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khalligan

Getting rotated points from GL

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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!

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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.

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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

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