You need to have only one Simulate node plugged into an ICETree node at a time. This means that if the Simulate Rigid Bodies node is plugged into the ICETree node, you don’t need the Simulate Particles node to also be plugged in.
The Simulation Regions and Nodes
An object’s construction stack is divided into different regions so that operators can be evaluated in different ways that are appropriate to the task that you’re doing. Traditionally, there are regions for Modeling, Animation, Shape Modeling, and Secondary Shape Modeling. If you’re working with simulated ICE trees, however, there are two more construction regions that are created: Simulation and Post Simulation.
The Simulation and Post Simulation regions are used only for ICE simulations and appear automatically when you create a Simulated ICE Tree node (operator). They appear directly below the Secondary Shape Modeling region.
By default, the initial state of a simulation is the result of the operators in the regions that are below the Simulation region on the first frame at which the simulation environment is active (see The ICE Simulation Environment). When the simulation is active, the regions below the Simulation region are not re-evaluated.
This means that if you have any operators in the Animation region, those operators are not evaluated again after the simulation is active. If you want to return to using the animation operators instead of the simulation, simply select and delete the Simulation marker from the stack so that there’s no Simulation region.
For more information, see ICE Trees and the Construction Stack.
The Simulation Region and the ICETree Node
When you create a simulated ICE tree, a Simulation region is created in the object’s construction history and the ICETree operator is placed in that region. The simulated ICETree node (operator) is exactly the same as the one you get with a regular ICE Tree except that it’s in the Simulation region by default.
To automatically create a simulated ICETree node, you can emit particles using the ICE > Create > Emit Particles from Selection command on the Simulation toolbar, or you can open an ICE Tree and choose Create > Simulated ICE Tree from its command bar.
Any operators that are in the Simulation region are time dependant. Currently, only particles and deformation operators can have simulated ICETree nodes. Simulated means that the result of the current frame depends on the result of the previous frame rather than on the results of the construction regions that are below it.
This is what differentiates a simulation from other types of modifications. For example, if you use a Push deformation on a sphere, at each frame the Push operator gets the undeformed sphere and applies the push. If you move one frame forward, the Push gets the undeformed geometry again and applies the same push.
The only way to modify the push deformation over time is to animate its parameters. However, if the Push operator is in the Simulation region, at each frame it takes the results of the sphere’s pushed points from the previous frame and pushes them out more. As you play the simulated push, you see the sphere gradually inflating, without any animation being defined. The only way to return to the undeformed sphere is to go back to the first frame.
Operators in this region are applied on top of the simulation, but are not used for defining shapes in the Secondary Shape Modeling region. You could use the Post-Simulation region to apply a deformation, such as a lattice, on top of a particle simulation.
The Post-Simulation region is always evaluated, even if there isn’t an active simulation.
The Simulate Particles and Simulate Rigid Bodies Nodes
In the world of ICE, there are two nodes that allow you to simulate particles: the Simulate Particles node and the Simulate Rigid Bodies node. These are their stories.
Simulate Particles Node
The Simulate Particles node is the “standard” particles node that updates the position and velocity of each particle at each frame by doing the following. It:
• Calculates the point accelerations based on mass and force.
• Updates the point velocities based on acceleration.
• Updates the point positions based on velocity.
• Sets the simulated frame fraction to 1. The SimulatedFrameFraction attribute defines the fraction of a frame that has already been simulated for a point. Setting this value to 1 prevents points from being resimulated by other Simulate nodes in the same tree (such as the Simulate Rigid Bodies node) or in other trees on the same object.
The Emitter compounds automatically set the SimulatedFrameFraction to (1 – Age/SimulationStep) so that on its first frame of existence, a particle moves only for a distance proportional to its age. As a result, it appears as if particles have been emitted continuously throughout the frame instead of all at once.
For information on using the Simulate Particles node for particle emissions, see Creating ICE Particle Emissions.
The Simulate Rigid Bodies Node
The Simulate Rigid Bodies node is similar to Simulate Particles in that it simulates the particles, but it also lets you make particles into PhysX rigid bodies, which are objects that do not deform in a collision. Particles can collide with each other, based on their shape, and with other objects that are set as obstacles.
Unlike the Simulate Particles node, the Simulate Rigid Bodies node does not have the SimulateFrameFraction attribute because the PhysX dynamics engine doesn’t work on frame fractions to detect collisions: it uses whole frames only.
For information on using this node, see ICE Rigid Bodies.
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You need to have only one Simulate node plugged into an ICETree node at a time. This means that if the Simulate Rigid Bodies node is plugged into the ICETree node, you don’t need the Simulate Particles node to also be plugged in. |
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