Yep,

You can do negative displacement Dave........

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> 50% grey (lighter) == positive displacement

50% == no displacement

< 50% grey (darker) == negative displacement

You can also play with alpha offset to adjust this value.

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The problem I have with mental ray displacement is that like most things in Mental Ray it is just to fricken convoluted!

What is the point of having a displacement node for instance that controls NOTHING! There are like half a dozen adjustable settings in the displacement node and I have never seen any of them used for anything!

You control offset distance with color balance, and displacement tesselation with a displacement approximation node.

But still have to set the bounding box using the shape node's Maya displacement section.

Also mental ray has all these freaking knobs and dials but do little to document how to adjust them properly or even describe what their function is for that matter. You go to one dial and you have to increment it in units of a billion to see any effect and then you go to the next dial and if you increment it by .0000001 you've gone out of range!

Verses V-Ray ... You give it a map, tell it max displacement for white, and give it an optional water level. No bounding box, no arcane mystical render tesselation computations. It just works and better still it breaks the task into bite size chunks that even a 32bit machine can handle!

I can sum up all the problems that I have with Mental Ray in one statement....

They take every possible rendering task you can perform in mental ray and then give you so much control over every little arcane internal variable, that in the end you have no control at all!