Velocity is the rate of change of a point’s position in a direction (vector). If the velocity does not change in the span of one second, the point travels along the length of that vector in one second.
Speed is simply the amplitude of the velocity (the length of the velocity vector) without the direction. You can set the particle direction using other parameters—see ICE Particle Direction. Particle speed is measured as the distance that the particles travel per second.
You can set a particle’s speed when it’s emitted, as well as have the particles inherit the velocity of an animated emitter. Different speeds help you to create different effects, from explosive flying debris to gently swirling snowflakes.
To modify the speed, you can change the speed over the particle’s lifetime. You can also randomize or turbulize the initial speed value to have particles randomly emitted at different speeds.
Adding a force to a particle simulation is another way in which you can alter a particle’s speed—see ICE Forces for more information.
Setting the Particle’s Initial Speed
You can determine the speed of the particles at emission time. The particle’s speed stays the same over its lifetime until some force changes it.
To set the particle’s initial speed
1. Create a particle emission—see Creating ICE Particle Emissions.
2. In the Emit compound’s property editor, set the Speed of the particle. This value is the number of Softimage units that the particles travel per second.
If this value is set to 0, the particles are emitted, but simply remain on the emitting object. This is an easy way to make particles accumulate on the emitter.
The Rate of particles can also affect the speed: emitting 20,000 particles per second slows is slower than emitting only 100 particles per second.
Displaying the Particle Velocity
To help you visualize the velocity of each particle, you can create a display attribute using PointVelocity, then choose to display this as vectors, vector trails, numeric values, vectors+length, etc. For information, see Creating Particle Display Attributes.
Inheriting the Particle Emitter’s Velocity
Particles can inherit the velocity of an animated emitter object at emission time by using the Inherit Emitter Velocity compound.
To use the particle emitter’s velocity
1. Create a particle emission—see Creating ICE Particle Emissions.
2. In the ICE tree’s preset manager, click the Task tab and select Particles.
3. Select the Inherit Emitter Velocity compound from the Emission Control group, and drag it into the graph area.
4. Plug this compound’s Execute on Emit output into the Emit compound’s Execute on Emit port.
5. In the Inherit Emitter Velocity property editor, set the percentage of the emitter’s velocity that the particles will use at emission time.
Modifying the Particle Velocity Over Time
The Modulate Velocity Over Time compound changes the particle velocity over its lifetime using a profile curve. This lets you easily make the particles accelerate or decelerate exactly how you want them to.
Forces also change the velocity of particles over time, such as gravity, wind, or drag. However, if you want to create non-physically accurate results, this compound gives you the control over how the particle velocity changes.
To vary the speed over time
1. Create a particle emission—see Creating ICE Particle Emissions.
2. In the ICE tree’s preset manager, click the Task tab and select Particles.
3. Select the Modulate Velocity Over Time compound from the Modifiers group, and drag it into the graph area.
4. Plug this compound’s Execute output into a port on the ICETree node, or into any Execute port of other compounds such as Slide on Surface, Bounce on Surface, or Stick to Surface.
5. In the Modulate Velocity Over Time property editor, set the profile curve of how the particle’s speed changes over time.
Turbulizing the Particle Velocity
The Turbulize Particle Velocity compound applies Simplex noise to add turbulence to the particle speed. Adding noise to the Speed value emits the particles at different speeds, which helps the particles look more natural for certain types of effects.
To add turbulence to the particle’s speed
1. Create a particle emission—see Creating ICE Particle Emissions.
2. In the ICE tree’s preset manager, click the Task tab and select Particles.
3. Drag the Modifiers > Turbulize Particle Velocity compound into the ICE view.
4. Plug this compound’s Execute output into a port on the ICETree node.
You can also plug it into any Execute port of other compounds such as Slide on Surface, Bounce on Surface, or Stick to Surface, such as to make the particle speed turbulize when the particles collide and bounce off an obstacle.
5. In the Turbulize Particle Velocity property editor, you can set the turbulence properties including its scale, strength on each of the XYZ axes, complexity (fractal-like detail), and its animation speed.
For more information on the parameters, click the ? icon in the property editor or see Turbulize Particle Velocity.
There are several compounds on the Task > Particle tab that let you add randomness or turbulence (noise) to the Speed value so that the particles are not all emitted at the same speed. Adding randomness or noise helps the particles look more natural for certain types of effects.
These compounds include:
• Emission Control > Randomize Emitter Value and Turbulize Emitter Value
• Modifiers > Randomize Value by Range and Turbulize Value by Range
• Modifiers > Randomize Around Value and Turbulize Around Value
See Modifying ICE Particle Values for information on each one of these compounds.
There are two particle attributes that are used to define particle velocity: Init_PointVelocity and PointVelocity. These attributes are used in several compounds, but you can also use them on their own in an ICE tree by specifying them in the Get Data or Set Data nodes, as described in Using ICE Particle Attributes.
For more information on attributes in general and a list of all available ICE attributes, see ICE Attributes.
• Init_PointVelocity: A particle’s original PointVelocity value when it is emitted. This attribute it set by the Emit compounds, but you can change its value using the Speed parameter in the Emit property editor (see Setting the Particle’s Initial Speed).
The Init_PointVelocity attribute is also used by the Move Toward Goal and Modulate Velocity Over Time compounds.
• PointVelocity: The velocity of points. This attribute is automatically updated by the Simulate Particles and Simulate Rigid Bodies nodes based on the particle’s Force and Mass attributes.
The PointVelocity attribute is used by the Align on Velocity, Turbulize Particle Velocity, Inherit Emitter Velocity, and Test Particle Velocity compounds.
You can get and set the PointVelocity attribute with the Get Particle Velocity and Set Particle Velocity compounds, which are simply the Get Data and Set Data nodes with the PointVelocity attribute already specified.
You can also use the Set Particle Speed compound, which uses the PointVelocity attribute and a Resize Vector node to determine number of units travelled.
And you can also use the Get Particle Movement compound, which gets the PointVelocity value from a particle for the current frame. You can then output the value as a 3D vector.
Autodesk Softimage v.7.5