Shot_06 AO Testing

Lighting design is definitely important for a piece of animation, not only improving the storytelling but also making the scene more realistic and believable. However I do not have enough time to render Global illumination and Final Gethering as I want to put more focus on my research and practice working rather than rendering. So I need to find another way to improve its effect in term of Ambient Occlusion, like the picture below. 

According to Wikipedia, http://en.wikipedia.org/wiki/Ambient_occlusion, Ambient occlusion is a shading method used in 3D computer graphics which helps add realism to local reflection models by taking into account attenuation of light due to occlusion. Ambient occlusion attempts to approximate the way light radiates in real life, especially off what are normally considered non-reflective surfaces.

Unlike local methods like Phong shading, ambient occlusion is a global method, meaning the illumination at each point is a function of other geometry in the scene. However, it is a very crude approximation to full global illumination. The soft appearance achieved by ambient occlusion alone is similar to the way an object appears on an overcast day.

Ambient Occlusion

diffuse only

combined ambient and diffuse
Obviously there are more details and more beautiful depth after combination.

Ambient occlusion is most often calculated by casting rays in every direction from the surface. Rays which reach the background or “sky” increase the brightness of the surface, whereas a ray which hits any other object contributes no illumination. As a result, points surrounded by a large amount of geometry are rendered dark, whereas points with little geometry on the visible hemisphere appear light.

Ambient occlusion is related to accessibility shading, which determines appearance based on how easy it is for a surface to be touched by various elements (e.g., dirt, light, etc.). It has been popularized in production animation due to its relative simplicity and efficiency. In the industry, ambient occlusion is often referred to as "sky light."

The ambient occlusion shading model has the nice property of offering a better perception of the 3d shape of the displayed objects. This was shown in a paper where the authors report the results of perceptual experiments showing that depth discrimination under diffuse uniform sky lighting is superior to that predicted by a direct lighting model.

The occlusion at a point on a surface with normal can be computed by integrating the visibility function over the hemisphere Ω with respect to projected solid angle:

                                   
where is the visibility function at , defined to be zero if is occluded in the direction and one otherwise, and is the infinitesimal solid angle step of the integration variable . A variety of techniques are used to approximate this integral in practice: perhaps the most straightforward way is to use the Monte Carlo method by casting rays from the point and testing for intersection with other scene geometry (i.e., ray casting). Another approach (more suited to hardware acceleration) is to render the view from by rasterizing black geometry against a white background and taking the (cosine-weighted) average of rasterized fragments. This approach is an example of a "gathering" or "inside-out" approach, whereas other algorithms (such as depth-map ambient occlusion) employ "scattering" or "outside-in" techniques.
In addition to the ambient occlusion value, a "bent normal" vector is often generated, which points in the average direction of unoccluded samples. The bent normal can be used to look up incident radiance from an environment map to approximate image-based lighting. However, there are some situations in which the direction of the bent normal is a misrepresentation of the dominant direction of illumination, e.g.,

In this example the bent normal N_b has an unfortunate direction, since it is pointing at an occluded surface.

 

In this example, light may reach the point p only from the left or right sides, but the bent normal points to the average of those two sources, which is, unfortunately, directly toward the obstruction.

Reference:

http://en.wikipedia.org/wiki/Ambient_occlusion