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# Basic rotation matrices are clockwise !

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4 replies to this topic

### #1hbdevelop1  Members

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 ?

### #2weeska  Members

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.

### #3Hodgman  Moderators

Posted 11 January 2013 - 01:18 AM

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

### #40r0d  Members

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 ?

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.

### #5Mercile55  Members

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 ?

it should be:

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

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Guest, the last post of this topic is over 60 days old and at this point you may not reply in this topic. If you wish to continue this conversation start a new topic.