Create a surface flow

The surface flow effect emits particles that flow along the surface of any NURBS model. You can use this effect to create flowing fluids, gases, or instanced objects. The flow effect automatically adjusts to any changes in the shape of the surface.

To create a surface flow

  1. Select one or more NURBS surfaces.
  2. Choose Effects > Flow > Create Surface Flow > .
  3. Set controls in the option window as needed.

    For controls related to creation, see Set surface flow creation controls.

    For controls that you can edit later, see Editing a surface flow .

  4. Click Create in the Create Surface Flow Effect Options window (See Surface Flow Options).

    An emitter and flow manipulators appear on the surface.

    Note:

    It may take a long time to create a surface flow. Creation time is affected by the number of objects in the scene, by the number of other flows already existing, and by the number of flow manipulators you specify.

  5. Play the animation.
  6. Render the scene with software or hardware rendering, whichever is appropriate for the render type of the particles.

Set surface flow creation controls

The creation controls apply only at the time of creation. You cannot modify these controls later.

Editing a surface flow

You can edit these attributes after you create the Surface Flow effect by selecting the surface flow selection handle and opening the Channel Box or the Attribute Editor, Extra Attributes section.

Note:

You cannot directly rotate or translate manipulators, because they automatically adjust to any changes in the shape of the surface. However, you can edit the Location attributes to move them along the surface.

Display Sub Manips

Turns visibility on or off for the flow sub manipulators.

Show True Manip Shape

By default, the manipulators are displayed as smooth bands that wrap around the base surface in exactly the shape that you specify. The actual shape that the effect uses depends on the Manipulator Resolution value that you set at the time you created the surface flow.

To see that shape, turn on the Show True Manip Shape attribute. This does not affect the way that the surface flow moves the particles. It simply lets you see how accurate the simulation is trying to be.

Smooth Sub Manips

You can use this option to smooth the transition of sub manipulators between flow manipulators that are spread apart or moved together. If you haven’t changed the location of flow manipulators, this option has no effect, because the default positions are smooth already.

Display Min Loft, Display Max Loft, Display Edge Loft

When Visibility is turned on (in the Display section of the SurfaceFlowAttribute Editor), you can choose to display the flow loft’s inner edge, outer edge, or the distance of the edge by turning on Display Min Loft, Display Max Loft, and Display Edge Loft respectively.

Emitter Rate

Sets the degree of particle emission in the flow. The emission is applied to the surface area of the first flow manipulator. An emission rate of 0 (zero) means no particles appear.

Random Speed

Sets the amount of random changes in the particle speed. Use this attribute to create a more natural effect.

Random Radius

Sets the amount of random changes in the particle emission radius, so that particles spread beyond the flow manipulator loft. Use this attribute to create a more natural effect.

U or V Location

Controls the position of each flow manipulator, measured in U or V. You can move manipulators anywhere along the surface in any order.

Typically, you adjust the manipulator locations to affect the particle speed. Particles slow down between manipulators that are close and speed up between manipulators that are distant.

You may also need to adjust the location of a manipulator if it is placed at the end of converging threads. For example, a manipulator at the end of a cone would be invisible because the surface at that point is zero.

These values are always between 0 and 1, regardless of the parameterization of the original surface. Therefore, a value of 0.5 will be approximately in the center of the surface.

Min U or V, Max U or V

Controls the beginning and end positions of each flow manipulator, measured in U or V. Adjust these values if you want the flow to cover only part of the surface. For example, you can break up a surface into two adjacent flows by ending one flow (Max setting) at the same point where the second flow begins (Min setting).

These values are always between 0 and 1, regardless of the parameterization of the original surface. Therefore, a value of 0.5 will be approximately in the center of the surface.

Min Distance, Max Distance

Controls the distance between the surface and the top and bottom of each flow manipulator. By adjusting these values, you can affect not only the proximity to the surface, but also the thickness of the flow.

Min Age Ratio, Max Age Ratio

Specifies what part of the total particle lifespan occurs in the flow—a ratio of flow lifespan to total lifespan. For example, if you set the Max Age Ratio to 0.5, the particles flow along the surface for half their lifespan and then continue beyond the last flow manipulator.

The age ratio also indirectly affects particle speed. In the above example, the particles only have half their lifespan to reach the end of the flow, so they speed up.

Goal Weight

Sets how much all particles in the flow are attracted to the flow manipulators. Values can range from 0 (no weight) to 1 (full weight).

Edit particle lifespan

To edit the particle lifespan, you must select the particle node, not the flow group node, because there may be several particles per flow.

Particle Lifespan

Sets when all particles in the flow die. This option controls the total lifespan, but the Min and Max Age Ratio control the lifespan within the flow. As a result, Particle Lifespan may affect the particle speed in order to meet the requirements of the Min and Max Age Ratio settings.

Connect flows on separate surfaces

You can connect flows from separate surfaces so they share the same particles. The workflow is different depending on whether you create flows at the same time or start with an existing flow.

To create connected flows

  1. Select the surfaces in the order you want the particles to flow.
  2. Choose Effects > Flow > Create Surface Flow > .
  3. In the options window, turn off Create Particles Per Flow.
  4. In the Control Resolution option, specify the number of flow manipulators per surface.
  5. Click Create.

    The effect creates one flow per surface, with the particles flowing in the order of selection.

    Note:

    By default, the Min and Max Age Ratio settings are evenly divided among the flows. As a result, a gap appears if the surfaces are separated. Edit the Min and Max Age Ratio settings if you want the particles to fill the gap.

To connect an existing flow

  1. Select the particle from the existing flow, along with the surfaces or flows you want to connect. (The selection order doesn’t matter.)
  2. Choose Effects > Flow > Create Surface Flow.

    There will be one flow per surface, but the particles may not flow correctly. Correct the flow of particles using the following steps.

  3. For all flows after the starting flow, change the Emitter Rate to 0 (zero).
  4. In all the flows, adjust the Min and Max Age Ratio settings so the particle life span extends evenly among all flows.

    The following table gives example settings for two flows.

    Flow Min Age Ratio Max Age Ratio

    A

    0

    .5

    B

    .5 or higher

    1

Delete flows

Because surface flows create expressions in the affiliated particle objects, you should not delete flows using the key or Edit > Delete option. Follow the instructions below.

  1. Select the flow or flows you want to delete.
  2. Choose Effects > Flow > Delete Selected Flow > .
  3. Edit the Delete Surface Flow Effect Options as needed.
  1. Click Delete.

Resolve incorrect deletion

If you accidentally delete a flow with the key or Edit > Delete option, follow these instructions to clean up the nodes in your scene.

  1. Delete the particles associated with the flow.
  2. Remove the shaders for the particles by opening the HyperShade or Hypergraph and choosing Delete Unused.
Parent topic: Maya Effects