Making ICE Particles into Rigid Bodies
When you make particles into rigid bodies, the particles from the same point cloud can collide among themselves, or with other particles from other point clouds. As long as all point clouds are defined as rigid bodies and are in the same simulation environment, they will interact with each other.
To make particles into rigid bodies
1. Create a particle emission—see Creating ICE Particle Emissions.
2. Open the ICE tree and click the Tool tab in the preset manager on the left side.
3. Select the Simulate Rigid Bodies compound from the Simulation group, and drag it into the graph area.
4. Plug the Simulate Rigid Bodies’ Simulate output into a port on the ICETree node.
5. If the Simulate Particles node is plugged into the ICETree node, you can unplug it or delete it because you should use only one type of Simulate node at a time in a tree.
The only time you need the Simulate Particles node to be plugged in addition to the Simulate Rigid Bodies node is if some of the particles are acting as rigid bodies while others are acting as “regular” particles, and you switch between each type of simulation at some point (by using an If node and setting a condition for switching).
6. Open the Simulate Rigid Bodies property editor and set the collision properties for the particles—see Setting the Elasticity and Friction.
Making Particles from Multiple Point Clouds Collide
To make particles collide with particles from other point clouds
1. Create another particle emission (a different point cloud) and make its particles into rigid bodies as described above.
2. Drag this point cloud’s name from an explorer into another point cloud’s ICE tree to create a Get Data node for it.
3. Plug the point cloud’s Value output into an Obstacle > Geometry port in the Simulate Rigid Bodies node.
You can also add the emitter objects and other objects as obstacles by plugging their Value outputs into more Obstacle > Geometry ports (see ICE Particle Collisions with Obstacles).
Collision Geometry for Rigid Body Particles
A bounding shape is automatically assigned to the particle when it becomes a rigid body, using whatever bounding shape is closest to the current particle shape type that is set in the Emit compound: bounding sphere, capsule, or box. Bounding shapes provide a quick solution for calculating particle collisions because they don’t have to calculate detailed geometry.
• Bounding spheres are used for the Sphere and Blob shapes. Points are approximated by very small spheres.
• Bounding capsules are used for the Capsule, Cone, Cylinder, and Segment shapes.
• Bounding boxes are used for the Box, Disc, and Rectangle shapes.
• If you’re using instanced geometry on the particles (see ICE Particle Instances (Objects as Particle Shapes)), a bounding sphere is used, not the object’s actual geometry. This is done to make the calculation time faster.
You can also set the CollisionShape attribute to change the shape of the collision geometry for the particles to one of the three bounding shapes described above (Box, Sphere, and Capsule). For example, if you want to use a box instead of a sphere to better approximate the shape of an instanced geometry, you can set the CollisionShape value to be a box.
This attribute is not exposed in a property editor or a compound, but you can set it in a Set Data node. See Using ICE Particle Attributes for more information.
If you don’t define the CollisonShape attribute, the Shape attribute is used instead (see The Shape Attribute).
Determining the Collision Size of the Rigid Body Particles
Rigid body particles have a thin skin surrounding them. This usually helps to prevent objects from going through one another without colliding, or prevent problems when using grids and flat objects as obstacles. However, the skin is also why you may notice small interpenetrations with the rigid bodies.
To solve this problem, there are two particle attributes that you can set to determine the particle rigid body’s size: CollisionSize and CollisionScale.
• The CollisionSize attribute is simply the particle rigid body radius in Softmage units.
• The CollisionScale attribute is the size of the particle rigid body shape on each of its XYZ axes. This lets you scale the rigid body by different values on different axes for oblong collision shapes that cannot be defined by Size or CollisionSize alone.
The final size of a particle for collisions is the product of CollisionSize and CollisionScale. With these two attributes, you can set a particle body’s size to be bigger than it actually is and counter the interpenetration caused by the skin. Making the CollisionSize value larger increases the chance of collisions while keeping the actual particle size small.
If you don’t set the CollisionSize and CollisionScale attributes, the particle’s Size and Scale attributes determine the rigid body’s physical size. If neither CollisionSize nor Size is defined, then 0.001 is used. If neither CollisionScale nor Scale is defined, then (1, 1, 1) is used. See ICE Particle Size for information on the Size and Scale attributes.
The CollisionSize and CollisionScale attributes are not exposed in a property editor or a compound, but you can select them in a Set Data node. See Using ICE Particle Attributes for more information.
Tips for Creating Successful Interparticle Collisions
• Don’t let the particles go so fast that they bounce off each other too easily. You can control rambunctious particles this way. See ICE Particle Speed.
• If the particles are bouncing around too much when they collide, lower the collision Elasticity value—see Setting the Elasticity and Friction.
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