Depending on the feather’s state - condensed or uncondensed - you have different attributes. When you uncondense a feather, you will lose several important attributes like barborient. Since barborient´is essential for the grooming process, you should keep it.

• One method is to add a Feather Match Uncondensed SOP to terminate the feather network. The node’s first input is connected to the Feather Template from Shaft SOP. All attributes, coming from the template node, are transferred.

• Another approach is to choose the Feather Uncondense SOP’s and open the Attribute and Group Copy section. There, go to Point Attributes and append the attributes you want to transfer. Note that attribute lists are space-separated, e.g. width barborient.

You can also add an Attribute Create SOP to create and promote attributes.

When you work with attributes and overrides, you have to deal with signatures. A parameter always expects a value in a certain format, e.g. as an integer or a vector. This differentiation is essential, because you often have to specify the signature yourself. Signatures are also sometimes called data types. A few common types are listed in the table below.

This standard workflow illustrates how to manipulate attributes and connect the result to a node to drive a certain parameter. The basis for this guide is an arbitrary feather like the one shown below: it’s an uncondensed down from the Creating an atlas page and separated into shaft, left and right barbs.

Here’s the relevant part of an uncondensed feather, split into left and right barbs.

The first example uses an Attribute Noise SOP for randomizing the barbs' length. The method you're going to use here is actually what happens inside the Guide Process SOP. The difference is that you have direct access to all noise parameters and you can also customize attribute names.

1. On the Guide Process SOP’s Operations tab, set Operation to Set Length.

2. Go to the Length parameter’s first dropdown and change it to Guide Attribute. You can see a new Length Attrib parameter with length as default.

3. Add an Attribute Noise SOP and place it between barbs_left and guideprocess1 to connect it.

4. On the the General section, change Attribute Names to length and make sure that the signature dropdown is set to Float.

5. Go to the Noise Value section. From the Range Values dropdown, choose Min/Max.

6. Set Min Value to, for example, 0.5 and Max Value to 1. Depending on the shape and scale of your own feather, you might need different values.

   Both values are multiplied with Guide Attribute SOP’s Length parameter. If you change Length, you also have to adjust the minimum and maximum parameters here.

   Depending on the feather’s shape, you will often hardly see any differences as shown in the middle image below. Sometimes the feather appears wider or you observe deformations near the vanes' endpoints.

7. To get random barbs, open the Noise Pattern section. Element Size defines the noise’s size. Compared to the tiny barbs, the default value of 1 is huge. To get more variation, decrease the value. In the image below, the value is 0.002.

The left image shows the original state. In the middle you can see the impact of Min Value and Max Value, and on the right the randomized barbs.

An Attribute Wrangle SOP lets you write custom VEX code to manipulate attributes and their values. It’s a very versatile method, but also requires knowledge of a scripting language.

1. Add an Attribute Wrangle SOP and place it between barbs_left and guideprocess1 to connect it.

2. On the VEXpression field, enter @length = fit(rand(@ptnum), 0, 1, 0.5, 1.1);

The script assigns random values to the @length attribute. The rand() function creates a random value between 0 and 1. To get different random numbers for each barb, the current point number @ptnum is used as a seed.

The fit() function remaps a range of values.

• The first value is the random number.

• 0, 1 defines the possible value range from the rand() function.

• 0.5, 1.1 are the remapped values that are applied to the barbs. So, each barb will have a random @length attribute between 0.5 and 1.1. The values represent the noise node’s Min Value and Max Value parameters

The result will look differently from the Attribute Noise method, because the method of calculating the random numbers is also different.

// Create custom parameter
float min_value = chf("min_value");
float max_value = chf("max_value");

// Use the input to drive @length
@length = fit(rand(@ptnum), 0, 1, min_value, max_value);

Another way to maintain or create attributes at any point in your network is to add an Attribute Create SOP. You can create new attributes or override the existing ones like the shaft’s width or the feather’s name.

The Attribute Create SOP lets you define multiple attributes through the Number of Attributes parameter. This option is very convenient, because you can manage all custom attributes inside a single node.

This point attribute is a quaternion (vector4) and one the centerpieces of Houdini’s feather system. As the name suggests, the attribute controls the orientation of a feather’s barbs. When you uncondense a feather, the attribute is deleted and has to be transferred.

Names are important when you create a feather atlas, because they help to distinguish the feathers. You can display the names in the viewport to have direct visual control over your template. There are several methods to apply names.

• Use the Feather Template from Shape SOP’s Name parameter.

• Add a Name SOP.

• Use the Feather Template from Shape SOP’s third Label Points input.

Note that all name attributes are deleted when you uncondense a feather. The Feather Uncondense SOP node changes the feather geometry and creates large amounts of points. The node is therefore very selective with attributes, but you can transfer the name attribute.

The id primitive attribute is only created with uncondensed feathers. If the attribute is missing, check if the Feather Uncondense SOP’s Attribute and Group Copy ▸ Create Output Id From Primitive Number Curve Id Missing is turned on. Each feather gets its own id attribute and the barbs' curves inherit the attribute.

If you want to enumerate the barbs, you need a custom primitive attribute. The most convenient way to do this is to add an Enumerate SOP after the Feather Uncondense SOP. For Attribute enter a name like index or barbid.

By default, the Feather Template from Shape SOP’s Add Barb UV Attributes is turned on and creates a uv attribute. This attribute is even kept when you uncondense the feather.

If you don’t want to work with the default UV set, you can create custom UVs. For more information, read the Feather texturing and UVs page.

You can adjust the shaft’s width through the Feather Template from Shape SOP. On the Shaft section, you can see Width Root and Width Tip. The values, you enter there, are written to the shaft’s points in the form of a width attribute.

The width attribute is kept when you uncondense the feather.

Houdini’s feather system creates a couple of default groups. Note that only some are kept throughout the entire network, while others are deleted, e.g. after uncondensing a feather.