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General Trigonomic Algorithm question...

Posted on 2004-03-28
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Last Modified: 2010-04-17
Okay so here's a question. I need an algorithm for rotating a point (p2) about an axis (p1). Here's the spaghetti code I have so far.

Radius = SquareRoot((p2.y - p1.y) ^ 2 + (p2.x - p1.x) ^ 2)
Radians = ArcCos((p1.y - p2.y) / radius) ' Angle between Vector(p1,p2) and unit vector (p1, {0, 1}). Rotating counter clockwise.
Radians = Radians + (2 * ((2 * PI) / 360)) 'Add two degrees to current angle of line
p2.y = (Cos(Radians) * Radius) + p1.y
p2.x = (Cos(Radians) * Radius) + p1.x

The code works fine for angles < 180 degrees. But as soon as I get to say 182 the line moves to an angle of 2 degrees counter clockwise from the unit vector... anyone know a better algorithm for this or how to fix this one to be able to run a full rotation? I need to be able to "add" angles to lines.
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Question by:CyrexCore2k
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mikkelp earned 125 total points
ID: 10702241
Hi

given a point A = (a1,a2) relative to the point 0 = (0,0) in the plane and a counter clockwise rotation of R degrees, your rotation is given by

A' = (a1 x cos R - a2 x sin R, a1 x cos R + a2 sin R)  // counterclockwise
A'' = (a1 x cosR + a2 x sin R, a1 x cos R - a2 sin R)  // clockwise

so to rotate any one point Q about the point P, find the vector D = (Q-P), rotate D R degrees and Add P.

p2' = p1 + ((p2-p1) rotated R degrees)

check this link
http://www.mapleapps.com/categories/maple_tools/animations/html/clock1.html
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by:CyrexCore2k
ID: 10705363
I'm sorry I've never done anything like this before. Do you think you could write out the spaghetti code?

In principle I understand how this works but for some reason when I try to write it out it all goes sour.
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by:CyrexCore2k
ID: 10709648
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