#### 3.3.6 Standard Particle Affectors

Ogre comes preconfigured with a few particle affectors. New ones can be added by creating plugins: see the Plugin_ParticleFX project as an example of how you would do this (this is where these affectors are implemented).

- Linear Force Affector
- ColourFader Affector
- ColourFader2 Affector
- Scaler Affector
- Rotator Affector
- ColourInterpolator Affector
- ColourImage Affector
- DeflectorPlane Affector
- DirectionRandomiser Affector

#### Linear Force Affector

This affector applies a force vector to all particles to modify their trajectory. Can be used for gravity, wind, or any other linear force. It’s extra attributes are:

force_vector

  Sets the vector for the force to be applied to every particle. The magnitude of this vector determines how strong the force is.
  format: force_vector <x> <y> <z>
  example: force_vector 50 0 -50
  default: 0 -100 0 (a fair gravity effect)

force_application

  Sets the way in which the force vector is applied to particle momentum.
  format: force_application <add|average>
  example: force_application average
  default: add
  The options are:

  average

    The resulting momentum is the average of the force vector and the particle’s current motion. Is self-stabilising but the speed at which the particle changes direction is non-linear.

  add

    The resulting momentum is the particle’s current motion plus the force vector. This is traditional force acceleration but can potentially result in unlimited velocity.

To create a linear force affector, include a section like this within your particle system script:

    affector LinearForce
    {
        // Settings go here
    }

Please note that the name of the affector type (’LinearForce’) is case-sensitive.

#### ColourFader Affector

This affector modifies the colour of particles in flight. It’s extra attributes are:

red

  Sets the adjustment to be made to the red component of the particle colour per second.
  format: red <delta_value>
  example: red -0.1
  default: 0

green

  Sets the adjustment to be made to the green component of the particle colour per second.
  format: green <delta_value>
  example: green -0.1
  default: 0

blue

  Sets the adjustment to be made to the blue component of the particle colour per second.
  format: blue <delta_value>
  example: blue -0.1
  default: 0

alpha

  Sets the adjustment to be made to the alpha component of the particle colour per second.
  format: alpha <delta_value>
  example: alpha -0.1
  default: 0

To create a colour fader affector, include a section like this within your particle system script:

    affector ColourFader
    {
        // Settings go here
    }

#### ColourFader2 Affector

This affector is similar to the ColourFader Affector, except it introduces two states of colour changes as opposed to just one. The second colour change state is activated once a specified amount of time remains in the particles life.

red1

  Sets the adjustment to be made to the red component of the particle colour per second for the first state.
  format: red <delta_value>
  example: red -0.1
  default: 0

green1

  Sets the adjustment to be made to the green component of the particle colour per second for the first state.
  format: green <delta_value>
  example: green -0.1
  default: 0

blue1

  Sets the adjustment to be made to the blue component of the particle colour per second for the first state.
  format: blue <delta_value>
  example: blue -0.1
  default: 0

alpha1

  Sets the adjustment to be made to the alpha component of the particle colour per second for the first state.
  format: alpha <delta_value>
  example: alpha -0.1
  default: 0

red2

  Sets the adjustment to be made to the red component of the particle colour per second for the second state.
  format: red <delta_value>
  example: red -0.1
  default: 0

green2

  Sets the adjustment to be made to the green component of the particle colour per second for the second state.
  format: green <delta_value>
  example: green -0.1
  default: 0

blue2

  Sets the adjustment to be made to the blue component of the particle colour per second for the second state.
  format: blue <delta_value>
  example: blue -0.1
  default: 0

alpha2

  Sets the adjustment to be made to the alpha component of the particle colour per second for the second state.
  format: alpha <delta_value>
  example: alpha -0.1
  default: 0

state_change

  When a particle has this much time left to live, it will switch to state 2.
  format: state_change <seconds>
  example: state_change 2
  default: 1

To create a ColourFader2 affector, include a section like this within your particle system script:

    affector ColourFader2
    {
        // Settings go here
    }

#### Scaler Affector

This affector scales particles in flight. It’s extra attributes are:

rate

  The amount by which to scale the particles in both the x and y direction per second.

