Standard Attribute Reference

Standard Attributes

Here is a comprehensive List of all commonly used attributes, with a brief description and usages.

Basic Simulation Attributes

The Initialize and Update Contexts use Simulation Attributes to apply behavior to simulated elements such as Particles or Particle Trails.

Name	Type	Description	Default Value
position	Vector3	The position of a simulated element, expressed in the corresponding System Space, in System Units	(0,0,0)
velocity	Vector3	The current self-velocity of the simulated element, expressed as a 3D Vector in the corresponding System Space, in System Units per Second.	(0,0,0)
age	float	The age of the simulated element, since its spawn, expressed in Seconds.	0.0
lifetime	float	The life expectancy of a simulated element, expressed in Seconds.	0.0
alive	bool	Whether the simulated element is alive or not.	true
size	float	The size of a simulated element, in system Units.	0.1

Advanced Simulation Attributes

Some attributes are a bit more advanced and will be used by default in most simulations. However, you can alter these attributes to enhance your behaviors

Name	Type	Description	Default Value
mass	float	The mass of a particle in Kg/dm^3	1.0 (defaults to 1kg per liter of water)
angle	Vector3	Variadic: Euler rotation of a simulated element, expressed as a Vector of Degrees Values.	(0,0,0)
angularVelocity	Vector3	Variadic: Euler rotation speed of a simulated element, expressed as a Vector of Degrees per second values.	(0,0,0)
oldPosition	Vector3	Deprecated: This attribute is a storage Helper if you want to back-up current position of a simulated element, before integrating its velocity.	(0,0,0)
targetPosition	Vector3	This attribute has various purposes: it can be a storage Helper if you want to store a position to reach, then compute a vector in order to reach this Target Position. In Line Renderers, this attribute can be also used to set the end of each line particle.	(0,0,0)

Rendering Attributes

Rendering Attributes are not used in simulation but are useful when you want to render a simulated element.

Name	Type	Description	Default Value
color	Vector3	The R,G, and B Components of the Rendered Element.	1,1,1
alpha	float	The alpha Component of the Rendered Element	1
size	float	The uniform size of a rendered element, in system Units, applied to its unit representation	0.1
scale	Vector3	The Non-uniform Scale multiplier of a rendered element, applied to its unit representation	(1,1,1)
pivot	Vector3	The Origin position of a rendered element, in its unit representation	(0,0,0)
texIndex	float	The animation frame used for sampling Flipbook UVs for a rendered element.	0.0
axisX	Vector3	The computed Right axis of the rendered Element.	(1,0,0)
axisY	Vector3	The computed Up axis of the rendered Element.	(0,1,0)
axisZ	Vector3	The computed Forward axis of the rendered Element.	(0,0,1)

System Attributes

System Attributes provide information about system values. These attributes are available as Read Only, which means you can only read them using the Get <Attribute> Operator.

Name	Type	Description	Default Value
particleID	uint	A unique ID that refers to 1 particle	0
seed	uint	A unique seed used for random number computations.	0
spawnCount	uint	A SpawnEvent attribute available as Source Attribute in Spawn Contexts, that describes how many particles were spawned this frame.	(0,0,0)
spawnTime	float	A SpawnEvent attribute available as Source Attribute in Spawn Contexts, that contains a Spawn Context internal time (when exported using a Set Spawn Time Spawn Block)	0.0
particleIndexInStrip	uint	The index in the Particle Strip Ring Buffer where is located this element.	0

Attribute Usage and Implicit Behavior

Some attributes combinations are used in various implicit cases during simulation and rendering. Here is a list of the usages and an explanation of their relationships.

Velocity and Position : Integration

During an Update simulation : any system that uses the velocity attribute will integrate the velocity to the positions, every frame.

Velocity integration basically used the following formula: position += velocity * deltaTime

Automatic Velocity Integration can be disabled by selecting the Update Context, then setting the Integration enum to None.

Age, Lifetime and Alive

Setting a Lifetime attribute to a particle in an Initialize Context, will implicitly add the following behaviors to an Update Context:

• Particle will age using the formula age += deltaTime
• Particles will be reaped using the formula : alive = (age < lifeTime)

Automatic particle aging can be disabled by selecting the Update Context, then setting the Age Particles value to False.

Automatic particle reaping can be disabled by selecting the Update Context, then setting the Reap Particles value to False.

Immortal Particles: Particles with no lifetime will be considered as immortal. You can still kill them explicitly by setting their alive attribute to false.

Angle and Angular Velocity : Angular Integration

During an Update simulation : any system that uses the angularVelocity attribute will integrate the angular Velocity to the angles, every frame.

Angular Velocity integration basically used the following formula: angle += angularVelocity * deltaTime

Automatic Velocity Integration can be disabled by selecting the Update Context, then setting the Angular Integration Enum to None.

AxisX, AxisY and AxisZ

These three attributes define the unit representation 3d coordinate system. While these axis are expected to be normalized, you can use non-normalized lengths and non-orthogonal angles between all three axis.

Most of the time, these axis are set using the Orient Block (Output Context Only).

Size, Scale and Pivot

In order to apply scaling and rotation to simulated elements, Visual Effect Graph uses 3 attributes:

• size (float) : uniform size of the particle.
• scale(Vector3) : per-axis size of the particle.
• pivot (Vector3) : pivot position in the unit representation.

The Pivot of a particle is computed in an unit box of size 1 : the unit representation. By default, it is (0,0,0), the center of the box. You can change its Value to adjust the center of the box. Every face is located at -0.5 or 0.5 in each axis.

The pivot representation can be also generalized in 3D, with the Z Axis being used for depth.

Using the Z-Component for Quad Particles using Camera Facing will pull the element towards the camera position or plane, or push it away so it appears more in front, or behind its actual position.

Scaling of a particle applies two multipliers the unit representation box:

• The uniform size
• The non-uniform scale

You can use these attributes to perform uniform and non-uniform scaling independently. For example, use the particle scale to compute an initial random scale, and use the size attribute to animate every element, keeping its ratio.