Hair and Fur Dynamics

Intro:

So I have a few buddies doing a lot of fur and hair grooms right now. Here ya go...Hope it helps :).

Setting Up a Hair/Fur Simulation

The easiest way to setup a hair/fur sim is to select your mesh, and click on the groom tools in the fur/hair tool shelf. You'll need to start with Plant Guide and work from there. Keep in mind that when working with the shelf tools  everything is going to appear on your object level. When you click the Plant Guides tool, two Object Nodes should appear. One being Guide Groom, and the other Hair Generate.

The Hair Generate node's core is pretty simple. You can think of it as a more complicated version of copying a bunch of lines to points. But you also have the option to regulate the width and colors of your hairs.

Everything done in hair generation is done with curves. This allows for Houdini to easily layer multiple lines/hairs and understand how their attributes flow across the skin.

The Guide Groom node creates guides for your character or animal. You use it to set up the initial direction and length of your fur. You can also chose which groups from your character you'd like to generate guides on.

Grooming Hair and Fur

While grooming your fur, you'll start to notice that your guides are based around the direction of your normals. So if your normals are pointing up, you are going to have a harder time painting your guides down. Try playing with your normals in an Attribute VOP before starting your groom.

Make sure you are working on a still version of your mesh. You don't want to be clicking between frames and watching your guides explode as your character moves away from your brush. Time shift your character still, and work from there.

Generate your fur color on the UVs of your object. It will be easier to control the final look that way.

While working on your guide, you'll notice red curves on the surface of your object. These represent the hairs on your object you can style. You'll want to start by setting the direction of your fur. You can do this by playing with the Wind Direction. Set Guide Length to lengthen or shorten hairs. Turning on Randomize will always give your fur a better look.

Using the Bend Guides Tools will also improve your guides. This tool does exactly what it sounds like. It allows you to bend your fur in whichever direction you please. You can also add clumping to your fur with the Clump Guides tool. As well as adjusting Clump Size, and the iterations.

However, there is one thing the Guide Groom node can't do. It can't render your hairs. The Hair Generate node can. So make sure you add it to your scene. You can also add clumping to your fur 

 

Simulating Fur Dynamics

You might start to get confused when you first start to try and simulate fur and hair. Generally, creating the guides and grooming are the easiest parts. For simulating hairs you generally have two options.

The Guide Simulate tool will help you simulate your fur. This tool adds a simulation to your guides and also adds a Guide Simulate Object. Then it begins to deform your guides along your animated skin.

You also have the option of using vellum to simulate hairs. But when it comes to a high amount of hairs spread over a large amount of space, Vellum might have some difficulties calculating the motion of your simulation. Therefore, try and simulate parts of your creation rather than shoving the whole thing down vellum's throat.

 

Fun Fur Facts

All animals usually have three types of hair that cover their body:

 

Guard hairs: These hairs are protective or "guard" outer hairs of a mammal's coat. They are firm and glossy. These hairs let the animal know when it is about to run into something.


Vibrissae or whiskers: These are usually very stiff and long hairs. They have many nerves at their base, and are very painful to the animal if damaged. These hairs give the animal senses about its immediate surroundings. Facial whiskers are the most common.


Under hairs: These hairs are the main hairs that cover the animal. They can have any type of length or appearance. Such as being short, curly, or wooly. Insulation is the main feature for these hairs.

Some other facts:

  • Deer have hollow hairs so in winter they have more warmth.

  • Pigments in the inner layers of hair filter harmful ultraviolet radiation, and regulate body temperature. Dark hairs absorb heat which warms the animal. Light colors reflect the sun, which keeps the animal cool.

  • Animals commonly trapped for their furs include raccoon, beaver, skunk, and muskrat.

Short and Long Hair Tips and Tricks

  • Grooming should always be done on a static mesh, not an animated one.

  • Mantra uses your created SOP network to do the grooming, and the cooking of the network at render time.

  • The Curve Advect tool is useful for painting the initial directions of your fur.

  • By adding a mask to the Hair-Clump SOP you can create different amplitudes and frequencies in your curly or clumpy hair.

  • You can use as many Hair Generate or Guide Deform nodes as you need to groom different parts of your simulation.

  • Use the Isolate Groom Parts tool to hide hairs and guides that you are not working on. This will help create less confusion in your scene.

  • By varying segment counts per curve, you can create a great sense of natural variation in your fur setup. Longer hairs should have more segment counts.

  • If anything goes weird check your rest attributes. (aka @rest)

References:

Hair and fur: https://www.sidefx.com/docs/houdini/fur/index.html

Add Dynamics shelf toolhttps://www.sidefx.com/docs/houdini/shelf/furdynamics.html

FX REFERENCES

References for VFX artistshttp://www.sylvain-nouveau.com/reffx/hair-cloth/

Fur and Hair in Houdinihttps://rmanwiki.pixar.com/display/RFH23/Fur+and+Hair+in+Houdini

Houdini software: Tips and tricks for Houdini 17.5https://www.creativebloq.com/advice/houdini-software/2

Guide hair dynamics?https://forums.odforce.net/topic/24451-guide-hair-dynamics/

HoudiniHairhttps://www.tokeru.com/cgwiki/index.php?title=HoudiniHair

Implementation of Artistic Curly Hair Dynamics in Houdinihttps://www.semanticscholar.org/paper/Implementation-of-Artistic-Curly-Hair-Dynamics-in-Mohsen/2dd24beada776740f28d72c1e63be5a6bcb7533e

Karma hair is not smoothly renderedhttps://www.sidefx.com/forum/topic/76187/

Guard hairshttps://www.sidefx.com/forum/topic/75837/

Weird render issue with hair and switchhttps://www.sidefx.com/forum/topic/75549/

Fur workflow: https://www.sidefx.com/docs/houdini/fur/workflow.html