Z fighting in baking AO map

Artistic Support Materials and Textures
1

I’m trying to bake an AO map from a detailed mesh to a simple mesh. (It is a shipping contianer, baked to a box). I am using the following settings:
normalized
clear
selected to active
distance: 0.1
Bias 0.04

However, the bake looks like it is having z-fighting issues:

40ftFail.png512×512 67.3 KB

I’ve been playing with the settings but that didn’t yield a working solution. The settings mentioned above are working for my 20ft container mesh, but not for my 40ft container mesh.

2

play with bias setting. by the way, attach your blend file for closer inspection.

3

Have you both objects selected ?

4

I do have both objects selected.
After a lot of experimenting with varying results, I finally had something working. Still not sure what was going wrong though.
In some cases the deeper areas would not show up in the AO map (but be white instead) asif they had been filled up with white. In some cases the higher areas would show black; And in some cases I would get that weird z-fighting.
Finally it gave me good results with :
distance: 0.5
bias: 0.1

I’m not allowed to share the original blend file, but here’s a simple blend file that can reproduce the results. I found out that if bias >= distance, you get this z fighting.BiasBug.blend (465 KB)

5

I can reproduce it on your blend, indeed if bias is equal or is superior to distance the artifacts appear, as long as bias is strictly lower than distance, the bake is good.

Looking up ao and bias on the blender website and i found there
http://www.blender.org/development/release-logs/blender-233/ambient-occlusion/

The bias setting allows to control how smooth faces will still appear smooth in AO rendering. The bias denotes the angle (in radians) a hemisphere will be made more narrow. Values of 0.05 to 0.1 typically work good.

I have no idea if the bias >= distance =artifact is a bug or an expected behavious (and bias should never be equal or superior to distance as i can’t find that mentionned anywhere) as while distance is understandable it’s always been a bit unclear to me what “Bias towards further away from the object (in blender units)” exactly means in combination to distance, but in any case, you could report this to the bug tracker and see what the devs will say about this :
http://projects.blender.org/tracker/?group_id=9&atid=498