Hair and Fur Dynamics Part 2

Intro:

A while back I wrote an article on Hair and Fur Dynamics in Houdini. (Feel free to check that out.) This is the sequel. I hope you enjoy it. In the meantime this one is for all the CFX artists I know. Olatz and Carlos you know what’s up…;)

Hair vs Fur

What’s the difference between hair and fur? Aren’t they the same? No. Depending on the animal and location of them, they will be categorized differently.

All mammals have hair on their bodies. This includes humans, whales, pigs, cats, dogs, and other mammalians. Hair feels different than fur. It’s purpose on the body is also different.

Fur can also be found on mammals and other animals. It can be found as thick or sparse follicles across an animal's body. Fur is defined by its density and coverage across an animal. The thicker the patches of fur are , the more likely the strands will be referred to as fur. The thinner they are, the more likely the strands will be referred to as hair. Humans have very thin layers of hair across our bodies. This is why we don’t call our chest hair fur.

Most animals have a mix between hair and fur on their bodies.

Now for some fun animal examples…..

Kangaroo Fur

Most people I think in North America haven’t pet or seen a kangaroo outside a zoo. But from experience I can tell you their fur feels like a medium haired dog. They also act like dogs. But they are dogs that can box you if they're angry.

Kangaroo fur doesn’t tangle easily. It is soft and short in nature, and you can see their joints defined pretty well through their fur. It is light brown in nature, and contains white patches on their tails and feet.

Depending on the species of the kangaroo, the fur might appear differently. There are two known kangaroo species. The Eastern Grey Kangaroo (Macropus Giganteus) and the Red Kangaroo (Macropus Rufus).

Their fur will also appear differently depending on the gender. For example, female red kangaroos will have a grey, blue, and white coat. While the males have dark red ones.

Kangaroos shed their fur in the summer months, and grow thicker coats in the winter.

Kangaroo skin is very strong and lightweight in nature. It holds the follicles of the fur in place, and makes it hard to rip out.

Polar Bear Fur

Polar bear fur is a far cry from a kangaroo's fur.

Polar bears have white fur so that they can camouflage themselves with their environment. Their fur works so effectively at this, they can almost seem invisible in their habitat.

The coat of a polar bear is double layered for insulation. The color of polar bear fur is very interesting because the fur itself contains no white pigment in it. The hair is transparent, and appears white from the way it refracts light. All the hairs making up it’s fur are hollow, and the skin under the fur is black.

The top layer of the bear’s fur is made up of uneven guard hairs. Under it is a dense layer of underfur. The under fur prevents heat loss.

The guard hairs on a polar bear are about 10 centimeters long and the undercoat is about 4 centimeters long.

Depending on how dirty the animal is, the more yellow the fur might look. But the angle of the sunlight and how much accumulated oil is in the fur.

The more wet a polar bear is, the colder the bear becomes. This makes the bear dependent on its thick layer of fat under it’s skin. Their fat layer can be up to 4 and a half inches thick.

From what I was able to dig up from some very brave scientists. The guard hair of the bear feels like cat whiskers that are wiry and coarse. The undercoat of the fur is wavy and soft.

Houdini Fur Dynamics

Now onto Houdini!

Houdini 16 introduced some pretty amazing fur and hair tools to the world. Most of these tools you can find on the tabs of the shelf tools.

Most of the hair system is SOP based, but the hairs can be simulated through DOPS. Hair tools at their most basic form are just a bunch of different ways to copy lines to a series of points. However, the hair and fur tools are very object container heavy. However, you can create your own custom setups in SOPs.

The best way to proceed in a hair setup is to first generate some low-res hair guides, groom the guides, and then generate the hair. The hair gen SOP will help you generate your guides. The Guide process SOP is a curve deformer. It has two inputs. One for the guide curves, and the other for the geo(skin) of the character. It also lets you set the length of the hair, the curl, and how it flows across the skin.

The final hair gen SOP in your setup is for generating the final hair count. It also allows you to set the width of the hairs and the color. Overall, the entire system needs you to set up the skin, the guides, and the final hair for your animal.

