Particle Interaction
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## Particle Interaction

by Kenneth Styrberg

`Relevant to Blender v2.34`

### Introduction

The Blender particle system allows particles to interact in two ways - force fields and deflection.

Force fields

There are two variants of force fields: a Standard `Force field` that behaves like a gravity force field, and the `Vortex field`. A vortex field gives more of a tornado effect, with particles spiralling around the vortex center. Force fields are available for all object types. Currently a point-based field with a spherical fall-off is implemented.

Force fields can be set to any object. Particles will then be: attracted with a negative `Strength` value, or repelled if you use a positive `Strength` value. There is also a `Fall-off` parameter which defines how much the strength diminishes with distance from the object's origin.

Deflection

This allows you to set any mesh object as a particle deflector. Particles will then bounce on the surface of the mesh. You can control how much the particles bounce with the `Damping` value, add some randomness to the bounce with `Rnd Damping`, and you can define the percentage of particles which pass through the mesh with the `Permeability` parameter.

### Note

Make sure that the normals of the mesh surface are facing towards the particles for correct deflection.

### Interface

The particle interaction settings are controlled via the `Particle Interaction` panel in Object context (F7).

Figure 20.34. Particle Interaction panel.

Force/Vortex field

- `Strength` - The strength of the field effect.

- `Fall-off` - How much the strength diminishes with distance.

When adding a force/vortex field to an object, the object will get a small graphic indicating that there is a particle interaction connected to the object. The force field will have small grey circles (Figure 20.35, “Force field indicator.”), and vortex fields will have a spiral drawn (Figure 20.36, “Vortex field indicator.”).

Figure 20.35. Force field indicator.

Figure 20.36. Vortex field indicator.

The lower section of the panel in Figure 20.34, “Particle Interaction panel.” handles the settings for deflection.

### Note

You will not see any extra graphic indicators with deflectors as you do with force fields.

Deflection parameters

- `Damping` - Controls the amount of bounce that surfaces will have.

  0.0 - No damping, particles will have maximum bounce.
  1.0 - Maximum damping, particles will not bounce at all.

- `Rnd Damping` - Adds a random element to the bounce. For example, with a `Damping` of 1.0 and a `Rnd Damping` of 0.5, the damping will vary between 1.0 and 1.5.

- `Permeability` - Percentage of particles passing through the mesh.

  0.0 - No particles pass through.
  1.0 - All particles pass through the deflector.

If you set up a particle deflector you'll have to make sure sufficient keys are available for Blender to calculate the collisions with sufficent detail. If you see particles moving through your deflector or bouncing in the wrong positions, then there might be problem with too few keys or your particle/deflector is moving too fast.

### Note

You can animate moving deflectors but particles can leak through the mesh if the deflector moves to fast or if the mesh is complicated. This can be partly solved by increasing the `Keys` parameter for the particle emitter.

After changing any parameters, you will need to select your particle emitter and go back to the `Effects` tab and press RecalcA button. (Figure 20.37, “Effects panel.”).

More keys means longer calculation times and usage of more memory. See the section called “Introduction” for how to set up particle emitters.

Figure 20.37. Effects panel.

For all parameters for force fields and deflectors, `Ipo` keys can be inserted. The `Ipo` curves are edited as `Object Ipo` types in the `Ipo` window. See Chapter 14, Animation of Undeformed Objects for more on `Animation` and `Ipo`.

### Example

Here is a small example demonstrating particle deflectors.

1. Start with default scene with a cube object.
2. In top view, add a `Circle` Mesh and accept default 32 vertices. Press TAB to exit EditMode. Switch to side view and move circle up 5 units.
3. With circle selected, press S, and scale it down to 0.1 in all directions.
4. Now press F7 and go to the `Effects` tab. Press NEW Effect and in the `Popup-list` select `Particles`. You should see the same picture as in Figure 20.37, “Effects panel.”.
5. Increase particle life from default 50 to 100 by changing the `Life` field to 100. In the force field in the lower section of the tab, set the `Force Z` to -0.100. If you press ALT-A you should see an animation of particles moving down towards our cube. See Figure 20.38, “Example frame 40.”. So far, the particles travel straight through without noticing the cube. This will change!

Figure 20.38. Example frame 40.

6. Now select our cube. With the `Effect` context still active, go to the `Particle Interaction` tab and set the `Deflector` settings as pictured in Figure 20.39, “Example Deflection settings.”.

Figure 20.39. Example Deflection settings.

7. Now select the circle and press RecalcA in the `Effects` tab. If you play the animation, ALT-A, you will see the particles bounce off the cube straight up and down. The problem is that the bounce is a bit away from the cube. This is an effect of too few `Keys`. In the `Keys` field enter 50, and press RecalcA. Play the animation and the particles will now bounce on the cube, or close enough.
8. Select the cube and tilt it 10 degrees in either direction. Select the circle and recalculate the particles. Now play the animation and you will see the particles bounce off in the direction you tilted the cube! (Figure 20.40, “Example end result.”).

Figure 20.40. Example end result.

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