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