Furtility and Fur and Hair in Films

Over the past few years, hair and fur in film has dramatically improved. It’s come a far away from the Stuart Little movies of the early 2000s. Twenty years ago, Sony Imageworks was the place to be for rendering and grooming mouse fur. Using a combination of RenderMan and some new surface shader technology, they were able to create Stuart’s fur. Most of their technology for the process was showcased at SIGGRAPH 2000.

Stuart Little might not have aged well computer graphics wise, but it was a big deal at the time as he was the first digital character to lead a live action movie. In the following sequels the Sony team would also have their first shot at simulating feathers. Which was also a very new concept in the VFX field.

Rendering and simulating fur was also extremely heavy for computers back in the day. So heavy that doing preview renders of the fur was only done if the VFX team really needed to see the final result. It was only done if it was absolutely necessary. Most of the work for fur was done in shaders, as that was the easiest way to fake lighting, and produce faster and better results.

Jumping to now, fur is much easier to set up, as well as render and shade. There is also a major push to make fur and hair simulation more accurate and have better interactions with characters.

We now can create films like Zootopia that can showcase over 800,000 different character models. As well as characters that contain over 2.5 million hairs on their bodies Zootopia is a great example of the progress of hair and fur technology, as it also shows how much research you can develop for different animals in film. For the film, VFX artists and supervisors visited animal parks and zoos to see creatures in person. They even looked at fur under microscopes to get a better idea of how it was built. As well as how translucent and opaque hairs can be.

They also developed new ways to animate fur on a character's body. Disney in the past would normally push noise patterns through the fur, and watch it move. But for Zootopia, they were able to animate every strand of hair through their XGen software. They were also able to resurrect an older software called iGroom in the process of making the characters. Then they rendered everything in Hyperion.

Shading for hair in Renderman has also come a long way. As of Renderman 19, there is something called a Marschner Hair shader. It allows for backscattering of light for hair, and glints to occur. When it was released, it was the only shader of its kind that did that. Hair development has been going on for a long time at Pixar, but light scatter techniques would start around 2003. Renderman 21.5 now contains two other shaders called PxrSurface and PxrMarschnerHair. Both of these are improvements to their regular shaders.

Other shaders for other render engines, such as Arnold, have a huge range of options when it comes to coloring hair. The Standard hair shader is very versatile, and even allows you to control how much Melanin is in your character's hair. As well as an Index of refraction.

But as we know, every studio has their own take on how to make hair and fur sims. As well as each artist. I personally know someone who was doing grooms through quarantine, and took advantage of his furry supervisor catching mice around the house. The cat was so proud of it’s work it would drop its catches by his feet while he was working, and he would take a look at the mice. He would look at their fur, how the strands looked on the location of the creatures, and incorporate the ideas into his current work. Then the mice would get a proper send off. This is why all studios need a resident cat supervisor. It’s not just artists who advance visual effects.

Let’s talk about Furtility.

Furtility is a tool used by MPC to generate hair sims. You’ve probably seen the work this tool has created in The Lion King, and in The Jungle Book remake. It’s a procedural tool that samples character meshes to generate fur. It uses something called GOPs(Geometry Operators) to better integrate the shape of curls, length, and other aspects of fur. Overall, it’s come a long way, and MPC has no plans to stop using it.

Jumping forward in time, shader wise. There is an excellent lecture here from the MPC team on The Lion King. They talk about shading hair, and their techniques. You can check out the lecture: HERE.

References

Australian Bush Store : Kangaroo Products

http://www.australianbushstore.com/australian-bush-store/kangaroo-products2/kangaroo-skins-1

Kangaroo Skins

https://www.skinnys.com.au/kangaroo-skins/

Kangaroo leather

https://en.wikipedia.org/wiki/Kangaroo_leather

Coat and Skin Appearance in the Healthy Cat

https://vcahospitals.com/know-your-pet/coat-and-skin-appearance-in-the-healthy-cat

Cat coat genetics

https://en.wikipedia.org/wiki/Cat_coat_genetics

Hair vs. Fur