Cavity and AO maps - difference?

Hi folks,

From the interwebs:

"Cavity and Ambient Occlusion maps are specialty textures that can be generated in ZBrush using the masking tools. Cavity maps are essentially the blue channel of a normal map and represent dark shading in the crevices of the model. They can be useful as a multiply layer over a diffuse color map to accentuate details or as a diffuse map in Maya to help punch up the high-frequency details in a render. Figure 9.67 shows a cavity map. It is essentially a black-and-white map in which black represents the recesses and white is the high points.

Cavity mapping is intended to help mimic Ambient Occlusion, but it is not by nature an Ambient Occlusion effect. It also has several other uses. Ambient Occlusion works by casting rays out from the surface of a 3D model. If a ray hits another surface before dying out, then a grayscale value is recorded that represents how far that ray traveled before being occluded. This is how Ambient Occlusion maps generate darker shadows in tight recesses like inside ears or under the brow ridge."

If i understand right, this quote is saying that a cavity map is just a “painted on effect”, whereas an AO map will actually interact and change the shading depending on the angle it is viewed from? Or is AO also just a painted on effect? So, not interactive, in the same way as a bump or normal map?

Thanks!

If you bake an AO map it isn’t interactive. It’s just a greyscale map of ambient lighting. Some renderers can mask it based on the amount of direct light hitting the surface.

The difference between AO and cavity is that cavity tends to cover smaller details and isn’t actually dependant on lighting just the… cavities higher frequency of geometric detail. For example, if you’d bake a cavity and AO on a head mesh the cavity map will have pores and wrinkles, but it won’t have shadows in places that woud actually get occluded, like the inside of the mouth, nostrils, ears and eye sockets. In cavity map there also isn’t any interaction between meshes that are close by that should have AO.

Cavity on left and AO on the right:
image

Easiest way to imagine AO is that it’s just the lighting you would get if you put a object into a evenly lit room, there’s no concrete shadows, just the areas get darker that light can’t easily reach. It isn’t dependant on the viewing angle. Generally there’s only visible difference based on the viewing angle if the material is glossy. AO bakes are 100% diffuse.

Edit: Small correction on cavity map - some bakers use same technique as AO for cavities, but with a smaller ray distance - so only smaller scale details get represented.

The quoted part is not even remotely true. Read the discussion at https://polycount.com/discussion/72813/my-cavity-map-notes/p1 .

And check out http://wiki.polycount.com/wiki/Cavity_map .

Thanks, i think i get it!