To understand the basic idea of the workflow, described above, start with a single feather. Later you will learn how to use this method with a complete feather atlas.

1. On the obj level, press ⇥ Tab to open the tab menu. From there, add a Geometry OBJ. Double-click the node to dive inside.

2. Open the tab menu again and choose the Feather Template entry. This tool creates a network with three nodes and draws a complete feather. The feather will be the basis for the simulation.

3. Go to Resolution ▸ Barb Density and increase the value to 500 to get more barbs.

4. Terminate the network stream with a Null SOP and rename it to FEATHERGEO.

The idea is to create a low-res version of the feather to speed up the simulation. Later you’ll transfer the motion from the proxy to the high-res feather.

1. Add a Feather Resample SOP to decrease the number of barbs for the proxy. Connect its first input with the output of the FEATHERGEO null.

2. Turn on all three Resample toggles.

3. For Shaft ▸ Segments, enter 12. The actual number depends on the feather’s complexity, but values between 10 and 20 should work in many cases.

4. To turn the “skeleton” into a surface, put down a Feather Surface SOP. Connect its first input with the first output of the resample node.

5. When you turn on the surface node’s blue Display/Render flag, you’ll see an enlarged shaft. To fix this, open the Surface section and decrease Shaft Width, e.g. to 0.005.

6. Now, open the Output Groups tab and turn on Create Shaft Group.

   The shaft group’s points will remain static during the simulation and so you must separate them from the rest of the feather.

The remeshing process is optional. For highly detailed feathers, however, remeshing can improve your results, because you can define the simulation’s level of detail.

1. Put down a Remesh SOP and connect its first input with the first output of the surface node. Remeshing does not only convert the surface’s quads into triangles, you can also use it to control the number of triangles.

2. Right now, the mesh’s resolution is very coarse. To get a denser mesh, go to Element Sizing and set Target Size to 0.04. You can change this value if you need more or less resolution for the Vellum simulation.

   With smaller settings you get more polygons and more accurate simulation results. The flip side is that the simulation can take significantly longer.

The shaft is of particular importance, because it will act like an anchor to hold the feather in place. Currently, the shaft group is a primitive group. However, for the Vellum simulation you need points. In this step you add the points to the shaft group.

1. Add a Group Promote SOP and connect its input with the output of the remesh node.

2. For Group Name and New Name, enter shaft.

   When you open the Geometry Spreadsheet pane, you can see entries with an orange label. A closer look reveals that these points now belong to group:shaft.

3. To complete this part of the network, lay down a Group Expand SOP and connect its input with the output of the promote node. The idea behind this node is add some barb points to the shaft group. This will care for a more smoother and more realistic behavior. This node is optional, but it’s a convenient method add new points without having to change the original point selection.

4. Use the Steps parameter to control the expansion of the shaft group. Higher values add more points to the group.

For demonstration purposes and a better impression of the points in the image, the Remesh SOP’s Target Size was decreased to 0.02.

The feather surface is simulated as a sheet of cloth. If you're not familiar with Vellum, we recommend reading the Vellum Cloth chapters for more information.

1. From the tab menu, add a Vellum Configure Cloth tool. This helper creates a preconfigured Vellum Conatraints SOP. Connect its first input with the output of the Group Expand SOP. If you don’t need static shaft points, connect the node to the output of the Remesh SOP.

2. If you want the shaft to be static, go to the Pin to Animation section. For Pin Points. enter shaft - the group with the shaft’s points. Otherwise proceed with the default settings.

3. Lay down a Vellum Solver SOP. Connect its three inputs with the three outputs of the constrains node. Start with the default settings on the Solver tab. If you observe problems like instabilities or intersecting polygons, increase Substeps and Constraint Iterations. The simulation will take longer, but it’ll also be more accurate.

Forces ¶

The solver has some built-in forces on the Forces tab, but sometimes you can’t achieve the desired effect. To create noisy turbulence, for example, double-click the solver node to dive inside. There you can see a FORCE output.

1. Add a POP Force DOP and connect its output to the FORCE node’s input. Instead of the POP Force node you can also use a POP Wind DOP. This node combines noise and wind.

2. On the Noise tab you’ll find a wide range of parameter to control the force. With Amplitude you can define the force’s strength.

3. Return to the solver level.

The last step transfers the simulation data to the high-res feather. For this process you need a total of three input sources.

1. Add a Feather Deform SOP.

2. To transfer the data, connect the

   • first input with the output of the Feather Template from Shape SOP to get the high-res feather

   • fourth input with the first output of the Vellum Constraints SOP to read out the surface rest position

   • fifth input with the first output of the Vellum I/O SOP for the motion data.

3. From the transfer node’s Deformer Type dropdown, choose Surfaces.

4. On the playbar, click the Play button to see the motion of the high-res feather.

5. Terminate your network with a Null SOP. You should also apply a name, e.g. FEATHERGEO_SIM.

With enough resolution and appropriate settings it’s possible to achieve convincing animations with a high level of detail.

It’s not always necessary to perform a complete simulation through Vellum. Sometimes you can also use procedural animation methods to quickly achieve results. The advantage with this approach is that doesn’t differ very much from the simulation method presented above. The basic workflow is the same: you create a surface from a resampled feather, add the animation, and transfer the motion back to the high-res version.

Here’s the network you're going to create. Lets assume that you have created a feather and turned it into a low-res surface already.

1. The shaft should remain static and you have to separate it from the vanes. Select the Feather Surface SOP and open the Output Groups section. Turn on Create Shaft Group to create the shaft primitive group.

2. To turn the primitive group into a points group, add a Group Promote SOP and connect its input with the first output of the surface node.

3. For both Group Name and New Name enter shaft.

4. Put down a Group Invert SOP and connect its input with the output of the Group Promote SOP.

   With Group Invert you select the points of the feather’s vanes. The reason why you need this node is that the Vellum solver provides a Pin Point option, where you can just add the point group you want to remain static. This approach doesn’t have such an option and you must define an explicit group for the vanes.

5. On the invert node, set Group Name to shaft, while the New Name will be vanes.

The previous chapters describe how to simulate a single feather, but in many cases you’ll add wind or turbulence to an entire groom.

This setup in this example is a mixture of the basic scattering project and the simulation method described on this page. The main difference to the single feather approach is that you're working with a complete groom, where a larger amount of instanced feathers is distributed over an object. Here you’ll learn how to connect feather, groom and simulation networks.

The scatter groom is described here for your convenience, so you don’t have to jump to the appropriate chapter page.

1. For the groom, add an object of your choice, e.g. a Sphere SOP.

2. Set Uniform Scale to 0.5.

3. Lay down a Hair Generate SOP and connect its first input with the sphere’s output.

4. For Distribution ▸ Total Count, enter a value of 1000. This parameter control the number of feathers on the object.

5. Go to the Unguided Hairs section and set Length to 0.25 to create longer feathers.

6. A Guide Tangent Space SOP is required to align the feathers along the base object’s (character’s) normals. Connect the node’s first input with the hair generator’s first output.