Rust Using Occlusion
By Neil Blevins
Created On: April 8th 2008
Updated On: Feb 24th 2026
Software: Blender or 3dsmax (vray)

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 adjacent 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.



Blender Example

Here's the shader for Blender...



First, lets make a teapot and ground surface, and use Cycle's Ambient Occlusion node for our occlusion.



We will use this node 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 a photo of real rust in the color slot...



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



Now, use the occlusion map as the mask between the two other materials as part of a Mix Shader. 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 and soft. This is because your occlusion map is too even. Ideally, you'd like to take the edge between the rust and paint and roughen it up.

My first thought was "why not use the occlusion node to define an areas where noise appears?" You can do this by creating a Noise Texture node, apply a Color Ramp to make it more contrasty, then hook that up to slot A of a Mix Node, make slot B white, and then in the Factor slot place your occlusion node.



Here's the resulting mask



This is good for some sorts of blurry rust.



But if you look at the second photo in our reference, 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 using a Color Ramp node, which makes the edge harder. But this just makes the shape of the occlusion harder, and you get an ugly edge.





You could also try adjusting the flags in the Color Ramp node hooked into the Noise to clamp it further, but this also doesn't solve the problem, you still see that occlusion edge.





The solution is instead of clamping the noise further and clamping the occlusion, then combining them, combine the noise and the occlusion, and then clamp the result.

Here's the nodes...



Here's the resulting mask...



And the final result...



This is much better looking.

You can also try different types of procedural noise, mixing several noises, or even bitmaps to replace the noise all together.

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

Here's the blend file, Blender 5.0: occlusion_rust_blender.zip

3dsmax (vray) Example

First, lets make a teapot and ground surface, and use vray's VrayDirt Map for our occlusion.



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 a photo of real rust in the color slot...



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 between the rust and paint and roughen it up.

My first thought was "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 good for some sorts of blurry rust.



But if you look at the second photo in our reference, 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 Falloff value. 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.

You can also try different types of procedural noise, mixing several noises, or even bitmaps to replace the noise all together.

Here's an example of that...



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

Here's the max file, 3dsmax 2022: occlusion_rust_3dsmax_vray.zip

To Bake or Not To Bake

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...
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