Archived

This topic is now archived and is closed to further replies.

Getting rotated points from GL

This topic is 5020 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Recommended Posts

I need to get the points from a rotation from the modelview matrix i have code like
::glPushMatrix();
  ::glTranslatef(1, 1, 0);
  ::glRotatef(180, 1, 0, 0);
  ::glTranslatef(0, 0, 0);
  float mm[16];
  ::glGetFloatv(GL_MODELVIEW_MATRIX, mm); 
::glPopMatrix();
Im rotating 180 about the X axis at (1, 1, 0). I think in rotations in 3D graphics if i can remember the matrices are cumulative so I think you must specify the operations in reverse. My modelview matrix is as follows: [ 1 0 0 (*x) 0 ] [ 0 -0.50000 -0.86625 (*y) 0 ] [ 0 0.86625 -5.00000 (*z) 0 ] [ 1 0.50000 9.13398 1 ] My understanding is that *x, *y, *z elements in the matrix should contain the new rotated x, y and z coordinates, but instead there all zeros. I know C stores matrices funny, but the bottom right element in this matrix should be a 1, so, it looks right there (ie. doing a transpose wont help). Can anyone help, All the best, Keith!

Share this post


Link to post
Share on other sites
To rotate a point by a matrix, you need to postmultiply it by the 3x3 rotation submatrix. Ie, if M is your rotation submatrix then v''=M*v

Google for the matrix and quaternion FAQ if you don''t know how to do that. Oh, and remember that OpenGL matrices are stored in column major format.

Share this post


Link to post
Share on other sites
I got it going after, i forgot to call
::glLoadIdentity();

after
::glPushMatrix();

And yeah the elements 12, 13, 14 are the x,y, z coordinates due to GL storing matrices in Column Major format.

Thanks for the help,
Keith

Share this post


Link to post
Share on other sites