A matrix and df_dx

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Hi, 

First of all, I would like to thank you for your open source. 

While I was looking at your code, I got some questions regarding A matrix and 
df_dx in implicit solver. 

In you code, number of non-zero elements in A matrix is always equal to the 
total number vertices(total_points). I believe it should have more non-zero 
elements. 

In terms of Jacobian matrix df_dx, let's say Kij is an element of Jacobian 
matrix and Kij = df_i / dx_j (i, j are vertex indices). The element of Jacobian 
matrix becomes non-zero when vertex i and j are linked by an edge(spring). In 
case i == j, Kii is always non-zero and Kii = -sum of Kij where j is linked to 
i and j != i. 

Implicit solver is supposed to handle very large stiffness such as 10^4 with 
natural gravity value.

Thank you.

Original issue reported on code.google.com by [email protected] on 21 Mar 2012 at 5:32