Ambient Occlusion
By Neil Blevins
Created On: Nov 9th 2004
Updated On:
June 19th 2005

Go here to read this tutorial in Russian.

Ambient Occlusion is a calculation that takes any particular point and figures out how occluded this point is by the objects around it. Renderers such as Brazil do ambient occlusion as part of it's skylight calculation. A skylight is a light emitted from a sphere around your scene (like the sky outside), and then after the ambient occlusion value is calculated for a point, the light is multiplied by this value to give you the final light intensity at a particular spot. The result is an evenly lit scene with soft shadows (or darkening) in cracks and crevices and under objects, much like how shadows look on a cloudy day. Note in this example the plane cuts out any light coming from under the spherical object.

To use skylight in many renderers is as simple as pressing the skylight button and you're done, but it's always a good idea to have some idea what's going on under the hood, so this tutorial is just a brief discussion on how ambient occlusion is calculated, and what this does visually to an image.

First, lets pick a point we'd like to render. Lets call this point A. Next, we shoot out a number of rays from A in a hemispherical direction around the face normal (since it's a ground plane, the face normal is pointing up), and see how often these rays are occluded by nearby objects. So in this example, we shoot out 5 rays (in a real render we generally shoot way more than 5 rays, but I'm choosing 5 for simplicity). Rays 1, 2 and 3 do not hit any object on their way up to the hemisphere. Rays 4 and 5 hit a nearby object (the sphere), and so these rays are occluded. So in this image, we shot out 5 rays, and 2 are occluded, so our ratio of occluded rays vs total rays is 2/5.

Now lets look at point B, which is closer on our plane to our sphere. We shoot again 5 rays, ray 1 and 2 are not occluded by anything, 3, 4 and 5 hit the sphere and so are occluded. So this point is given a value of 3/5.

Now imagine doing this again and again, point after point. For each point we take the ratio of occluded rays over the total number of rays. The closer this value is to 0, the less it's occluded, the closer to 1, the more occluded it is. So in the example above, point A would be closer to black (less occluded), point B would be closer to white (more occluded).

In our 3d world, that gives us the following image...

Now multiply our skylight by the inverse of the ambient occlusion image, and we get the skylight image from above.

So next time you're doing an outdoor scene, I highly recommend a little skylight (as well as some other lights, such as a bright yellow directional light for your sun) to make the image more realistic.

Note: the actual math behind implementing this technique is far more complex, and each renderer does it a slightly different way, but this should give you the basic idea.

Also note, renderers that include a "skylight" but not an "ambient occlusion shader" generally do not include a maximum distance value in their ambient occlusion calculation, which specifies a cutoff value for how far to shoot the ray before declaring it's a non occluded ray. This is useful for doing ambient occlusion indoors. But since a skylight is supposed to be a simulation of a sky, and not a more general purpose ambient occlusion shader, many do not include this feature. This causes some confusion between what a skylight does, and what ambient occlusion is. So as a rule of thumb, a skylight is a light that includes an ambient occlusion calculation generally with no max dist parameter, whereas an ambient occlusion shader just provides an occlusion image with no light, and generally has options such as a max dist value. Ideal of course is to have both abilities.

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