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mass-spring baraff-witkin implicit solver

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Hi

I was wondering if anyone knows of a simple, say 3x3 matrix, example of formulating the mass-spring implicit solver as described in 1998 Baraff-Witkin paper "Large steps in Cloth Simulation"?

I'm trying to code equation (6) of the paper, which is a linear [A]{dv}= but don't undertsnad how I don't end up simply with a diagonal matrix.

The identity matrix is diagonal. The inverse mass-matrix gives a diagonal term for constant mass and the derivatives df/dv and df/fx at a particular point, i, can be computed at each time step.

Without the non-diagonal elements I can't see how I arrive a coupled system such as when solving FE stiffness matrices.

Thanks for any help.

Graham

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It looks like the non-diagonal parts are the derivatives. The derivative of a vector valued function on a vector input at a point is a non-diagonal matrix: http://en.wikipedia.org/wiki/Jacobian_matrix

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