| In <1pscti$aqe@travis.csd.harris.com> srp@travis.csd.harris.com (Stephen Pietrowicz) writes: |
| >How do you go about orienting all normals in the same direction, given a |
| >set of points, edges and faces? |
| This algorithm works well for me: |
| Algorithm to attempt to find outward-facing normals: |
| First, mark all faces as UNKNOWN. |
| Then create an edge dictionary that allows you to find all of the |
| faces sharing a given edge (where an edge is two integers representing |
| the two shared vertices). |
| Pick an arbitrary face and mark it COUNTER_CLOCKWISE. Using the edge |
| dictionary, orient all surrounding faces based on the orientation of |
| this face. And recurse for all surrounding faces, consistently |
| orienting the entire surface. |
| Find the average of the vertices in this surface. Using that point, |
| calculate a volume measurement, taking into account the face's |
| orientation. If the volume turns out to be positive, assume the faces |
| are oriented correctly. If it is negative, reverse their orientations |
| (mark them CLOCKWISE). |
| If any faces are still UNKNOWN after this, choose another face |
| and go through the algorithm again. |
| At the end, faces marked CLOCKWISE must have their indices reversed |
| before facet normals are found. |
| (Note: if you are running on Silicon Graphics machines and buy the |
| IRIS Inventor 3D toolkit developers package you have the source to |
| this algorithm-- see /usr/src/Inventor/tools/ivnorm/. If you're |
| not... sorry, I can't give out the source, and even if I could it |
| relies heavily on Inventor). |
| --gavin (gavin@sgi.com, (415)390-1024) |
|
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