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IVfxBaseDriver

15.22 Interface

Inheritance 104 descendants

Class Documentation

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A VFX driver: one node of the expression graph that computes the animated values of the component VFX system (VfxComponents). A driver returns a float (IVfxFloatDriver), a two-component vector (IVfxVector2Driver), a three-component vector or RGB colour (IVfxVector3Driver), or a four-component vector or RGBA colour (IVfxVector4Driver). Drivers nest: most take other drivers as operands, so data builds an expression such as "a random value between 2 and 5, multiplied by a curve over the particle's age".

Where drivers are used. A component property that takes a driver value is a dynamic property: VfxFloatDynamicProperty, VfxVector2DynamicProperty, VfxVector3DynamicProperty or VfxVector4DynamicProperty. It holds the top node of the expression. The lifetime, spawn, physics, render, light and effector components use dynamic properties for their numeric values. The graph is separate from the logic drivers of materials and from the legacy IVfxMaterialDriver. A Vfx*LogicDriver node, such as VfxFloatLogicDriver, reads a logic driver into the graph.

Evaluation

The holder of a value evaluates it for one particle at a time, or once for the emitter. Nothing caches a result: every evaluation walks the whole expression again.

When Evaluated for Values
Particle spawn each new particle, once the start values of the physics modifiers
Every update each live particle the physics modifier values, the render and geometry values
Every update the emitter's first particle the light and effector values
Spawn and lifetime steps the emitter durations, delays, the emission rate

An evaluation for the emitter has no particle. In it, the particle property drivers (such as VfxPropertyFloatDriver) return their default, and the particle time sources 2, 3 and 4 read 0.

Unset operands. Every driver checks its operands, and an operand that is not set never crashes the client. Each driver page states the result for a missing operand: either a fixed fallback, or no result. When a driver writes no result, the value is the one that its holder starts from, which the holder's page states (for example 0 for an effector strength, white for a light colour).

Setup

Once per emitter definition, when the system is prepared, the client walks every driver value of the emitter:

  1. Each random node takes one or more random slots (see Random values).
  2. Each Vfx*LogicDriver node registers its logic driver for the logic pass (see Logic drivers).
  3. Each node reports which output components it defines. The material render component reads this mask.

Random values

The ranged random drivers (VfxUniformRangedRandomDriver, VfxSquareRootRangedRandomDriver) and the probability tables of the curve drivers (VfxFloatAnimatedWithRandomDriver and its variants) read a random number between 0 and 1 from their slot. The property that holds the value decides the scope of its slots:

  • Per particle: the numbers are drawn when the particle spawns and stay fixed for the particle's life.
  • Per emitter: the numbers are drawn once, when the emitter instance is created, and every particle reads the same numbers.

A random node that the setup pass does not reach has no slot. It draws a new number on every evaluation, so its value changes on every update and differs between particles. The Factor operand of the lerp drivers is such a place, see VfxFloatLerpDriver.

Logic drivers

Once per frame, for each emitter, the client evaluates every registered logic driver and stores the result. A Vfx*LogicDriver node returns the stored value, so every particle of the emitter reads the same value in one frame. If the logic driver or the object that it is evaluated for is missing, the stored value is 0 in every component.

The logic pass probably runs after the emitter updates of the same frame, so an emitter update can read the value of the previous frame. The order of the two passes is not fully established.

Interval evaluation

A physics modifier evaluates some of its values over the particle's last step, from the particle's age at the start of the step to its age at the end. A curve driver that is the top node of the value (VfxFloatAnimatedWithRandomDriver and its vector and colour variants) samples its curve at both ends of the step. Any other top node evaluates the whole expression once, at the middle of the step, so a curve inside another node is sampled at the middle. How a curve driver combines its two samples is not established.

Time sources

frequency on the curve and easing drivers and Time on VfxFloatTimeDriver pick one of these clocks:

Value Name Returns
0 Emitter age fraction The emitter's age divided by its duration, at most 1.
1 Emitter age Seconds since the emitter started.
2 Particle age fraction The particle's age divided by its lifetime, at most 1. A particle with a lifetime of 0 reads 0, and a particle with a negative (unlimited) lifetime reads about 0.
3 Particle age Seconds since the particle spawned.
4 Particle birth fraction The particle's spawn time, in seconds after the emitter start, divided by the emitter duration. The duration is not checked: 0 gives an infinite value and a negative duration a negative value.
5 Ramp progress Progress, 0-1, of an emitter ramp timer. What starts the ramp is not established.
6 Outro progress Progress, 0-1, through the emitter's ending phase. 0 before that phase, 1 once the emitter has finished.
7 Game time The game clock, in seconds.
other - 0

Comparators

Comparator on the VfxCondition*Driver classes, such as VfxConditionFloatDriver, picks the comparison. The result is 1 when the comparison holds and 0 when it does not.

Value Name Holds when
0 Equal abs(Left - Right) <= 1e-6
1 NotEqual Equal does not hold
2 Less Left < Right
3 LessOrEqual Left <= Right
4 Greater Left > Right
5 GreaterOrEqual Left >= Right
other - never

The vector conditions compare each component on its own and return 1 or 0 per component.