what is ambient occlusion? what can it be used for?

[Trapezium]

As an analogy
So there's this copper golf ball someone lost and it oxidizes and there is a patina formed. I find the ball and rub it for good luck before every golf game. In The patina that was occluded from my rubbing stays and the copper where I've rubbed is revealed. Anyway, the shader used for Ambient Occlusion in Mental Ray is called Dirtmap so it makes sense I guess.

[dbuchhofer]

Long john.. It doesn't, but you can use the Ambient occlusion pass to mask out the areas that would get more 'dirt' or less 'reflection', or more 'anything'. It is a utility, used in some small way that one uses the Falloff map

Ambient occlusion is just a way of figuring out how much a mesh has the possibility to self shadow itself, with the idea that if it has a self shadow, it is an area that is not as exposed to (light/elements/air/water/etc) as it could be.. A way to measure concavity. and using in the dirt example .. the exposure of a certain point determines the amount of the mixed texture.. more self shadowing, deeper the hole or crevice is.. more of the texture blends in.. less.. etc.

It is a utility rendering.. used in fakes of various effects.

[Da_elf]

there is a plugin called dirtreyes not sure if its been updated to max5 or 6. was a cool plugin that analised the model and where there were folds it put dirt in there. its the same thing as the ambient occlusion.