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Attaching Objects to Particles

Attaching objects to particles allows you to use objects as part of a particle simulation. You can attach any geometric object, a light, or a camera to particles to create many different effects.

For example, you could attach a blood cell object to some particles flowing through an artery and lights to others. Then attach a camera to one particle and its interest to another, and take a wild ride through the blood stream!

Depending on what type of simulation you’re creating, there are two ways in which you can attach objects to particles:

• Create instances of objects that are attached to the particles at render time, such as the bats attached to the particles in the image below.

or

• Create particle clusters and then constrain an object to the clusters. This is useful if you have particles that don’t constantly get born and die, such as with a static cloud whose particles live forever. The cluster can be an individual particle or a group of particles.

Creating Instances of Objects to Attach to Particles

You can automatically create instances of objects for each particle at render time. You can select a group of objects and instantiate them on particles. Each object in the group is assigned randomly onto a particle and stays with that particle for its lifetime. This is because the instance is based on the unique ID of the particle and not its index (see About Particle IDs).

The local transformation of the object’s tree is ignored when instancing so that you can scale the source objects to 0 or translate them out of view. The instances’ translation is always inherited from the particle’s position and the particle’s rotation and scaling can be inherited as well.

Instancing takes the objects at the current time, so if you need multiple objects to be all the same at different times (for example a flock of birds), you have to clone that object multiple times and shift it along its cycle.

To create instances

1. Select the objects you want to instance and create a group that includes them (press Ctrl+g).

2. Select the particle type to which you want to attach the instances and open its Particle Type property editor.

3. On the Instancing page, select Enable to activate instancing.

4. Click the Pick button beside the Instance Group text box. In the explorer, pick the group you want to instance.

5. If you want the instances to inherit rotation from the particle, select Rotation > From Particle; otherwise, select None.

6. For Scaling, there are four different options for how the instanced object inherits the particle’s size:

- By particle size scales the object uniformly by using the size of the particle as the scale factor.

- Non-uniform to particle scales the object to fit the particle’s bounding box using non-uniform scaling. This makes the object completely fill in the bounding box.

- Uniform to particle scales the object to fit the particle’s bounding box using uniform scaling. This makes the object fit in the bounding box, but without changing its shape.

- None: the instanced object stays its current size and is not influenced by the particle size.

7. The instanced objects are displayed as nulls and/or bounding boxes in the viewport. Draw a render region to see the object geometry on the particles.

Inheriting the Particles’ Orientation

When you constrain or attach objects to particles, the objects inherit only the position of the particle. However, if the particles are rotated, there are two things you can do to have the objects also inherit the particles’ orientation:

• Select the Align on Velocity option on the Particle Type > General property page to align the particle type’s orientation to the velocity of the particle (see Rotating Particles).

• Activate the Tangency and Normal options for the Object to Cluster constraint (see Object to Cluster Constraints [ Animation ] for more information).

An attached cannot object inherit the particles’ size and color, but you could do this via a scripted particle event. You create a particle event that uses a script that sets the size and color of the attached object based on certain particle attributes. See Scripting a Particle Event for more information.

Constraining Objects to Particle Clusters

Because particles are points, you can create clusters of them and then constrain objects to those clusters. This method is useful if you don’t have too many objects to constrain and you’re using a static cloud.

To constrain objects to particle clusters

1. Create a particle cloud and select Live Forever for the particle type’s Life span (see Setting the Particle’s Lifetime). This prevents the particle from dying, so its ID stays active throughout the length of the simulation.

2. Create an initial state for the particle cloud at a frame in the simulation that you’d like to be frame 1. See Creating an Initial State for Particles for information on how to do this.

Initial states allow you to forego creating a pre-roll to get to the frame you want for frame 1. Initial states also help to deal with objects constrained to particles that haven’t been emitted yet. If you don’t set the initial state, these objects hang around until their particle appears.

3. In Point mode, select the particles you want to use for a cluster and click the Cluster button on the Edit panel. You can also select a single particle to which you want to constrain an object.

Do this step for each cluster of particles you want to create.

4. Create the object you want to attach, including the correct number of instances for the number of clusters to which you want to attach it.

5. Select the object you want to attach to a cluster or particle and choose Constrain > Object to Cluster. Pick the particle cluster from the explorer.

Do this for each object that you want to attach to each particle cluster.

If you have many objects to attach to many clusters, you may want to write a script to do this.

6. Play the simulation and see the objects attached to the particle clusters.

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