How can I calculate a vertex normal for a hard edge?

Posted by K.G. on Game Development See other posts from Game Development or by K.G.
Published on 2012-03-23T03:04:03Z Indexed on 2012/03/23 5:39 UTC
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vector

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linear-algebra

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normals

Here is a picture of a lovely polygon:

Box of awesome

Circled is a vertex, and numbered are its adjacent faces. I have calculated the normals of those faces as such (not yet normalized, 0-indexed):

Vertex 1 normal 0:   0.000000   0.000000    -0.250000
Vertex 1 normal 1:   0.000000   0.000000    -0.250000
Vertex 1 normal 2:  -0.250000   0.000000     0.000000
Vertex 1 normal 3:  -0.250000   0.000000     0.000000
Vertex 1 normal 4:   0.250000   0.000000     0.000000

What I'm wondering is, how can I determine, taken as given that I want this vertex to represent a hard edge, whether its normal should be the normal of 1/2 or 3/4? My plan after I glanced at the sketch I used to put this together was "Ha! I'll just use whichever two faces have the same normal!" and now I see that there are two sets of two faces for which this is true.

Is there a rule I can apply based on the face winding, angle of the adjacent edges, moon phase, coin flip, to consistently choose a normal direction for this box?

For the record, all of the other polygons I plan to use will have their normals dictated in Maya, but after encountering this problem, it made me really curious.

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