To create a scale affector, include a section like this within your particle system script:

    affector Scaler
    {
        // Settings go here
    }

#### Rotator Affector

This affector rotates particles in flight. This is done by rotating the texture. It’s extra attributes are:

rotation_speed_range_start

  The start of a range of rotation speeds to be assigned to emitted particles.
  format: rotation_speed_range_start <degrees_per_second>
  example: rotation_speed_range_start 90
  default: 0

rotation_speed_range_end

  The end of a range of rotation speeds to be assigned to emitted particles.
  format: rotation_speed_range_end <degrees_per_second>
  example: rotation_speed_range_end 180
  default: 0

rotation_range_start

  The start of a range of rotation angles to be assigned to emitted particles.
  format: rotation_range_start <degrees>
  example: rotation_range_start 0
  default: 0

rotation_range_end

  The end of a range of rotation angles to be assigned to emitted particles.
  format: rotation_range_end <degrees>
  example: rotation_range_end 360
  default: 0

To create a rotate affector, include a section like this within your particle system script:

    affector Rotator
    {
        // Settings go here
    }

#### ColourInterpolator Affector

Similar to the ColourFader and ColourFader2 Affectors, this affector modifies the colour of particles in flight, except it has a variable number of defined stages. It swaps the particle colour for several stages in the life of a particle and interpolates between them. It’s extra attributes are:

time0

  The point in time of stage 0.
  format: time0 <0-1 based on lifetime>
  example: time0 0
  default: 1

colour0

  The colour at stage 0.
  format: colour0 <r> <g> <b> [<a>]
  example: colour0 1 0 0 1
  default: 0.5 0.5 0.5 0.0

time1

  The point in time of stage 1.
  format: time1 <0-1 based on lifetime>
  example: time1 0.5
  default: 1

colour1

  The colour at stage 1.
  format: colour1 <r> <g> <b> [<a>]
  example: colour1 0 1 0 1
  default: 0.5 0.5 0.5 0.0

time2

  The point in time of stage 2.
  format: time2 <0-1 based on lifetime>
  example: time2 1
  default: 1

colour2

  The colour at stage 2.
  format: colour2 <r> <g> <b> [<a>]
  example: colour2 0 0 1 1
  default: 0.5 0.5 0.5 0.0

[...]

The number of stages is variable. The maximal number of stages is 6; where time5 and colour5 are the last possible parameters. To create a colour interpolation affector, include a section like this within your particle system script:

    affector ColourInterpolator
    {
        // Settings go here
    }

#### ColourImage Affector

This is another affector that modifies the colour of particles in flight, but instead of programmatically defining colours, the colours are taken from a specified image file. The range of colour values begins from the left side of the image and move to the right over the lifetime of the particle, therefore only the horizontal dimension of the image is used. Its extra attributes are:

image

  The start of a range of rotation speed to be assigned to emitted particles.
  format: image <image_name>
  example: image rainbow.png
  default: none

To create a ColourImage affector, include a section like this within your particle system script:

    affector ColourImage
    {
        // Settings go here
    }

#### DeflectorPlane Affector

This affector defines a plane which deflects particles which collide with it. The attributes are:

plane_point

  A point on the deflector plane. Together with the normal vector it defines the plane.
  default: plane_point 0 0 0

plane_normal

  The normal vector of the deflector plane. Together with the point it defines the plane.
  default: plane_normal 0 1 0

bounce

  The amount of bouncing when a particle is deflected. 0 means no deflection and 1 stands for 100 percent reflection.
  default: bounce 1.0

#### DirectionRandomiser Affector

This affector applies randomness to the movement of the particles. Its extra attributes are:

randomness

  The amount of randomness to introduce in each axial direction.
  example: randomness 5
  default: randomness 1

scope

  The percentage of particles affected in each run of the affector.
  example: scope 0.5
  default: scope 1.0

keep_velocity

  Determines whether the velocity of particles is unchanged.
  example: keep_velocity true
  default: keep_velocity false