# APEX Basic FS Users Guide¶

## Introduction¶

The Basic FS module provides several kinds of field samplers. Field samplers are used to create forces which move particles along a particular way. Currently Basic FS module contains Attractor, Jet, Noise, Vortex and Wind field samplers. User can combine these field samplers to achieve desired behaviour.

## Common Field Sampler Properties¶

All field samplers share some properties to allow setting up particular interaction mode between them and particles. These properties are Field Drag Coefficient and Field Weight.

If Field Drag Coefficient is 0, then:

newParticleVelocity = fieldWeight * fieldVelocity + (1 - fieldWeight) * oldParticleVelocity;

(so it’s just a linear blending of field velocity and old particle velocity) – this is so called direct velocity mode.

If Field Drag Coefficient is > 0, then:

newParticleVelocity = dragWeight * fieldVelocity + (1 - dragWeight) * oldParticleVelocity, where dragWeight = min(fieldWeight * fieldDragCoeff * elapsedTime / particleMass, 1)

This formula simulates some simple drag force from real physics (this is so called drag force mode – it usually produce some time delay for changing particle velocity and also it should vary with particle mass)

## Attractor Field Sampler¶

Attractor Field Sampler acts as a sphere creating a force which attracts particles to this sphere. These are the parameters for this type of effect:

• Boundary Fade Percentage - Works similarly as the same property on the grid
• Const Field Strength - How strong the attractor is pulling the particles. A negative number will repel particles.

## Jet Field Sampler¶

Jet Field Sampler Actor affects the Turbulence grid’s velocities and causes the particles to be pushed in a single direction. Outside the grid, jets act completely differently. It acts directly on any particles and moves them around in a torus shape. Some of important parameters of a Jet Field Sampler are:

• Grid Shape Radius - the radius of the jet’s volume of influence (a capsule).
• Grid Shape Height - the height of the jet’s volume of influence (a capsule).
• Grid Boundary Fade Percentage - this number controls how the influence of the jet falls off as you travel out from its central axis. For example, a value of 0.01 means that the jet’s field strength is at full strength up until the outermost 1% of the capsule’s volume. Then, in this last 1%, the field strength goes from full strength to zero at the boundary of the capsule. This is useful for “softening the edges” of the jet’s influence.
• Field Strength - the strength of the jet.

## Noise Field Sampler¶

Noise Field Sampler creates noise in a volume represented by a box. Noise affects particles by applying force or velocity. There are several important parameters to setup noise:

• Noise Type - CURL or SIMPLEX. By default use CURL type. This is divergence free type noise.
• Noise Strength - coefficient of the noise strength.
• Noise Space Period - the size of vortices in the noise field.
• Noise Time Period - how fast the noise vortices would change direction.
• Noise Octaves - how many octaves does noise spectrum have.

## Vortex Field Sampler¶

Vortex Field Sampler has a capsule shape. Particles that are inside this shape receive propulsion with a force propotional to a distance between a particle and the capsule axis. There are three components of the velocity - rotational, radial and lift.

## Wind Field Sampler¶

Wind Field Sampler creates a force which represents wind influencing the particles. This field sampler has no shape or volume, hence it affects all particles in case no filtering is set. Wind Field Sampler is able to create a variable direction and strength of wind force. These settings are avaiable through asset properties such as Field Direction, Field Direction Deviation Angle, Field Direction Oscillation Period, Field Strength, Field Strength Deviation Percentage, Field Strength Oscillation Period.