passing on percentage values, such as the Age Percentage property.
• Vector: This is a 3D vector without a unit. These can be UVW coordinates, for example.
• Vector - Meter: This is a 3D vector with a distance unit that can be used, for example, to transmit positions or flight directions. Because a distance unit is included, this value is automatically subject to automatic adjustment by the project scaling, for example.
• Color A: This is a 4D vector format that can be used to manage the RGB color values and the alpha value.
• Quaternion: This special format is for storing the orientation of a particle system.

By activating the Auto option, an automatic change of the user property can be activated. However, this is only available for the data types integer, bool and Floating Point time. The saved Integer value is automatically increased by 1 in each simulation image. With a floating point time value, an increase by the time period elapsed per simulation image (delta time) takes place in each simulation image. The boole data type is automatically reset to 0 or false in each new simulation screen.

Finally, the Remove button also allows you to delete a user property that is no longer required. However, all conditions and modifiers that used this property will then no longer work and must be reconfigured. To temporarily deactivate individual properties, you can also deactivate the corresponding Used options instead.

Additional user properties can be added at any time using the Add custom property button, which can then be used by the emitters, conditions and modifiers mentioned above.

Note that the user properties for the particles are always managed by the active simulation scene of your simulation. You can therefore also add these properties directly to a simulation scene object (see Simulate menu in Cinema 4D) and then only make them available to the subordinate simulation elements there. The normal Scene Presets (see Edit menu in Cinema 4D) and their particle simulation presets are responsible for all particle elements that are not subordinate under a separate Simulate Scene object.

Usage example for user properties

Some emitters and modifiers already offer a selection of particle properties that can be passed directly to user properties. For example, you will find an option to write the UVW coordinates of the emitter shape in the output area on the Base emitter. Let's use this to color the particles.
To do this, first create a new user data property for particles in the Simulation > Particles area of the scene presets by first clicking the button for Add custom property and then entering e.g., EmitterUVW in the name field. Select Vector as the type, as we are dealing with three values without a unit for the UVW coordinates.

Then create a Base emitter and use it to activate the cube as the emitter shape, for example, to create U, V, and W components. With 2D emitter shapes, the W component would otherwise always be 0. In the output settings of the emitter, you will find two data sets that can be transferred from the particles to your own particle user data. We activate the option for the UVW coordinates there. To the right of the text input field for the name of the user property, you will find an arrow icon which you can use to open a list of all available variables (see orange frame in the following image). Simply select your EmitterUVW variable from this list. Alternatively, you can also enter the name EmitterUVW directly in the text field.

From now on, the emitter will automatically write the UVW coordinates of each newly created particle to the EmitterUVW user property. We can then read these out using Math modifiers, for example, and pass them to any other property of the particle, such as the color of the particle. The following figure illustrates this.

Using three Math modifiers, we transfer the stored U, V and W components (since the UVW values were stored as a vector, these can be extracted from the X, Y and Z components of the vector) to the R, G and B components of the particle colors. Also pay attention to the multiplier of the Math modifiers, as this is set to 0 by default. To convert UVW coordinates directly to RGB values, this multiplier should be set to 1. The right half of the figure above shows the result.