For cycles renders, why is there an AO option on the World Properties Tab?

Something I have been a little confused about … the effect on a cycles render of the AO option in the World Properties Tab.

I always understood that cycles was a physics based renderer. And with the AO option disabled, it appears to produce a proper AO-like effect by deepening the shadows in the corners etc. With the AO option enabled, the image brightens and the shading changes but I can’t quite tell what it is doing. Its certainly different than the AO effect in the Blender Renderer.

Does anyone understand this? ( or even get what I’m rambling about :slight_smile: )

Thanks

Wayne

AO is often used to add ambient lighting without adding a whole lot to the render time, the global option is also often used so as to get a usable AO in the compositor with which to darken crevices and corners (which in this latter case will work even if the power is set to 0).

The world AO option will calculate AO for everything and just add the result to the final image. It functions as an ambient lamp with built-in AO. It’s meant mainly as an alternative to GI and fill lighting for test renders and certain stylistic looks. If you have GI enabled, it will correctly add the indirect lighting and contact shadows that AO is designed to fake. If you want extra AO for whatever reason (it’s physically wrong, but can occasionally look good anyhow), the AO pass is a better way to go.

Thanks to you both for the explanation.