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Chronix

Rotating to "sit" a unit on a triangle

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Hi guys, I'm trying to figure out how to rotate a unit model (RTS game) to "sit" on a triangle of any orientation. I have the 3 points that make the current triangle up, how do I work out how much to rotate on x/y/z to align properly on it? Maths isn't my strong point, and I have to keep hammering away at things like this to figure it out, so any help would be appreciated! :)) Thanks a lot

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Naive suggestion...

Take the surface normal of the triangle and the "up" vector.
Dot product gives the angle between the two vectors.
Cross product gives a rotation axis.

If the dot product is close to zero, do nothing.

Otherwise, normalize the result of the cross product, then do an arbitrary-axis rotation using acos(dot product) as the amount of rotation. Depending upon ordering of the cross product, you might need to negate the angle.

Or as pseudo code:
float angle = dotProduct(surfaceNormal, upVector);
if (angle > 0.001) {
vector axis = crossProduct(surfaceNormal, upVector);
float radians = acos(angle);
matrix = ArbitraryAxisRotation(axis, radians);
}

Caveat: I'm a programmer, not a mathematician, so at no time do I know what I'm talking about.

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