General quadrics and transformation matrices

[Chris Theis]

Hello all,

I'm currently facing a problem and I think I have found a solution to it. However, I'd be interested to hear your opinion or hints on this.

I have users which can define a number of different bodies (e.g. cylinders, spheres, ellipsoids etc.) by providing the coefficients of the general quadratic surface equation:

ax^2 + 2bxy + 2cxz + 2dx + ...

which in general can be written also using a coefficient matrix Q and a row vector v:

v Q v^t

My solid modeler already provides various different objects like cylinders, ellipsoids etc. but they are defined in a canonical system and carry a transformation matrix to place them at the right position for rendering.

In order to treat bodies defined by general quadrics I have to do two things:

1.) Determine which kind of surface has been defined
2.) Obtain the transformation matrix which will transform the respective surface to the position/orientation which is implicitly included in the quadrics coefficients.

After doing a bit of algebra I came up with the following idea for 2.)

I thought to tackle the issue via principal axis transformation. So I would first try to determine the eigenvalues and the eigenvectors for the coefficient matrix Q. The matrix which is represented by the eigenvectors in its columns should transform my coefficient matrix into the canonical system.

If I take the inverse of this eigenmatrix then this should be my transformation matrix, or am I wrong here? The eigenvalues would eventually determine the magnitude of parameters like the radius for spheres & cylinders, the major or minor halfaxes for ellipsoids etc.

I'd appreciate to hear your thoughts on this or if somebody has a simpler solution I'd be happy to discuss this as well.

Thanks a lot
Chris