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Basic rotation matrices are clockwise !


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#1 hbdevelop1   Members   -  Reputation: 113

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Posted 10 January 2013 - 08:18 PM

Hello,

I have noticed that the basic 3D rotation matrices in  http://en.wikipedia.org/wiki/Rotation_matrix

rotate vectors clockwise and not counter-clockwise, when the axis about which they occur points toward me !


For example: the following operation yields the vector (0,1,0)
[1      0      0     ]   [0]
[0  cos(90)  sin(90) ] X [0]
[0  -sin(90) cos(90) ]   [1]


Am I missing something ?

Thank you for your help



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#2 weeska   Members   -  Reputation: 255

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Posted 11 January 2013 - 12:53 AM

This Rotation Matrix rotates a vector around the x-axis by 90 degrees. Your vector initially points toward the z-axis and after the rotation it points in y-direction. I think this is counter-clock-wise as it's written in the article.

Edited by weeska, 11 January 2013 - 12:54 AM.


#3 Hodgman   Moderators   -  Reputation: 27668

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Posted 11 January 2013 - 01:18 AM

I'm guessing this depends on whether you're using the left or right handed convention.



#4 0r0d   Members   -  Reputation: 797

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Posted 11 January 2013 - 02:20 AM

Hello,

I have noticed that the basic 3D rotation matrices in  http://en.wikipedia.org/wiki/Rotation_matrix

rotate vectors clockwise and not counter-clockwise, when the axis about which they occur points toward me !


For example: the following operation yields the vector (0,1,0)
[1      0      0     ]   [0]
[0  cos(90)  sin(90) ] X [0]
[0  -sin(90) cos(90) ]   [1]


Am I missing something ?

Thank you for your help

 

I think the problem is that the given matrix is a row-major matrix, which means row vectors.  You're trying to use it as a column-major w/ column vector.

 

So the operation you want to do is really:

 

[0 0 1] X [1      0      0     ] = [0 -1 0]
          [0  cos(90)  sin(90) ]
          [0  -sin(90) cos(90) ]

 

Remember when you look at a matrix it's very important if it's row-major or column-major and right-handed vs left-handed.  The above matrix gives you the correct result if it's right-handed row-major.



#5 Mercile55   Members   -  Reputation: 136

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Posted 20 January 2013 - 07:40 PM

Hello,

I have noticed that the basic 3D rotation matrices in  http://en.wikipedia.org/wiki/Rotation_matrix

rotate vectors clockwise and not counter-clockwise, when the axis about which they occur points toward me !


For example: the following operation yields the vector (0,1,0)
[1      0      0     ]   [0]
[0  cos(90)  sin(90) ] X [0]
[0  -sin(90) cos(90) ]   [1]


Am I missing something ?

Thank you for your help

Your example gives (0,-1,0).

it should be:

[1      0      0     ]   [0]
[0  cos(90) -sin(90) ] X [0]
[0  sin(90)  cos(90) ]   [1]






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