Ambient Occlusion (AO) determines the degree to which each visible surface point is exposed and darkens it accordingly. Ambient Occlusion (AO) offers, within certain limits, a fast alternative to GI.
Note: As an alternative to using AO as a global effect on the whole Rendered scene, Ambient Occlusion can be applied as a shader effect to specific Surface Channels.
See Ambient Occlusion (CineRender Surface Channel).
Let’s say your scene is surrounded on all sides by a sky. The AO shader will determine to what extent each visible area sees the sky.
Corner areas, holes, and areas between objects placed very close to each other will see less of the sky and will therefore be darkened in accordance with the AO settings.
Color
Use the Color option to define the color gradient that AO assigns, dependent on exposure. Normally, it will be a simple custom black to white gradient but other colors can also be defined.
Be aware of the fact that these gradients will be treated as grayscale, depending on the material channel.
See also Adjusting Gradient Parameters.
General
•Minimum Ray Length: Determines how the gradient defined in Color will be rendered between exposed and non-exposed areas. The closer the Minimum Ray Length value is to the Maximum Ray Length value, the further the gradient will be pushed towards the edges, which are defined by the maximum ray length.
This value shouldn’t really be tampered with. Leave it set to its default setting of 0.
•Maximum Ray Length: This value defines to which distance the surfaces see each other. Where geometry meets, as with the floor and wall or the sphere and floor, even small rays will fall upon surfaces, which is enough to darken those areas.
If high values are used, a much larger distance will be included within which objects will be able to see each other. This results in a softer, more homogeneous darkening that will cause longer render times.In general, lower values are recommended.
•Dispersion: During each AO calculation, several rays (Samples) will be emitted for each point within a virtual hemisphere in the scene. These samples check to see if any geometry lies within the Maximum Ray Length. Dispersion determines to what extent these samples will be taken into account on the hemisphere’s surface. A value of 0% takes only the hemisphere’s zenith (vertically over the sphere) into account. A value of 100% will take the entire hemisphere into account.
Accuracy, Minimum Samples and Maximum Samples (Ambient Occlusion)
These settings are responsible for AO quality.
•Low quality is accompanied by grainy results. This is not necessarily bad. In fact, in some cases, this can be of high aesthetic value.
•If you prefer homogeneous, seamless transitions, you should select higher quality settings, which will also increase render times.
In simple terms: Samples are required to render AO. The more samples that are used, the more homogeneous (less grainy) the rendering will be - the longer the render times will be as well. Fewer samples, on the other hand, result in shorter render times.
Of course a maximum number of samples can be calculated for the entire scene. This would take very, very long to calculate and it does not make any sense since a scene contains many areas in which a relatively small number of samples would suffice.
That’s what the Minimum Samples and Maximum Samples settings are for. They are used to affect the critical and less critical areas of your scene in different ways.
The Accuracy setting determines where and in what amount samples must be distributed in order to achieve the best possible result. In critical areas, the maximum amount of samples will be used.
Hence, the Accuracy setting has the most influence in critical areas (since higher values in these areas lead to more samples) and less influence in less critical areas, which use the Minimum Samples value.
•Contrast: Use this setting to adjust the AO effect’s contrast. Negative values can also be entered!
•Use Sky Environment: AO can work without a separate light source if the Physical or HDRI Sky is used as Illumination. If the Environment’s Sky checkbox is active, the reflected sky color will be multiplied onto the final AO image.
Note: For this to have an effect, the Ambient Occlusion texture must activated in the Environment or Luminance surface channels.
See Ambient Occlusion (CineRender Surface Channel).
•Evaluate Transparency: When this option is active, transparency will be analyzed on the basis of the Transparency or Alpha surface channels. In doing so, the degree of transparency will be taken into consideration: A semi-transparent sphere will still cause some AO in spite of this option being active.
•Self Shadowing Only: When this option is active, separate objects will not see each other, they will only see themselves.