3D polygon front/back orientation in ArcScene

[Mark_W]

Hello forum,

I've created 3D polygons in AutoCAD, converted them to shapefiles, and added them as a layer in ArcScene. The polygons are all there, and generally, the 3D geologic model I'm creating looks pretty good. My problem is that when I use the lighting in ArcScene, my 3D polygons are inconsistent in their front/back orientation. This orientation greatly affects the perception of the polygon's color. I've been able to flip some of the objects orientation by recreating them in CAD, but even this method seems to be random. I've looked for an editable shapefile field to flip the individual (problematic) polygons in ArcScene, but have had no success so far. Any advice on this subject would be greatly appreciated.

Thank you,

[GuthCAD Software Support]

Hi Mark,

I am not really up on how Arccene determines which is back and front of a surface. (If anybody definitively knows please post here!) Possibilities I can think of are:

1. Order of (direction of) vertices in the polygon.
2. A shapefile field value to indicate an upward normal vector, or to indicate how 'up' is related to the vertices direction.
3. Just treat the surface facing the eye position as the front.

My first advice would be to check the direction of vertices of your polygons in AutoCAD. Taking a bird's eye view of the terrain, the vertices of polygons with their front surfaces facing up should all be in a clockwise direction.

Let me know how you go.

best regards,
Paul G.

[chrisgnicholas]

I was just wondering if Cad2Shape can create true ESRI 'PolygonZ' multipatches , with proper orientation, etc.

thanks!
Chris Nicholas

[GuthCAD Software Support]

Hi Chris,

Yes, the software definitely creates true ESRI PolygonZ and Multipatch shapefiles. In CAD2Shape check the 'Polygon' and '3D' items on the main dialog to get PolygonZ output. For 'Multipatch' just check the 'Multipatch' item.

The 'ESRI Shapefile Technical Description' is available at the ESRI web site here.
www.esri.com/library/whitepapers/pdfs/shapefile.pdf

The ESRI Shapefile Technical Description makes no mention of 'orientation' with regards to PolygonZ or Multipatch shapes (or any type of shape for that matter).

best regards,
Software Support

[chrisgnicholas]

thanks for this reply! Also, My reading of the Shapefile spec doesn't talk at all about traditional "texture coordinates", as per OpenGL texture mapping, UV, or otherwise, as per: http://en.wikipedia.org/wiki/UV_Mapping

The reason I ask is that a customer wants us to give them a 3D model in ESRI multipatch format, but I can't understand how to give them texture map coordinates.

thanks!
Chris Nicholas