yeti_dirt

CG fur always has the tendency to look very clean and soft. To break up the structure and to give a bit more realism I like to mix it with a bit of dirt, e.g. some leafs or sticks in the fur of a character walking through the woods. Here’s a quick rundown on what I’m doing to achieve this using Yeti and Renderman within Maya.Let’s start with a simple sphere and some clumpy fur on it to get started. This is my base fur setup.

 

What I’m doing next is to create another YetiNode on the same Mesh. This will be (one of) my dirt layers. In this example I’d like to have some leafs between the fur strands. For this I’m going to use polyPlanes with textures on them. So to distribute those planes along my sphere I need to first of all get some guide strands lain out. To make the planes take the direction of the fur I have to create a custom comb attribute which I’ll call “dirt_dir” in this example. So after comb’ing the grown fur with the groom I’m going to create yet another comb which has a comb attribute called “dirt_dir” in it.

 

Next I’ll import the planes which are supposed to hold the leafs and instance them on the fur strands. For the alignment I’m going to choose my “dirt_dir” attribute to make the planes align along the fur strands. For this to work I have to set the “Instance To” to “Elements.

 

To get a bit more variation and randomness one can also change the alignment, scale and twist variation as well as tick the “Deform” checkbox in the “Objects” tab.
Alright, now that this is setup you can assign a simple Shader which holds a leaf texture for example and render and you’ll have a bit of dirt between the fur. However I personally like to have a bit more control over things in comp, so let’s take this a step further. I’m going to set up an AOV which holds the color and alpha of the leafs respectively, but the actual fur render will not contain the leafs at all. To set this up, I just map the leaf’s color to the diffuse and the leaf’s alpha to the mask input of a GPSurface Shader. Also I will turn up the transparency to full white because I don’t want the leafs to appear in the actual beauty render.

 

For more information on how to set up custom AOV’s in Slim there’s a nice explanation over at pixar.

When rendered and everything is set up correctly something like this should come out:

 

 

 

Of course with a bit more sophisticated than just planes with textures on them one can get really nice results while still maintaining a lot of control. One thing I might add is that the color AOV of the dirt  should actually be rendered without the fur being visible. Otherwise as you can see it will be held out by the fur aswell, which could lead to premultiplication issues in comp. In this example it’s not really obvious but there might be cases where you should be aware of it.