Simple point rotation question

OpenGL
1

Hi

I want to rotate a point around z axis.
Here is the way I am doing it.

My point is x = 1, y = 2, z = 3

With opengl the transformation i did that as following

But some tutorials it say to do it as following

Here, I cannot understand why is that.
Could you please explain this thing a bit?

Thank you

2

It depends which direction you want to rotate around the axis. One will rotate it clockwise, the other anti-clockwise. This is because Sin(-a) = -Sin(a) and Cos(-a) = Cos(a), so the 2nd matrix could be written as:


[  cos(-30) sin(-30) 0 0 ]
[ -sin(-30) cos(-30) 0 0 ]
[         0        0 1 0 ]
[         0        0 0 1 ]

which when compared to the first matrix, will show it will rotate in the opposite direction.

3

Do it as the romans do, that is use the matrix that rotates counterclockwise, which means that you use the second matrix you have provided, with a possible negative argument to sin() or cos() functions.

4

Thank you for the reply and the clear explanation.