I have been trying to understand "Fast Simulation of Mass-Spring Systems" by Liu et al., which can be found here. There is one part in the methods section that is confusing me. Just after equation 12, it states:
Similarly, Si ∈ Rs is the i-th spring indicator, i.e., Si,j = δi,j
The subscript i is for each spring in the system. But I am not sure what δ is supposed to be - it isn't mentioned anywhere else in the paper. I also don't know what the subscript j is for, or what a spring indicator is. My guess was that S is a diagonal matrix, with the deflection of each spring along the diagonal. But this doesn't work in my code. Having S as just the identity matrix works better.
Does anyone know what the S matrix is supposed to be?