Fast Ambient Occlusion in 3ds Max
Ambient occlusion adds great realism to your renders, but can be bit cumbersome to set up in Max, particular if you're using a mix of mental ray shaders and standard shaders. You could create an ambient occlusion effect by using lights, but it can be slow to render and tricky to get the right look. So if you want to skip all that, you can simply use a material override in your render options to get the job done - it's super fast and gives you the added bonus of having the ambient occlusion in a separate pass. As a separate pass you can adjust and manipulate to your heart's desire without affecting the rest of your render.
Please note that this tutorial uses 3ds Max 2011 and Photoshop CS5. Earlier versions should work too, but there might be slight differences in interface.
Step 1: Create a mental ray material
First you have to make sure you have assigned mental ray in your Render Setup menu (press F10) (Fig.02).
Then open your Material Editor (press M) and select an empty material slot. Give it a new name, like "AO material" and click the Standard button to select the mental ray shader in the Material/Map Browser (Fig.03).
You should now see the mental ray shader. Under Basic Shaders click the Surface parameter and select Ambient/Reflective Occlusion in the Material/Map Browser (Fig.04).
The shader should look something like this now (Fig.05).
Step 2: Assign to Render
Open the Render Setup menu (F10), then click the Processing tab, find the Material Override option and click Enable. You can now drag and drop the material you just created onto the Material:None button. Chose Instance to make sure the material updates when you change the settings (Fig.06).
Click Render (Fig.07)!
And you will probably see a more or less completely white render. The default values assigned to the material are not optimal for what we want to do. Please note: if your render is black, try to set your exposure control to No Exposure Control in the Rendering > Exposure Control menu.
Find the Max Distance value and increase to let say 100. And the render again (Fig.08).
Keep playing around with the values to get the quality you need. Add higher samples value to reduce grain, just keep in mind this increases render time. (Increase to 32, 64 or even 128). When you're happy with the result save the image and open it in Photoshop.
Step 3: Add AO Layer in Photoshop
To avoid the Ambient occlusion layer darkening your image in general, you might want to adjust the image levels (Ctrl + L) before you apply it to your picture. A quick auto adjust should do the trick (Fig.09).
Now all you have to do is to drag the Ambient Occlution Layer on top of your final render and choose Multiply. And voilà, you have Ambient Occlusion that was super quick to render with the added advantage of being easy to tweak and adjust (Fig.10).
The final result can be seen in Fig.11.
And for comparison, the render before Ambient Occlusion layer was applied (Fig.12).
Thanks for reading!