I need AO education

[CliveG]

OK I give up, I can't find out what it is on a lot of imported models that causes the AO pass to fill whole chunks of some of these models heavily in shades of grey. Yet if I remodel the things myself, they are fine, only triggering dirt at the intersections as expected.

 

Not normals or anything obvious to a newbie.

 

I know the answer is to remodel it all myself anyway... but I'd still like to learn what it is that causes an AO pass to go mad on some other peoples work?

 

I'm Max and Vray by the way and am wondering whether this may be to do with models from other packages perhaps?

 

Any ideas?

[Fooch]

Normals --> flip them

Or single face at one bit --> shell them

[Scott Schroeder]

Scale of the model hurts too. If they created it in something other than what is listed in your system units, the geometry will be scaled incorrectly which,in turn, leads to incorrect AO. Say if you are working in meters and the model was created using mm or generic units, or even feet and inches. It's like taking a 4x4x4 cube and scaling it to 400x400x400. Max still tends to see the box at 4x4x4 during render as that is the original scale transform still on the box. Try it, make a 4x4x4 cube and scale it 8000 times. You'll still see the sliders on the cube showing 4x4x4 and your UVW map modifier will still come in at 4x4x4. Thus, your AO will be way off given the massive size of the scaled cube.

 

One way to test this is to apply a UVW map to an object of generaly known size. For example (in feet and inches) an 8' high by 12' long wall. If I apply a UVW map to this, I should get 8'x12'. If I get 800' or 8" or anything else, I know my geometry has the wrong scale to it.

 

You work around this by first resetting the transform and scale in your hierarchy, then you must reset the X-form from the Utilities panel. Collapse to a mesh, and now your geometry should have the correct scale transforms on it.

[erickdt]

You'll also get AO shading like that (with whole portions of objects getting grayed out) with coincident faces. This is mostly due to sloppy modeling where whoever created the model has duplicates of the same object on top of one another. You can go through and delete these additional objects and it should solve your problem.

 

E

[dmv79]

If it's coming out black, you may also try checking "emit light on back side" on the vray light mtl (helps with the flips). If you are working with the "invert normals" in vray dirt on a thin object (things get grey and you can see the occlusion effects), then it's the back face occluding the front face... there is not much you can do other than detach faces and work around excluding/including objects from the dirt map.