Ambient Occlusion Pass(Soft Shadows in the Nooks and Crannies to Replicate Photorealistic Lighting)

In this tutorial we are going to go over some advanced lighting techniques for an Ambient Occlusion Pass. This is a very simple idea that will help make the scenes that you want to look a bit more photorealistic appear to have those soft shadows that we see in the real world where light just doesn't quite make it. The corners of the walls, behind furniture or under objects. These shadows creep out just enough to ground everything and make it look a lot more realistic.

First you need a scene, this method is often done with realistic architectural renderings to make them feel more realistic but we can simulate it with any scene even one made up of standard primitives. For the purpose of demonstration I have quickly modeled out and minimally furnished a room.

Now the modeling, texturing, and lighting of this scene are all separate ideas and will be discussed in their own tutorials as well as in class. If you aren't sure how to texture, light, and model a scene, then perhaps you should go back to some of the more basic ideas before you get to Ambient Occlusion. If you do have a good basic knowledge of the program and the ideas listed above, then stick with this one and we will create an ambient occlusion pass which we can composite together with another image to create the final piece of work.

The room is simple enough, and as a daylight scene most of the light in this image is indirect lighting thanks to things like Final Gather which you can learn more about in the "Photometric Lighting Basics" tutorial. With a lot of indirect lighting in 3D scenes, our shadows end up very minimal and leaves some things like depth, and grounding hard to see.

Once you have a scene rendered out with all its textures, lights, and models we can then use a program like Photoshop to composite the image above with a completely different render which we will create now.

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First things first make sure that mental ray is set in your render setup before continuing or you won't find the correct textures for our tutorial.

Then open up your material editor (M) and find an empty texture slot so we can create our Ambient Occlusion Pass.

Click on the empty map button next to your diffuse color, or in the Maps rollout.

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In the Map Browser, find the listing for Ambient/Reflective Occlusion (3dsmax). It will have a yellow icon next to it which tells us that it's used with mental ray.

Then click the OK button at the bottom after you have selected it.

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It will take you into the AO texture settings:

The first thing we want to notice is the Samples in this dialogue. The higher the number the less noisy or speckled the shadows will look. I normally bump this up quite a bit higher than the default of 16.

The next number is the Max Distance. If you leave this at 0.0 then it tells the texture to add the occlusion to everything for as far away as it can. By giving it a distance we can get some good occlusion near the camera and not as much in the far distance where it wouldn't make sense for us to be able to see it. Try some different numbers here based on how big your scene is.

That is all we will worry about for now, you can click on the Go To Parent button to get back to the top level of your texture.

Next we are going to go into our scene and get rid of any lights we currently have set up. Ambient occlusion utilizes the ambient light in a scene so it will look much better if we leave the default lighting as our only light source.

Go to your Select By Name button -

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Then proceed to select all the lights in your scene.

Click the OK button once you have them all, and then hit the Delete Key on your keyboard to get rid of them.

Then open up the Render Setup (F10) and go to the Processing Tab.

Click the checkbox next to the Enable under Material Override. Then drag and drop the Ambient Occlusion texture we just made in the Material: None box, just like the image above.

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This way we don't have to retexture everything in our scene, the renderer will automatically use this new material no matter what we have in our scene.

Next go to the Indirect Illumination tab and turn off Final Gather by unchecking the Enable Box towards the top of these settings.

Depending on the scene as well, you may want to hide anything in the scene like glass at this point that is clear and shouldn't need any AO shadows, as well as change your environment (8) color to White:

Once you have done this click on the Render button. It might take a couple minutes to render based on how high you set your Samples at in the AO texture, once it's finished you should have a nice grayscale image of your scene with the models and the ambient occlusion textures only. Notice the shadows in this image, and see where you have some soft darker shadows where there weren't any in our first render with all the lights and textures present.

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Compare the two images, and once you are finished looking at the differences, open them both up in Photoshop.

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<-----Once open in Photoshop, drag the Ambient Occlusion render on top of the fully lit and textured render as a layer like the image here.

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Then select the layer with your AO Pass, and change the type from Normal to Multiply, and reduce the opacity on the layer to somewhere between 50 and 70 percent depending on the scene.

---------------------> The Multiply layer adds only the dark parts of the AO Pass, and leaves the lighter areas see through so our shadows bleed onto the image below the layer and lets the original colors and textures come through.

If your scene is a little dark still, you can always add an adjustment Curves layer and play with the curves until it is to your liking like below.

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The half and half circle is your adjustment layers.

Select Curves...

The Curves dialogue box will show up and you can click in the middle of the diagonal line and pull it back and forth to lighten or darken parts of your image.

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This and playing with your Levels... adjustments as well as any other post work you wish to do to your final 3D render will give you a composite combining the shadows from Ambient Occlusion with the fully textured and lit render.

Scene with the AO pass composited on top.

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Original Render without the AO pass.Check out the differences in the back room with the stairs, the corner near the doorway, under the table legs and chair, and behind the lanterns and picture on the wall. Things are given a bit more depth and are grounded within the render which makes them seem more 3 dimensional and realistic. Subtle but effective.

See below for some more examples of Ambient Occlusion Passes:

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without AO Pass...

with the AO Pass.

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without the AO Pass....

with the AO Pass.

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And for those 3D Modelers out there, try seeing what your models will look like with an ambient occlusion texture instead of those basic standard textures just to show off those modeling skills.

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