Displacement Map Problem, Mesh Distortion

[angryoreo]

I am trying to put a pattern using a displacement map on a object. When i render the model distorts like an hour glass shape. I have attached an image showing my problem.

http://img246.imageshack.us/my.php?image=displaceproblemqa0.jpg

http://img246.imageshack.us/img246/9829/di...eproblemqa0.jpg

When i put the displacement map on a single plane it works fine, but on the pillar which is a cube like shape, it distorts for reasons unknown to me. i just want to put this displacement on the sides of the pillar keeping it a rectangle shape. The UV's aren't messed up either. Thanks..
Any help is appreciated.

[joojaa]

Well the data does not move over hard edges like you would expect. instead each face would deform saparately and cause something not many of us dont want. In general theres a huge conceptual problem displacing polygonal models, indeed surfaces that are'nt continious , or even just the discrete nature of polygonssince it is usually not what people intend to happen.

[angryoreo]

QUOTE(Joojaa @ 12/17/08, 07:45 PM) [snapback]297227[/snapback]
Well the data does not move over hard edges like you would expect. instead each face would deform saparately and cause something not many of us dont want. In general theres a huge conceptual problem displacing polygonal models, indeed surfaces that are'nt continious , or even just the discrete nature of polygonssince it is usually not what people intend to happen.

Thanks Joojaa, so there might not be a problem with what i have done?? i notice when i change the mental ray approximation settings, the cube shape gets even more curved, so i tried playing with the settings but i couldn't get the pillar to look square.

[joojaa]

well see theres 2 planes perpendicular to each other. like this:

CODE
+--
|
|

now if you start to move subdivisions of that plane you get something like:

CODE
  |
  |

---

  |
  |

which obviously form direction of a plane looks ok.

Because the planes are separate* it does not help how much you subidvide if your texture reports movement at the seams even by inetrpolation as the map must be 0 offset on both sides of the pilars uvmap edges so you must have some tetxure slack area outside the uv map taht does not move the texture or the edge wont line up.

However if you round the edge it doe snot really maytter since its a smooth surface and it aligns nevertheless.

*The fact that the 3d software reopots them being unison does not mean thay are inside the rasterisation step. because they are not. common sense of mathematics overrides whatever you think will happen

[angryoreo]

oh ok, thanks jooja for explaining what those mental ray approximation settings are doing. I think ill just work around this by cutting up the single plane poly where it worked and then join the four poly's together.