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Krak

Trig in 3D Rotations?

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Say I was going to rotate a 3D field...like a fighting arena, on its y axis. I know that the matrix for y rotations involves trigonomic functions...but for that to happen, you need a right triangle and a reference angle. My question is: if it's not clearly a triangle...and it's something like a character, or an entire level....how you do determine its reference angle, and the size and location of the triangle?

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Define everything by points, say the midpoint of a character, the middle of a level etc. Once you have everything as a point, you can find the angles between them (3 points defines a triangle). I usually use the origin (0,0) as the reference point, so the other 2 are just the objects:


A(-6,1)
O(0,0)

B(-3,3)

Need A-O-B. Law of Cosines:
AB2 = AO2 + BO2 - 2 * AO * BO * cos(A-O-B)

Where AO = sqr(62 + 12)
BO = sqr(32 + 32)
AB = sqr(32 + 22)
(distance formula to find distance between points)
(don't forget this still works with 3 dimensions: dist=sqr(x2+y2+z2)!

Therefore,
cos(A-O-B) = AB2 -AO2 - BO2/-2 * AO * BO

And voila! You have your angle!
...I hate trig.

If I got that wrong, someone please correct me. It's been awhile! [lol]

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The relation to a right triangle can be seen by rotating the coordinate axes, i.e. (1,0,0), (0,1,0) and (0,0,1). What happens to (1,0,0) tells you what the first column should be assuming it is a matrix times a vector. So a rotation by t around the z axis take (1,0,0) to (cos(t),sin(t)) and (0,1,0) to (-sin(t),cos(t)).

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