CG Education

Ambient Occlusion Rust
By Neil Blevins
April 8th 2008

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, so while it's usually used for lighting (like producing shadows in a skydome), Ambient Occlusion 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, 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 ambient 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 ao 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 ambient 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 ao 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 ao, 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 ao 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 ao edge.

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

So take your mix map, the one that mixes between the non clamped noise and the non clamped ao, 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 or the Essential Textures by Digimation 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 using max's "Render To Texture" feature to render out your ao rust into maps, which you can then edit in photoshop. It's a nice way to get part of the way there procedurally, and then add the final details by hand. But if you have a LOT of objects, this isn't recommended since you'll have to uv map all the objects and keep around a ton of maps assigned to the objects. Also, if you have to do modeling edits on your object after shading them, you'll mess up the results of your ao baking. Another reason you may wish to use the "Render To Texture" feature is that ao is a slow process to capture, and it's a lot faster to read a precomputed bitmap than it is to calculate ao on the fly.

Here's the max file that made the image above, max2008, using mentalray: ambient_occlusion_rust.zip