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Displacements and Maya - object size change causes problem

ZBrush Usage Questions
#1

Hi there,
I made a perfectly fine model with displacement maps in ZBrush which looks terrific in Maya mental ray render…
The displacement map is set to alpha gain and alpha offset 2.2 and -1.1, the object has an Approximation node.

My problem is that as soon as I scale down the model, all the displacement maps are being influenced and I get a much more exaggerated and inflated result.
How would I go about having the displacement maps not being influenced by the size of the object?

Thanks you.

#2

Displacement maps are created for the specific cage mesh that they’ll be applied to. They’re calculated based upon the assumption that the model will be subdivided and smoothed by the renderer, and then displaced from that point to match the original high-res version.

What all of this means is that if the cage changes, the map will no longer be accurate. It’s like following directions. You can’t end at the correct point if you started from the wrong one.

#3

You’re usually going to get better Maya-specific answers (including displacement questions) if you put them to the Autodesk forum instead of here at ZBC. That’s not a jab at aurick’s answer, he’s absolutely correct. I don’t get the impression from your post that you’ve altered your mesh though, aside from uniformly scaling it in Maya.

LAMRUG has this to say:

In order to support mental ray’s displacement algorithm a proper displacement bound value ‘max displace’ needs to be supplied for any object. This is automatically derived from Maya’s displacement map settings’ ‘bounding box scale’ attribute on shape nodes by default. However, this value can be set directly via a dynamic attribute ‘miMaxDisplace’ (float) created on the shape node, which is recognized by the plug-in and exported without any further checks. Furthermore, a global default ‘max displace’ value can be forced via a dynamic attribute ‘maxDisplace’ (float) created on the mental ray options node (miDefaultOptions).

#4

Thanks for the replies guys,
I just posted on 2 Maya forums, hope I’ll get a more specific answer there.

I can imagine why the displacement changes according to size but I still need a solution to it. I know there should be a simple way, something like having it be influenced locally and not by world or something like that.