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camera parameters from GL projection matrix

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I am trying to construct the 3x4 Camera Projection matrix that maps a 3D point, X, to the 2d image coords, x. i.e.: x = PX (basically what gluProject does but in 1 matrix) where x and X are in homogeneous coords. This matrix is constructed from: P = K.[R|t] Where R is a 3x3 rotation matrix, and t the translation vector. The matrix K is a 3x3 that contains the intrinsic camera parameters, basically the focal length and the coordinates of the camera center: fx 0 xo 0 fy y0 0 0 1 I can get x0 and y0 from the viewport matrix, as I can R and t from the modelview matrix. My question is how can I get the camera focal length from the projection matrix? I have found no documentation convering this apart from pointers to the 3dgamedev faq which seems to no longer exist... Thanks.

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