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ThunderSoul
06-09-2005, 08:17 AM
Hello...
I have found a formula to rotate a vector about an arbitrary axis at
http://www.cise.ufl.edu/~cmccann/seniorp...or%20rotation\' (http://www.cise.ufl.edu/~cmccann/seniorproj/ArbitraryAxisRotation.pdf#search=\'arbitrary%20vec tor%20rotation\')
... I wish to find the counterclockwise rotation around the arbitrary vector (u, 0, w); I'm setting v to 0 since my arbitrary vector involves rotations not including the y-axis.
Since I don't have the matrix to invert it and only the final vector formula, I thought I could rotate the vector (u, 0, w) by 180 degrees around the y axis getting (-u, 0, -w). Plugging those values in didn't hold...
Am I doing something wrong or is there another way to do it?

Thank you,
-TS...

Ehsan Kamrani
06-09-2005, 10:03 AM
Thundersoul,
These formulas help you to understand the meaning of the OpenGL architecture. Do you want to use from the OpenGL to write your programs?
As i said before, to rotate around a vectore (vx, vy, vz ) you can use from the following function:
glRotatef( angle, vx, vy, vz );

In OpenGL, we use from the homogeneous coordinate system. It means that we can use from both the points and vectors in our application. the fourth component(w) specifies the type of your primitive--Vector or point.to rotate a point around a vector set its w component to 1.to rotate a vector around a vector set its w component to 0.

As an example, here's a code that rotates the point p(x,y,z,1) around a vector( vx, vy, vz )

GLfloat p[] = { x,y,z, 1};

....
glRotatef( angle, vx,vy, vz );
glVertex4fv(p);

And here's a code that rotates the vector p(x,y,z,0) around a vector( vx, vy, vz )

GLfloat p[] = { x,y,z, 0};

....
glRotatef( angle, vx,vy, vz );
glVertex4fv(p);

-Ehsan-

powerpad
06-09-2005, 11:45 AM
Maybe it helps what I wrote in the Math forum.

By the way please do not cross post.