Occlusion Rust
By Neil Blevins
Created On: April 8th 2008
Updated On: Dec 25th 2013

Go here to read this tutorial in Russian.

This lesson outlines a technique for using Ambient Occlusion to produce rust on your objects. First, read up on what Ambient Occlusion is here.

If you look at photos of rust, one thing you'll quickly notice is how a lot of rust appears in areas that are hidden from the elements (wind, rain, etc). Areas where objects intersect. Hard to reach areas that remain damp because they don't get sun to dry them off.

Ambient Occlusion is a technique to determine how much of a particular surface sees of the sky (Or for the more technical minded, its the amount a particular point sees of a hemisphere centered at the point and oriented by the face normal (unless you have a cutoff distance, in which case it's far more likely you'll be simply calculating how close you are to adjacenet surfaces)).

So while Ambient Occlusion is used primarily for lighting (like producing shadows in a skydome), the same technique is also perfect for defining the areas of your object that would naturally rust.

First, lets make a teapot and ground surface, and use mentalray's "Ambient/Reflection Occlusion" Map (note, pretty much all raytracers have a similar map, for example, brazil 2 has a "Brazil2 Occlusion" Map that does exactly the same thing, or Vray has the "Vray Dirt" Map, so feel free to use whatever renderer you'd prefer for this lesson).

We will use the map to blend between a rust surface and a yellow paint surface.

Here's the yellow paint by itself...

Here's the rust by itself, fashioned using fractal noise in the bump and the color is a photo manipulation of real rust...

And here's the ambient occlusion map results...

Now, use the occlusion map as the mask between the two other materials as part of a Blend Material. You get the following results.

So this is ok, the rust is showing up in the occluded areas of the mesh. But the rust appears too even. This is because your occlusion map is too even. Ideally, you'd like to take the edge and roughen it up. My first thought was to use the Warp Texture plugin. Warp Texture was written by John Burnett, and what it does is takes any map and distorts it using another map. However, the warp map not only doesn't work with mentalray, but it also won't work for ambient occlusion because the warp texture cannot warp a raytraced effect.

The next logical thing to try is, why not use the occlusion map to define an areas where noise appears? You can do this by placing a fractal noise in the first slot of a mix map, make the second slot white, and then in the mix slot place your occlusion map.

Here's the resulting mask

This is better, and is good for some sorts of rust (specifically rust that's generally blurry).

But if you look at the second photo, some rust has a much more defined edge. How do we get this sort of effect?

One way to try and fix that is to clamp your occlusion, which makes the edge harder. You can do this by changing the spread spinner (for this example, I used a spread of 0.05 instead of the default 0.5). But this just makes the shape of the occlusion harder, and you get an ugly edge.

You could also try clamping your fractal noise, but this also doesn't solve the problem, you still see that occlusion edge.

After a lot of discussion on cgtalk, Zap Anderson came up with a decent solution to the problem. Instead of clamping the noise and the occlusion separately, then combining them, combine the noise and the occlusion, and then clamp the result.

So take your mix map, the one that mixes between the non clamped noise and the non clamped occlusion, and adjust the output curve to a clamped value...

Here's the resulting mask...

And the final result...

This is much better looking.

A few notes... Use different types of procedural noise. Darktree by DarkSim have some noises that are more complex than the standard fractal noise. Also, instead of a single noise, you can try mixing several noises together at different sizes to get variety. Here's an example of that...

Also note, while this technique is good as a building block for further rust, or for objects you will see off in the distance, it is not a recommended way to add rust to hero objects, since the results are not easy to control, and may not show the variety in rust that a hero object requires.

Also, you could try texture baking to render out your occlusion rust into maps. Visit my Texture Baking: Baking Patterns To Use In An Incompatible Render tutorial for an example of baking maps out to textures. A number of reasons to do this:
There are a few disadvantages to baking out your occlusion though...
Here's the max file that made the image above, max2008, using mentalray: occlusion_rust.zip

If you want to learn how to do this same sort of effect using mentalray in Maya, check this thread out...

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