The ICE forces can be found in the Forces group on the Task > Particles tab on the preset manager in the ICE tree.
There is a standard procedure to use all of these forces in the ICE tree—see Creating and Applying Forces to ICE Simulations.
The Add Forces compound is a hub for the forces in your ICE tree: it adds up and blends the effect of all forces that are plugged into it, then applies it to the simulated objects.
If no other force is plugged into the Add Forces compound, you can open its property editor and create a simple velocity force in any global direction (XYZ).
Gravity is the most common type of force that you will use, for obvious reasons. In a larger cosmic sense, it is the attraction between all objects that have mass. However, on Earth, we are usually only concerned about the Earth’s gravity effect on us and objects. Gravity actually applies a force that defines an acceleration over time.
Gravity is the same for all objects regardless of their mass. Objects of different mass will fall at the same speed: if you drop a bowling ball and a beach ball from the same height, they will hit the ground at the same time if there is no air resistance (drag).
However, everything changes at the moment there is air resistance/friction or when the object collides with another: that’s where mass, energy, and momentum play a major role. Gravity moves simulated objects based on their mass, and is directly proportional to an object’s mass. The more massive an object, the stronger the gravity force applied to it.
To have the correct gravitational behavior from the objects, the size of the objects in the scene must be taken into consideration. Depending on the scale of objects in your scene, you may need to adjust this value to get objects falling as they should.
The drag force adds flow resistance to the movement of an object in the opposite direction to its velocity, as in a fluid. This force give a more direct level of control when you want to slow down particles or other simulated objects. For example, you could create the effect of particles moving under water using the drag force or create a slow-motion effect of particles as they are being emitted.
The drag force depends on the density or thickness of the fluid; for example, moving through air is easier than moving through water. It also depends on the velocity of the object and on its size (most importantly, the size of the object that is perpendicular to the velocity), which is why long, thin objects are aerodynamic. Drag is proportional to the square of the velocity: if you move twice as fast, the drag will be four times as strong. The degree to which the drag force slows down an object depends on its mass: heavier objects decelerate less because they have more momentum.
The drag force is opposite to an object’s velocity relative to the fluid’s velocity. For example, an object can’t remain still in a strong current: even if its velocity is zero, its velocity relative to the current is not.
You can control the drag’s strength, as well as set the drag type to be either a simple velocity-based or a physically accurate drag:
• Physically Accurate drag takes into account the size and mass of the particle. The Strength value needs to be smaller to achieve results that are similar to physics in real life.
• Simple Velocity Based drag slows down the particles or objects by a ratio each frame. For example, a setting of 0.1 slows down particles or objects by 10% each frame. It generates a force that is proportional to the mass so that the particle’s Mass value (in the Emit compound) has no effect. You may want to use this option if you don’t want to create physically accurate simulations.
• The Strength determines how much velocity is lost by the particle each frame. A value of 1 stops the particle completely in one frame; a value of 0.1 means the particle loses 10% of its speed each frame.
The wind force is a directional force with velocity and strength. It generates a force that speeds up particles or objects to a target velocity.
You can set the wind’ velocity, which is the speed to which the particles or objects try to reach, the strength, and the direction.
If there are no other forces at work on a particle simulation except wind, particles will eventually reach the speed of the wind’s velocity setting which, in some cases, can actually slow down the particle velocity at emission. This is because particles have an initial velocity of their own when they’re emitted.
The Coagulate force attracts points toward their neighbors to form clusters. Once the points get within a certain range of each other, the friction (drag) slows them down.
This force is useful for blood splatter, mercury or liquid metal forming, water droplets. You may want to try using this force with the particle shape set as blobs to create these types of effects.
You can set the cutoff distance (the greatest distance in which the coagulation force is applied), the attraction strength value, and the coagulation size and friction. You can use a profile curve to define the strength based on the distance the points are from each other.
The Neighboring Particles force attracts particles to each other when they get within a certain range, in a manner similar to the Coagulate force. However, there is no friction between the particles so they don’t stay clumped together as with the Coagulate force, they keep moving.
You can set the cutoff distance (the greatest distance in which the force is applied), the attraction strength value, and a use profile curve to define the strength based on the distance the particles are from each other.
The Null Controller force uses the location of a null to control the center, direction, and size of the force. The points are either attracted to or repelled from this null in a manner that is similar to how particles move toward or away from a goal object.
To use this force, you must create a null and drag its name into the ICE tree to create a node for it. Then you plug its Out Name output into the Null Name port of the Null Controller Force compound.
Changing the icon shape of the null (to something like Rings, Square, or Circle) changes the behavior of this force. This is because the force value that is generated is modulated from the center of the null towards the edges of its shape using the profile curve. You can also set this force’s direction on any local axis of the null.
You can change the null icon shape by opening its property editor and selecting a Primary Icon shape from the list.
The Point force attracts particles/objects to or repels them from a position in space that you define.
You can set the force’s strength, location of its center, maximum distance in which it has an effect, and whether the force attracts or repels the particles or objects (direction).
The Surface force attracts particles/objects to or repels them from an object’s surface, much like a magnet attracts/repels iron filings. This makes it easy to produce effects such as those created by magnets, black holes, or gravity.
While this force is similar to creating goals for particles, the particles with this force keep moving around (“swarming”) the surface object instead of trying to assume its shape, as they do with goal objects.
To use this force, you must create an object with surface geometry (such as a polygon mesh or NURBS surface), and drag its name into the ICE tree to create a node for it. Then you plug its Value output into the Surface port of the Surface Force compound.
You can set the force’s strength, the offset distance of particles or objects from the surface, the maximum distance around the surface in which the force has an effect, and whether the particles or objects are attracted to or repelled from the surface (direction).
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