Volume axis curves let you move objects, including particles and nParticles, in various directions along the curve, as well as define a radius around that curve in which the axis field is active. You can select an existing NURBS curve (in addition to the object to which you want to apply the field) when creating a volume curve field. Otherwise, a default curve is created for the volume axis field.

The overall radius of the volume axis field on a curve is set using Section Radius. Using the Curve Radius ramp, you can vary Section Radius along the length of your volume axis curve. When a position marker is added to the Curve Radius ramp, a Radius Scale indicator is placed on the curve in the scene view. These indicators show the points on your NURBS curve where Curve Radius is scaled to specific values. See Curve Radius

To create a volume axis curve

1. From the Dynamics or nDynamics menu set, select Fields > Volume Curve.
   
2. In the Attribute Editor, click the volumeAxisField tab, and then modify the settings. See the following:
   • To set Volume Control Attributes
   • To set Volume Speed Attributes

To create volume axis curve from an existing NURBS curve

1. Select the NURBS curve you want to use as a volume axis curve, and then -select the objects, such as particle, nParticle, or nCloth objects, you want the field to influence with the volume axis curve.
2. From the Dynamics or nDynamics menu set, select Fields > Volume Curve.

   In the scene view, dotted rings appear around the NURBS curve to indicate that the curve is now a volume axis curve.

   
3. In the Attribute Editor, click the volumeAxisField tab, and then modify the settings. See the following:
   • To set Volume Control Attributes
   • To set Volume Speed Attributes

To connect a volume axis curve to particle and nParticle objects

1. In the scene view, select a volume axis curve.
2. In the Outliner, select the objects, such as particle, nParticle, or nCloth objects you want the field to influence with the volume axis curve, and then -select (Windows and Linux) or -select (Mac OS X) the volume axis field.
3. Select Fields > Affect Selected Object(s).

To set Volume Control Attributes

1. In the scene view, select the volume axis curve you want to edit.
2. In the Attribute Editor, click the VolumeAxisfield tab.
3. In the Volume Control Attributes section, set Section Radius to define the radius of the volume field.
4. To set volume trapping properties to keep objects from exiting the volume field, do the following:
   • Set Trap Inside to apply a force to the volume field that keeps objects inside the volume.
   • Set Trap Radius to specify the radius (as a local space distance) from the volume boundary within which the Trap Inside force is applied.
   • To create ends on volume curves, turn on Trap Ends.

For information about other Volume Control Attributes, see Volume Control Attributes.

To set a Curve Radius ramp

1. In the scene view, select the volume axis curve for which you want to set a Curve Radius ramp.
2. In the Attribute Editor, click the volumeAxisField tab.
3. In the Volume Control Attributes section, under Curve Radius, set the ramp options by doing the following:
   • Click the ramp to create a Selected Position for a Section Radius value. The left position on the ramp represents Section Radius at the base of the curve, while the right position on the ramp represents a Section Radius at the tip of the curve.
     

     When a position marker is added to the Curve Radius ramp, a Radius Scale indicator is placed on the curve in the scene view. These indicators show the points on your NURBS curve where Curve Radius is scaled to specific values. For more information, see Curve Radius.

     
   • Set an Interpolation method to determine how Section Radius blends between the positions on the ramp. See Interpolation.

To set Volume Speed Attributes

1. In the scene view, select the volume axis curve you want to edit.
2. In the Attribute Editor, click the VolumeAxisfield tab.
3. In the Volume Speed Attributes section, set the following:
   • Away From Axis to specify the speed at which particles or other objects move away from the central axis of the curve.
   • Along Axis to specify the speed at which particles or other objects move along the central axis of the curve.
   • Around Axis to specify the speed at which particles or other objects move along the central axis of all volumes.

For information about other Volume Speed Attributes, see Volume Speed Attributes.