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If you are using a version of Blender up to 2.45 click here to access the old documentation.

Particles, Soft Bodies and Cloth objects can interact with their environment (other objects or particle systems). They may collide with them or be moved by forces (Fields). All types of objects and particles can generate fields, only mesh objects can be used as deflecting objects. Only curve object can bear Curve Guides.

  • The objects need to share at least one common layer to have effect.
  • You may limit the effect on particles to a group of objects (in the Extras panel).
  • Fields can be generated by objects and/or by particles.

After changing the fields (Fields panel) or deflection (Collision panel) settings, you have to recalculate the particle, softbody or cloth system (Free Cache), this is not done automatically. You can clear the cache for all selected objects with CtrlB → Free cache selected.

Particles react to all kind of Force Fields, Soft Bodies only to Spherical/Wind/Vortex (they react on Harmonic fields but not in a useful way).

Mode: Object Mode

Panel: Object context → Physics sub-context → Fields

Hotkey: F7

Image 1a: Fields panel in the Physics sub-context for an object without particles.
Image 1b: Fields panel for an object with particles.

To create a field you have to select the object and change to the Physics sub-context.

  • Select the field type in the Fields panel. If the object has no particle system the panel looks like (Image 1a). An object can have only one field.
  • If the object has one or more particle systems you can select one of the particle systems instead. Each particle system may have up to two fields assigned (Image 1b).

The fields have many options in common, these common options are explained for the Spherical field.

Spherical field

Image 2a: Force field panel for Spherical fields.

The Spherical field is the simplest of the fields. It gives a constant force towards (positive strength) or away from (negative strength) the object’s center. Newtonian particles are attracted to a field with negative strength, and are blown away from a field with positive strength.

For Boids a field with positive strength can be used as a Goal, a field with negative strength can be used as Predator. Whether Boids seek or fly goals/predators depends on the Physics settings of the Boids.

Image 2b: Spherical field indicator.
Strength
The strength of the field effect. This can be positive or negative to change the direction that the force operates in. A force field’s strength is scaled with the force object’s scale, allowing you to scale up and down scene, keeping the same effects. You can animate this parameter (Object Ipos, FStrength channel).
Planar
The field is constant in the XY-plane, changes only in Z direction.
Fall-off
Here you can specify the shape of the force field (if the Fall-off Power is not nil).
Sphere
Spherical force field.
Fall-off (Power)
How the power of the force field changes with the distance from the force field. If r is the distance from the center of the object, the force changes with 1/rPower. A Fall-off of 2 changes the force field with 1/r2. You can animate this parameter (Object Ipos, FFall channel).
Pos
Fall-off only in the direction of the positive Z-Axis.
Max Dist/Use
Makes the force field only take effect within a specified maximum radius (shown by an additional circle around the object). You can animate this parameter (Object Ipos, FMaxD channel).
Min Dist/Use
The distance from the object center, up to where the force field is effective with full strength. If you have a Fall-off of 0 this parameter does nothing, because the field is effective with full strength up to Max Dist (or the infinity). Shown by an additional circle around the object.
Tube
Fall off results in a tube shaped force field. The force field can change longitudinally - Longitudinal, along the axis of the tube - or radially (Radial).
Cone
Fall off results in a cone shaped force field.


Other Field Types

Image 3a: Wind field indicator.
Wind
Wind gives a constant force in a single direction, along the force object’s local Z axis. The strength of the force is visualized by the spacing of the circles shown.
Noise
Random variation of the force. Nice, because you don’t have to use an IPO curve for that any more.


Image 3b: Vortex field indicator.
Vortex
Vortex fields give a spiraling force that twists the direction of points around the force object’s local Z axis. This can be useful for making a swirling sink, or tornado, or kinks in particle hair.


Magnetic
This field depends on the speed of the particles.
Harmonic
The source of the force field is the zero point of a harmonic oscillator (spring, pendulum). If you set the Damping parameter to 1, the movement is stopped in the moment the object is reached. This force field is really special if you assign it to particles. Normally every particle of the field system influences every particle of the target system. Not with Harmonic! Here every target particle is assigned to a field particle. So particles will move to the place of other particles, thus forming shapes. Tutorial: Particles forming Shapes.
Charge
It is similar to spherical field except it changes behavior (attract/repulse) based on the effected particles charge field (negative/positive), like real particles with a charge. This mean this field has only effect on particles that have also a Charge field (else, they have no “charge”, and hence are unaffected)!
Lennard-Jones
This field is a very short range force with a behavior determined by the sizes of the effector and effected particle. At a distance smaller than the combined sizes the field is very repulsive and after that distance it’s attractive. It tries to keep the particles at an equilibrium distance from each other. Particles need to be at a close proximity to each other to be effected by this field at all.

