When you take a closer look reference image, you’ll see that the barbs of the right vane have the same length and create a hard edge. On the left vane, however, you can identify small differences in the barbs' lengths.

1. Add a Guide Process: Set Length tool from the tab menu. Connect the guide node’s first input with the first output of the upstream Feather Clump SOP.

2. On the Operation tab, go to the second dropdown menu and choose Feather Barbs.

3. There are now Left and Right buttons. In this example, only the Left barbs should be changed.

4. When you turn on Randomize, you have access to the Min Length and Max Length parameters. Use the sliders to adjust the values interactively in the viewport.

On the right image you can see a possible result with Min Length set to 0.045 and Max Length set to 0.057. On the left, you can also see that the barbs at the feather’s tip are strongly curved and too long. One method to achieve a less exaggerated result is to decrease Operations ▸ Blend. With 0.7 the barbs' curvature is better, but still not as in the reference. To fix this, add a mask around the tip.

A mask protects certain areas of an object from becoming modified. The Guide Process SOP provides several methods and you can find them on the node’s Masking tab. You don’t have to choose one method, but you can also combine the different options.

The Bend operation is the other tool that’s available for left and right barbs. This mode pushes the barbs into a given Direction, e.g. towards the feather’s tip or bottom, or to bend the barbs to create a U-shaped feather profile. In this example, the barbs direction is only changed around the feather’s bottom. The reference image also shows that the barbs are differently shaped on the left and the right vane.

1. Add a Process Guide (Bend) tool from tab menu and connect the node’s first input with the first output of the previous upstream node. If you have followed the workflow guides up to here, the last node is also a Process Guide SOP.

2. On the Operations tab, and change Curves to Feather Barbs.

3. Click the Right button to turn it off.

4. From the Mode dropdown, choose Direction Constant.

   The Direction parameters let you define an axis and the default vector 0,0,-1 will push the barbs towards the tip. Right now, all feathers from the left vane are affected, so you need another mask. The process of adding a mask is the same as described in the Masking chapter above. Here you will most probably use a geometry mask. The mask’s Output Attribute is bendmaskleft, but feel free to use any other name.

   You might also end up with a very small object. In that case you have to decrease Additional Masks ▸ Geometry Mask ▸ Voxel Size on the Guide Mask SOP. Only then you will have a resolution that’s fine enough to restrict the mask to the desired area. Values between 0.005 and 0.01 should work.

5. Once you've created the new mask, you can start to adjust Angle and Random Angle to move the barbs.

6. Change the mask object’s position and scale if necessary.

For the right barbs near the bottom you need one more Guide Mask SOP with a mask object. The process of shaping the barbs is the same as described in this chapter. You can copy the nodes for the left barbs and use them as a starting point. Don’t forget to set the Output Attribute to bendmaskright. The final result could look as shown here.

The Frizz operation (and also the process node’s remaining modes) only works on curves, not on barbs. This means that the operator modifies the shaft’s points to displace the barbs.

For this feather, Frizz isn’t necessary, but you can test out the operation to see its effect.

1. Add a Guide Process (Frizz) tool from the tab menu and connect its first input with the first output of the last upstream node.

2. Increase Frequency and decrease Amplitude. You can see more clumps and stray barbs between the clumps. The barbs' midpoints are also displaced, because the shaft’s points are no longer aligned along the original curve.

3. If you want to avoid a uniform look, change Random Frequency and Random Amplitude.

Of course, you can again apply masks to the node.

Noise is a very effective method to randomize feathers. The Feather Noise SOP uses a 3-dimensional noise function to create a more organic look. The noise is applied along the feather’s tangent, bitangent and normal vectors (TBN matrix). It can be confusing that the node doesn’t use the common XYZ notation. The reason for using a TBN matrix is that the noise-based transformation has to be independent from the feather’s global orientation. However, TBN vectors have directions and introduce a local XYZ system for each shaft point, but this system doesn’t necessarily correspond with the global XYZ direction.

• If you consider the feather a plane, normal is the normal of this plane. Since feathers must be drawn in Houdini’s XZ space, normal corresponds with the Y axis.

• tangent defines the transformation along the shaft.

• bitangent defines the transformation along the barbs.

• The node’s three Amplitude parameters let you control the strength of the noise/displacement along each vector.

The following image illustrates this concept.

The Feather Noise SOP only works in conjunction with condensed feathers. You can’t differentiate between left and right barbs, but you're able to control noise frequency on barbs and shaft independently. The image below uses the following values.

1. Set Amplitude to 0.001. This is a global multiplier for the vector amplitudes.

2. Amplitude Normal and Amplitude Tangent are not required and set to 0.

3. To create small ripples in the lower half of the right vane, increase Shaft Frequency of 150.

The Guide Process SOP does not only work in conjunction with a single feather, but you can also merge several feathers and apply the node to the entire group. This is a fast and convenient way to randomize large feather sets through a single node.