The attributes on the bifrostMotionFieldContainer node can be used to affect the velocity of Bifröst liquid, foam, or aero particles based. The effect of the field is indicated by arrows in the viewport. See Control velocity with Bifröst motion fields.
Container Attributes
- Enable
- Enables evaluation of the entire container node.
- Evaluation Type
- Controls how the node is evaluated. Normally, this should not be changed.
- Simulation
- The node gets evaluated based on the previous frame.
- Property
- The node is used only to set values used by other nodes.
- Mesh Property
- The node is used to set values on polygon mesh objects, such as emitters, colliders, and other meshes used in a simulation.
- Per Frame
- The node is evaluated every frame, but does not depend on the previous frame. This mode can be used for animation or reading cache files.
Motion Field Properties
- Magnitude
- An overall multiplier for the total combined effect of all components of the motion field.
- Direction
- Applies acceleration in a specified direction. To control the effect, use the Field Direction attributes, or transform the field node in the scene.
- Drag
- Applies a force to emulate drag by pulling particles' velocities gradually toward the field velocity, similar to wind. This has the effect of speeding up particles that are moving more slowly than the field velocity and slowing down particles that are moving faster, unlike the other options which always add to the particles' velocities. Use the Drag attributes below to control the drag effect.
- Noise
- Applies random noise to particles' velocities. Use Noise Magnitude in the Turbulence and Noise attributes to control the effect.
- Turbulence
- Applies a divergence-free turbulence to particles' velocities, with no sources or sinks. This can be used to create vortices that evolve over time. Use the Turbulence and Noise attributes to control the effect.
- Geometry
- Affects particles' velocities based on the connected polygon meshes. Use the Geometry attributes to control the effect.
- Scale Affects Speed
- Whether the field node's world scaling factor affects the magnitude of the effect.
Boundary Countrols
- Enable Boundary
- Applies the effect based on an implicit shape, shown in wireframe in the viewport. Scale, rotate, and translate the bifrostMotionField node in the scene to define the affected volume.
- Boundary Shape
- Select a shape. Note that the shape affects some of the Field Direction options, even when Enable Boundary is off.
- Boundary Falloff
- Controls the exponent of the falloff curve from the full magnitude at the center of the shape to none at the boundary. In the case of a torus shape, the falloff is from the circular axis to the boundary. A value of 0.0 gives no falloff, 1.0 gives linear falloff, 2.0 gives quadratic falloff, and so on.
- Section Radius
- The "tube" radius, in the case of a torus shape.
- Enable Max Depth, Max Depth
-
Restricts the effect of the motion field to a layer of the specified depth below the surface of the fluid volume. For example, this can be useful to apply drag only at or near the surface of fast-moving water.
Note that this option does not require Enable Boundary to be activated.
- Invert Falloff
- Inverts the falloff based on the boundary shape. When this option is on and Boundary Falloff is 0.0, the effect applies outside the boundary shape, instead of inside. When Boundary Falloff is greater than 0.0, the effect has full magnitude at the boundary as well as outside it, and falls off to none at the shape's center.
Field Direction
These options apply when Direction is activated.
The axis used by some of the options depends on the Boundary Shape. In the case of a torus shape, the circular axis is used. When Enable Boundary is off or if the shape has no natural main axis, the default axis is +Y in the field node's object space.
Use negative magnitudes to reverse the direction.
- Direction Magnitude
- The acceleration along the Direction vector in m/s2. You can set this value to 0 to turn off the main directional force and still use the other options to control acceleration along other directions.
- Direction
- The direction of acceleration in the object space of the field node.
- Enable Concentric, Concentric
- Controls the acceleration in m/s2 from the center of the field node.
- Enable Along Axis, Along Axis
- Controls the acceleration in m/s2 in the direction of the axis.
- Enable Around Axis, Around Axis
- Controls the acceleration in m/s2 around the axis.
- Enable Away From Axis, Away From Axis
- Controls the acceleration in m/s2 away from the axis.
Drag
- Drag
- Controls how strongly the particles' velocities are pulled toward the motion field's velocity. At very high values, the particles's velocities quickly match the field velocity and then hover around that value.
- Normal Drag
- Similar to drag, but is applied based on the normal of the fluid surface. More drag is applied at the leading edge than at the back. This can help to break up liquid and flatten drops as they fall.
Turbulence and Noise
- Turbulence Magnitude
- The strength of the acceleration caused by the turbulence field.
- Turbulence Frequency
- The granularity of the turbulence. Higher values produce smaller, denser vortices.
- Turbulence Speed
- The rate at which the turbulence pattern evolves over time.
- Turbulence Scale
- Scales the turbulence pattern in space.
- Turbulence Offset
- Translates the turbulence pattern in space.
- Turbulence Time
- The time used to look up the current turbulence pattern. By default, this is driven by the scene time but you can disconnect it if you want to control the turbulence pattern's evolution independently.
- Turbulence Type
- The algorithm used to generate the underlying turbulence pattern.
- CurlNoise is based on a simple noise field.
- CurlFractal uses several iterations of noise at different scales and speeds. This creates a mix of larger and smaller vortices.
- Worldspace Turbulence
- Whether positions in the turbulence field are considered in world space or the local space of the field. Turn this option on if you want to:
- Scale the field without stretching the turbulence pattern.
- Translate or rotate the field without dragging the turbulence pattern along with it. For example, this lets you animate the field and have the boundary "swim" through the turbulence pattern.
- Noise Magnitude
- The magnitude of the noise when Noise is on.
Geometry
Controls how a connected mesh affects the particles' velocities.
- Enable Falloff, Min Distance, Max Distance
-
Controls whether the motion field considers the mesh's volume.
If Enable Falloff is on, you can use Min Distance and Max Distance to define the bandwidth of the falloff region at the mesh's surface. These values are signed distances to the mesh surface, where negative values are inside the mesh. If Enable Boundary is also on, then the effect is applied to the intersection of the mesh and the implicit shape.
If Min Distance is less than Max Distance, then the effect is applied inside the mesh's volume. There is a full effect below Min Distance, no effect above Max Distance, and a linear falloff between the two.
If Min Distance is greater than Max Distance, then the effect is applied outside the mesh's volume. There is a full effect above Min Distance, no effect below Max Distance, and a linear falloff between the two.
If Enable Falloff is off, you can still use other options such as Along Normal to make the mesh influence the particles, and the effect is applied everywhere.
- Along Normal
- The acceleration along the mesh's surface normals in m/s2.
- Friction
- Damps velocity perpendicular to the mesh's surface normals. Typically, you would use Enable Falloff to limit the effect to a band of fluid close to the mesh surface. Note that friction does not take moving meshes into account, but you can use some Inherit Velocity for that.
- Inherit Velocity
- The proportion of the mesh's animated motion that is added to the particles' velocities.
Velocity and Speed Clamp
- Enable Velocity Scale, Velocity Scale
- Controls the scaling factors applied to the X, Y, and Z components of the particles' velocities. Values greater than 1.0 can eventually result in very high speeds, so it's useful to also clamp speeds when enabling velocity scaling.
- Clamp Speed, Min Speed, Max Speed
- Limits the particles' speeds to the specified range.
Velocity Draw
- Draw Velocity Grid
- Helps to visualize the net effect of the motion field by displaying a grid of arrows in the viewport.
Note: The displayed arrows do not accurately reflect the effect of turbulence when Worldspace Turbulence is on.
- Velocity Grid Resolution
- The number of arrows displayed in each direction.
- Velocity Draw Scale
- Scaling factor for the arrow lengths.