Particles can have for example both a charge and a Lennard-Jones potential - which is probably something for the nuclear physicists amongst us.

Texture field

You can use a texture force field to create an arbitrarily complicated force field, which force in the 3 directions is color coded. Red is coding for the x-axis, green for the y-axis and blue for the z-axis (like the color of the coordinate axes in the 3D window). A value of 0.5 means no force, a value larger than 0.5 acceleration in negative axis direction (like -Z), a value smaller than 0.5 acceleration in positive axis direction (like +Z).

Texture mode
This sets the way a force vector is derived from the texture.
RGB
Uses the color components directly as the force vector components in the color encoded directions. You need an RGB texture for this, e.g. an image or a colorband. So a Blend texture without a colorband would not suffice.
Gradient
Calculates the force vector as the 3d-gradient of the intensity (grayscale) of the texture. The gradient vector always points to the direction of increasing brightness.
Curl
Calculates the force vector from the curl of the 3d-rgb texture (rotation of rgb vectors). This also works only with a color texture. It can be used for example to create a nice looking turbulence force with a color clouds texture with perlin noise.
Nabla
It is the offset used to calculate the partial derivatives needed for Gradient and Curl texture modes.
Use Object Coordinates
Uses the emitter object coordinates (and rotation & scale) as the texture space the particles use. Allows for moving force fields, that have their coordinates bound to the location coordinates of an object.
Root TexCo
The Root TexCo is useful for hair as it uses the texture force calculated for the particle root position for all parts of the hair strand.
2D
The 2D button disregards the particles z-coordinate and only uses particles x&y as the texture coordinates.

Remember that only procedural textures are truly 3D.

Examples:

  • A single colored texture 0.5/0.0/0.5 creates a force in the direction of the positive y-axis, e.g. hair is orientated to the y-axis.
  • A blend texture with colorband can be used to created a force “plane”. E.g. on the left side 0.5/0.5/0.5, on the right side 1.0/0.5/0.5 you have a force plane perpendicular to XY (i.e. parallel to Z). If you use an object for the coordinates, you can use the object to push particles around.
  • An animated wood texture can be used to create a wave like motion.


Curve Guide field

Image 4a: A Curve Guide field.

Curve objects can be the source of a Curve Guide field. You can guide particles along a certain path, they don’t affect Softbodys. A typical scenario would be to move a red blood cell inside a vein, or to animate the particle flow in a motor. You can use Curve Guides also to shape certain hair strands - though this may no longer be used as often now because we have the Particle Mode. Since you can animate curves as Softbody or any other usual way, you may build very complex animations while keeping great control and keeping the simulation time to a minimum.

The Curve needs the option CurvePath set to work. Blender sets this option when you assign a Curve Object this force field type automatically, but if you switch that off you have to restore it manually.

The option Curve Follow does not work for particles. Instead you have to set Angular Velocity (in the Physics panel of the Particle sub-context) to Spin and leave the rotation constant (i.e. don’t turn on Dynamic).

Curve Guides affect all particles on the same layer, independently from their distance to the curve. If you have several guides in a layer, their fields add up to each other (the way you may have learned it in your physics course). But you can limit their influence radius:

MaxDist/Use
The maximum influence radius. Shown by an additional circle around the curve object.
MinDist
The distance from the curve, up to where the force field is effective with full strength. If you have a Fall-off of 0 this parameter does nothing, because the field is effective with full strength up to MaxDist (or the infinity). MinDist is shown with a circle at the endpoints of the curve in the 3D window.
Fall-off
This setting governs the strength of the guide between MinDist and MaxDist. A Fall-off of 1 means a linear progression.

A particle follows a Curve Guide during it’s lifetime, the velocity depends from it’s lifetime and the length of the path. You can change that of course:

  • A time Ipo curve of the emitter change the speed of the particles along the path, but time may not run backwards.
Free
Fraction of particle life time, that is not used for the curve.
Additive
If you use Additive, the speed of the particles is also evaluated depending on the Fall-off.

The other settings govern the form of the force field along the curve.

Clump
The particles come together at the end of the curve (1) or they drift apart (-1).
Shape
Defines the form in which the particles come together. +0.99: the particles meet at the end of the curve. 0: linear progression along the curve. -0.99: the particles meet at the beginning of the curve.
Image 4b: Kink options of a curve guide. From left to right: Radial, Wave, Braid, Roll.
Animation

With the drop down box Kink, you can vary the form of the force field:

Curl
The radius of the influence depends on the distance of the curve to the emitter.
Radial
A three dimensional, standing wave.
Wave
A two dimensional, standing wave.
Braid
Braid.
Roll
A one dimensional, standing wave.

It is not so easy to describe the resulting shapes, I hope it’s shown clearly enough in (Image 4b).

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