= Using guide hairs =

NOTE:
    This tutorial is a detailed look into what goes on at the network level. When you use the [Add Fur|/shelf/addfur] tool on the shelf, guides are automatically created for you.

Using guide hairs provides you with more flexibility when creating fur. Guide hairs are generated when you create a line, copy it onto the points of the grid, and connect it to the guide geometry input of the fur surface node.

Although you can create fur without using guides, as described in [Basic fur setup|basicsetup], you will not have as much flexibility in terms of styling the fur. Additionally, using guides is the preferred workflow if you plan to animate your character at any point in time.

This procedure is a continuation of [Basic fur setup|basicsetup], as it demonstrates how to add guide hairs to your basic fur setup.

# Create a [Icon:SOP/line][Line SOP|Node:sop/line] in the `Fur` geometry object you created in [Basic fur setup|basicsetup].

    NOTE:
        You can increase the __Distance__ parameter to make the guide hairs longer.

# Use the [Icon:SOP/copy][Copy SOP|Node:sop/copy] to copy the line onto the grid to create the guide hairs. See [copying and instancing|/model/copy].

    Fur is created by interpolating between guide hairs.
    
# Connect the following in the network editor:

    * [Line|Node:sop/line] to the first input of the [copy|Node:sop/copy] node.
    
    * [AttribCreate|Node:sop/attribcreate] to the second input of the [copy|Node:sop/copy] node.
    
    * [Copy|Node:sop/copy] node to the second input of the [fur|Node:sop/fur] node.

    [Image:/images/fur/guides_nodes.png]
   
    The lines will be copied to the grid; however, they will not be perpendicular to the grid.
       
# To reposition the guide hairs perpendicular to the grid, they need to be oriented in the Z axis. Do this by changing the __Direction__ parameters in the [line|Node:sop/line] node. Make the Y value `0` and the Z value `1`.

    The guide hairs are copied onto the points on your grid, and the rest of the hairs are interpolated between the guide hairs.

    [Image:/images/fur/guides_points.png]

    You can specify the amount of strands that are interpolated by altering the __Density__ parameter on the [Fur SOP|Node:sop/fur].

    [Image:/images/fur/guides_fur.png]

# The fur procedural shader, that you created in [Basic fur setup|basicsetup], needs an _object_ containing the guide geometry (because the renderer only knows about objects). So we'll extract the guide geometry out into a new object and then reference it in the fur procedural shader.

    Create a [Icon:COMMON/null][Null|Node:sop/null] node in your geometry object between the [Icon:SOP/copy][copy|Node:sop/copy] node and the [Icon:SOP/fur][fur|Node:sop/fur] node and name it `GUIDE_OUT`.

    We'll use this Null as the target for extracting the geometry. The Null should always be the node connected to the Fur node's guide geometry input. This lets us insert nodes above the Null later.

    [Image:/images/fur/guides_null.png]

# Select the `GUIDE_OUT` node in the network editor, and select the grid in the viewport, then click [Icon:SHELF/extract_object][Extract|/shelf/extract] on the __Modify__ shelf tab.

    This will create a new geometry object that automatically references `GUIDE_OUT`.
    
# Click `obj` in the path at the top of the network editor to return to the object level, and rename your new geometry object `GUIDE_REFERENCE`.

# Select the `GUIDE_REFERENCE` node. In the parameter editor, click the __Render__ tab and turn off __Renderable__ to prevent double rendering. Also, turn off __Automatically Compute Normals__ on the __Geometry__ subtab of the __Render__ tab.

# Double-click the `Fur` object, then double-click the SHOP Network, and select the [fur procedural shader|/nodes/shop/vm_geo_fur] that you created in [Basic fur setup|basicsetup]. In the [parameter editor|/ref/panes/parms] of the fur procedural shader, set __Guides__ to your `GUIDE_REFERENCE` object by clicking the [Icon:BUTTONS/chooser_node] node chooser and selecting it from the tree.

You should already have a material applied, the procedural referenced for rendering, and lights set up in your scene from completing the [Basic fur setup|basicsetup] tutorial.

[Image:/images/fur/guides_render.png]

Remember, the [Fur SOP|Node:sop/fur] should only be used for visualization in the viewport. In order to render, it is highly recommended that you use a [fur procedural shader|/nodes/shop/vm_geo_fur], which will delay the generation of fur until render time. For more information on setting this up, see [Basic fur setup|basicsetup].