HI all,
I made normal map and export the model into 3dsmax.after some changes( make UVs and reduced faces),Now I cannot get it back to zbrush,and cannot render the displacement map.
now I only have the normal map,and redo all things will take me long time,
Normal maps do not contain the information that’s needed to be able to reconstruct the geometry. This is because they don’t actually contain any depth information.
Both normal maps and displacement maps are difference maps, but they map those differences in completely different and non-compatible ways. A displacement map can be used to create a normal map because the disp map has depth info. The normal map does not have that data, though, and so it’s a one-way ticket.
Hi Aurick,
Thanks for you reply
I found a little tool displacementCreator at www.ryanclark.net, it can convert the normal map to displacement map.
very cool!
really interesting tut on making normalmaps from photos on that site too.
thanks for finding this
I think for normal repeatetive textures zbrush provides alpha deformation which works very well when generated as displacement maps.
I still not able to understand the actual use of normal maps.
Excuse my little knowhow regarding this.
I mean what gives the normal maps edge over bump maps ?
Chris Dong, that tool has been around a bit, has some issues with the results. However if it works for you, give it a go. 
Normal maps better simulate reality than bump maps. A normal mapped ‘cavity’ or crater in a wall will look more realistic than a bump mapped one, for very little increase in computational complexity. There are some good articles out there on the differences.